Title: Effect of leg immersion in mild warm carbonated water on skin and muscle blood flow

Authors: Ogoh S, Washio T, Suzuki K, Ikeda K, Hori T, Olesen ND, Muraoka Y.

Journal: Physiol Rep. 2018 Sep;6(18):e13859. doi: 10.14814/phy2.13859. PMID: 30221833; PMCID: PMC6139710.

Link to full text: Effect of leg immersion in mild warm carbonated water on skin and muscle blood flow

Abstract: Leg immersion in carbonated water improves endothelial-mediated vasodilator function and decreases arterial stiffness but the mechanism underlying this effect remains poorly defined. We hypothesized that carbonated water immersion increases muscle blood flow. To test this hypothesis, 10 men (age 21 ± 0 years; mean ± SD) underwent lower leg immersion in tap or carbonated water at 38°C. We evaluated gastrocnemius muscle oxyhemoglobin concentration and tissue oxygenation index using near-infrared spectroscopy, skin blood flow by laser Doppler flowmetry, and popliteal artery (PA) blood flow by duplex ultrasound. Immersion in carbonated, but not tap water elevated PA (from 38 ± 14 to 83 ± 31 mL/min; P < 0.001) and skin blood flow (by 779 ± 312%, P < 0.001). In contrast, lower leg immersion elevated oxyhemoglobin concentration and tissue oxygenation index with no effect of carbonation (P = 0.529 and P = 0.495). In addition, the change in PA blood flow in response to immersion in carbonated water correlated with those of skin blood flow (P = 0.005) but not oxyhemoglobin concentration (P = 0.765) and tissue oxygenation index (P = 0.136) while no relations was found for tap water immersion. These findings indicate that water carbonation has minimal effect on muscle blood flow. Furthermore, PA blood flow increases in response to lower leg immersion in carbonated water likely due to a large increase in skin blood flow.

Title: Carbonate Ion-Enriched Hot Spring Water Promotes Skin Wound Healing in Nude Rats

Authors: Liang J, Kang D, Wang Y, Yu Y, Fan J, Takashi E.

Journal: PLoS One. 2015 Feb 11;10(2):e0117106. doi: 10.1371/journal.pone.0117106. PMID: 25671581; PMCID: PMC4324962.

Link to full text: Carbonate Ion-Enriched Hot Spring Water Promotes Skin Wound Healing in Nude Rats

Abstract: Hot spring or hot spa bathing (Onsen) is a traditional therapy for the treatment of certain ailments. There is a common belief that hot spring bathing has therapeutic effects for wound healing, yet the underlying molecular mechanisms remain unclear. To examine this hypothesis, we investigated the effects of Nagano hot spring water (rich in carbonate ion, 42°C) on the healing process of the skin using a nude rat skin wound model. We found that hot spring bathing led to an enhanced healing speed compared to both the unbathed and hot-water (42°C) control groups. Histologically, the hot spring water group showed increased vessel density and reduced inflammatory cells in the granulation tissue of the wound area. Real-time RT-PCR analysis along with zymography revealed that the wound area of the hot spring water group exhibited a higher expression of matrix metalloproteinases-2 and -9 compared to the two other control groups. Furthermore, we found that the enhanced wound healing process induced by the carbonate ion-enriched hot spring water was mediated by thermal insulation and moisture maintenance. Our results provide the evidence that carbonate ion-enriched hot spring water is beneficial for the treatment of skin wounds.

Title: A Novel Bathing Therapeutic Approach for Diabetic Foot Ulcers

Authors: Tappia PS, Hiebert B, Sanjanwala R, Komenda P, Sathianathan C, Arneja AS, Ramjiawan B.

Journal: Applied Sciences. 2021; 11(18):8402. https://doi.org/10.3390/app11188402

Link to full text: A Novel Bathing Therapeutic Approach for Diabetic Foot Ulcers

Abstract: Ineffective healing and treatment of foot ulcers can lead to an infection and gangrene of the wound area that ultimately results in the loss of the limb. The incidence of foot ulcers is higher in patients with diabetes, peripheral vascular disease and kidney disease. Accordingly, this study was undertaken to assess the ability of foot bathing in CO2-enriched water to heal foot ulcers. The design was a double-blinded, randomized, placebo-controlled study. Patients with at least one foot ulcer were randomized to receive either a treatment with bath therapy at 37 ± 0.5 °C containing either 1000–1200 ppm CO2-enriched tap water (the intervention) or non-carbonated tap water at 37 ± 0.5 °C (the control group). Treatment was conducted three times/week for 15 min per session for up to 16 weeks for a total of 48 treatment sessions. Before and at the end of every treatment month, wound size, wound area oxygenation and the ankle brachial index were measured. In addition, the McGill pain questionnaire was conducted. Blood was also collected at these time points (for a total of five collections) for the measurement of different biomarkers. While no significant differences (p < 0.05) in the group/time interaction effect were observed, a clear separation within the wound area reduction/wound area/oxygenated Hb outcomes was seen between placebo (control) and treatment (CO2) group. This pilot study is suggestive that bathing in CO2-enriched water may accelerate the healing of foot ulcers.

Title: Effect of Carbon Dioxide Therapy on Diabetic Foot Ulcer

Authors: Shalan, N. , Al-Bazzaz, A. , Al-Ani, I. , Najem, F. and Al-Masri, M.

Journal: Journal of Diabetes Mellitus, 5, 284-289. doi: 10.4236/jdm.2015.54035.

Link to full text: Effect of Carbon Dioxide Therapy on Diabetic Foot Ulcer

Abstract: Ulceration of diabetic foot represents one of the most concerning complications associated with uncontrolled blood sugar in diabetes mellitus. The aim of this study is to evaluate the condition of twenty-two diabetic patients with different degrees of ulceration in their feet after daily secessions of carbon dioxide therapy. Blood flow to the affected foot was measured by Doppler; also the size, color, degree of ulceration and sensation of the ulcerative area were all evaluated. Results showed improvement of blood flow to the affected foot as well as improvement in the sensation and color of the ulcerative area. It was concluded that carbon dioxide therapy of diabetic foot was promising and needed thorough investigation to be brought widely into application.

Title: Cognitive factors in carbon dioxide therapy

Authors: van den Hout MA, Griez E.

Journal: J Psychosom Res. 1982;26(2):209-14. doi: 10.1016/0022-3999(82)90038-1. PMID: 6804625.

Link to PubMed: Cognitive factors in carbon dioxide therapy

Abstract: The effects of CO2 inhalation on subjective state, heart rate and blood pressure were studied in two differently instructed groups, with use of air for placebo. Participants who were told that inhalation would produce a state of pleasant relaxation reported a statistically significant, confirming change when administered CO2, whereas air was found not to alter the subjective state appreciably. Those who had expected unpleasant feelings of tension did show a non-significant change in this respect after CO2 inhalation and no such reaction at all to the placebo. Heart rate and diastolic blood pressure dropped significantly after CO2 intake. Surprisingly, participants were subject to a significant decrease in heart rate upon inhalation of air when relaxation was expected. Finally, a number of theoretical inferences are made and the role of cognitive processes in the clinical use of CO2 therapy is underscored.

Title: Cardiovascular and subjective responses to inhalation of carbon dioxide. A controlled test with anxious patients

Authors: van den Hout MA, Griez E.

Journal: Psychother Psychosom. 1982;37(2):75-82. doi: 10.1159/000287556. PMID: 6812154.

Link to PubMed: Cardiovascular and subjective responses to inhalation of carbon dioxide. A controlled test with anxious patients

Abstract: Cardiovascular and subjective responses to inhalation of a 35% CO2 mixture were studied in a sample of anxious neurotics. Inhalation of air was used as a placebo control treatment. In comparison with the inhalation of air, CO2 produced a significant drop in diastolic blood pressure. A drop in heart rate frequency was observed both after inhalation of CO2 and air. CO2 inhalation did not produce an average decline in subjective feelings of distress. Explanations of these data are offered and they are compared with earlier findings. Some speculations are made on the clinical usefulness of CO2 inhalation in the treatment of anxious neurotics.

Title: Effect of carbon dioxide-enriched water and fresh water on the cutaneous microcirculation and oxygen tension in the skin of the foot.

Authors: Hartmann BR, Bassenge E, Pittler M.

Journal: Angiology. 1997 Apr;48(4):337-43. doi: 10.1177/000331979704800406. PMID: 9112881.

Link to PubMed: Effect of carbon dioxide-enriched water and fresh water on the cutaneous microcirculation and oxygen tension in the skin of the foot.

Abstract: The effects of immersion of the lower leg and foot in fresh water and in carbon dioxide (CO2)-enriched water (1200 mg CO2 per kg water; succinate+sodium bicarbonate: Actibath, KAO Tokyo) on cutaneous circulation, vasomotion and oxygen tension (PO2) were measured by laser Doppler flowmetry and transcutaneous oximetry. On the first of two consecutive days patients were randomly assigned to have the lower extremities immersed in either fresh water or CO2-enriched water under standardized conditions (temperature, 34 degrees C; depth, 35 cm; immersion time, twenty minutes) with concurrent measurement. On the second day patients were switched to the other bath type. For both sets of measurements probes were attached symmetrically to the dorsum of each foot. Included in the study were 18 patients with mild, bilateral, peripheral, occlusive arterial disease (intermittent claudication, femoral or iliac type). During immersion in CO2-enriched water the Doppler laser signal and vasomotion amplitude rose by 300%, while PO2 increased by 10%. These increases were still apparent during the latter part of the measurement period, following withdrawal of the limbs from the bath, while patients were seated and supine. During immersion in fresh water and thereafter the Doppler laser signal was unchanged and the PO2 increase was considerably less marked. The authors were thus able to demonstrate vasodilation and increased oxygen utilization (Bohr effect) resulting from topical CO2 application, and hence, that the use of topical CO2 has an objective basis.

Title: Effects of serial percutaneous application of carbon dioxide in intermittent claudication: results of a controlled trial

Authors: Hartmann BR, Bassenge E, Hartmann M.

Journal: Angiology. 1997 Nov;48(11):957-63. doi: 10.1177/000331979704801104. PMID: 9373047.

Link to full text: Effects of serial percutaneous application of carbon dioxide in intermittent claudication: results of a controlled trial

Abstract: In a prospective, controlled clinical trial, serial application of carbon-dioxide-enriched water was compared with fresh water. Twenty-four patients with peripheral arterial occlusive disease (stable claudication) were randomly allocated to one of two serial intervention groups, lower extremities immersed in either fresh water or in CO2-enriched water (1000 mg CO2/kg) water under standardized conditions (temperature, 33 degrees C; depth, 40 cm; immersion time, 30 min; five times a week over 4 weeks). The serial application of carbon-dioxide-enriched water increased arterial peak flow (reactive hyperemia), transcutaneous oxygen tension (basal value and half-recovery-time), and pain-free walking distance. The serial fresh water application did not change these values. The authors conclude that serial carbon dioxide application can be clinically effective in patients with arterial obstructions in the lower extremities.

Title: Vasomotor effects of transcutaneous CO2 in stage II peripheral occlusive arterial disease

Authors: Savin E, Bailliart O, Bonnin P, Bedu M, Cheynel J, Coudert J, Martineaud JP.

Journal: Angiology. 1995 Sep;46(9):785-91. doi: 10.1177/000331979504600904. PMID: 7661381.

Link to full text: Vasomotor effects of transcutaneous CO2 in stage II peripheral occlusive arterial disease

Abstract: Vasomotor effects of skin exposure to carbon dioxide (CO2) have been described in normal subjects. It was of interest, therefore, to determine whether percutaneous CO2 is of therapeutic benefit. In a randomized, double-blind study, 10 patients with lower limb arteriopathy (stage II) were investigated before and after local exposure for twenty minutes to CO2-rich spa gas or to water-vapor-saturated air at the same temperature as that CO2-rich spa gas. Brachial and femoral blood flows, brachial and posterior tibial artery pressures, heart rate, and chest and foot transcutaneous oxygen tensions (tcPO2) were determined. Femoral blood flow, tibial pressure, and foot tcPO2 were significantly increased after exposure of the skin to CO2-rich spa gas. This effect was not accompanied with systemic hemodynamic modifications. Water-vapor-saturated air had no effect. These results suggest that transfer of CO2 across the skin can have beneficial local vasomotor effects in patients with lower limb stage II arteriopathy.

Title: The effect of carbonated mineral water and mofette treatment in Baile Tusnad after ischemic stroke – a case report

Authors: Dogaru, G., Marieta, M., Bulboacă, A.E., Ciumărnean, L., & Stanescu, I.C.

Journal:

Link to full text: The effect of carbonated mineral water and mofette treatment in Baile Tusnad after ischemic stroke – a case report

Abstract: Carbon dioxide baths might represent an effective therapeutic method in the rehabilitation of coronary heart disease, myocardial infarction and stroke, as well as in the treatment of chronic venous insufficiency, inflammatory diseases and functional disorders. According to the World Health Organization, 5.5 million deaths from stroke were recorded in 2001, and about 15 million people survive stroke every year. Mortality from stroke is 11% for women and 8.4% for men. According to the European Association for Cardiovascular Prevention Rehabilitation, phase II and III cardiovascular rehabilitation is performed in Romania only in a proportion of 10%. The therapeutic effects of carbonated mineral waters are due to the action of carbon dioxide. This induces cutaneous vasodilation, with a decrease in blood pressure values. It also causes an increase of cardiac output, while reducing blood pressure and heart rate. Mofettes are natural emanations along the Harghita volcanic massif, which contain CO2 in concentrations of 90-98% with cutaneous vasodilator effects, increasing cerebral and muscle blood flow. The natural therapeutic factors in Baile Tusnad, consisting of carbonated mineral water baths, mofettes, climate therapy, along with medical physical culture, indicated in the rehabilitation treatment of post-stroke patients had a beneficial effect on clinical and functional symptomatology, improving the quality of gait and balance, functionality and exercise capacity in a patient who suffered stroke five years before and was followed up for three years, while she attended an annual medical rehabilitation program in Baile Tusnad. Continuing medical rehabilitation programs, in the absence of contraindications, in Romanian spa resorts for cardiovascular treatment, as well as conducting randomized clinical studies on the efficiency of these treatments is important.

Title: Effects of inhaled carbon dioxide and oxygen on cheyne-stokes respiration in patients with heart failure

Authors: Lorenzi-Filho G, Rankin F, Bies I, Douglas Bradley T.

Journal: Am J Respir Crit Care Med. 1999 May;159(5 Pt 1):1490-8. doi: 10.1164/ajrccm.159.5.9810040. PMID: 10228116.

Link to full text: Effects of inhaled carbon dioxide and oxygen on cheyne-stokes respiration in patients with heart failure

Abstract: We hypothesized that in patients with congestive heart failure (CHF), reductions in PaCO2 sensed at the peripheral chemoreceptors trigger central apneas during Cheyne-Stokes respiration (CSR-CSA), and that raising PaCO2 by inhalation of a CO2 would eliminate these events. The effects of CO2 inhalation on the frequency of apneas and hypopneas during stage 2 (S2) sleep were studied in 10 CHF patients with CSR-CSA. The time from the breath with the minimal end tidal fraction of CO2 (FETCO2) during hyperpnea until the onset of apnea correlated strongly with the lung to ear circulation time (LECT) (r2 = 0.90, p < 0.0001), a measure of lung to carotid body circulatory delay. Among the six patients who also inhaled O2, CO2 inhalation increased transcutaneous PCO2 (PtcCO2) (36.4 +/- 4.6 mm Hg versus 38 +/- 4.4 mm Hg, p < 0.002), abolished central apneas and hypopneas (43.0 +/- 8.4 per hour on air versus 1.6 +/- 2.6 per hour on CO2, p < 0.0001), and increased SaO2. In contrast, O2 inhalation causing a similar rise in SaO2 had no significant impact on either PtcCO2 or the frequency of central events. We conclude that central apneas in patients with CHF are triggered by a low PaCO2 most likely sensed at the peripheral chemoreceptors, and that inhalation of CO2 reverses central apneas by raising PaCO2.

Title: Acute effects on cardiovascular oscillations during controlled slow yogic breathing

Authors: Bhagat OL, Kharya C, Jaryal A, Deepak KK.

Journal: Indian J Med Res. 2017 Apr;145(4):503-512. doi: 10.4103/ijmr.IJMR_830_15. PMID: 28862183; PMCID: PMC5663165.

Link to full text: Acute effects on cardiovascular oscillations during controlled slow yogic breathing

Abstract: Background & objectives: Breathing exercises are believed to modulate the cardiovascular oscillations in the body. To assess the validity of the assumption and understand the underlying mechanism, the key autonomic regulatory parameters such as heart rate variability (HRV), blood pressure variability (BPV) and baroreflex sensitivity (BRS) were recorded during controlled slow yogic breathing. Alternate nostril breathing (ANB) was selected as the yogic manoeuvre. Methods: Twelve healthy volunteers (age 30±3.8 yr) participated in the study. ANB was performed at a breathing frequency of 5 breaths per minute (bpm). In each participant, the electrocardiogram, respiratory movements, beat-to-beat BP and end-tidal carbon dioxide were recorded for five minutes each: before, during and after ANB. The records were analyzed for HRV, BPV and BRS. Results: During ANB, HRV analysis showed significant increase in the standard deviation of all NN intervals, low-frequency (LF) component, LF/HF (low frequency/high frequency) ratio and significant decrease in the HF component. BPV analysis showed a significant increase in total power in systolic BPV (SBPV), diastolic BPV (DBPV) and mean BPV. BRS analysis showed a significant increase in the total number of sequences in SBPV and DBPV and significant augmentation of α-LF and reduction in α-HF. The power spectrum showed a dominant peak in HRV at 0.08 Hz (LF component) similar to the respiratory frequency. The acute short-term change in circulatory control system declined immediately after the cessation of slow yogic breathing (ANB) and remained elevated in post-ANB stage as compared to the pre-ANB. Interpretation & conclusions: Significant increase in cardiovascular oscillations and baroreflex recruitments during-ANB suggested a dynamic interaction between respiratory and cardiovascular system. Enhanced phasic relationship with some delay indicated the complexity of the system. It indicated that respiratory and cardiovascular oscillations were coupled through multiple regulatory mechanisms, such as mechanical coupling, baroreflex and central cardiovascular control.

Title: The effect of transcutaneous application of carbon dioxide (CO2) on skeletal muscle

Authors: Oe K, Ueha T, Sakai Y, Niikura T, Lee SY, Koh A, Hasegawa T, Tanaka M, Miwa M, Kurosaka M.

Journal: Biochem Biophys Res Commun. 2011 Apr 1;407(1):148-52. doi: 10.1016/j.bbrc.2011.02.128. Epub 2011 Mar 1. PMID: 21371433.

Link to PubMed: The effect of transcutaneous application of carbon dioxide (CO2) on skeletal muscle

Abstract: In Europe, carbon dioxide therapy has been used for cardiac disease and skin problems for a long time. However there have been few reports investigating the effects of carbon dioxide therapy on skeletal muscle. Peroxisome proliferators-activated receptor (PPAR)-gamma coactivator-1 (PGC-1α) is up-regulated as a result of exercise and mediates known responses to exercise, such as mitochondrial biogenesis and muscle fiber-type switching, and neovascularization via up-regulation of vascular endothelial growth factor (VEGF). It is also known that silent mating type information regulation 2 homologs 1 (SIRT1) enhances PGC-1α-mediated muscle fiber-type switching. Previously, we demonstrated transcutaneous application of CO2 increased blood flow and a partial increase of O2 pressure in the local tissue known as the Bohr effect. In this study, we transcutaneously applied CO2 to the lower limbs of rats, and investigated the effect on the fast muscle, tibialis anterior (TA) muscle. The transcutaneous CO2 application caused: (1) the gene expression of PGC-1α, silent mating type information regulation 2 homologs 1 (SIRT1) and VEGF, and increased the number of mitochondria, as proven by real-time PCR and immunohistochemistry, (2) muscle fiber switching in the TA muscle, as proven by isolation of myosin heavy chain and ATPase staining. Our results suggest the transcutaneous application of CO2 may have therapeutic potential for muscular strength recovery resulting from disuse atrophy in post-operative patients and the elderly population.

Title: Improved exercise ventilatory efficiency with nasal compared to oral breathing in cardiac patients

Authors: Eser P, Calamai P, Kalberer A, Stuetz L, Huber S, Kaesermann D, Guler S, Wilhelm M.

Journal: Front Physiol. 2024 Aug 6;15:1380562. doi: 10.3389/fphys.2024.1380562.PMID: 39165283; PMCID: PMC11334221.

Link to full text: Improved exercise ventilatory efficiency with nasal compared to oral breathing in cardiac patients

Abstract: Objectives: To assess whether nasal breathing improves exercise ventilatory efficiency in patients with heart failure (HF) or chronic coronary syndromes (CCS). Background: Exercise inefficient ventilation predicts disease progression and mortality in patients with cardiovascular diseases. In healthy people, improved ventilatory efficiency with nasal compared to oral breathing was found. Methods: Four study groups were recruited: Patients with HF, patients with CCS, old (age≥45 years) and young (age 20-40 years) healthy control subjects. After a 3-min warm-up, measurements of 5 min with once nasal and once oral breathing were performed in randomized order at 50% peak power on cycle ergometer. Ventilation and gas exchange parameters measured with spiroergometry were analysed by Wilcoxon paired-sample tests and linear mixed models adjusted for sex, height, weight and test order. Results: Groups comprised 15 HF, CCS, and young control and 12 old control. Ventilation/carbon dioxide production (V E/V CO2), ventilation (V E), breathing frequency (fR), and end-tidal oxygen partial pressure (PETO2) were significantly lower and tidal volume and end-tidal carbon dioxide partial pressure (PETCO2) significantly higher during nasal compared to oral breathing in all groups, with large effect sizes for most parameters. For patients with HF, median V E/V CO2 was 35% lower, fR 26% lower, and PETCO2 10% higher with nasal compared to oral breathing, respectively. Exercise oscillatory ventilation (EOV) was present in 6 patients and markedly reduced with nasal breathing. Conclusion: Nasal breathing during submaximal exercise significantly improved ventilatory efficiency and abnormal breathing patterns (rapid shallow breathing and EOV) in 80% of our patients with HF and CCS.

Title: Increase in carbon dioxide accelerates the performance of endurance exercise in rats

Authors: Ueha T, Oe K, Miwa M, Hasegawa T, Koh A, Nishimoto H, Lee SY, Niikura T, Kurosaka M, Kuroda R, Sakai Y.

Journal: J Physiol Sci. 2018 Jul;68(4):463-470. doi: 10.1007/s12576-017-0548-6. Epub 2017 Jun 10. PMID: 28601950; PMCID: PMC10717130.

Link to full text: Increase in carbon dioxide accelerates the performance of endurance exercise in rats

Abstract: Endurance exercise generates CO2 via aerobic metabolism; however, its role remains unclear. Exogenous CO2 by transcutaneous delivery promotes muscle fibre-type switching to increase endurance power in skeletal muscles. Here we determined the performance of rats running in activity wheels with/without transcutaneous CO2 exposure to clarify its effect on endurance exercise and recovery from muscle fatigue. Rats were randomised to control, training and CO2 groups. Endurance exercise included activity-wheel running with/without transcutaneous CO2 delivery. Running performance was measured after exercise initiation. We also analysed changes in muscle weight and muscle fibres in the tibialis anterior muscle. Running performance improved over the treatment period in the CO2 group, with a concomitant switch in muscle fibres to slow-type. The mitochondrial DNA content and capillary density in the CO2 group increased. CO2 was beneficial for performance and muscle development during endurance exercise: it may enhance recovery from fatigue and support anabolic metabolism in skeletal muscles.

Title: Effects of an artificially carbonated bath on athletic warm-up

Authors: Akamine T, Taguchi N

Journal: J Hum Ergol (Tokyo). 1998 Dec;27(1-2):22-9. PMID: 11579696.

Link to full text: Effects of an artificially carbonated bath on athletic warm-up

Abstract: The effects of an artificially carbonated bath (36 degrees C, CO2 300 ppm, 20 minutes) on the warm-up of swimmers was compared with those of a freshwater bath (36 degrees C, 20 minutes). Carbon dioxide is reported to have a vasodilatory effect on peripheral blood vessels of cutaneous and muscular tissue and to promote blood flow. We observed that the warm-up effects of a carbonated bath before swimming on the hematocrit, white blood cell, total plasma protein, and total cholesterol levels in venous blood were significantly increased more than those of a freshwater bath before swimming in recovery period (p < 0.05). Thus the carbonated bath tended to be more effective for increasing the concentrations of blood components. In the recovery period, the carbonated bath before swimming also resulted in significantly smaller changes in blood lactic acid and heart rate than those of a freshwater bath before swimming (p < 0.05). The decrease in electromyography of the M. rectus femoris during swimming suggested more efficient muscle activity after a carbonated bath. Therefore after a carbonated bath, swimmers should have a higher reserve left in the cardiovascular system, resulting in better performance during swimming and less accumulation of fatigue-related metabolites after swimming.

Title: Enhanced Athletic Recovery Using Cold-Water Immersion with Dissolved CO2 and H2

Authors: Yoshimura M, Fukuoka Y, Sawada Y, Ichikawa H, Nakamura M

Journal: International Journal of Physical Medicine & Rehabilitation

Link to full text: Enhanced Athletic Recovery Using Cold-Water Immersion with Dissolved CO2 and H2

Abstract: A Cold therapy is widely recognized as a prescription for acute-phase recovery in athletes. However, it has been suggested that vasoconstriction caused by cooling may, in fact, delay the recovery process after exercise. Therefore, we are researching a new cooling method that involves supplementing ice baths with carbon dioxide gas, which has vasodilatory effects, and hydrogen gas, which has anti-inflammatory properties. This new approach aims to improve muscle blood flow and performance while maintaining the benefits of cooling, such as pain alleviation. This mini review provides an overview of the acute recovery effects of this innovative method and its potential applications.

Title: Repeated-sprint training in hypoxia induced by voluntary hypoventilation improves running repeated-sprint ability in rugby players.

Authors: Fornasier-Santos C, Millet GP, Woorons X.

Journal: Eur J Sport Sci. 2018 May;18(4):504-512. doi: 10.1080/17461391.2018.1431312. Epub 2018 Feb 5. PMID: 29400616.

Link to PubMed: Repeated-sprint training in hypoxia induced by voluntary hypoventilation improves running repeated-sprint ability in rugby players.

Abstract: Purpose: The goal of this study was to determine the effects of repeated-sprint training in hypoxia induced by voluntary hypoventilation at low lung volume (VHL) on running repeated-sprint ability (RSA) in team-sport players. Methods: Twenty-one highly trained rugby players performed, over a 4-week period, seven sessions of repeated 40-m sprints either with VHL (RSH-VHL, n = 11) or with normal breathing (RSN, n = 10). Before (Pre-) and after training (Post-), performance was assessed with an RSA test (40-m all-out sprints with a departure every 30 s) until task failure (85% of the reference velocity assessed in an isolated sprint). Results: The number of sprints completed during the RSA test was significantly increased after the training period in RSH-VHL (9.1 ± 2.8 vs. 14.9 ± 5.3; +64%; p < .01) but not in RSN (9.8 ± 2.8 vs. 10.4 ± 4.7; +6%; p = .74). Maximal velocity was not different between Pre- and Post- in both groups whereas the mean velocity decreased in RSN and remained unchanged in RSH-VHL. The mean SpO2 recorded over an entire training session was lower in RSH-VHL than in RSN (90.1 ± 1.4 vs. 95.5 ± 0.5%, p < .01). Conclusion: RSH-VHL appears to be an effective strategy to produce a hypoxic stress and to improve running RSA in team-sport players.

Title: Application of carbon dioxide to the skin and muscle oxygenation of human lower-limb muscle sites during cold water immersion

Authors: Yoshimura M, Hojo T, Yamamoto H, Tachibana M, Nakamura M, Tsutsumi H, Fukuoka Y.

Journal: PeerJ. 2020 Aug 21;8:e9785. doi: 10.7717/peerj.9785. PMID: 32884861; PMCID: PMC7444506.

Link to full text: Application of carbon dioxide to the skin and muscle oxygenation of human lower-limb muscle sites during cold water immersion

Abstract: Background: Cold therapy has the disadvantage of inducing vasoconstriction in arterial and venous capillaries. The effects of carbon dioxide (CO2) hot water depend mainly on not only cutaneous vasodilation but also muscle vasodilation. We examined the effects of artificial CO2 cold water immersion (CCWI) on skin oxygenation and muscle oxygenation and the immersed skin temperature. Subjects and methods: Fifteen healthy young males participated. CO2-rich water containing CO2 >1,150 ppm was prepared using a micro-bubble device. Each subject's single leg was immersed up to the knee in the CO2-rich water (20 °C) for 15 min, followed by a 20-min recovery period. As a control study, a leg of the subject was immersed in cold tap-water at 20 °C (CWI). The skin temperature at the lower leg under water immersion (Tsk-WI) and the subject's thermal sensation at the immersed and non-immersed lower legs were measured throughout the experiment. We simultaneously measured the relative changes of local muscle oxygenation/deoxygenation compared to the basal values (Δoxy[Hb+Mb], Δdeoxy[Hb+Mb], and Δtotal[Hb+Mb]) at rest, which reflected the blood flow in the muscle, and we measured the tissue O2 saturation (StO2) by near-infrared spectroscopy on two regions of the tibialis anterior (TA) and gastrocnemius (GAS) muscles. Results: Compared to the CWI results, the Δoxy[Hb+Mb] and Δtotal[Hb+Mb] in the TA muscle at CCWI were increased and continued at a steady state during the recovery period. In GAS muscle, the Δtotal[Hb+Mb] and Δdeoxy[Hb+Mb] were increased during CCWI compared to CWI. Notably, StO2values in both TA and GAS muscles were significantly increased during CCWI compared to CWI. In addition, compared to the CWI, a significant decrease in Tsk at the immersed leg after the CCWI was maintained until the end of the 20-min recovery, and the significant reduction continued. Discussion: The combination of CO2 and cold water can induce both more increased blood inflow into muscles and volume-related (total heme concentration) changes in deoxy[Hb+Mb] during the recovery period. The Tsk-WI stayed lower with the CCWI compared to the CWI, as it is associated with vasodilation by CO2.

Title: Anaerobic performance after 3-day consecutive CO2-rich cold-water immersion in physically active males

Authors: FFujita M, Yoshimura M, Nakamura M, Hojo T, Fukuoka Y.

Journal: J Exerc Sci Fit. 2022 Apr;20(2):148-154. doi: 10.1016/j.jesf.2022.02.004. Epub 2022 Mar 4. PMID: 35356104; PMCID: PMC8921317.

Link to full text: Anaerobic performance after 3-day consecutive CO2-rich cold-water immersion in physically active males

Abstract:
Background objective: We investigated the effects of a 3-day consecutive CO2-rich cold (20 °C) water immersion (CCWI) following a high-intensity intermittent test (HIIT) on subjects' sublingual temperature (Tsub), blood lactate ([La]b), and heart rate (HR) compared to cold (20 °C) tap-water immersion (CWI) or passive recovery (PAS).

Methods: Thirty-two subjects were randomly allocated into three groups (CCWI, CWI, and PAS), each of which completed 4 consecutive days of cycling experiments. HR, Tsub, and [La]b were recorded on each day of exercise testing (immersion from Day 1 to Day 3 and Day 4). HIIT consisted of 8 sets of 20-sec maximum exercise at an intensity of 120% of VO2max with 10-sec passive rest. The mean and peak power, and peak pedal repetitions (PPR) within HIIT were averaged and the decline in PPR (ΔPPR) from Day 1 to Day 4 was measured.

Results: In CCWI and CWI, HR declined significantly following each immersion, with CCWI showing the larger reduction (p < 0.001). At Day 2, CCWI showed a significantly lower [La]b compared to PAS (p < 0.01). The changes in mean and peak power from Day 1 to Day 4 did not differ among the groups (p = 0.302). ΔPPR of HIIT was significantly correlated with the HR and [La]b values after immersions (ΔPPR-HR: r2 = 0.938, p < 0.001, ΔPPR-[La]b: r2 = 0.999, p < 0.001).

Conclusions: These findings indicate that CCWI is a promising intervention for maintaining peak performance in high-intensity intermittent exercise, which is associated with a reduction in [La]b and HR.

Title: The field study about the effects of artificial CO2-rich cool-water immersion after outdoor sports activity in a hot environment

Authors: Yoshimura M, Nakamura M, Hojo T, Arai A, Fukuoka Y.

Journal: J Exerc Sci Fit. 2023 Jul;21(3):268-274. doi: 10.1016/j.jesf.2023.05.001. Epub 2023 May 10. PMID: 37250065; PMCID: PMC10209124.

Link to full text: The field study about the effects of artificial CO2-rich cool-water immersion after outdoor sports activity in a hot environment

Abstract:
Background/objective: In our previous laboratory experiment (room temperature of 25 °C), CO2-rich cool-water immersion (CCWI) suppressed subjects' core body temperature even during repeated exercise. It is unclear whether the suppression of body temperature elevation would also continue after CCWI in a hot outdoor environment. Herein we investigated the thermal effects of CCWI after regular exercise training in heat on subjects' core temperature (Tcore), three skin temperatures (Tskin), heart rate (HR), and the rate of perceived ice (RPI).

Methods: Thirty-six subjects (25 males, 11 females) were randomly allocated into three groups (CCWI, CWI, and control). After training at their competitive clubs, each subject was immersed up to the chest in CCWI or CWI at 20 °C for 20 min, followed by a 60-min recovery period. Tcore, Tskin, HR, and RPI were measured at the initial rest, the end of immersion, and every 10 min during the recovery period.

Results: Compared to the control, the CCWI subjects' Tcore was significantly lower at 50-60 min after the end of immersion (p < 0.05). Tskin at abdominal and lower-leg regions during the recovery period was maintained at significantly lower values in the CWI and CCWI groups versus control (p < 0.05). The CCWI subjects maintained lower Tskin for a longer time than the CWI subjects.

Conclusions: These findings indicate that CCWI suppresses the rise in body temperatures more than CWI, even in a hot environment, suggesting that CCWI may be a more effective countermeasure against increasing body temperature in a hot outdoor environment.

Title: Breathing Practices for Stress and Anxiety Reduction: Conceptual Framework of Implementation Guidelines Based on a Systematic Review of the Published Literature

Authors: Bentley TGK, D’Andrea-Penna G, Rakic M, Arce N, LaFaille M, Berman R, Cooley K, Sprimont P.

Journal: Brain Sciences. 2023; 13(12):1612. https://doi.org/10.3390/brainsci13121612

Link to full text: Breathing Practices for Stress and Anxiety Reduction: Conceptual Framework of Implementation Guidelines Based on a Systematic Review of the Published Literature

Abstract: Anxiety and stress plague populations worldwide. Voluntary regulated breathing practices offer a tool to address this epidemic. We reviewed the peer-reviewed published literature to evaluate practice components associated with stress reduction effectiveness. We identified 58 clinical studies, comprising 41 RCTs and 17 single-arm, pre-post, non-randomized trials; 5407 total participants were included, with a mean age of 35.9 years and 43.92% female. Among the 72 included interventions, breathing practices significantly reduced stress/anxiety in 54 interventions. Findings were used to develop a conceptual framework of breath practice effectiveness.

While commonly emphasized factors such as specific breath pace, population, and group versus individual practice were not found to be associated with effectiveness, five core components were: avoiding fast-only and <5 min practices; and incorporating human-guided training during initial sessions, multiple sessions, and long-term practice. Following this simple, evidence-based framework can help maximize the stress reduction benefits of breathing practices among broad populations.

Title: Slow-Breathing Curriculum for Stress Reduction in High School Students: Lessons Learned From a Feasibility Pilot

Authors: Bentley TGK, Seeber C, Hightower E, Mackenzie B, Wilson R, Velazquez A, Cheng A, Arce NN, Lorenz KA.

Journal: Front Rehabil Sci. 2022 Jul 1;3:864079. doi: 10.3389/fresc.2022.864079. PMID: 36189008; PMCID: PMC9397716.

Link to full text: Slow-Breathing Curriculum for Stress Reduction in High School Students: Lessons Learned From a Feasibility Pilot

Abstract: Purpose: Nearly one in three US adolescents meet the criteria for anxiety, an issue that has worsened with the COVID-19 pandemic. We developed a video-based slow diaphragmatic breathing stress-reduction curriculum for high school students and evaluated its feasibility, tolerability, and preliminary effectiveness.

Methods: This cluster-randomized feasibility pilot compared 5-min slow diaphragmatic breathing for 5 weeks with treatment-as-usual control among four 12th-grade public high school classes. Students individually participated after school during COVID-19-related hybrid teaching, with slow diaphragmatic breathing three times/week and breath science education once/week. Feasibility was based on completion of breathing exercises, breath science education, and preliminary effectiveness assessments, and ease/tolerability was based on qualitative assessments. Results: Forty-three students consented to participate. Breath practice compliance ranged from 29 to 83% across classes and weeks, and decreased on average over the 5 weeks. Compliance with the breath science videos ranged from 43 to 86%, and that with the weekly STAI-State and CO2TT measures varied from 36 to 86%. Compliance with ease/tolerability assessments ranged from 0 to 60%. Preliminary effectiveness assessments' compliance varied across classes from 83 to 89% during baseline, and 29 to 72% at follow-up. The curriculum was rated as somewhat-to-definitely useful/beneficial, and definitely-to-very easy/tolerable. Students reported enjoying the diaphragmatic breathing, CO2TT, and breath science education; some found the extended exhales challenging and the curriculum and assessments time-consuming. Preliminary effectiveness analyses indicated no significant changes in STAI or CO2TT from baseline to followup or from before to after breathing exercises (p > 0.05 for all).

Conclusions: Implementation of this 5-week slow breathing curriculum was feasible and tolerable to this cohort. Compliance, tolerability, and effectiveness may be improved with in-class participation. Future research on simple and accessible slow-breathing exercises is warranted to address today's adolescent stress-management crisis.

Title: Effects of conscious connected breathing on cortical brain activity, mood and state of consciousness in healthy adults

Authors: Bahi, C., Irrmischer, M., Franken, K. et al.

Journal: Curr Psychol 43, 10578–10589 (2024). https://doi.org/10.1007/s12144-023-05119-6

Link to full text: Effects of conscious connected breathing on cortical brain activity, mood and state of consciousness in healthy adults

Abstract: Breathwork as a means of inducing non-ordinary states of consciousness is gaining traction as a potential therapeutic modality. We examined the effects of breathwork (in the form of connected breathing) on electroencephalography (EEG) and mood in 20 healthy participants (aged between 23 and 39 years (female = 11, Mage = 29). In addition, to compare with other means of inducing non-ordinary states of consciousness, we assessed the subjective effects of breathwork using the 11 Dimension Altered State of Consciousness questionnaire. EEG spectral power analysis of eyes closed rest recordings before and after the breathwork session showed a decrease in delta (1–4 Hz) and theta (4–8 Hz) frequencies in frontotemporal and parietal regions, respectively no changes were seen in Alpha (9–12 Hz) and Beta (12–30 Hz) bands. However, after decomposing the beta waves in Beta 1 (12–15 Hz), Beta 2 (15–20 Hz), Beta 3 (20–30 Hz), decreases in power were observed across Beta1 and Beta 2 in parietotemporal regions. Notably, the spectral power in gamma increased in experienced practitioners. Scores on the Profile of Mood States questionnaire showed a reduction in negative affect (anger, tension, confusion, and depression) and an increase in esteem. Scores on the 11D-ASC scale indicated that subjective experiences during breathwork were similar to those after medium to high doses of psilocybin, suggesting the occurrence of experiences of mystical quality. Present results indicate that breathwork changes brain activity and mood, and induces mystical experiences. These results are promising and suggest that such techniques could be useful to improve mental well-being.

Title: Breathwork interventions for adults with clinically diagnosed anxiety disorders: A scoping review

Authors: Banushi B, Brendle M, Ragnhildstveit A, et al.

Journal: Brain Sci. 2023 Feb 2;13(2):256. doi: 10.3390/brainsci13020256. PMID: 36831799; PMCID: PMC9954474.

Link to full text: Breathwork interventions for adults with clinically diagnosed anxiety disorders: A scoping review

Abstract: Anxiety disorders are the most common group of mental disorders, but they are often underrecognized and undertreated in primary care. Dysfunctional breathing is a hallmark of anxiety disorders; however, mainstays of treatments do not tackle breathing in patients suffering anxiety. This scoping review aims to identify the nature and extent of the available research literature on the efficacy of breathwork interventions for adults with clinically diagnosed anxiety disorders using the DSM-5 classification system. Using the PRISMA extension for scoping reviews, a search of PubMed, Embase, and Scopus was conducted using terms related to anxiety disorders and breathwork interventions. Only clinical studies using breathwork (without the combination of other interventions) and performed on adult patients diagnosed with an anxiety disorder using the DSM-5 classification system were included. From 1081 articles identified across three databases, sixteen were included for the review. A range of breathwork interventions yielded significant improvements in anxiety symptoms in patients clinically diagnosed with anxiety disorders. The results around the role of hyperventilation in treatment of anxiety were contradictory in few of the examined studies. This evidence-based review supports the clinical utility of breathwork interventions and discusses effective treatment options and protocols that are feasible and accessible to patients suffering anxiety. Current gaps in knowledge for future research directions have also been identified.

Title: Transcutaneous Carbon Dioxide Improves Contractures After Spinal Cord Injury in Rats

Authors: Inoue S, Moriyama H, Yakuwa T, Mizuno E, Suzuki R, Nomura M, Sakai Y, Akisue T.

Journal: Clin Orthop Relat Res. 2019 Aug;477(8):1934-1946. doi: 10.1097/CORR.0000000000000808. PMID: 31135536; PMCID: PMC7000005.

Link to full text: Transcutaneous Carbon Dioxide Improves Contractures After Spinal Cord Injury in Rats

Abstract: Background: Joint contractures are a major complication in patients with spinal cord injuries. Positioning, stretching, and physical therapy are advocated to prevent and treat contractures; however, many patients still develop them. Joint motion (exercise) is crucial to correct contractures. Transcutaneous carbon dioxide (CO2) therapy was developed recently, and its effect is similar to that of exercise. This therapy may be an alternative or complementary approach to exercise.

Question/purposes: Using an established model of spinal cord injury in rats with knee flexion contractures, we sought to clarify whether transcutaneous CO2 altered (1) contracture, as measured by ROM; (2) muscular and articular factors contributing to the loss of ROM; (3) fibrosis and fibrosis-related gene expression in muscle; and (4) the morphology of and fibrosis-related protein expression in the joint capsule.

Methods: Thirty-six Wistar rats were divided into three equal groups: caged control, those untreated after spinal cord injury, and those treated with CO2 after spinal cord injury. The rats were treated with CO2 from either the first day (prevention) or 15th day (treatment) after spinal cord injury for 2 or 4 weeks. The hindlimbs of rats in the treated group were exposed to CO2 gas for 20 minutes once daily. Knee extension ROM was measured with a goniometer and was measured again after myotomy. We calculated the muscular and articular factors responsible for contractures by subtracting the post-myotomy ROM from that before myotomy. We also quantified histologic muscle fibrosis and evaluated fibrosis-related genes (collagen Type 1, α1 and transforming growth factor beta) in the biceps femoris muscle with real-time polymerase chain reaction. The synovial intima's length was measured, and the distribution of fibrosis-related proteins (Type I collagen and transforming growth factor beta) in the joint capsule was observed with immunohistochemistry. Knee flexion contractures developed in rats after spinal cord injuries at all timepoints.

Results: CO2 therapy improved limited-extension ROM in the prevention group at 2 weeks (22° ± 2°) and 4 weeks (29° ± 1°) and in the treatment group at 2 weeks (31° ± 1°) compared with untreated rats after spinal cord injuries (35° ± 2°, mean difference, 13°; 39° ± 1°, mean difference, 9°; and 38° ± 1°, mean difference, 7°, respectively) (95% CI, 10.50-14.86, 8.10-10.19, and 4.73-9.01, respectively; all p < 0.001). Muscular factors decreased in treated rats in the prevention group at 2 weeks (8° ± 2°) and 4 weeks (14°± 1°) and in the treatment group at 2 weeks (14 ± 1°) compared with untreated rats (15° ± 1°, 4.85-9.42; 16° ± 1°, 1.24-3.86; and 17° ± 2°, 1.16-5.34, respectively; all p < 0.05). The therapy improved articular factors in the prevention group at 2 weeks (4° ± 1°) and 4 weeks (6° ± 1°) and in the treatment group at 2 weeks (8° ± 1°) compared with untreated rats (10° ± 1°, 4.05-7.05; 12° ± 1°, 5.18-8.02; and 11° ± 2°, 1.73-5.50, respectively; all p < 0.05). CO2 therapy decreased muscle fibrosis in the prevention group at 2 weeks (p < 0.001). The expression of collagen Type 1, α1 mRNA in the biceps femoris decreased in treated rats in the prevention group at 2 and 4 weeks compared with untreated rat (p = 0.002 and p = 0.008, respectively), although there was little difference in the expression of transforming growth factor beta (p > 0.05). CO2 therapy did not improve shortening of the synovial intima at all timepoints (all p > 0.05). CO2 therapy decreased transforming growth factor beta immunolabeling in joint capsules in the rats in the prevention group at 2 weeks. The staining intensity and Type I collagen pattern showed no differences among all groups at all timepoints.

Conclusion: CO2 therapy may be useful for preventing and treating contractures after spinal cord injuries. CO2 therapy particularly appears to be more effective as a prevention and treatment strategy in early-stage contractures before irreversible degeneration occurs, as shown in a rat model.

Title: Role of the medullary raphe nuclei in the respiratory response to CO2

Authors: Dreshaj IA, Haxhiu MA, Martin RJ.

Journal: Respir Physiol. 1998 Jan;111(1):15-23. doi: 10.1016/s0034-5687(97)00110-2. PMID: 9496468.

Link to PubMed: Role of the medullary raphe nuclei in the respiratory response to CO2

Abstract: We characterized the role of neurons within the midline of the medulla oblongata on phrenic and hypoglossal nerve responses to hypercapnia during early-development. Studies were performed on decorticate or anesthetized; vagotomized and mechanically ventilated 14-20 day old piglets. Reversible withdrawal of midline neuronal activity was induced by microinjections of lidocaine (2%, 300 nl; n = 10) and lesioning was caused by microinjections of the neurotoxic agent, ibotenic acid (n = 12), at the same sites. At any given end-tidal CO2, peak phrenic and hypoglossal activities after lidocaine were significantly lower than in the control period (P < 0.01). Similarly, 1-2 h after injections of ibotenic acid, both phrenic and hypoglossal nerve responses to CO2 were significantly lower than in the control period (P < 0.01). The results indicate for the first time that the medullary midline neurons are required for full expression of ventilatory responses to hypercapnia and raise the possibility that dysfunction of these nuclei may contribute to respiratory instability during early postnatal life.

Title: Smaller amygdala is associated with anxiety in patients with panic disorder

Authors: Hayano F, Nakamura M, Asami T, Uehara K, Yoshida T, Roppongi T, Otsuka T, Inoue T, Hirayasu Y.

Journal: Psychiatry Clin Neurosci. 2009 Jun;63(3):266-76. doi: 10.1111/j.1440-1819.2009.01960.x. PMID: 19566756.

Link to full text: Smaller amygdala is associated with anxiety in patients with panic disorder

Abstract: Aims: Anxiety a core feature of panic disorder, is linked to function of the amygdala. Volume alterations in the brain of patients with panic disorder have previously been reported, but there has been no report of amygdala volume association with anxiety.

Methods: Volumes of hippocampus and amygdala were manually measured using magnetic resonance imaging obtained from 27 patients with panic disorder and 30 healthy comparison subjects. In addition the amygdala was focused on, applying small volume correction to optimized voxel-based morphometry (VBM). State-Trait Anxiety Inventory and the NEO Personality Inventory Revised were also used to evaluate anxiety.

Results: Amygdala volumes in both hemispheres were significantly smaller in patients with panic disorder compared with control subjects (left: t = -2.248, d.f. = 55, P = 0.029; right: t = -2.892, d.f. = 55, P = 0.005). VBM showed that structural alteration in the panic disorder group occurred on the corticomedial nuclear group within the right amygdala (coordinates [x,y,z (mm)]: [26,-6,-16], Z score = 3.92, family-wise error-corrected P = 0.002). The state anxiety was negatively correlated with the left amygdala volume in patients with panic disorder (r = -0.545, P = 0.016).

Conclusions: These findings suggested that the smaller volume of the amygdala may be associated with anxiety in panic disorder. Of note, the smaller subregion in the amygdala estimated on VBM could correspond to the corticomedial nuclear group including the central nucleus, which may play a crucial role in panic attack.

Title: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Authors: Johnson PL, Fitz SD, Hollis JH, Moratalla R, Lightman SL, Shekhar A, Lowry CA.

Journal: J Psychopharmacol. 2011 Jan;25(1):26-36. doi: 10.1177/0269881109353464. Epub 2010 Jan 15. PMID: 20080924.

Link to PubMed: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Abstract: Inspiration of air containing high concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) mobilizes respiratory, sympathetic and hypothalamic-pituitary-adrenal axis responses and increases anxiety-like behaviour in rats and humans. Meanwhile the same stimulus induces panic attacks in the majority of panic disorder patients. However, little is known about the neural circuits that regulate these acute effects. In order to determine the effects of acute hypercarbic gas exposure on forebrain and brainstem circuits, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO(2) concentrations (from baseline concentrations up to 20% CO(2)) during a 5 min period. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure of functional cellular responses. Exposing rats to hypercarbic gas increased anxiety-related behaviour and increased numbers of c-Fos-immunoreactive cells in subcortical regions of the brain involved in: (1) the initiation of fear- or anxiety-associated behavioural responses (i.e. the dorsomedial hypothalamus, perifornical nucleus and dorsolateral and ventrolateral periaqueductal gray); (2) mobilization of the hypothalamic-pituitary-adrenal axis (i.e. the dorsomedial hypothalamus, perifornical nucleus and paraventricular hypothalamic nucleus); and (3) initiation of stress-related sympathetic responses (i.e. the dorsomedial hypothalamus, dorsolateral periaqueductal grey and rostroventrolateral medulla). These findings have implications for understanding how the brain senses changes in environmental CO(2) concentrations and the neural mechanisms underlying the subsequent adaptive changes in stress-related physiology and behaviour.

Title: Amygdalar atrophy in panic disorder patients detected by volumetric magnetic resonance imaging

Authors: Massana G, Serra-Grabulosa JM, Salgado-Pineda P, Gastó C, Junqué C, Massana J, Mercader JM, Gómez B, Tobeña A, Salamero M.

Journal: Neuroimage. 2003 May;19(1):80-90. doi: 10.1016/s1053-8119(03)00036-3. PMID: 12781728.

Link to PubMed: Amygdalar atrophy in panic disorder patients detected by volumetric magnetic resonance imaging

Abstract: It has been suggested that the pathophysiology of panic disorder (PD) may involve abnormalities in several brain structures, including the amygdala. To date, however, no study has used quantitative structural neuroimaging techniques to examine amygdalar anatomy in this disorder. Volumetric magnetic resonance imaging (MRI) studies of the amygdalas, hippocampi, and temporal lobes were conducted in 12 drug-free, symptomatic PD patients (six females and six males), and 12 case-matched healthy comparison subjects. Volumetric MRI data were normalized for brain size. PD patients were found to have smaller left-sided and right-sided amygdalar volumes than controls. No differences were found in either hippocampi or temporal lobes. These findings provide new evidence of changes in amygdalar structure in PD and warrant further anatomical and MRI brain studies of patients with this disorder.

Title: Neural Correlates of Competing Fear Behaviors Evoked by an Innately Aversive Stimulus

Authors: Mongeau R, Miller GA, Chiang E, Anderson DJ.

Journal: J Neurosci. 2003 May 1;23(9):3855-68. doi: 10.1523/JNEUROSCI.23-09-03855.2003. PMID: 12736356; PMCID: PMC6742203.

Link to full text: Neural Correlates of Competing Fear Behaviors Evoked by an Innately Aversive Stimulus

Abstract: Environment and experience influence defensive behaviors, but the neural circuits mediating such effects are not well understood. We describe a new experimental model in which either flight or freezing reactions can be elicited from mice by innately aversive ultrasound. Flight and freezing are negatively correlated, suggesting a competition between fear motor systems. An unfamiliar environment or a previous aversive event, moreover, can alter the balance between these behaviors. To identify potential circuits controlling this competition, global activity patterns in the whole brain were surveyed in an unbiased manner by c-fos in situ hybridization, using novel experimental and analytical methods. Mice predominantly displaying freezing behavior had preferential neural activity in the lateral septum ventral and several medial and periventricular hypothalamic nuclei, whereas mice predominantly displaying flight had more activity in cortical, amygdalar, and striatal motor areas, the dorsolateral posterior zone of the hypothalamus, and the vertical limb of the diagonal band. These complementary patterns of c-fos induction, taken together with known connections between these structures, suggest ways in which the brain may mediate the balance between these opponent defensive behaviors.

Title: Panic and the Brainstem: Clues from Neuroimaging Studies

Authors: Perna G, Guerriero G, Brambilla P, Caldirola D.

Journal: CNS Neurol Disord Drug Targets. 2014;13(6):1049-56. doi: 10.2174/1871527313666140612112923. PMID: 24923341.

Link to PubMed: Panic and the Brainstem: Clues from Neuroimaging Studies

Abstract: One of the most influential theories has conceived unexpected panic attack (PA) as a primal defensive reaction to threat within the internal milieu of the body. This theory is based on findings suggesting the involvement of dysfunctional respiratory regulation and/or abnormally sensitive central neural network of carbon dioxide (CO2)/hydrogen ion (H+) chemoreception in PA. Thus, unexpected PA may be related to phylogenetically older brain structures, including the brainstem areas, which process basic functions related to the organism's internal milieu. The brainstem represents a crucial area for homeostatic regulation, including chemoreception and cardio-respiratory control. In addition, the midbrain dorsal periaqueductal gray may be involved in the unconditioned defense reactions to proximal threats, including internal physical stimuli. Our aim was to specifically consider the potential involvement of the brainstem in panic disorder (PD) by a comprehensive review of the available neuroimaging studies. Available data are limited and potentially affected by several limitations. However, preliminary evidence of a role of the brainstem in PD can be found and, secondly, the brainstem serotonergic system seems to be involved in panic modulation with indications of both altered serotonergic receptors and 5-HT transporter bindings. In conclusion, our review suggests that the brainstem may be involved in psychopathology of PD and supports the relevant role of subcortical serotonergic system in panic pathogenesis.

Title: fMRI of fearful facial affect recognition in panic disorder: the cingulate gyrus-amygdala connection

Authors: Pillay SS, Gruber SA, Rogowska J, Simpson N, Yurgelun-Todd DA.

Journal: J Affect Disord. 2006 Aug;94(1-3):173-81. doi: 10.1016/j.jad.2006.04.007. Epub 2006 Jun 16. PMID: 16782207.

Link to PubMed: fMRI of fearful facial affect recognition in panic disorder: the cingulate gyrus-amygdala connection

Abstract: Background: This study investigated cingulate cortex (CC) and amygdala response to fearful facial affect recognition in patients with panic disorder (PD) as measured by BOLD fMRI during the presentation of static facial images.

Methods: Eight patients with PD and eight controls were studied. Scanning was performed on a GE Signa 1.5-T scanner. Echo planar and high-resolution MR images were acquired.

Results: Controls produced greater CC activation compared to patients with PD in response to fearful faces. Furthermore, patients with PD produced less amygdala activation than controls in response to fearful faces. During the neutral face condition, overall activation for the CC was significantly greater in PD patients although anterior cingulate cortex (ACC) activation was not as markedly different between both groups. There were no between group differences in amygdala activation on exposure to the neutral face. Only left CC activation was significantly correlated negatively with HAM-A in PD patients in the fearful facial affect condition.

Limitations: Although comparable to similar studies, the sample size is small enough to warrant further investigation. Also, the effects of medication need to be considered when interpreting these results.

Conclusions: Patients with PD activate the ACC and amygdala significantly less than controls when asked to identify fearful facial affect during fMRI. The higher the anxiety, the lower the left CC activation. Thus, chronic hyperarousal in PD may diminish attentional resources and emotional response reflected in reduced ACC and amygdala activation. Even if these are medication effects, the differences from controls are clinically relevant.

Title: Neuroanatomical Correlates of a Lactate-Induced Anxiety Attack

Authors: Reiman EM, Raichle ME, Robins E, Mintun MA, Fusselman MJ, Fox PT, Price JL, Hackman KA.

Journal: Arch Gen Psychiatry. 1989 Jun;46(6):493-500. doi: 10.1001/archpsyc.1989.01810060013003. PMID: 2786401.

Link to PubMed: Neuroanatomical Correlates of a Lactate-Induced Anxiety Attack

Abstract: Positron emission tomographic measurements of regional blood flow were used to assess local neuronal activity in patients with panic disorder and in normal control subjects before and during the infusion of sodium lactate. A new technique for the analysis of positron emission tomographic data was employed to identify significant changes in regional blood flow associated with lactate infusion in the panicking patients, nonpanicking patients, and controls. Lactate-induced panic was associated with significant blood flow increases bilaterally in the temporal poles; bilaterally in insular cortex, claustrum, or lateral putamen; bilaterally in or near the superior colliculus; and in or near the left anterior cerebellar vermis. Lactate infusion was not associated with significant changes in regional blood flow in the nonpanicking patients or control subjects. Thus, the identified regions seemed to be involved in an anxiety attack.

Title: Amygdala circuitry mediating reversible and bidirectional control of anxiety

Authors: Tye KM, Prakash R, Kim SY, Fenno LE, Grosenick L, Zarabi H, Thompson KR, Gradinaru V, Ramakrishnan C, Deisseroth K.

Journal: Nature. 2011 Mar 17;471(7338):358-62. doi: 10.1038/nature09820. Epub 2011 Mar 9. PMID: 21389985; PMCID: PMC3154022.

Link to full text: Amygdala circuitry mediating reversible and bidirectional control of anxiety

Abstract: Anxiety--a sustained state of heightened apprehension in the absence of immediate threat--becomes severely debilitating in disease states. Anxiety disorders represent the most common of psychiatric diseases (28% lifetime prevalence) and contribute to the aetiology of major depression and substance abuse. Although it has been proposed that the amygdala, a brain region important for emotional processing, has a role in anxiety, the neural mechanisms that control anxiety remain unclear. Here we explore the neural circuits underlying anxiety-related behaviours by using optogenetics with two-photon microscopy, anxiety assays in freely moving mice, and electrophysiology. With the capability of optogenetics to control not only cell types but also specific connections between cells, we observed that temporally precise optogenetic stimulation of basolateral amygdala (BLA) terminals in the central nucleus of the amygdala (CeA)--achieved by viral transduction of the BLA with a codon-optimized channelrhodopsin followed by restricted illumination in the downstream CeA--exerted an acute, reversible anxiolytic effect. Conversely, selective optogenetic inhibition of the same projection with a third-generation halorhodopsin (eNpHR3.0) increased anxiety-related behaviours. Importantly, these effects were not observed with direct optogenetic control of BLA somata, possibly owing to recruitment of antagonistic downstream structures. Together, these results implicate specific BLA-CeA projections as critical circuit elements for acute anxiety control in the mammalian brain, and demonstrate the importance of optogenetically targeting defined projections, beyond simply targeting cell types, in the study of circuit function relevant to neuropsychiatric disease.

Title: Neurobiology of panic and pH chemosensation in the brain

Authors: Wemmie JA.

Journal: Dialogues Clin Neurosci. 2011;13(4):475-83. doi: 10.31887/DCNS.2011.13.4/jwemmie. PMID: 22275852; PMCID: PMC3263394.

Link to full text: Neurobiology of panic and pH chemosensation in the brain

Abstract: Panic disorder is a common and disabling illness for which treatments are too frequently ineffective. Greater knowledge of the underlying biology could aid the discovery of better therapies. Although panic attacks occur unpredictably, the ability to provoke them in the laboratory with challenge protocols provides an opportunity for crucial insight into the neurobiology of panic. Two of the most well-studied panic provocation challenges are CO(2) inhalation and lactate infusion. Although it remains unclear how these challenges provoke panic animal models of CO(2) and lactate action are beginning to emerge, and offer unprecedented opportunities to probe the molecules and circuits underlying panic attacks. Both CO(2) and lactate alter pH balance and may generate acidosis that can influence neuron function through a growing list of pH-sensitive receptors. These observations suggest that a key to better understanding of panic disorder may He in more knowledge of brain pH regulation and pH-sensitive receptors.

Keywords: CO2; chemo-sensation; lactate, brain pH; panic disorder; provocation challenge.

Title: Behavioral and cardiovascular effects of 7.5% CO2 in human volunteers

Authors: Bailey JE, Argyropoulos SV, Kendrick AH, Nutt DJ.

Journal: Depress Anxiety. 2005;21(1):18-25. doi: 10.1002/da.20048. PMID: 15782425.

Link to PubMed: Behavioral and cardiovascular effects of 7.5% CO2 in human volunteers

Abstract: The study of carbon dioxide (CO2) inhalation in psychiatry has a long and varied history, with recent interest in using inhaled CO2 as an experimental tool to explore the neurobiology and treatment of panic disorder. As a consequence, many studies have examined the panic-like response to the gas either using the single or double breath 35% CO2 inhalation or 5-7% CO2 inhaled for 15-20 min, or rebreathing 5% CO2 for a shorter time. However, this lower dose regime produces little physiological or psychological effects in normal volunteers. For this reason we have studied the effects of a higher concentration of CO2, 7.5%, given over 20 min. Twenty healthy volunteers were recruited to a double blind, placebo-controlled study where air and 7.5% CO2 were inhaled for 20 min. Cardiovascular measures and subjective ratings were obtained. When compared to air, inhaling 7.5% CO2 for 20 min increases systolic blood pressure and heart rate, indicating increased autonomic arousal. It also increases ratings of anxiety and fear and other subjective symptoms associated with an anxiety state. The inhalation of 7.5% CO2 for 20 min is safe for use in healthy volunteers and produces robust subjective and objective effects. It seems promising as an anxiety provocation test that could be beneficial in the study of the effects of anxiety on sustained performance, the discovery of novel anxiolytic agents, and the study of brain circuits and mechanisms of anxiety.

Title: The 35% CO2 challenge in panic disorder: optimization by receiver operating characteristic (ROC) analysis

Authors: Battaglia M, Perna G.

Journal: J Psychiatr Res. 1995 Mar-Apr;29(2):111-9. doi: 10.1016/0022-3956(94)00045-s. PMID: 7666379.

Link to PubMed: The 35% CO2 challenge in panic disorder: optimization by receiver operating characteristic (ROC) analysis

Abstract: To test the power of the 35% CO2 test as a challenge for panic disorder (PD), and to set an ideal threshold of discrimination between patients and controls, we analysed by receiver operating characteristic (ROC) analysis the responses of 91 out-patients with PD and 46 controls who inhaled a 35% CO2/65% O2 gas mixture. ROC analysis confirmed that the CO2 challenge discriminates well between PD patients and controls, with 86% probability to classify them correctly on the basis of subjective anxiety after the test. A relatively modest increment in subjective anxiety (i.e. an absolute increment of 20 units, or a relative increment of 26% of subjective anxiety) proved to be the ideal threshold to separate the two groups of our sample.

Title: On the psychotropic effects of carbon dioxide

Authors: Colasanti A, Esquivel G, Schruers KJ, Griez EJ.

Journal: Curr Pharm Des. 2012;18(35):5627-37. doi: 10.2174/138161212803530745. PMID: 22632467.

Link to PubMed: On the psychotropic effects of carbon dioxide

Abstract: It has been well established that the inhalation of Carbon Dioxide (CO2) can induce in humans an emotion closely replicating spontaneous panic attacks, as defined by current psychiatry nosology. The purpose of this review is to provide a critical summary of the data regarding CO2's psychopharmacological properties and underlying mechanisms. The authors review the literature on the human and animal response for the exposure of exogenous CO2 focusing on five points of interest: 1) the early history of the use of CO2 as an anesthetic and therapeutic agent, 2) the subjective effects of breathing CO2 at different concentrations in humans, 3) the use of CO2 in experimental psychiatric research as an experimental model of panic, 4) the pharmacological modulation of CO2-induced responses, and 5) the putative neurobiological mechanisms underlying the affective state induced by CO2. The authors conclude with an evolutionary-inspired notion that CO2 might act as an agent of a primal emotion serving a homeostatic function, in the control of respiration and acid-base balance.

Title: Body fluids after CO2 inhalation: insight into panic mechanisms?

Authors: Griez E, van den Hout MA, Verstappen F.

Journal: Eur Arch Psychiatry Neurol Sci. 1987;236(6):369-71. doi: 10.1007/BF00377427. PMID: 3119345.

Link to PubMed: Body fluids after CO2 inhalation: insight into panic mechanisms?

Abstract: Blood gases and electrolyte fluctuations were studied in arterial blood, after a single 35% CO2-65% O2 inhalation, which is known to trigger panic attacks in patients with panic disorder. The immediate effect of this maneuver was a brief hypercapnic acidosis followed by a slight alkalotic rebound, with shifts in Ca2+ and K+. The possible effect of these changes on neuronal membrane excitability is discussed, referring to recent experimental findings in panic provocation.

Title: A Long-Term Prospective Evaluation of First-Degree Relatives of Panic Patients Who Underwent the 35% CO2 Challenge

Authors: Perna G, Cocchi S, Allevi L, Bussi R, Bellodi L.

Journal: Biol Psychiatry. 1999 Feb 1;45(3):365-7. doi: 10.1016/s0006-3223(98)00030-4. PMID: 10023515.

Link to PubMed: A Long-Term Prospective Evaluation of First-Degree Relatives of Panic Patients Who Underwent the 35% CO2 Challenge

Abstract: This follow-up study investigated the potential priming effect of the 35% CO2 challenge on the development of anxiety disorders and/or panic attacks in healthy first-degree relatives of panic patients across a period of 3-4 years subsequent to the challenge. Thirty-one relatives who underwent the 35% CO2 challenge 3-4 years before and 14 relatives, free from psychiatric diagnoses in the same period, were directly reevaluated for the presence of anxiety disorders and panic attacks. None developed anxiety disorders and only 1, among relatives previously tested with the 35% CO2 challenge, reported sporadic panic attacks. The 35% CO2 challenge is a safe research paradigm in the investigation of healthy subjects with a familial vulnerability to panic, and CO2 hypersensitivity might be considered a trait marker of an underlying familial vulnerability to panic disorder.

Title: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Authors: Roberson-Nay R, Beadel JR, Gorlin EI, Latendresse SJ, Teachman BA.

Journal: J Behav Ther Exp Psychiatry. 2015 Jun;47:68-76. doi: 10.1016/j.jbtep.2014.10.013. Epub 2014 Nov 15. PMID: 25496936; PMCID: PMC4324118.

Link to full text: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Abstract: Carbon dioxide (CO2) hypersensitivity is hypothesized to be a robust endophenotypic marker of panic spectrum vulnerability. The goal of the current study was to explore the latent class trajectories of three primary response systems theoretically associated with CO2 hypersensitivity: subjective anxiety, panic symptoms, and respiratory rate (fR). Participants (n = 376; 56% female) underwent a maintained 7.5% CO2 breathing task that included three phases: baseline, CO2 air breathing, and recovery. Growth mixture modeling was used to compare response classes (1…n) to identify the best-fit model for each marker. Panic correlates also were examined to determine class differences in panic vulnerability. For subjective anxiety ratings, a three-class model was selected, with individuals in one class reporting an acute increase in anxiety during 7.5% CO2 breathing and a return to pre-CO2 levels during recovery. A second, smaller latent class was distinguished by elevated anxiety across all three phases. The third class reported low anxiety reported during room air, a mild increase in anxiety during 7.5% CO2 breathing, and a return to baseline during recovery. Latent class trajectories for fR yielded one class whereas panic symptom response yielded two classes. This study examined CO2 hypersensitivity in one of the largest samples to date, but did not ascertain a general population sample thereby limiting generalizability. Moreover, a true resting baseline measure of fR was not measured. Two classes potentially representing different risk pathways were observed. Implications of results will be discussed in the context of panic risk research.

Title: Five percent carbon dioxide challenge: valid analogue and marker of panic disorder?

Authors: Sanderson WC, Wetzler S.

Journal: Biol Psychiatry. 1990 Apr 1;27(7):689-701. doi: 10.1016/0006-3223(90)90584-o. PMID: 2109638.

Link to PubMed: Five percent carbon dioxide challenge: valid analogue and marker of panic disorder?

Abstract: The administration of 5% carbon dioxide (CO2) to patients with panic disorder (PD) induces a behavioral response similar to a naturally occurring panic attack. This article reviews the literature on the nature and incidence of this response. We conclude that the 5% CO2 challenge test is a valid and useful laboratory analogue of naturally occurring panic attacks, and shows promise as a marker to identify a subset of PD patients. Though further research on reliability, validity, and dose-response effects must be conducted, the CO2 challenge test provides important information regarding the phenomenology of panic states.

Title: Fear Reactivity to Bodily Sensations Among Heavy Smokers and Nonsmokers

Authors: Abrams K, Zvolensky MJ, Dorflinger L, Galatis A, Blank M, Eissenberg T.

Journal: Exp Clin Psychopharmacol. 2008 Jun;16(3):230-9. doi: 10.1037/1064-1297.16.3.230. PMID: 18540783.

Link to PubMed: Fear Reactivity to Bodily Sensations Among Heavy Smokers and Nonsmokers

Abstract: Individuals who smoke are more likely to experience panic attacks and develop panic disorder than those in the general population. One possible explanation is that smokers may experience a heightened fear response to somatic disturbances. To date, few laboratory studies have tested this hypothesis directly. The present study examined 24 adult heavy smokers (10 females) in 12-hr nicotine withdrawal and 24 adult nonsmokers (12 females) on subjective and physiological reactivity to a 4-min carbon dioxide rebreathing challenge. Results indicate that, despite an attenuated acceleration in respiration during the challenge, smokers experienced a significantly greater increase in self-reported panic symptoms than nonsmokers. In addition, smokers reported significantly greater trait levels of suffocation fear prior to the challenge. Findings are discussed with respect to the role of smoking in panic vulnerability.

Title: Interoceptive Fear Conditioning and Panic Disorder: The Role of Conditioned Stimulus–Unconditioned Stimulus Predictability

Authors: Acheson DT, Forsyth JP, Moses E.

Journal: Behav Ther. 2012 Mar;43(1):174-89. doi: 10.1016/j.beth.2011.06.001. Epub 2011 Jun 15. PMID: 22304889.

Link to PubMed: Interoceptive Fear Conditioning and Panic Disorder: The Role of Conditioned Stimulus–Unconditioned Stimulus Predictability

Abstract: Interoceptive fear conditioning is at the core of contemporary behavioral accounts of panic disorder. Yet, to date only one study has attempted to evaluate interoceptive fear conditioning in humans (see Acheson, Forsyth, Prenoveau, & Bouton, 2007). That study used brief (physiologically inert) and longer-duration (panicogenic) inhalations of 20% CO(2)-enriched air as an interoceptive conditioned (CS) and unconditioned (US) stimulus and evaluated fear learning in three conditions: CS only, CS-US paired, and CS-US unpaired. Results showed fear conditioning in the paired condition, and fearful responding and resistance to extinction in an unpaired condition. The authors speculated that such effects may be due to difficulty discriminating between the CS and the US. The aims of the present study are to (a) replicate and expand this line of work using an improved methodology, and (b) clarify the role of CS-US discrimination difficulties in either potentiating or depotentiating fear learning. Healthy participants (N=104) were randomly assigned to one of four conditions: (a) CS only, (b) contingent CS-US pairings, (c) unpaired CS and US presentations, or (d) an unpaired "discrimination" contingency, which included an exteroceptive discrimination cue concurrently with CS onset. Electrodermal and self-report ratings served as indices of conditioned responding. Consistent with expectation, the paired contingency and unpaired contingencies yielded elevated fearful responding to the CS alone. Moreover, adding a discrimination cue to the unpaired contingency effectively attenuated fearful responding. Overall, findings are consistent with modern learning theory accounts of panic and highlight the role of interoceptive conditioning and unpredictability in the etiology of panic disorder.

Title: A genetic study of the acute anxious response to carbon dioxide stimulation in man

Authors: Battaglia M, Ogliari A, Harris J, Spatola CA, Pesenti-Gritti P, Reichborn-Kjennerud T, Torgersen S, Kringlen E, Tambs K.

Journal: J Psychiatr Res. 2007 Dec;41(11):906-17. doi: 10.1016/j.jpsychires.2006.12.002. Epub 2007 Jan 24. PMID: 17254605.

Link to PubMed: A genetic study of the acute anxious response to carbon dioxide stimulation in man

Abstract: People with panic disorder-agoraphobia and their relatives often react anxiously to CO(2)-enriched gas mixtures. Available data are not suited to disentangle genetic from common environmental causes of familial aggregation of CO(2) reactivity, nor provide quantitative estimations of the sources of trait variation. Three-hundred-forty-six twin pairs belonging to the general population-based Norwegian NIPH Mental Health Study underwent self-assessments of anxiety and of DSM-IV panic symptoms after inhalation of a 35%CO(2)-65%O(2) mixture. Two thresholds were employed - at sample's 75th and 90th percentiles of responses - to define provoked panic attacks and to calculate polychoric correlations. Variance components were estimated by structural equation modelling (SEM). For definitions of responses based on the sum of all 13 panic symptoms, SEM could not discriminate between shared environmental versus genetic causes of familial resemblance for provoked attacks. For definitions of responses based on global anxiety, or on the sums of those symptoms (dyspnea, dizziness, palpitations) with highest variance post-CO(2), the best-fitting models indicated additive genetic factors as the sole causes for within-family resemblance. Best-fit heritability estimates ranged from 0.42 to 0.57. Genetic and idiosyncratic environmental factors explain most of individual differences in reactivity to hypercapnia. Within-family similarities for this trait are largely explained by genetic determinants.

Title: A Genetically Informed Study of the Association Between Childhood Separation Anxiety, Sensitivity to CO2, Panic Disorder, and the Effect of Childhood Parental Loss

Authors: Battaglia M, Pesenti-Gritti P, Medland SE, Ogliari A, Tambs K, Spatola CA.

Journal: Arch Gen Psychiatry. 2009 Jan;66(1):64-71. doi: 10.1001/archgenpsychiatry.2008.513. PMID: 19124689.

Link to PubMed: A Genetically Informed Study of the Association Between Childhood Separation Anxiety, Sensitivity to CO2, Panic Disorder, and the Effect of Childhood Parental Loss

Abstract: Context: Childhood separation anxiety disorder can predate panic disorder, which usually begins in early adulthood. Both disorders are associated with heightened sensitivity to inhaled CO(2) and can be influenced by childhood parental loss. Objectives: To find the sources of covariation between childhood separation anxiety disorder, hypersensitivity to CO(2), and panic disorder in adulthood and to measure the effect of childhood parental loss on such covariation. Design: Multivariate twin study. Participants: Seven hundred twelve young adults from the Norwegian Institute of Public Health Twin Panel, a general population cohort. Main outcome measures: Personal direct assessment of lifetime panic disorder through structured psychiatric interviews, history of childhood parental loss, and separation anxiety disorder symptoms. Subjective anxiety response to a 35% CO(2)/65% O(2) inhaled mixture compared with compressed air (placebo). Results: Our best-fitting solution yielded a common pathway model, implying that covariation between separation anxiety in childhood, hypersensitivity to CO(2), and panic disorder in adulthood can be explained by a single latent intervening variable influencing all phenotypes. The latent variable governing the 3 phenotypes' covariation was in turn largely (89%) influenced by genetic factors and childhood parental loss (treated as an identified element of risk acting at a family-wide level), which accounted for the remaining 11% of covariance. Residual variance was explained by 1 specific genetic variance component for separation anxiety disorder and variable-specific unique environmental variance components. Conclusions: Shared genetic determinants appear to be the major underlying cause of the developmental continuity of childhood separation anxiety disorder into adult panic disorder and the association of both disorders with heightened sensitivity to CO(2). Inasmuch as childhood parental loss is a truly environmental risk factor, it can account for a significant additional proportion of the covariation of these 3 developmentally related phenotypes.

Title: CO2-Induced Panic Attacks: A Twin Study

Authors: Bellodi L, Perna G, Caldirola D, Arancio C, Bertani A, Di Bella D.

Journal: Am J Psychiatry. 1998 Sep;155(9):1184-8. doi: 10.1176/ajp.155.9.1184. PMID: 9734540.

Link to PubMed: CO2-Induced Panic Attacks: A Twin Study

Abstract: Objective: The authors investigated the role of genetic factors in 35% CO2-induced panic attacks. Method: Ninety twins recruited from the general population were challenged with one-vital-capacity inhalations of 35% CO2-65% O2. Probandwise concordance rates were calculated and rates compared for monozygotic and for dizygotic twins. Results: A significantly higher concordance was found for 35% CO2-induced panic attacks among monozygotic than dizygotic twins (55.6% versus 12.5%). Conclusions: These results suggest a relevant role of genetic factors in 35% CO2-induced panic attacks.

Title: Panic disorder subtypes: differential responses to CO2 challenge

Authors: Biber B, Alkin T.

Journal: Am J Psychiatry. 1999 May;156(5):739-44. doi: 10.1176/ajp.156.5.739. PMID: 10327907.

Link to PubMed: Panic disorder subtypes: differential responses to CO2 challenge

Abstract: Objective: The purpose of this study was to investigate the possibility of a differential sensitivity to CO2 in patients diagnosed with panic disorder subtypes that were defined by the presence of prominent respiratory symptoms. Method: The authors used a 35% CO2 and 65% O2 mixture as a challenge agent. Fifty-one unmedicated subjects with DSM-III-R panic disorder, who were divided into respiratory (N = 28) and nonrespiratory (N = 23) subtypes by their symptom profiles, underwent a CO2 challenge procedure. Patients in the two groups were compared with regard to physiological and psychological measures, pulmonary function tests, panic rates, and smoking habits. Results: The patients in the respiratory group were significantly more sensitive to CO2 than were the patients in the nonrespiratory group. The respiratory group also had higher scores on the Panic and Agoraphobia Scale and had a longer duration of illness; both of these factors can be indicators of illness severity. In addition, the respiratory group's higher cigarette consumption (mean = 12.46 package-years, SD = 2.49) may have been a contributory factor not only for illness severity but also for the pathogenesis of panic disorder. Conclusions: The CO2 challenge procedure appears to be a good dissection tool in the understanding of different subtypes of panic disorder. Moreover, there may be a more specific association with prominent respiratory symptom subtype and CO2 hypersensitivity.

Title: Experiential, autonomic, and respiratory correlates of CO2 reactivity in individuals with high and low anxiety sensitivity

Authors: Blechert J, Wilhelm FH, Meuret AE, Wilhelm EM, Roth WT.

Journal: Psychiatry Res. 2013 Oct 30;209(3):566-73. doi: 10.1016/j.psychres.2013.02.010. Epub 2013 Mar 13. PMID: 23489596.

Link to PubMed: Experiential, autonomic, and respiratory correlates of CO2 reactivity in individuals with high and low anxiety sensitivity

Abstract: Psychometric studies indicate that anxiety sensitivity (AS) is a risk factor for anxiety disorders such as panic disorder (PD). To better understand the psychophysiological basis of AS and its relation to clinical anxiety, we examined whether high-AS individuals show similarly elevated reactivity to inhalations of carbon dioxide (CO2) as previously reported for PD and social phobia in this task. Healthy individuals with high and low AS were exposed to eight standardized inhalations of 20% CO2-enriched air, preceded and followed by inhalations of room air. Anxiety and dyspnea, in addition to autonomic and respiratory responses were measured every 15 s. Throughout the task, high AS participants showed a respiratory pattern of faster, shallower breathing and reduced inhalation of CO2 indicative of anticipatory or contextual anxiety. In addition, they showed elevated dyspnea responses to the second set of air inhalations accompanied by elevated heart rate, which could be due to sensitization or conditioning. Respiratory abnormalities seem to be common to high AS individuals and PD patients when considering previous findings with this task. Similarly, sensitization or conditioning of anxious and dyspneic symptoms might be common to high AS and clinical anxiety. Respiratory conditionability deserves greater attention in anxiety disorder research.

Title: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Authors: Blechert J, Wilhelm FH, Meuret AE, Wilhelm EM, Roth WT.

Journal: Biol Psychol. 2010 Apr;84(1):104-11. doi: 10.1016/j.biopsycho.2010.01.002. Epub 2010 Jan 12. PMID: 20064582; PMCID: PMC2891596.

Link to full text: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Abstract: Inhalation of carbon dioxide (CO₂) enriched air triggers anxiety in panic disorder (PD) patients, which is often interpreted as a sign of biological vulnerability. However, most studies have not measured respiration in these tasks. We compared patients with PD (n=20) and social phobia (SP, n=19) to healthy controls (n=18) during eight inhalations of 20% CO₂, preceded and followed by two inhalations of room air, while continuously measuring subjective anxiety and dyspnea as well as autonomic and respiratory variables. PD patients showed increased reactivity and delayed recovery during CO₂ inhalations for most measures. Unlike both other groups, the PD group's tidal volume responses did not habituate across CO₂ inhalations. However, PD patients did not differ from SP patients on most other measures, supporting a continuum model of CO₂ sensitivity across anxiety disorders. Both patient groups showed continued reactivity during the last air inhalations, which is unlikely to be due to a biological sensitivity.

Title: The 35% CO2 challenge test in patients with social phobia

Authors: Caldirola D, Perna G, Arancio C, Bertani A, Bellodi L.

Journal: Psychiatry Res. 1997 Jun 16;71(1):41-8. doi: 10.1016/s0165-1781(97)00038-3. PMID: 9247980.

Link to PubMed: The 35% CO2 challenge test in patients with social phobia

Abstract: Panic disorder (PD) and social phobia (SP) share many clinical, demographic and biological characteristics. To investigate the relationships between the two disorders, the responses to inhalation of a 35% carbon-dioxide (CO2) and 65% oxygen (O2) gas mixture were assessed. Sixteen patients with PD, 16 patients with SP, 13 patients with both SP and PD, seven patients with SP who experienced sporadic unexpected panic attacks and 16 healthy control subjects inhaled one vital capacity of 35% CO2 or compressed air. A double-blind, randomized, crossover design was used. PD patients and SP patients showed similar anxiogenic reactions to 35% CO2, both stronger than seen in control subjects. Patients with both disorders and SP patients with sporadic unexpected panic attacks reacted similarly to subjects with PD or SP alone. These results suggest that PD and SP share a common hypersensitivity to CO2 and thus might belong to the same spectrum of vulnerability.

Title: Carbon Dioxide-Induced Emotion and Respiratory Symptoms in Healthy Volunteers

Authors: Colasanti A, Salamon E, Schruers K, van Diest R, van Duinen M, Griez EJ.

Journal: Neuropsychopharmacology. 2008 Dec;33(13):3103-10. doi: 10.1038/npp.2008.31. Epub 2008 Mar 19. PMID: 18354390.

Link to full text: Carbon Dioxide-Induced Emotion and Respiratory Symptoms in Healthy Volunteers

Abstract: A number of evidences have established that panic and respiration are closely related. Clinical studies indicated that respiratory sensations constitute a discrete cluster of panic symptoms and play a major role in the pathophysiology of panic. The aim of the present study was to explore the phenomenology of an experimental model of panic in healthy volunteers based on the hypothesis that: (1) we can isolate discrete clusters of panic symptoms, (2) respiratory symptoms represent a distinct cluster of panic symptoms, and (3) respiratory symptoms are the best predictor of the subjective feeling of panic, as defined in the DSM IV criteria. Sixty-four healthy volunteers received a double inhalation of four mixtures containing 0, 9, 17.5 and 35% CO(2,) respectively, in a double-blind, cross-over, random design. An electronic visual analog scale and the Panic Symptom List (PSL) were used to assess subjective 'fear/discomfort' and panic symptoms, respectively. Statistical analyses consisted of Spearman's correlations, a principal component factor analysis of the 13 PSL symptoms, and linear regressions analyses. The factor analysis extracted three clusters of panic symptoms: respiratory, cognitive, and neurovegetative (r(2)=0.65). Respiratory symptoms were highly related to subjective feeling of fear/discomfort specifically in the CO(2)-enriched condition. Moreover, the respiratory component was the most important predictor of the subjective feeling of 'fear/discomfort' (beta=0.54). The discrete clusters of symptoms observed in this study were similar to those elicited in panic attacks naturally occurring in patients affected by panic disorder. Consistent with the idea that respiration plays a crucial role in the pathophysiology of panic, we found that respiratory symptoms were the best predictors the subjective state defined in the DSM IV criteria for panic.

Title: Hypersensitivity to carbon dioxide as a disease-specific trait marker

Authors: Coryell W.

Journal: Biol Psychiatry. 1997 Feb 1;41(3):259-63. doi: 10.1016/s0006-3223(97)87457-4. PMID: 9024948.

Link to PubMed: Hypersensitivity to carbon dioxide as a disease-specific trait marker

Abstract: There is now substantial evidence that an abnormal threshold for suffocation alarm underlies panic disorder. Because this disorder is highly familial, evidence of an abnormal suffocation threshold may be apparent in high-risk individuals before they develop clinical illness. To explore this possibility, we used a single inhalation of 35% CO2 vs. air to evaluate 11 subjects who had at least one first-degree relative with DSM-III-R panic disorder, 13 who had at least two relatives treated for mania or for depression (HR-AD), and 15 low-risk controls who had no family history of panic disorder, affective disorder, or alcoholism (LR-C). All were aged 18-34 and had no history of panics or of any Research Diagnostic Criteria disorder. Five (45.5%) of the subjects at high risk for panic disorder, but none of the LR-C subjects (p = .007), nor any of the HR-AD subjects (p = .011), developed a panic attack following inhalation of the CO2 mixture.

Title: Anxiety responses to CO2 inhalation in subjects at high-risk for panic disorder

Authors: Coryell W, Pine D, Fyer A, Klein D.

Journal: J Affect Disord. 2006 May;92(1):63-70. doi: 10.1016/j.jad.2005.12.045. Epub 2006 Mar 9. PMID: 16527360.

Link to PubMed: Anxiety responses to CO2 inhalation in subjects at high-risk for panic disorder

Abstract: Background: A number of reports have shown that patients with panic disorder have greater anxiety responses to the inhalation of enhanced carbon dioxide mixtures than do well controls or patients with other psychiatric illnesses. Three earlier studies have shown that well individuals who have first-degree relatives with panic disorder also experience more anxiety following CO(2) than do controls without such a family history. The following was undertaken to confirm and extend these findings. Methods: Well subjects at high risk for panic disorder (

Title: The 35% CO2 Inhalation Procedure: Test–Retest Reliability

Authors: Coryell W, Arndt S.

Journal: Biol Psychiatry. 1999 Apr 1;45(7):923-7. doi: 10.1016/s0006-3223(98)00241-8. PMID: 10202581.

Link to PubMed: The 35% CO2 Inhalation Procedure: Test–Retest Reliability

Abstract: The subjective response to a single-breath, 35% carbon dioxide challenge test shows promise as a tool for the study of panic disorder and may comprise a trait marker for that disorder. Little has been done to measure the reliability of test results, however. Subjects took a single breath at 35% CO2 and completed a self-rating of anxiety symptoms immediately thereafter. This procedure was repeated after a mean interval of 29 days. One group, considered at high risk for panic disorder, consisted of well, first-degree relatives of individuals treated for panic disorder. The control group included well subjects at high risk for affective disorder and subjects who had family histories negative for both affective disorder and panic disorder. On both testing occasions, subjects at high risk for panic disorder had symptom scores that were significantly higher than those of control subjects. Group differences in the portions who experienced a panic attack were dependent on the symptom threshold used to define an attack. A lower threshold was optimal with the second testing and a single, positive test result appeared to be more meaningful than a single negative result. The majority of individual symptom ratings were highly correlated across tests. Ratings for "smothering sensations," in particular, correlated highly across tests and consistently discriminated high-risk from control subjects. The sources of test result variability are unclear and warrant more investigation before the tests can be clinically useful. Research efforts should seek optimal thresholds to define positive test results within given data sets.

Title: Restoring Acid-Sensing Ion Channel-1a in the Amygdala of Knock-Out Mice Rescues Fear Memory But Not Unconditioned Fear Responses

Authors: Coryell MW, Wunsch AM, Haenfler JM, Allen JE, McBride JL, Davidson BL, Wemmie JA.

Journal: J Neurosci. 2008 Dec 17;28(51):13738-41. doi: 10.1523/JNEUROSCI.3907-08.2008. PMID: 19091964; PMCID: PMC2651157.

Link to full text: Restoring Acid-Sensing Ion Channel-1a in the Amygdala of Knock-Out Mice Rescues Fear Memory But Not Unconditioned Fear Responses

Abstract: Acid-sensing ion channel-1a (ASIC1a) contributes to multiple fear behaviors, however the site of ASIC1a action in behavior is not known. To explore a specific location of ASIC1a action, we expressed ASIC1a in the basolateral amygdala of ASIC1a-/- mice using viral vector-mediated gene transfer. This rescued context-dependent fear memory, but not the freezing deficit during training or the unconditioned fear response to predator odor. These data pinpoint the basolateral amygdala as the site where ASIC1a contributes to fear memory. They also discriminate fear memory from fear expressed during training and from unconditioned fear. Furthermore, this work illustrates a strategy for identifying discrete brain regions where specific genes contribute to complex behaviors.

Title: The effects of acceptance versus control contexts on avoidance of panic-related symptoms

Authors: Eifert GH, Heffner M.

Journal: J Behav Ther Exp Psychiatry. 2003 Sep-Dec;34(3-4):293-312. doi: 10.1016/j.jbtep.2003.11.001. PMID: 14972675.

Link to full text: The effects of acceptance versus control contexts on avoidance of panic-related symptoms

Abstract: The present study compared the effects of creating an acceptance versus a control treatment context on the avoidance of aversive interoceptive stimulation. Sixty high anxiety sensitive females were exposed to two 10-min periods of 10% carbon dioxide enriched air, an anxiogenic stimulus. Before each inhalation period, participants underwent a training procedure aimed at encouraging them either to mindfully observe (acceptance context) or to control symptoms via diaphragmatic breathing (control context). A third group was given no particular training or instructions. We hypothesized that an acceptance rather than control context would be more useful in the reduction of anxious avoidance. Compared to control context and no-instruction participants, acceptance context participants were less avoidant behaviorally and reported less intense fear and cognitive symptoms and fewer catastrophic thoughts during the CO(2) inhalations. We discuss the implications of our findings for an acceptance-focused vs. control-focused context when conducting clinical interventions for panic and other anxiety disorders.

Title: The effects of acute exercise and high lactate levels on 35% CO2 challenge in healthy volunteers

Authors: Esquivel G, Schruers K, Kuipers H, Griez E.

Journal: Acta Psychiatr Scand. 2002 Nov;106(5):394-7. doi: 10.1034/j.1600-0447.2002.01333.x. PMID: 12366475.

Link to PubMed: The effects of acute exercise and high lactate levels on 35% CO2 challenge in healthy volunteers

Abstract: Objective: To test the possible antipanic effects of acute exercise in healthy volunteers exposed to an inhalation of 35% CO2 challenge. Method: Twenty healthy subjects in a randomized separate group design, performed exercise in a bicycle ergometer reaching >6 mm of blood lactate and a control condition of minimal activity in the same fashion with no lactate elevation. Immediately afterwards an inhalation of a vital capacity using a mixture of 35% CO2/65% O2 through a mask was given on both conditions. Results: Subjects under the exercise condition reported less panic symptoms than controls after a CO2 challenge on the diagnostic statistical manual-IV (DSM-IV) Panic Symptom List but no difference on the Visual Analogue Anxiety Scale. Conclusion: Subjects under the exertion condition had lactate levels comparable with those of lactate infusions but an inhibitory rather than accumulative effect was seen when combined with a CO2 challenge.

Title: To inhale or not to inhale: Conditioned avoidance in breathing behavior in an odor—20% CO2 paradigm

Authors: Fannes S, Van Diest I, Meulders A, De Peuter S, Vansteenwegen D, Van den Bergh O.

Journal: Biol Psychol. 2008 Apr;78(1):87-92. doi: 10.1016/j.biopsycho.2008.01.003. Epub 2008 Jan 31. PMID: 18316154.

Link to PubMed: To inhale or not to inhale: Conditioned avoidance in breathing behavior in an odor—20% CO2 paradigm

Abstract: This study investigated breathing behavior in an odor-CO2-inhalation fear conditioning paradigm. A differential conditioning paradigm was applied in 55 participants. Both acquisition and extinction consisted of three CS+ and three CS- trials. Diluted ammonia and butyric acid served as conditional odor cues (CSs); inhalation of 20% CO2-enriched as US. The US was presented 10s after CS+ onset and both stimuli co-terminated 30s later. Subjective anxiety and US-expectancy were measured online upon presentation of the CSs. Respiratory behavior showed a biphasic pattern during CS+ acquisition trials. Participants paradoxically lowered their ventilation first; an increased ventilation was observed only towards the end of the trial. Extinction of this breathing inhibition was found. Participants avoiding the CO2 during acquisition did not show a reduction in fear from acquisition to extinction, whereas Non-avoiders did. We conclude that paradoxical decreases in ventilation constitute a relevant behavioral index of fear in CO2-inhalation paradigms.

Title: Emotional avoidance: an experimental test of individual differences and response suppression using biological challenge

Authors: Feldner MT, Zvolensky MJ, Eifert GH, Spira AP.

Journal: Behav Res Ther. 2003 Apr;41(4):403-11. doi: 10.1016/s0005-7967(02)00020-7. PMID: 12643964.

Link to PubMed: Emotional avoidance: an experimental test of individual differences and response suppression using biological challenge

Abstract: The present study examined the affective consequences of response inhibition during a state of anxiety-related physical stress. Forty-eight non-clinical participants were selected on the basis of pre-experimental differences in emotional avoidance (high versus low) and subjected to four inhalations of 20% carbon dioxide-enriched air. Half of the participants were instructed to inhibit the challenge-induced aversive emotional state, whereas the other half was instructed to simply observe their emotional response. Participants high in emotional avoidance compared to those low in emotional avoidance responded with greater levels of anxiety and affective distress but not physiological arousal. Individuals high in emotional avoidance also reported greater levels of anxiety relative to the low emotional avoidance group when suppressing compared to observing bodily sensations. These findings are discussed in terms of the significance of emotional avoidance processes during physical stress, with implications for better understanding the nature of panic disorder.

Title: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Authors: Forsyth JP, Lejuez CW, Finlay C.

Journal: J Behav Ther Exp Psychiatry. 2000 Jun;31(2):103-21. doi: 10.1016/s0005-7916(00)00014-8. PMID: 11132115.

Link to PubMed: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Abstract: Increasingly carbon dioxide-enriched air is being used as an aversive unconditioned stimulus in laboratory examinations of anxiety. Yet, little is known about the stability of the autonomic and subjective effects of this stimulus across repeated inhalations and sessions. We examined whether repeated administrations of high concentrations of CO2-enriched air produced either habituation, stability, or sensitization across several autonomic and self-report indices within one session (Experiment 1) and then several sessions (Experiment 2) of exposure. Results suggest that non-clinical participants do not habituate to CO2 within sessions, but do show habituation on cardiac and subjective report of anxiety across sessions. Individual difference factors such as anxiety sensitivity and suffocation fear seem to moderate some of these effects, including self-reported distress and anxiety in response to the challenge. These results support the use of CO2 as a panicogenic aversive stimulus in laboratory models of fear onset and in clinical settings for interoceptive exposure treatments of panic.

Title: The absence of relation between anxiety sensitivity and fear conditioning using 20% versus 13% CO2-enriched air as unconditioned stimuli

Authors: Forsyth JP, Palav A, Duff K.

Journal: Behav Res Ther. 1999 Feb;37(2):143-53. doi: 10.1016/s0005-7967(98)00113-2. PMID: 9990745.

Link to PubMed: The absence of relation between anxiety sensitivity and fear conditioning using 20% versus 13% CO2-enriched air as unconditioned stimuli

Abstract: Anxiety sensitivity has been implicated as a risk factor in the development and maintenance of anxiety and fear-related disorders. Indeed, persons who score high on the anxiety sensitivity index (ASI) are generally more responsive to biological challenge procedures such as CO2-inhalation that directly evoke the feared bodily events. One would expect, therefore, that persons high on anxiety sensitivity should be more conditionable and hence more likely to acquire fears, than persons low on anxiety sensitivity when CO2-enriched air is used as an unconditioned stimulus (UCS). Undergraduates (N = 96), scoring high, medium and low on the ASI received 8 repeated 20-s inhalations of either 20 or 13% CO2-enriched air (UCSs) paired with one of three CSs differing in fear-relevance (snake, heart and flowers). Several autonomic and self-report measures were assessed. Contrary to expectation, electrodermal and cardiac conditioned responses failed to discriminate between ASI groups. Yet, SUDS and severity and frequency of DSM-IV panic symptoms varied reliably as a function of anxiety sensitivity. Overall, the findings suggest that anxiety sensitivity is related to subjective fear-related complaints, but not autonomic responding and conditionability. We discuss clinical and theoretical implications for understanding the place fo anxiety sensitivity in fear onset.

Title: High-Dose Carbon Dioxide Challenge Test in Anxiety Disorder Patients

Authors: Gorman JM, Papp LA, Martinez J, Goetz RR, Hollander E, Liebowitz MR, Jordan F.

Journal: Biol Psychiatry. 1990 Nov 1;28(9):743-57. doi: 10.1016/0006-3223(90)90510-9. PMID: 2124151.

Link to full text: High-Dose Carbon Dioxide Challenge Test in Anxiety Disorder Patients

Abstract: Many investigators have shown that panic disorder patients and possibly social phobics are hypersensitive to the anxiogenic effects of inhaled carbon dioxide (CO2). In this study we administered double-breath inhalation of 35% CO2 and 65% oxygen (O2) to panic disorder patients, social phobics, and normal controls. At baseline, panic disorder patients were characterized by higher pulse, anxiety score, and evidence of hyperventilation. Panic patients and social phobics panicked more often to 35% CO2 than to room air; normal controls did not have a higher rate of panic to CO2 than to room air. However, we did not find significant group differences in anxiety level, physiological measures, or biochemical measures in response to CO2 breathing compared with room air breathing. These results confirm earlier reports of baseline hyperventilation in panic disorder patients. However, 35% CO2 may be too high a dose to differentiate respiratory responses of patients compared with normals.

Title: Anxiogenic effects of CO2 and hyperventilation in patients with panic disorder

Authors: Gorman JM, Papp LA, Coplan JD, Martinez JM, Lennon S, Goetz RR, Ross D, Klein DF.

Journal: Am J Psychiatry. 1994 Apr;151(4):547-53. doi: 10.1176/ajp.151.4.547. PMID: 8147452.

Link to PubMed: Anxiogenic effects of CO2 and hyperventilation in patients with panic disorder

Abstract: Objective: Previous studies have indicated that patients with panic disorder are more likely than normal subjects to have acute panic attacks during inhalation of CO2, but methodological objections have been raised. In this study the authors attempted to address three of these methodological problems by ensuring that raters who assessed whether panic attacks occurred were blind to subjects' diagnoses, by randomizing the order of administration of 5% CO2 and hyperventilation, and by challenging a greater number of subjects with 7% CO2. Method: Patients with panic disorder and normal subjects underwent 20-minute inhalations of 5% CO2 and 7% CO2 and 15 minutes of room-air hyperventilation. Ratings of panic/no panic during each condition were made separately by an assessor blind to diagnosis and by the subject. Scores on four panic rating scales were also recorded before and after each intervention. Results: Room-air hyperventilation caused panic attacks in a small number of patients; the difference in panic rate between patients and comparison subjects was statistically significant by the subjects' but not by the raters' assessment. Panic rates during 5% CO2 and 7% CO2 were significantly greater among the patients by both assessments; the panic rate was greatest during 7% CO2. Order of administration did not significantly affect panic rates for hyperventilation and 5% CO2. Conclusions: Panic patients were clearly more sensitive to the anxiogenic effects of CO2 than comparison subjects, and CO2 was a more potent anxiogenic stimulus than room-air hyperventilation. Seven percent CO2 discriminated best between patients and comparison subjects and should be the focus of further research.

Title: Response to hyperventilation in a group of patients with panic disorder

Authors: Gorman JM, Askanazi J, Liebowitz MR, Fyer AJ, Stein J, Kinney JM, Klein DF.

Journal: Am J Psychiatry. 1984 Jul;141(7):857-61. doi: 10.1176/ajp.141.7.857. PMID: 6428243.

Link to PubMed: Response to hyperventilation in a group of patients with panic disorder

Abstract: Twelve patients with DSM-III diagnoses of panic disorder and four normal volunteers were studied during hyperventilation of room air and increased ventilation stimulated by 5% CO2. Patients also underwent sodium lactate infusion. Eight of the patients had a panic attack during sodium lactate infusion, seven during CO2 inhalation, and three during room-air hyperventilation. Since CO2 increases the firing of the locus ceruleus, and since sodium lactate infusion is known to increase cerebral CO2, these findings are consistent with other evidence implicating the locus ceruleus in panic anxiety.

Title: The Effect of Successful Treatment on the Emotional and Physiological Response to Carbon Dioxide Inhalation in Patients with Panic Disorder

Authors: Gorman JM, Martinez J, Coplan JD, Kent J, Kleber M.

Journal: Biol Psychiatry. 2004 Dec 1;56(11):862-7. doi: 10.1016/j.biopsych.2004.08.016. PMID: 15576063.

Link to PubMed: The Effect of Successful Treatment on the Emotional and Physiological Response to Carbon Dioxide Inhalation in Patients with Panic Disorder

Abstract: Background: A number of studies have shown that patients with panic disorder are more likely to have panic attacks during carbon dioxide inhalation than are normal comparison subjects. Some studies have shown that antipanic medications can reduce the anxiogenic response to carbon dioxide, but none have shown if this is the case for cognitive behavioral therapy or if successful treatment reduces the respiratory physiologic response to carbon dioxide. Methods: Twenty-five patients with panic disorder and 13 normal comparison subjects underwent baseline testing with 5% and 7% carbon dioxide inhalation. The patients were then retested after at least 12 weeks of treatment with either antipanic medication or cognitive behavioral therapy. Comparison subjects were retested after a similar interval. Results: Successful treatment resulted in lower panic rates, and reduced anxiogenic response. Treatment had no effect, however, on the respiratory physiologic response. Conclusions: There is dissociation in treatment response between the subjective and objective responses to carbon dioxide inhalation in panic disorder patients, with the former but not the latter showing positive change. We hypothesize that the strengthening of higher cortical control over subcortical fear-related structures, whether via medication or cognitive behavioral therapy treatment, results in less anxiety and fear in response to provoked symptoms reminiscent of naturally occurring panic.

Title: Ventilatory physiology of patients with panic disorder

Authors: Gorman JM, Fyer MR, Goetz R, Askanazi J, Liebowitz MR, Fyer AJ, Kinney J, Klein DF.

Journal: Arch Gen Psychiatry. 1988 Jan;45(1):31-9. doi: 10.1001/archpsyc.1988.01800250035006. Erratum in: Arch Gen Psychiatry 1991 Feb;48(2):181. PMID: 2827599.

Link to PubMed: Ventilatory physiology of patients with panic disorder

Abstract: Thirty-one patients with DSM-III panic disorder or agoraphobia with panic attacks, 13 normal controls, and 12 patients with other anxiety disorders were studied during ventilatory challenge with room air hyperventilation and 5% carbon dioxide inhalation. Patients also underwent sodium lactate infusion. Among the patients with panic disorder, 58% panicked with sodium lactate, 39% with 5% CO2, and 23% with room air hyperventilation. Of the other patients, four panicked with sodium lactate, none with 5% CO2, and one with room air hyperventilation. One normal control panicked with both sodium lactate and 5% CO2. Panic with CO2 was associated with an exaggerated ventilatory response and increases in plasma norepinephrine level and diastolic blood pressure. Patients with panic disorder may have hypersensitive CO2 receptors that, when triggered, evoke a subjective panic associated with an exaggerated ventilatory response and consequent hypocapnic alkalosis.

Title: CO2 Vulnerability in Panic Disorder

Authors: Griez EJ, Lousberg H, van den Hout MA, van der Molen GM.

Journal: Psychiatry Res. 1987 Feb;20(2):87-95. doi: 10.1016/0165-1781(87)90001-1. PMID: 3107009.

Link to PubMed: CO2 Vulnerability in Panic Disorder

Abstract: The immediate effects of a single inhalation of a 35% CO2 mixture in oxygen were examined in 12 patients with panic disorders and 11 normal control subjects. Compared to a placebo air inhalation, the CO2 inhalation provoked short-lived autonomic panic symptoms in both patients and normals; it also elicited high subjective anxiety in patients with panic disorders. The latter rated the overall CO2-induced state as very similar to a real-life panic attack.

Title: Carbon Dioxide Inhalation Induces Dose-Dependent and Age-Related Negative Affectivity

Authors: Griez EJ, Colasanti A, van Diest R, Salamon E, Schruers K.

Journal: PLoS One. 2007 Oct 3;2(10):e987. doi: 10.1371/journal.pone.0000987. PMID: 17912364; PMCID: PMC1991589.

Link to full text: Carbon Dioxide Inhalation Induces Dose-Dependent and Age-Related Negative Affectivity

Abstract: Background: Carbon dioxide inhalation is known to induce an emotion similar to spontaneous panic in Panic Disorder patients. The affective response to carbon dioxide in healthy subjects was not clearly characterized yet. Methodology/principal findings: Sixty-four healthy subjects underwent a double inhalation of four mixtures containing respectively 0, 9, 17.5 and 35% CO(2) in compressed air, following a double blind, cross-over, randomized design. Affective responses were assessed according to DSM IV criteria for panic, using an Electronic Visual Analogue Scale and the Panic Symptom List. It was demonstrated that carbon dioxide challenges induced a dose dependent negative affect (p<0.0001). This affect was semantically identical to the DSM IV definition of panic. Older individuals were subjectively less sensitive to Carbon Dioxide (p<0.05). Conclusions/significance: CO(2) induced affectivity may lay on a continuum with pathological panic attacks. Consistent with earlier suggestions that panic is a false biological alarm, the affective response to CO(2) may be part of a protective system triggered by suffocation and acute metabolic distress.

Title: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Authors: Johnson PL, Fitz SD, Hollis JH, Moratalla R, Lightman SL, Shekhar A, Lowry CA.

Journal: J Psychopharmacol. 2011 Jan;25(1):26-36. doi: 10.1177/0269881109353464. Epub 2010 Jan 15. PMID: 20080924.

Link to PubMed: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Abstract: Inspiration of air containing high concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) mobilizes respiratory, sympathetic and hypothalamic-pituitary-adrenal axis responses and increases anxiety-like behaviour in rats and humans. Meanwhile the same stimulus induces panic attacks in the majority of panic disorder patients. However, little is known about the neural circuits that regulate these acute effects. In order to determine the effects of acute hypercarbic gas exposure on forebrain and brainstem circuits, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO(2) concentrations (from baseline concentrations up to 20% CO(2)) during a 5 min period. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure of functional cellular responses. Exposing rats to hypercarbic gas increased anxiety-related behaviour and increased numbers of c-Fos-immunoreactive cells in subcortical regions of the brain involved in: (1) the initiation of fear- or anxiety-associated behavioural responses (i.e. the dorsomedial hypothalamus, perifornical nucleus and dorsolateral and ventrolateral periaqueductal gray); (2) mobilization of the hypothalamic-pituitary-adrenal axis (i.e. the dorsomedial hypothalamus, perifornical nucleus and paraventricular hypothalamic nucleus); and (3) initiation of stress-related sympathetic responses (i.e. the dorsomedial hypothalamus, dorsolateral periaqueductal grey and rostroventrolateral medulla). These findings have implications for understanding how the brain senses changes in environmental CO(2) concentrations and the neural mechanisms underlying the subsequent adaptive changes in stress-related physiology and behaviour.

Title: Neural Pathways Underlying Lactate-Induced Panic

Authors: Johnson PL, Truitt WA, Fitz SD, Lowry CA, Shekhar A.

Journal: Neuropsychopharmacology. 2008 Aug;33(9):2093-107. doi: 10.1038/sj.npp.1301621. Epub 2007 Dec 5. PMID: 18059441; PMCID: PMC3065200.

Link to full text: Neural Pathways Underlying Lactate-Induced Panic

Abstract: Panic disorder is a severe anxiety disorder characterized by susceptibility to induction of panic attacks by subthreshold interoceptive stimuli such as 0.5 M sodium lactate infusions. Although studied for four decades, the mechanism of lactate sensitivity in panic disorder has not been understood. The dorsomedial hypothalamus/perifornical region (DMH/PeF) coordinates rapid mobilization of behavioral, autonomic, respiratory and endocrine responses to stress, and rats with disrupted GABA inhibition in the DMH/PeF exhibit panic-like responses to lactate, similar to panic disorder patients. Utilizing a variety of anatomical and pharmacological methods, we provide evidence that lactate, via osmosensitive periventricular pathways, activates neurons in the compromised DMH/PeF, which relays this signal to forebrain limbic structures such as the bed nucleus of the stria terminalis to mediate anxiety responses, and specific brainstem sympathetic and parasympathetic pathways to mediate the respiratory and cardiovascular components of the panic-like response. Acutely restoring local GABAergic tone in the DMH/PeF blocked lactate-induced panic-like responses. Autonomic panic-like responses appear to be a result of DMH/PeF-mediated mobilization of sympathetic responses (verified with atenolol) and resetting of the parasympathetically mediated baroreflex. Based on our findings, DMH/PeF efferent targets such as the C1 adrenergic neurons, paraventricular hypothalamus, and the central amygdala are implicated in sympathetic mobilization; the nucleus of the solitary tract is implicated in baroreflex resetting; and the parabrachial nucleus is implicated in respiratory responses. These results elucidate neural circuits underlying lactate-induced panic-like responses and the involvement of both sympathetic and parasympathetic systems.

Title: Sex differences in response to a panicogenic challenge procedure: an experimental evaluation of panic vulnerability in a non-clinical sample

Authors: Kelly MM, Forsyth JP, Karekla M.

Journal: Behav Res Ther. 2006 Oct;44(10):1421-30. doi: 10.1016/j.brat.2005.10.012. Epub 2005 Dec 20. PMID: 16364237.

Link to PubMed: Sex differences in response to a panicogenic challenge procedure: an experimental evaluation of panic vulnerability in a non-clinical sample

Abstract: Epidemiological studies show that women are twice as likely as men to develop panic disorder (PD) during their lifetimes. Data from retrospective studies also suggest that women are more distressed by panic and other negative emotional states than men, and that this tendency may precede the development of PD. The present prospective study sought to expand this work by evaluating sex differences in the predisposition to panic in individuals without PD or other forms of psychopathology. Participants (N=96; 52 women) were exposed to 12 panicogenic inhalations of 20% CO2-enriched air while physiological (e.g., heart rate, electrodermal response, Frontalis EMG) and self-report (e.g., subject distress, fear, frequency and severity of DSM-IV panic symptoms) response domains were monitored. As expected, magnitude of autonomic responding failed to reliably discriminate between women and men. Yet, women reported more fear and panic immediately following the challenge procedure relative to men, and this sex difference persisted when assessed again 30 min post-challenge. Implications of these results are discussed broadly in the context of biopsychosocial accounts of sex differences in the clinical presentation of PD.

Title: Carbon dioxide inhalation as a human experimental model of panic: The relationship between emotions and cardiovascular physiology

Authors: Leibold NK, Viechtbauer W, Goossens L, De Cort K, Griez EJ, Myin-Germeys I, Steinbusch HW, van den Hove DL, Schruers KR.

Journal: Biol Psychol. 2013 Oct;94(2):331-40. doi: 10.1016/j.biopsycho.2013.06.004. Epub 2013 Jun 28. PMID: 23816952.

Link to PubMed: Carbon dioxide inhalation as a human experimental model of panic: The relationship between emotions and cardiovascular physiology

Abstract: Inhaling carbon dioxide (CO2)-enriched air induces fear and panic symptoms resembling real-life panic attacks, the hallmark of panic disorder. The present study aimed to describe the emotional and cardiovascular effects evoked by inhaling CO2, taking shortcomings of previous studies into account. Healthy volunteers underwent a double inhalation of 0, 9, 17.5, and 35% CO2, according to a randomized, cross-over design. In addition to fear, discomfort, and panic symptom ratings, blood pressure and heart rate were continuously monitored. Results showed a dose-dependent increase in all self-reports. Systolic and diastolic blood pressure rose with increasing CO2 concentration, whereas heart rate results were less consistent. Diastolic blood pressure and heart rate variation correlated with fear and discomfort. Based on this relationship and the observation that the diastolic blood pressure most accurately mimicked the degree of self-reported emotions, it might serve as a putative biomarker to assess the CO2-reactivity in the future.

Title: Avoidance of 20% carbon dioxide-enriched air with humans

Authors: Lejuez CW, O'Donnell J, Wirth O, Zvolensky MJ, Eifert GH.

Journal: J Exp Anal Behav. 1998 Jul;70(1):79-86. doi: 10.1901/jeab.1998.70-79. PMID: 9684345; PMCID: PMC1284669.

Link to full text: Avoidance of 20% carbon dioxide-enriched air with humans

Abstract: Four college students were exposed to a Sidman avoidance procedure to determine if an avoidance contingency involving 20% carbon dioxide-enriched air (CO2) would produce and maintain responding. In Phase 1, two conditions (contingent and noncontingent) were conducted each day. These conditions were distinguished by the presence or absence of a blue or green box on a computer screen. In the contingent condition, CO2 presentation were delivered every 3 s unless a subject pulled a plunger. Each plunger pull postponed CO2 presentations for 10 s. In the noncontingent condition, CO2 presentations occurred on the average of every 5 min independent of responding. Following stable responding in Phase 1, condition-correlated stimuli were reversed. In both conditions, plunger response rate was high during the contingent condition and low or zero during the noncontingent condition. Furthermore, subjects avoided most CO2 presentations. However, CO2 presentations did not increase verbal reports of fear. Overall, the results from the present study suggest that CO2 can be used effectively in basic studies of aversive control and in laboratory analogues of response patterns commonly referred to as anxiety.

Title: Examination of mask disturbance behavior during a carbon dioxide-enriched air challenge

Authors: Marshall-Berenz EC, Gonzalez A, Leyro TM, Zvolensky MJ.

Journal: J Behav Ther Exp Psychiatry. 2011 Sep;42(3):253-7. doi: 10.1016/j.jbtep.2011.01.004. Epub 2011 Jan 12. PMID: 21349246; PMCID: PMC3086949.

Link to full text: Examination of mask disturbance behavior during a carbon dioxide-enriched air challenge

Abstract: Background and objectives: The present investigation examined mask disturbance behavior in the context of a biological challenge. Method: Participants included 128 adults (63.3% women; M(age) = 23.2, SD = 8.9) who underwent a 10% carbon dioxide-enriched air challenge. Results: Mask disturbance behavior, including complete or partial mask removal, was common during the challenge. Moreover, mask removal behavior during the challenge was significantly related to a greater increase in anxiety pre- to post-challenge, as well as greater levels of self-reported avoidance of a future challenge administration. However, mask touching and lifting behaviors were not related to other challenge criterion variables. Limitations: The current study consisted primarily of young adults and did not include individuals with a panic disorder diagnosis. Replication and extension of the current findings is recommended. Conclusions: Results suggest that mask removal behavior during the challenge may represent a possible behavioral marker of panic-relevant risk, although further work is necessary to determine its usefulness as a challenge measure.

Title: Shared Variance Among Self-Report and Behavioral Measures of Distress Intolerance

Authors: McHugh RK, Daughters SB, Lejuez CW, Murray HW, Hearon BA, Gorka SM, Otto MW.

Journal: Cognit Ther Res. 2011 Jun;35(3):266-275. doi: 10.1007/s10608-010-9295-1. PMID: 23894216; PMCID: PMC3721199.

Link to full text: Shared Variance Among Self-Report and Behavioral Measures of Distress Intolerance

Abstract: Distress intolerance may be an important individual difference variable in understanding maladaptive coping responses across diagnostic categories. However, the measurement of distress intolerance remains inconsistent across studies and little evidence for convergent validity among existing measures is available. This study evaluated the overlap among self-report and behavioral measures of distress intolerance in four samples, including an unselected sample, a sample of patients with drug dependence, and two samples of cigarette smokers. Results suggested that the self-report measures were highly correlated, as were the behavioral measures; however, behavioral and self-report measures did not exhibit significant associations with each other. There was some evidence of domain specificity, with anxiety sensitivity demonstrating strong associations with somatic distress intolerance, and a lack of association between behavioral measures that elicit affective distress and those that elicit somatic distress. These findings highlight a potential divergence in the literature relative to the conceptualization of distress intolerance as either sensitivity to distress or as the inability to persist at a task when distressed. Further research is needed to elucidate the conceptualization and measurement of distress intolerance to facilitate future clinical and research applications of this construct.

Title: History of suffocation, state-trait anxiety, and anxiety sensitivity in predicting 35% carbon dioxide-induced panic

Authors: Monkul ES, Onur E, Tural U, Hatch JP, Alkın T, Yücel B, Fidaner H.

Journal: Psychiatry Res. 2010 Sep 30;179(2):194-7. doi: 10.1016/j.psychres.2009.06.015. Epub 2010 May 16. PMID: 20478634.

Link to PubMed: History of suffocation, state-trait anxiety, and anxiety sensitivity in predicting 35% carbon dioxide-induced panic

Abstract: The aim of this study was to examine the effects of history of suffocation, state-trait anxiety, and anxiety sensitivity on response to a 35% carbon dioxide (CO₂) challenge in panic disorder patients, their healthy first-degree relatives and healthy comparisons. Thirty-two patients with panic disorder, 32 first-degree relatives, and 34 healthy volunteers underwent the 35% CO₂ challenge. We assessed baseline anxiety with the Anxiety Sensitivity Index (ASI) and State-Trait Anxiety Inventory (STAI1), and panic symptoms with the Panic Symptom List (PSL III-R). A history of suffocation was associated with greater risk of CO₂ reactivity in the combined sample. Patients had more anxiety sensitivity and state and trait anxiety than relatives and healthy comparisons; the difference between relatives and healthy comparisons was not significant. In female patients, trait anxiety predicted CO₂-induced panic. Having a CO₂-sensitive panic disorder patient as a first-degree relative did not predict CO₂-induced panic in a healthy relative. History of suffocation may be an important predictor of CO₂-induced panic. Trait anxiety may have a gender-specific relation to CO₂ reactivity.

Title: Acute panicogenic, anxiogenic and dissociative effects of carbon dioxide inhalation in patients with post-traumatic stress disorder (PTSD)

Authors: Muhtz C, Yassouridis A, Daneshi J, Braun M, Kellner M.

Journal: J Psychiatr Res. 2011 Jul;45(7):989-93. doi: 10.1016/j.jpsychires.2011.01.009. Epub 2011 Feb 15. PMID: 21324483.

Link to PubMed: Acute panicogenic, anxiogenic and dissociative effects of carbon dioxide inhalation in patients with post-traumatic stress disorder (PTSD)

Abstract: Background: Increased anxiety and panic to inhalation of carbon dioxide (CO(2)) has been described in patients with anxiety disorders, especially panic disorder, compared to healthy subjects. Post-traumatic stress disorder (PTSD) has been hypothesised to resemble panic disorder and is currently classified as an anxiety disorder in DSM-IV. However, there are only very few data available about the sensitivity of patients with PTSD to CO(2). Methods: In 10 patients with PTSD, 10 sex- and age-matched healthy subjects and 8 patients with panic disorder we assessed anxiety, panic, dissociative and PTSD symptoms before and after a single vital capacity inhalation of 35% CO(2). Results: Patients with PTSD showed an increased anxiety, panic and dissociative reaction to the inhalation of 35% CO(2) compared to healthy participants. PTSD subjects' responses were indistinguishable from those of panic patients. Additionally, PTSD-typical symptoms like post-traumatic flashbacks were provoked in patients with PTSD after the inhalation of CO(2). Conclusions: In our sample, PTSD was associated with an increased CO(2) reactivity, pointing to an increased susceptibility of PTSD patients to CO(2) challenge.

Title: Panicogens in patients with Post-Traumatic Stress Disorder (PTSD)

Authors: Muhtz C, Wiedemann K, Kellner M.

Journal: Curr Pharm Des. 2012;18(35):5608-18. doi: 10.2174/138161212803530817. PMID: 22632476.

Link to PubMed: Panicogens in patients with Post-Traumatic Stress Disorder (PTSD)

Abstract: Symptom provocation has proved its worth for understanding the pathophysiology of diseases and in general for the development of new therapeutic approaches in the medical field. In the research of anxiety disorders, investigations using experimentally induced panic attacks by various agents, such as sodium lactate, carbon dioxide, cholezystokinine-tetrapetid etc., have a long tradition and allow the exploration of usually naturally occuring spontaneous psychopathological phenomena under controlled conditions. Post-Traumatic Stress Disorder (PTSD) is a prevalent disorder that can develop following exposure to an extreme traumatic event. In DSM-IV it is currently classified as an anxiety disorder and shares phenomenological similarities with panic disorder. The use of panicogenic challenge tests is also an interesting neurobiological approach to learn more about the nature of PTSD and may be a possibility to develop new therapeutic strategies for the treatment of PTSD symptoms. Not only panic anxiety, but also flashbacks and other dissociative symptoms can be provoked by several panicogens in PTSD. The purpose of this review is to evaluate studies using panicogens in PTSD. Methodological short-comings of current studies and needed directions of further research are discussed.

Title: 35% Carbon dioxide and breath-holding challenge tests in panic disorder: a comparison with spontaneous panic attacks

Authors: Nardi AE, Valença AM, Mezzasalma MA, Lopes FL, Nascimento I, Veras AB, Freire RC, de-Melo-Neto VL, Zin WA.

Journal: Depress Anxiety. 2006;23(4):236-44. doi: 10.1002/da.20165. PMID: 16528718.

Link to PubMed: 35% Carbon dioxide and breath-holding challenge tests in panic disorder: a comparison with spontaneous panic attacks

Abstract: Respiration and its control mechanisms may represent an important system involved in abnormal anxiety. Our aim was to compare the demographic and clinical features of patients with panic disorder (PD) with agoraphobia (DSM-IV) who had a panic attack after both the 35% carbon dioxide (CO(2)) test and the breath-holding test (CPA group), and compare them with PD patients who did not have a panic attack after both tests (NPA group). We examined 76 patients with PD who were administered a 35% CO(2)test and a breath-holding test within a 1-week interval. Anxiety scales were applied before and after each test. A panic attack was induced in 50 (65.8%) patients during the CO(2)test (chi(2) = 28.44, df = 1, P<.001) and in 40 (52.6%) patients during the breath-holding test (chi(2) = 15.35, df = 1, P = .036). All patients who had a panic attack during the breath-holding test also had a panic attack during the CO(2)test (n = 40; CPA group). Twenty-six (34.2%) patients with PD did not have a panic attack after both respiratory tests (NPA group). The CPA group had more (chi(2) = 21.67, df = 1, P = .011) respiratory PD subtype. In the CPA group, the disorder started earlier (Mann-Whitney, P<.001), had a higher familial prevalence of PD (chi(2) = 18.34, df = 1, P = .028), and had more previous depressive episodes (chi(2) = 23.59, df = 1, P<.001). Our data suggest that there is an association between respiratory PD subtype and the response to respiratory challenge tests: CO(2)and breath-holding. The CPA may be confirmed as a subgroup of respiratory PD subtype.

Title: Sex differences in panic-relevant responding to a 10% carbon dioxide-enriched air biological challenge

Authors: Nillni YI, Berenz EC, Rohan KJ, Zvolensky MJ.

Journal: J Anxiety Disord. 2012 Jan;26(1):165-72. doi: 10.1016/j.janxdis.2011.10.008. Epub 2011 Nov 6. PMID: 22115836; PMCID: PMC3254702.

Link to full text: Sex differences in panic-relevant responding to a 10% carbon dioxide-enriched air biological challenge

Abstract: The current study examined sex differences in psychological (i.e., self-reported anxiety, panic symptoms, and avoidance) and physiological (i.e., heart rate and skin conductance level) response to, and recovery from, a laboratory biological challenge. Participants were a community-recruited sample of 128 adults (63.3% women; M(age)=23.2 years, SD=8.9) who underwent a 4-min 10% CO(2)-enriched air biological challenge. As predicted, women reported more severe physical panic symptoms and avoidance (i.e., less willingness to participate in another challenge) and demonstrated increased heart rate as compared to men above and beyond the variance accounted for by other theoretically relevant variables (recent panic attack history, neuroticism, and anxiety sensitivity). Additionally, women demonstrated a faster rate of recovery with respect to heart rate compared to men. These results are in line with literature documenting sex-specific differences in panic psychopathology, and results are discussed in the context of possible mechanisms underlying sex differences in panic vulnerability.

Title: The Relationships between Adverse Events, Early Antecedents, and Carbon Dioxide Reactivity as an Intermediate Phenotype of Panic Disorder

Authors: Ogliari A, Tambs K, Harris JR, Scaini S, Maffei C, Reichborn-Kjennerud T, Battaglia M.

Journal: Psychother Psychosom. 2010;79(1):48-55. doi: 10.1159/000259417. Epub 2009 Nov 18. PMID: 19923875.

Link to PubMed: The Relationships between Adverse Events, Early Antecedents, and Carbon Dioxide Reactivity as an Intermediate Phenotype of Panic Disorder

Abstract: Background: Although adverse events have been consistently described to precede and potentially precipitate the onset of panic disorder, there is no information about their ability to alter the individual reactivity to inhaled carbon dioxide, a putative intermediate phenotype of susceptibility to panic disorder. Method: Seven-hundred twelve subjects belonging to the general population-based Norwegian Institute of Public Health Twin Panel underwent a 35% CO(2)/65% O(2) inhalation challenge test and interview-based lifetime assessments of DSM-IV panic disorder, separation anxiety disorder, childhood parental separation/loss, major life events, adverse events of suffocative nature and common stressful life events. Regression models were applied to predict global subjective anxiety and DSM-IV panic symptoms after 35% CO(2)/65% O(2) inhalation. Results: The responses to the challenge measured as semicontinuous variables were predicted by symptoms of childhood separation anxiety, childhood parental loss, common stressful events, major life events, suffocative events and the female gender. The role of most of these predictors was confirmed and held true after the exclusion of subjects with lifetime panic attacks/disorder from the analyses. Conclusions: Several factors which have been reported by previous clinical studies to influence the individual susceptibility to develop panic disorder seem to affect the individual reactivity to inhaled carbon dioxide in people from the general population. Some elements of risk may impact simultaneously upon the individual liability to panic and exaggerated sensitivity to hypercapnia.

Title: Respiratory Psychophysiology of Panic Disorder: Three Respiratory Challenges in 98 Subjects

Authors: Papp LA, Martinez JM, Klein DF, Coplan JD, Norman RG, Cole R, de Jesus MJ, Ross D, Goetz R, Gorman JM.

Journal: Am J Psychiatry. 1997 Nov;154(11):1557-65. doi: 10.1176/ajp.154.11.1557. PMID: 9356564.

Link to PubMed: Respiratory Psychophysiology of Panic Disorder: Three Respiratory Challenges in 98 Subjects

Abstract: Objective: Respiratory abnormalities may play a central role in the pathophysiology of panic disorder. The current study was undertaken to examine the respiratory response in the largest series of subjects to date during three respiratory challenges that used improved methodology. Method: Fifty-nine patients with DSM-III-R panic disorder and 39 normal volunteers were challenged with 5% and 7% CO2 inhalation and room air hyperventilation separated by room air breathing with continuous spirometry. Results: Patients with panic disorder were more sensitive to the anxiogenic effects of CO2 than were normal subjects, and CO2 was a more potent stimulus to panic than hyperventilation. Patients increased their respiratory rate more quickly during CO2 inhalation than did comparison subjects, and this increase preceded the panic attacks. Patients who panicked in response to 5% CO2 demonstrated continued rise in end-tidal CO2, while the end-tidal CO2 of the comparison groups stabilized. Low end-tidal CO2 and high variance in minute ventilation at baseline predicted panic attacks during CO2 inhalation. Following CO2 or hyperventilation challenges, respiratory rate dropped sharply, while tidal volume remained elevated longer in patients than in comparison subjects. Conclusions: The findings confirm the greater behavioral and physiological sensitivity of patients with panic disorder to CO2 inhalation and identify a series of respiratory abnormalities. Panic attacks in panic disorder may be explained by inefficient compensatory mechanisms, primarily of respiratory rate.

Title: Diagnostic and substance specificity of carbon-dioxide-induced panic

Authors: Papp LA, Klein DF, Martinez J, Schneier F, Cole R, Liebowitz MR, Hollander E, Fyer AJ, Jordan F, Gorman JM.

Journal: Am J Psychiatry. 1993 Feb;150(2):250-7. doi: 10.1176/ajp.150.2.250. PMID: 8123056.

Link to PubMed: Diagnostic and substance specificity of carbon-dioxide-induced panic

Abstract: Objective: The authors assessed the substance and diagnostic specificity of carbon-dioxide-induced panic since, in addition to the specific biochemical effects of inhaled carbon dioxide (CO2), simple physiologic distress is also frequently implicated as a panicogenic factor during respiratory challenge studies with CO2 in patients with anxiety disorders. Method: Eighteen patients with panic disorder, 20 with social phobia, and 23 psychiatrically normal subjects inhaled a mixture of 35% CO2 and 65% O2 for 30 seconds through a face mask. They also breathed for 30 seconds through a valve reducing the diameter of the airway. A double-blind, counterbalanced, randomized design was used. Results: In spite of important similarities between the two interventions, including the induction of equal amounts of subjective respiratory distress, carbon dioxide inhalation was significantly more potent than increased airway resistance in provoking panic in the anxiety disorder patients. The patients with panic disorder were significantly more sensitive to CO2 than were the patients with social phobia or the normal subjects. Conclusions: Carbon dioxide inhalation appears to have a specific panicogenic effect in panic patients that goes beyond simple breathlessness.

Title: Psychophysiological responses to CO2 inhalation

Authors: Pappens M, De Peuter S, Vansteenwegen D, Van den Bergh O, Van Diest I.

Journal: Int J Psychophysiol. 2012 Apr;84(1):45-50. doi: 10.1016/j.ijpsycho.2012.01.008. Epub 2012 Jan 20. PMID: 22265938.

Link to PubMed: Psychophysiological responses to CO2 inhalation

Abstract: Inhalation of CO(2)-enriched air has been used as a laboratory model for a number of anxiety disorders, such as general anxiety disorder and panic disorder. Because studies describing psychophysiological responses to this challenge are scarce, the present studies investigated skin conductance level, eyeblink startle, self-reported anxiety and fractional end-tidal carbon dioxide during inhalation of CO(2)-enriched air. In study 1, thirty-five healthy volunteers inhaled 7.5% CO(2) for 2min. In study 2, twenty healthy volunteers inhaled 20% CO(2) for 30s. Control groups (N=20 in each study) inhaled room air during the same time periods. Compared to room air breathing, both CO(2)-mixtures were associated with increases in skin conductance levels, self-reported anxiety and fractional end-tidal CO(2.) Eyeblink startles were inhibited during CO(2) compared to room air breathing in both experiments. Our findings suggest that inhalation of CO(2)-enriched air is associated with a circa-strike defensive response pattern, corroborating its application as an interoceptive, panic-relevant stimulus in fear research.

Title: Carbon dioxide/oxygen challenge test in panic disorder

Authors: Perna G, Battaglia M, Garberi A, Arancio C, Bertani A, Bellodi L.

Journal: Psychiatry Res. 1994 May;52(2):159-71. doi: 10.1016/0165-1781(94)90085-x. PMID: 7972572.

Link to PubMed: Carbon dioxide/oxygen challenge test in panic disorder

Abstract: The effects of a single inhalation of a 35% CO2/65% O2 gas mixture were examined in 71 patients with panic disorder with or without agoraphobia and 44 normal control subjects. Compared with the placebo condition, inhalation of air, the CO2/O2 mixture elicited a clear anxiety reaction only in panic disorder patients, who experienced a sudden rise of subjective anxiety as well as of several panic symptoms. Respiratory symptoms and the fear of dying best distinguished the patients from the control subjects. Baseline anxiety was not the key factor in explaining this differential reaction. The clinical features of panic disorder (namely, frequency of panic attacks, agoraphobia, anticipatory anxiety, and duration of illness) were not significantly related to the response to the challenge test, suggesting that CO2 reactivity might be a trait marker of panic disorder.

Title: 35% CO2 challenge in panic and mood disorders

Authors: Perna G, Barbini B, Cocchi S, Bertani A, Gasperini M.

Journal: J Affect Disord. 1995 Mar 14;33(3):189-94. doi: 10.1016/0165-0327(94)00088-q. PMID: 7790671.

Link to PubMed: 35% CO2 challenge in panic and mood disorders

Abstract: 20 patients with Panic Disorder (PD), 19 patients with Mood Disorder (MD) and 20 healthy controls inhaled one vital capacity of 35% CO2-65% O2 gas mixture and of compressed air in a double-blind, random, cross-over design. Only PD patients showed a strong reaction to 35% CO2 while MD patients and controls did not react significantly. These results support the specificity of the 35% CO2 challenge in PD patients and suggest that PD and MD are separate disorders.

Title: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Authors: Perna G, Bertani A, Caldirola D, Gabriele A, Cocchi S, Bellodi L.

Journal: J Clin Psychopharmacol. 2002 Jun;22(3):300-8. doi: 10.1097/00004714-200206000-00011. PMID: 12006901.

Link to PubMed: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Abstract: Carbon dioxide (CO2) inhalation induces acute anxiety and panic attacks in patients with Panic Disorder (PD). Anti-panic drugs decrease CO2 reactivity after the first days of treatment; however, the clinical meaning of this finding has not yet been established. This study investigated the effects of treatment with tricyclic antidepressants and selective serotonin re-uptake inhibitors (SSRIs) on CO2 reactivity and compared the relationships between 35% CO2 hyperreactivity modulation and short-term clinical outcome. One hundred twenty-three patients with PD with or without agoraphobia who were hyperreactive to CO2 were randomly assigned to treatment groups with imipramine, clomipramine, paroxetine, sertraline, or fluvoxamine. A double-blind, randomized design was applied. Each patient received the 35% CO2 challenge on days 0, 7, and 30. The severity of clinical symptomatology was measured on days 0 and 30. Decreased hyperreactivity to 35% CO2 in all five treatment groups was already evident after the first week. The decrease in CO2 reactivity at the end of treatment was proportional to the degree of clinical improvement. Multiple regression analyses showed that the decrease in CO2 reactivity after the first week was a significant predictor for good clinical outcome after one month. The results of this study confirm evidence that psychoactive drugs effective in the treatment of PD decrease CO2 hyperreactivity. They also suggest that precocious modulation of CO2 reactivity might fairly reliably predict short-term clinical outcome in patients with "respiratory" PD.

Title: Sensitivity to 35% CO2 in healthy first-degree relatives of patients with panic disorder

Authors: Perna G, Cocchi S, Bertani A, Arancio C, Bellodi L.

Journal: Am J Psychiatry. 1995 Apr;152(4):623-5. doi: 10.1176/ajp.152.4.623. PMID: 7694916.

Link to PubMed: Sensitivity to 35% CO2 in healthy first-degree relatives of patients with panic disorder

Abstract: Objective: The authors tested the hypothesis that hyperreactivity to CO2 in healthy subjects represents an underlying familial vulnerability to panic disorder. Method: One vital-capacity inhalation of 35% CO2 and 65% O2 was administered to each of 84 patients with panic disorder, 23 healthy first-degree relatives of probands with panic disorder, and 44 healthy subjects with no family history of panic disorder. Results: The first-degree relatives of the probands with panic disorder reacted significantly more than the healthy subjects and significantly less than the probands. Conclusions: These findings suggest an association between family history of panic disorder and hyperreactivity to 35% CO2 in healthy subjects.

Title: Differential carbon dioxide sensitivity in childhood anxiety disorders and nonill comparison group

Authors: Pine DS, Klein RG, Coplan JD, Papp LA, Hoven CW, Martinez J, Kovalenko P, Mandell DJ, Moreau D, Klein DF, Gorman JM.

Journal: Arch Gen Psychiatry. 2000 Oct;57(10):960-7. doi: 10.1001/archpsyc.57.10.960. PMID: 11015814.

Link to PubMed: Differential carbon dioxide sensitivity in childhood anxiety disorders and nonill comparison group

Abstract: Background: To examine the relationship between respiratory regulation and childhood anxiety disorders, this study considered the relationship between anxiety disorders and symptoms during carbon dioxide (CO(2)) exposure, CO(2) sensitivity in specific childhood anxiety disorders, and the relationship between symptomatic and physiological responses to CO(2). Methods: Following procedures established in adults, 104 children (aged 9-17 years), including 25 from a previous study, underwent 5% CO(2) inhalation. The sample included 57 probands with an anxiety disorder (social phobia, generalized anxiety disorder, separation anxiety disorder, and panic disorder) and 47 nonill comparison subjects. Symptoms of anxiety were assessed before, during, and after CO(2) inhalation. Results: All children tolerated the procedure well, experiencing transient or no increases in anxiety symptoms. Children with an anxiety disorder, particularly separation anxiety disorder, exhibited greater changes in somatic symptoms during inhalation of CO(2)-enriched air, relative to the comparison group. During CO(2) inhalation, symptom ratings were positively correlated with respiratory rate increases, as well as with levels of tidal volume, minute ventilation, end-tidal CO(2), and irregularity in respiratory rate during room-air breathing. Conclusions: Childhood anxiety disorders, particularly separation anxiety disorder, are associated with CO(2) hypersensitivity, as defined by symptom reports. Carbon dioxide hypersensitivity is associated with physiological changes similar to those found in panic disorder. These and other data suggest that certain childhood anxiety disorders may share pathophysiological features with adult panic disorder.

Title: Response to 5% carbon dioxide in children and adolescents: relationship to panic disorder in parents and anxiety disorders in subjects

Authors: Pine DS, Klein RG, Roberson-Nay R, Mannuzza S, Moulton JL 3rd, Woldehawariat G, Guardino M.

Journal: Arch Gen Psychiatry. 2005 Jan;62(1):73-80. doi: 10.1001/archpsyc.62.1.73. PMID: 15630075.

Link to PubMed: Response to 5% carbon dioxide in children and adolescents: relationship to panic disorder in parents and anxiety disorders in subjects

Abstract: Background: Carbon dioxide (CO(2)) sensitivity is postulated to be a familial risk marker of panic disorder (PD). Exaggerated responses to CO(2) inhalation have been reported in adults with PD and their unaffected adult relatives, as well as in clinic-referred children with anxiety disorders.

Objective: To test in a family-based design whether CO(2) hypersensitivity is a familial risk marker for PD and associated with current anxiety disorders in children and adolescents.

Setting and participants: One hundred forty-two offspring (aged 9-19 years) of parents with PD, major depressive disorder, or no disorder. Forty-five (32%) had a current anxiety disorder, excluding specific phobia.

Design and main outcome measures: Parents and offspring received diagnostic assessments. Offspring underwent 5% CO(2) inhalation at home. Panic symptoms and panic attacks were rated with the Acute Panic Inventory at baseline, while anticipating CO(2) delivery ("threat"), and during CO(2) inhalation. Respiratory rate and volume were measured with spirometry.

Results: No group differences were found in Acute Panic Inventory ratings at baseline or in respiratory measures during threat. Risk for PD was not associated with CO(2) sensitivity (panic symptoms and respiratory physiologic response). During CO(2) inhalation, offspring with anxiety disorders, relative to offspring without anxiety disorders, experienced significantly more panic symptoms and panic attacks, as well as elevated respiratory rates. During threat, panic symptoms were significantly and independently associated with both parental PD and offspring anxiety disorders.

Conclusions: No support was obtained for CO(2) hypersensitivity as a familial risk marker for PD in children and adolescents. Links between childhood anxiety disorders and CO(2) sensitivity were replicated. Familial risk for PD in children and adolescents may be associated with vulnerability to anticipatory anxiety.

Title: Cognitive mediation in the affective component of spontaneous panic attacks

Authors: Rapee R, Mattick R, Murrell E.

Journal: J Behav Ther Exp Psychiatry. 1986 Dec;17(4):245-53. doi: 10.1016/0005-7916(86)90059-5. PMID: 3100580.

Link to PubMed: Cognitive mediation in the affective component of spontaneous panic attacks

Abstract: Single inhalations of a 50% carbon dioxide/50% oxygen gas mixture were administered to 16 subjects with spontaneous panic attacks and to 16 social phobics who did not experience such attacks. Half of each diagnostic category was randomly allocated to either a no explanation condition in which minimal instructions on expected outcome were provided, or an explanation condition in which all possible sensations were described and attributed to the effects of the gas. Subjects with panic attacks who were given no explanation reported a greater proportion of catastrophic cognitions, greater panic, and a greater similarity of the overall experience to a naturally-occurring panic attack than those with panic attacks who received a full explanation. In contrast, both groups of social phobics reported similar effects to each other, regardless of the explanation given. The results provide support for cognitive mediation in the "panic" component of spontaneous panic attacks.

Title: Response to hyperventilation and inhalation of 5.5% carbon dioxide-enriched air across the DSM-III-R anxiety disorders

Authors: Rapee RM, Brown TA, Antony MM, Barlow DH.

Journal: J Abnorm Psychol. 1992 Aug;101(3):538-52. doi: 10.1037//0021-843x.101.3.538. PMID: 1500611.

Link to PubMed: Response to hyperventilation and inhalation of 5.5% carbon dioxide-enriched air across the DSM-III-R anxiety disorders

Abstract: Anxiety disorder patients (n = 198; under criteria of the Diagnostic and Statistical Manual of Mental Disorders; rev. 3rd ed.; American Psychiatric Association, 1987) and nonanxious control subjects (n = 25) underwent challenges of 90 s of voluntary hyperventilation and 15 min of 5.5% carbon dioxide in air. Panic disorder subjects showed a greater subjective response to both challenges than did subjects with other anxiety disorders, who in turn responded more than did control subjects. Furthermore, subjects with panic disorder as an additional diagnosis tended to report more subjective response than did anxiety disorder subjects without panic disorder. The best prechallenge predictor of response to each procedure was a measure of fear of physical symptoms. The findings support previous results that have pointed to a greater fear or anxiety-inducing effect of these challenge procedures in panic disorder patients, as compared with other subjects.

Title: Evidence for distinct genetic effects associated with response to 35% CO₂

Authors: Roberson-Nay R, Moruzzi S, Ogliari A, Pezzica E, Tambs K, Kendler KS, Battaglia M.

Journal: Depress Anxiety. 2013 Mar;30(3):259-66. doi: 10.1002/da.22038. Epub 2013 Jan 24. PMID: 23349098; PMCID: PMC4096694.

Link to full text: Evidence for distinct genetic effects associated with response to 35% CO₂

Abstract: Background: Carbon dioxide (CO2 ) hypersensitivity represents an individual difference response to breathing CO2 enriched air. People with a history of panic attacks or panic disorder are particularly prone to anxious response, suggesting that CO2 hypersensitivity is a robust risk marker of panic spectrum vulnerability.

Methods: Twin pairs (n = 346) from the general population-based Norwegian NIPH Mental Health Study completed a measure of anxiety before and after vital capacity inhalation of 35% CO2 air and before and after inhalation of regular air. Three hypotheses regarding genetic factors for CO2 hypersensitivity were examined: (1) a single set of genetic risk factors impacts anxiety before exposure to CO2 and these same genes constitute the only genetic influences on anxiety in response to CO2 , (2) the genetic effects on pre-CO2 anxiety are entirely different from the genetic effects on anxiety in response to exposure to CO2 (i.e., new genetic effects), and (3) pre-CO2 anxiety influences anxiety in response to CO2 as well as unique genetic factors that become activated by respiratory stimulation.

Results: Our results support the latter hypothesis for response to 35% CO2 , with additive genetic and unique environmental factors best fitting the data. Evidence of new genetic effects was observed, accounting for 20% unique variance in post 35% CO2 anxiety response. New genetic effects were not observed for anxiety ratings made post regular air where only preregular air anxiety ratings explained significant variance in this outcome.

Conclusions: These data suggest that there are distinct genetic factors associated with responsivity to respiratory stimulation via 35% CO2 .

Title: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Authors: Roberson-Nay R, Beadel JR, Gorlin EI, Latendresse SJ, Teachman BA.

Journal: J Behav Ther Exp Psychiatry. 2015 Jun;47:68-76. doi: 10.1016/j.jbtep.2014.10.013. Epub 2014 Nov 15. PMID: 25496936; PMCID: PMC4324118.

Link to full text: Examining the latent class structure of CO2 hypersensitivity using time course trajectories of panic response systems

Abstract: Background and objectives: Carbon dioxide (CO2) hypersensitivity is hypothesized to be a robust endophenotypic marker of panic spectrum vulnerability. The goal of the current study was to explore the latent class trajectories of three primary response systems theoretically associated with CO2 hypersensitivity: subjective anxiety, panic symptoms, and respiratory rate (fR).

Methods: Participants (n = 376; 56% female) underwent a maintained 7.5% CO2 breathing task that included three phases: baseline, CO2 air breathing, and recovery. Growth mixture modeling was used to compare response classes (1…n) to identify the best-fit model for each marker. Panic correlates also were examined to determine class differences in panic vulnerability.

Results: For subjective anxiety ratings, a three-class model was selected, with individuals in one class reporting an acute increase in anxiety during 7.5% CO2 breathing and a return to pre-CO2 levels during recovery. A second, smaller latent class was distinguished by elevated anxiety across all three phases. The third class reported low anxiety reported during room air, a mild increase in anxiety during 7.5% CO2 breathing, and a return to baseline during recovery. Latent class trajectories for fR yielded one class whereas panic symptom response yielded two classes.

Limitations: This study examined CO2 hypersensitivity in one of the largest samples to date, but did not ascertain a general population sample thereby limiting generalizability. Moreover, a true resting baseline measure of fR was not measured.

Conclusions: Two classes potentially representing different risk pathways were observed. Implications of results will be discussed in the context of panic risk research.

Keywords: Anxiety; Carbon dioxide hypersensitivity; Latent class; Panic; Respiratory; Risk.

Title: The influence of an illusion of control on panic attacks induced via inhalation of 5.5% carbon dioxide-enriched air

Authors: Sanderson WC, Rapee RM, Barlow DH.

Journal: Arch Gen Psychiatry. 1989 Feb;46(2):157-62. doi: 10.1001/archpsyc.1989.01810020059010. PMID: 2492423.

Link to PubMed: The influence of an illusion of control on panic attacks induced via inhalation of 5.5% carbon dioxide-enriched air

Abstract: The current study tested the notion that a sense of control can mitigate anxiety and panic attacks caused by the inhalation of 5.5% carbon dioxide (CO2)-enriched air. Twenty patients with panic disorder inhaled a mixture of 5.5% CO2-enriched air for 15 minutes. All patients were instructed that illumination of a light directly in front of them would signal that they could decrease the amount of CO2 that they were receiving, if desired, by turning a dial attached to their chair. For ten patients, the light was illuminated during the entire administration of CO2. For the remaining ten patients, the light was never illuminated. In fact, all patients experienced the full CO2 mixture, and the dial was ineffective. When compared with patients who believed they had control, patients who believed they could not control the CO2 administration (1) reported a greater number of DSM-III-revised panic attack symptoms, (2) rated the symptoms as more intense, (3) reported greater subjective anxiety, (4) reported a greater number of catastrophic cognitions, (5) reported a greater resemblance of the overall inhalation experience to a naturally occurring panic attack, and (6) were significantly more likely to report panic attacks. These data illustrate the contribution of psychologic factors to laboratory induction of panic attacks through inhalation of 5.5% CO2-enriched air.

Title: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Authors: Schmidt NB, Trakowski JH, Staab JP.

Journal: J Abnorm Psychol. 1997 Nov;106(4):630-8. doi: 10.1037//0021-843x.106.4.630. PMID: 9358693.

Link to PubMed: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Abstract: Inhalations of high concentrations of carbon dioxide (CO2) reliably produce panic attacks in patients with panic disorder. The present study evaluated whether cognitive-behavioral treatment (CBT) for panic disorder would extinguish CO2-induced panic and whether changes in panic and arousal-related cognitions were associated with the induction of panic. Patients with panic disorder (N = 54) were assigned to 1 of 3 experimental conditions: CBT with respiratory training (CBT-R), CBT without respiratory training (CBT), or delayed treatment. Participants received 5 repeated vital-capacity inhalations of 35% CO2/65% O2 prior to and following either 12 treatment sessions or a 12-week waiting period. During pretreatment assessments, 74% of patients experienced a panic attack during at least 1 inhalation. At posttreatment, only 20% of treated participants (CBT-R = 19%, CBT = 22%), compared with 64% of untreated participants, panicked. Forty-four percent of treated participants, compared with 0% of untreated participants, reported no anxiety during all posttreatment inhalations. Anxiety sensitivity as well as panic appraisals regarding the likelihood of panic and self-efficacy with coping with panic were significantly related to fearful responding to the CO2 challenge.

Title: Biological Challenge Manipulation of PCO2 Levels: A Test of Klein's (1993) Suffocation Alarm Theory of Panic

Authors: Schmidt NB, Telch MJ, Jaimez TL.

Journal: J Abnorm Psychol. 1996 Aug;105(3):446-54. doi: 10.1037//0021-843x.105.3.446. PMID: 8772015.

Link to PubMed: Biological Challenge Manipulation of PCO2 Levels: A Test of Klein's (1993) Suffocation Alarm Theory of Panic

Abstract: D.F. Klein (1993) proposed that patients with panic disorder (PD) have a hypersensitive suffocation monitor that predisposes them to experience panic attacks under certain conditions. The suffocation alarm theory predicts differential emotional responding to biological challenges that affect arterial partial pressure of carbon dioxide (PCO2). These PD patients should exhibit (a) lower fear and less likelihood of panic in response to biological challenges that lower PCO2 levels (e.g., hyperventilation), and (b) increased fear and greater likelihood of panic in response to biological challenges that raise PCO2 levels (e.g., inhalation of 35% CO2 gas). The following indicators of the suffocation monitor were assessed: (a) severity of dyspnea symptoms, (b) frequency of dyspnea symptoms, (c) heightened respiration rate, and (d) lowered PCO2 levels. Ratings of physiological and subjective responding, as well as panic, were obtained during both a hyperventilation and a 35% CO2 challenge. None of the classification methods predicted differential emotional responding to hyperventilation versus 35% CO2 challenge.

Title: Genetic moderation of CO2-induced fear by 5-HTTLPR genotype

Authors: Schruers K, Esquivel G, van Duinen M, Wichers M, Kenis G, Colasanti A, Knuts I, Goossens L, Jacobs N, van Rozendaal J, Smeets H, van Os J, Griez E.

Journal: J Psychopharmacol. 2011 Jan;25(1):37-42. doi: 10.1177/0269881110372543. Epub 2010 Jun 28. PMID: 20584994.

Link to PubMed: Genetic moderation of CO2-induced fear by 5-HTTLPR genotype

Abstract: Inhalation of an increased concentration of carbon dioxide (CO(2)) has been shown to induce a state of negative affect in healthy subjects that is closely related to the clinical phenomenon of panic. It has been suggested that the vulnerability to CO(2) is moderated by differences in serotonin (5-HT) activity, caused by a functional polymorphism in the promoter region of the 5-HT transporter (5-HTTLPR) gene. Our aim was to examine the relationship between bi- and tri-allelic 5-HTTLPR genotype and the affective response to different dosages of inhaled CO(2) in healthy volunteers. Ninety-six subjects performed a double inhalation of four mixtures containing, respectively, 0%, 9%, 17.5% and 35% CO(2), following a double-blind, cross-over, randomized design. Affective responses were measured with a visual analogue scale for fear and the Panic Symptom List. 5-HTTLPR genotype was expressed as LL, SL and SS. Subjects with the SL and SS genotype reported less fear than LL subjects. A significant interaction effect was found between genotype and CO(2) dosage: the SS genotype showed lower fear scores than the LL genotype, particularly in the 17.5% CO(2) dose condition. The present study suggests that the dose-dependent fear reaction to CO(2) is moderated by a polymorphism in the 5-HT transporter gene, particularly at intermediate CO(2) dosages. It also underscores the usefulness of the introduction of an intermediate phenotype related to panic to reveal an underlying genetic vulnerability otherwise staying elusive. These results are in line with current theories on the role of 5-HT in both panic and respiration.

Title: Symptom profiles of natural and laboratory panic attacks

Authors: Schruers KR, van de Mortel H, Overbeek T, Griez E.

Journal: Acta Neuropsychiatr. 2004 Apr;16(2):101-6. doi: 10.1111/j.0924-2708.2004.0084.x. PMID: 26984003.

Link to PubMed: Symptom profiles of natural and laboratory panic attacks

Abstract: Background: Little accurate information is available about the symptomatology of real-life panic attacks and about how well they are reproduced by an experimental model such as the 35% CO2 challenge.

Method: Real-life panic symptoms were assessed in a group of 67 panic disorder patients, using daily life monitoring. Panic symptoms elicited by a 35% CO2 challenge were assessed in 61 panic disorder patients, and their frequency was compared with the real-life symptoms.

Results: The most frequent real-life symptoms were palpitations, dizziness and trembling. The 35% CO2 challenge reproduced well the majority of real-life symptoms.

Conclusion: The findings suggest that the 35% CO2 challenge is a marker for spontaneous panic attacks, which are considered the core of panic disorder.

Title: Effects of 7.5% CO2 challenge in generalized anxiety disorder

Authors: Seddon K, Morris K, Bailey J, Potokar J, Rich A, Wilson S, Bettica P, Nutt DJ.

Journal: J Psychopharmacol. 2011 Jan;25(1):43-51. doi: 10.1177/0269881110364270. Epub 2010 Mar 16. PMID: 20233897.

Link to PubMed: Effects of 7.5% CO2 challenge in generalized anxiety disorder

Abstract: We have previously developed a putative model of generalized anxiety disorder in healthy volunteers using a 20-minute 7.5% carbon dioxide (CO(2)) inhalation challenge. The aim of this study was to validate the 7.5% CO(2) paradigm by assessing its effects in patients with generalized anxiety disorder in a test-retest design. Twelve medication-free generalized anxiety disorder patients attended our lab for two study days. On each study day placebo (compressed air) and 7.5% CO(2) mixture were randomly administered over 20 min, at least 30 min apart, in a single blind, randomized, placebo-controlled cross-over design. Subjective ratings, cardiovascular measures and cortisol levels were collected throughout. CO(2) challenge significantly increased ratings for anxiety and other subjective symptoms associated with generalized anxiety disorder, compared with air. It also significantly increased systolic blood pressure on day 2, indicating increased autonomic arousal. There was no change between the two test days in mean anxiety rating scores, and there also appeared to be a correlation for individual scores on a number of the subjective measures. In conclusion, 20 min of 7.5% CO(2) gas inhalation increases anxiety responses in patients with generalized anxiety disorder, and this is reliable over time.

Title: Respiratory sensations in subjects who lack a ventilatory response to CO2

Authors: Shea SA, Andres LP, Shannon DC, Guz A, Banzett RB.

Journal: Respir Physiol. 1993 Aug;93(2):203-19. doi: 10.1016/0034-5687(93)90006-v. PMID: 8210759.

Link to PubMed: Respiratory sensations in subjects who lack a ventilatory response to CO2

Abstract: An urge to breath is perceived during breath hold and hypercapnia (termed 'air hunger') and during heavy exercise (often termed 'shortness of breath'). To better understand the neural mechanisms responsible for these sensations we studied five patients (8-17 years old) with congenital central hypoventilation syndrome (CCHS) who lack ventilatory response to CO2. CCHS patients reported no respiratory discomfort during CO2 inhalation or during maximal breath hold which was of much longer duration than age-matched controls. However, all 3 CCHS patients who exercised heavily reported some sensations akin to shortness of breath (they increased breathing nearly as much as controls). Our results are consistent with two possibilities. First, the air hunger of hypercapnia and breath hold is caused by projection to the forebrain of respiratory chemoreceptor afferents which bypass the respiratory centers, while exercise shortness of breath is caused by direct projections of limb afferents or locomotory center activity. Second, air hunger and shortness of breath share the same origin--projection of increased brain stem respiratory center motor activity (corollary discharge) to the forebrain.

Title: The interplay between physical activity and anxiety sensitivity in fearful responding to carbon dioxide challenge

Authors: Smits JA, Tart CD, Rosenfield D, Zvolensky MJ.

Journal: Psychosom Med. 2011 Jul-Aug;73(6):498-503. doi: 10.1097/PSY.0b013e3182223b28. Epub 2011 Jun 23. PMID: 21700713; PMCID: PMC3131468.

Link to full text: The interplay between physical activity and anxiety sensitivity in fearful responding to carbon dioxide challenge

Abstract: Objective: Physical activity may confer protective effects in the development of anxiety and its disorders. These effects may be particularly strong among individuals who have elevated levels of anxiety sensitivity (AS; i.e., the fear of somatic arousal), an established cognitive-based risk factor for anxiety and its disorders. The present study performed a laboratory test of the interplay between physical activity and AS.

Methods: The participants were adults free of Axis I psychopathology (n = 145) who completed measures of physical activity and AS before undergoing a recurrent 20% carbon dioxide-enriched air (CO(2)) challenge.

Results: Consistent with the hypothesis, physical activity was significantly related to CO(2) challenge reactivity among persons with elevated levels of AS, at high levels of physical activity (p < .001) but not at low levels of physical activity (p = .90). Also consistent with hypothesis, irrespective of the level of physical activity, physical activity did not relate significantly to CO(2) challenge reactivity among persons with normative levels of AS (p = .28).

Conclusions: These findings provide novel empirical insight into the role that physical activity may play in terms of resiliency for the development of anxiety disorders. Specifically, the protective effects of physical activity may only be evident at higher doses and among persons who are at increased risk of developing anxiety disorders because they have elevated AS.

Title: Unexpected arousal, anxiety sensitivity, and their interaction on CO₂-induced panic: further evidence for the context-sensitivity vulnerability model

Authors: Telch MJ, Harrington PJ, Smits JA, Powers MB.

Journal: J Anxiety Disord. 2011 Jun;25(5):645-53. doi: 10.1016/j.janxdis.2011.02.005. Epub 2011 Mar 3. PMID: 21474277.

Link to PubMed: Unexpected arousal, anxiety sensitivity, and their interaction on CO₂-induced panic: further evidence for the context-sensitivity vulnerability model

Abstract: The present experiment tested several predictions derived from the context-sensitivity vulnerability model of panic. Participants (N=79) scoring either high or low in anxiety sensitivity (AS) and with no history of unexpected panic were randomly assigned to one of two instructional sets: expected arousal (EA) or expected relaxation (ER). All participants were administered inhalation of room air and 35% CO(2) in a counterbalanced order. Consistent with theoretical predictions, High-AS participants who received ER instructions showed greater emotional responding compared to High-AS participants who received EA instructions, while instructional set did not affect responding among Low-AS participants. Panic attacks were observed in 52% of the High-AS-ER group compared to 17%, 5%, and 5% in the High-AS-EA, Low-AS-ER, and Low-AS-EA groups respectively. These findings are consistent with the theory's assertion that dispositional tendencies, such as anxiety sensitivity potentiate the panicogenic effects of threat-relevant context variables.

Title: Emotional reactivity to a single inhalation of 35% carbon dioxide and its association with later symptoms of posttraumatic stress disorder and anxiety in soldiers deployed to Iraq

Authors: Telch MJ, Rosenfield D, Lee HJ, Pai A.

Journal: Arch Gen Psychiatry. 2012 Nov;69(11):1161-8. doi: 10.1001/archgenpsychiatry.2012.8. PMID: 23117637.

Link to PubMed: Emotional reactivity to a single inhalation of 35% carbon dioxide and its association with later symptoms of posttraumatic stress disorder and anxiety in soldiers deployed to Iraq

Abstract: Context: The identification of modifiable predeployment vulnerability factors that increase the risk of combat stress reactions among soldiers once deployed to a war zone offers significant potential for the prevention of posttraumatic stress disorder (PTSD) and other combat-related stress disorders. Adults with anxiety disorders display heightened emotional reactivity to a single inhalation of 35% carbon dioxide (CO(2)); however, data investigating prospective linkages between emotional reactivity to CO(2) and susceptibility to war-zone stress reactions are lacking.

Objective: To investigate the association of soldiers' predeployment emotional reactivity to 35% CO(2) challenge with several indices of subsequent war-zone stress symptoms assessed monthly while deployed in Iraq.

Design, setting, and participants: Prospective cohort study of 158 soldiers with no history of deployment to a war zone were recruited from the Texas Combat Stress Risk Study between April 2, 2007, and August 28, 2009.

Main outcome measures: Multilevel regression models were used to investigate the association between emotional reactivity to 35% CO(2) challenge (assessed before deployment) and soldiers' reported symptoms of general anxiety/stress, PTSD, and depression while deployed to Iraq.

Results: Growth curves of PTSD, depression, and general anxiety/stress symptoms showed a significant curvilinear relationship during the 16-month deployment period. War-zone stressors reported in theater were associated with symptoms of general anxiety/stress, PTSD, and depression. Consistent with the prediction, soldiers' emotional reactivity to a single inhalation of 35% CO(2)-enriched air before deployment significantly potentiated the effects of war-zone stressors on the subsequent development of PTSD symptoms and general anxiety/stress symptoms but not on the development of depression, even after accounting for the effects of trait anxiety and the presence of past or current Axis I mental disorders.

Conclusion: Soldiers' emotional reactivity to a 35% CO(2) challenge may serve as a vulnerability factor for increasing soldiers' risk for PTSD and general anxiety/stress symptoms in response to war-zone stressors.

Title: Objective and subjective measures in recovery from a 35% carbon dioxide challenge

Authors: Niccolai V, van Duinen MA, Griez EJ.

Journal: Can J Psychiatry. 2008 Nov;53(11):737-44. doi: 10.1177/070674370805301105. PMID: 19087467.

Link to PubMed: Objective and subjective measures in recovery from a 35% carbon dioxide challenge

Abstract: Objectives: Because hyperventilation, dyspnea, and a feeling of choking are often core features of a panic attack, respiration has been one of the most widely studied physiological parameters in panic disorder (PD) patients. A respiratory subgroup of PD, with distinct etiological pathways, has also been suggested. Investigation of the recovery phase following a respiratory challenge may be a reliable way to establish respiratory impairment in PD patients. The objective of the present study was to investigate the recovery phase from a 35% carbon dioxide challenge in PD patients and in healthy controls, and to test the hypothesis of a different respiratory pattern in patients, compared to control subjects.

Methods: Eleven nonmedicated PD patients with or without agoraphobia, 11 medicated PD patients, and 11 control subjects took part in a 35% carbon dioxide and 65% oxygen inhalation challenge. Respiratory rate, partial pressure of carbon dioxide, heart rate, and blood pressure were recorded during the baseline phase (10 minutes) and the recovery phase (10 minutes). Visual Analogue Scale of Anxiety and Panic Symptom List scores were collected pre- and post-challenge.

Results: Nonmedicated patients had increased variability in respiratory rate and partial pressure of carbon dioxide during recovery, compared with control subjects and medicated PD patients. Also, PD patients tended to have higher heart rates and to need more time to recover from the challenge than control subjects.

Conclusions: Results suggest that PD patients have less effective homeostatic control after their physiological equilibrium has been disrupted by a respiratory stressor.

Title: Reactivity to a 35% CO2 challenge in healthy first-degree relatives of patients with panic disorder

Authors: Van Beek N, Griez E.

Journal: Biol Psychiatry. 2000 May 1;47(9):830-5. doi: 10.1016/s0006-3223(99)00265-6. PMID: 10812042.

Link to PubMed: Reactivity to a 35% CO2 challenge in healthy first-degree relatives of patients with panic disorder

Abstract: Background: The effects of a 35% CO2 challenge were examined in healthy first-degree relatives of panic disorder patients and in healthy control subjects matched for age and gender.

Methods: One single inhalation of a 35% CO2/65% O2 challenge was administered to 50 first-degree relatives of panic disorder patients and 50 control subjects.

Results: The first-degree relatives were more reactive to the 35% CO2 challenge than the control subjects.

Conclusions: These findings indicate that being a member of a family with a panic disorder patient is, in itself an important factor in CO2 hypersensitivity among subjects who have never experienced a panic attack. Both panic disorder patients and their first-degree relatives have a tendency to be more reactive to the CO2 challenge.

Title: Reliability of the 35% carbon dioxide panic provocation challenge

Authors: Verburg K, Pols H, de Leeuw M, Griez E.

Journal: Psychiatry Res. 1998 May 8;78(3):207-14. doi: 10.1016/s0165-1781(98)00009-2. PMID: 9657425.

Link to PubMed: Reliability of the 35% carbon dioxide panic provocation challenge

Abstract: The objective of this study was to determine the test-retest reliability of the 35% carbon dioxide (CO2) panic provocation challenge. Thirty patients with panic disorder were included in this study. Twenty-four patients were challenged twice, with 1 week between the two challenges. Six patients dropped out after the first test. The 35% CO2 challenge appeared to have a good test-retest reliability; both on induced subjective anxiety, measured on a Visual Analogue Scale for Anxiety (VAS-A), and induced panic symptoms, measured with a Panic Symptom List. Assessing the state of anxiety immediately after the challenge gave the most reliable results. Calculating increase in anxiety from the pre- and post-scores on the VAS-A rendered less reliable scores. This study completes a series of studies in which the criteria for an ideal model of panic are tested for the 35% CO2 challenge. Apart from an absolute specificity for panic disorder, the challenge meets these criteria.

Title: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Authors: Ziemann AE, Allen JE, Dahdaleh NS, Drebot II, Coryell MW, Wunsch AM, Lynch CM, Faraci FM, Howard MA 3rd, Welsh MJ, Wemmie JA.

Journal: Cell. 2009 Nov 25;139(5):1012-21. doi: 10.1016/j.cell.2009.10.029. PMID: 19945383; PMCID: PMC2808123.

Link to full text: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Abstract: The amygdala processes and directs inputs and outputs that are key to fear behavior. However, whether it directly senses fear-evoking stimuli is unknown. Because the amygdala expresses acid-sensing ion channel-1a (ASIC1a), and ASIC1a is required for normal fear responses, we hypothesized that the amygdala might detect a reduced pH. We found that inhaled CO(2) reduced brain pH and evoked fear behavior in mice. Eliminating or inhibiting ASIC1a markedly impaired this activity, and localized ASIC1a expression in the amygdala rescued the CO(2)-induced fear deficit of ASIC1a null animals. Buffering pH attenuated fear behavior, whereas directly reducing pH with amygdala microinjections reproduced the effect of CO(2). These data identify the amygdala as an important chemosensor that detects hypercarbia and acidosis and initiates behavioral responses. They also give a molecular explanation for how rising CO(2) concentrations elicit intense fear and provide a foundation for dissecting the bases of anxiety and panic disorders.

Title: The effects of offset control over 20% carbon-dioxide-enriched air on anxious responding

Authors: Zvolensky MJ, Eifert GH, Lejuez CW, McNeil DW.

Journal: J Abnorm Psychol. 1999 Nov;108(4):624-32. doi: 10.1037//0021-843x.108.4.624. PMID: 10609427.

Link to PubMed: The effects of offset control over 20% carbon-dioxide-enriched air on anxious responding

Abstract: Control over the offset of repeated administrations of 20% carbon-dioxide-enriched air was assessed in nonclinical participants (n = 30) reporting elevated levels of anxiety sensitivity--a population at an increased risk for experiencing panic attacks and possibly developing panic disorder. In Phase I, participants were randomly assigned to 1 of 2 conditions: one that permitted offset control over gas inhalation and one that did not. These conditions were reversed in Phase II. Across phases, a lack of offset control resulted in greater self-reported anxiety compared with having control, although no significant differences were observed for heart rate. Whereas all participants demonstrated a Stroop interference effect for general (e.g., coffin) compared with specific (e.g., dizzy) physical threat word types prior to the first experimental phase, this effect persisted only for participants who had offset control in Phase I. We discuss these results in relation to the differential effects of offset control, with implications for better understanding anxious responding during elevated bodily arousal.

Title: Offset control during recurrent 20% carbon dioxide-enriched air induction: relation to individual difference variables

Authors: Zvolensky MJ, Eifert GH, Lejuez CW.

Journal: Emotion. 2001 Jun;1(2):148-65. doi: 10.1037/1528-3542.1.2.148. PMID: 12899194.

Link to PubMed: Offset control during recurrent 20% carbon dioxide-enriched air induction: relation to individual difference variables

Abstract: Although control over aversive events maintains a central role in contemporary models of anxiety pathology, particularly panic disorder, there is little understanding about the emotional consequences of specific types of control processes. In the present study, offset control over 8 20% carbon dioxide-enriched air administrations was experimentally manipulated in a large nonclinical population (n = 96) varying in anxiety sensitivity (high or low) and gender. Dependent measures included self-reported anxiety, affective reports of valence, arousal, emotional control, and physiological indices of heart rate and skin conductance. High anxiety-sensitive participants who lacked offset control reported significantly greater elevations in self-reported anxiety, emotional displeasure, arousal, and dyscontrol relative to their yoked counterparts with offset control. In contrast, low anxiety-sensitive individuals responded with similar levels of cognitive and affective distress regardless of the offset control manipulation. Although the provocation procedure reliably produced bodily arousal relative to baseline, at a physiological level of analysis, no significant differences emerged across conditions. These findings are discussed in relation to offset control during recurrent interoceptive arousal, with implications for better understanding anxiety about abrupt bodily sensations.

Title: Cognitive therapy versus interoceptive exposure as treatment of panic disorder without agoraphobia

Authors: Arntz A.

Journal: Behav Res Ther. 2002 Mar;40(3):325-41. doi: 10.1016/s0005-7967(01)00014-6. PMID: 11863242.

Link to PubMed: Cognitive therapy versus interoceptive exposure as treatment of panic disorder without agoraphobia

Abstract: Cognitive therapy (CT) and interoceptive exposure (IE) as treatments of panic disorder without agoraphobia were compared in a sample of 69 patients, randomly allocated to condition. There were no significant differences between treatments as to reductions in panic frequency, daily anxiety levels and a composite questionnaire score, at posttest after the 12-session treatment, and at both follow-ups (4 weeks, 6 months). In both conditions, high percentages of patients were panic free at post and follow-up tests (range 75-92%). Although the reduction in idiosyncratic beliefs about the catastrophic nature of bodily sensations was equally strong in both conditions, post-treatment beliefs correlated strongly with symptoms at post and follow-up tests in the CT condition, but not in the IE condition. Reduction of beliefs may be essential in CT, but not in IE. This suggests that the two treatments utilize different change mechanisms.

Title: Anxiety sensitivity and interoceptive exposure: a transdiagnostic construct and change strategy

Authors: Boswell JF, Farchione TJ, Sauer-Zavala S, Murray HW, Fortune MR, Barlow DH.

Journal: Behav Ther. 2013 Sep;44(3):417-31. doi: 10.1016/j.beth.2013.03.006. Epub 2013 Apr 2. PMID: 23768669; PMCID: PMC3727659.

Link to full text: Anxiety sensitivity and interoceptive exposure: a transdiagnostic construct and change strategy

Abstract: Recent findings support the relevance of anxiety sensitivity (AS) and interoceptive exposure (IE) across emotional disorders. This study (a) evaluated levels of AS across different anxiety disorders, (b) examined change in AS over the course of transdiagnostic psychological intervention, and its relationship with outcome, and (c) described the implementation of IE to address AS with patients with different anxiety disorders. Participants (N=54) were patients who received treatment with the Unified Protocol for Transdiagnostic Treatment of Emotional Disorders (UP) in two consecutive treatment trials. Participants completed a measure of AS at pre- and posttreatment, and multiple occasions during treatment. Symptom severity was assessed at pre- and posttreatment, and clinical information related to physical symptoms and IE were collected as part of routine clinical practice. Elevated AS was observed at pretreatment across diagnoses and decreases in AS were observed from pre- to posttreatment. Similar changes occurred across the diagnostic categories, notably coinciding with the introduction of IE. Change in AS was correlated with reduced symptom levels at posttreatment and 6-month follow-up. Patients with different anxiety disorders endorsed similar physical symptoms and practiced similar IE exercises with similar effects. Results provide preliminary support for the usefulness of IE as a treatment strategy across the spectrum of anxiety disorders, and additional support for the transdiagnostic relevance of AS.

Title: Efficacy of interoceptive exposure therapy combined with trauma-related exposure therapy for posttraumatic stress disorder: a pilot study

Authors: Wald J, Taylor S.

Journal: J Anxiety Disord. 2007;21(8):1050-60. doi: 10.1016/j.janxdis.2006.10.010. Epub 2007 Jan 10. PMID: 17270392.

Link to PubMed: Efficacy of interoceptive exposure therapy combined with trauma-related exposure therapy for posttraumatic stress disorder: a pilot study

Abstract: The aim of this case series was to examine efficacy of interoceptive exposure (IE) combined with trauma-related exposure therapy (TRE) for posttraumatic stress disorder (PTSD). Seven participants completed treatment consisting of four weekly sessions of IE followed by eight weekly sessions of TRE (four sessions of imaginal exposure and four sessions of in vivo exposure). Assessments were conducted at pretreatment, posttreatment, 1- and 3-month follow-up. Outcome measures included PTSD symptoms, anxiety sensitivity, posttraumatic cognitions, anxiety, and depression. Five of the seven participants showed pre- to posttreatment improvements on these measures, and two participants showed less symptom reduction. Results at the 1-month follow-up showed that treatment gains were generally maintained in five (of the seven) participants and four of these individuals no longer met PTSD diagnostic criteria. Four individuals completed the 3-month follow-up and their symptoms and diagnostic status remained unchanged. These preliminary findings are promising. The next step in this line of research is to conduct a randomized, controlled trial to further to examine the efficacy, tolerability, and mechanisms of using IE in the treatment of PTSD.

Title: Responses to interoceptive exposure in people with posttraumatic stress disorder (PTSD): a preliminary analysis of induced anxiety reactions and trauma memories and their relationship to anxiety sensitivity and PTSD symptom severity

Authors: Wald J, Taylor S.

Journal: Cogn Behav Ther. 2008;37(2):90-100. doi: 10.1080/16506070801969054. PMID: 18470740.

Link to PubMed: Responses to interoceptive exposure in people with posttraumatic stress disorder (PTSD): a preliminary analysis of induced anxiety reactions and trauma memories and their relationship to anxiety sensitivity and PTSD symptom severity

Abstract: A growing body of evidence suggests that anxiety sensitivity (AS; fear of arousal-related sensations) plays a role in posttraumatic stress disorder (PTSD). Consistent with this, evidence indicates that interoceptive exposure (IE), which is a method for reducing AS, reduces PTSD symptoms. Clinical observations from our treatment studies indicate that IE triggers both anxiety and trauma memories in people with PTSD. The primary aim of this study was to describe the anxiety responses to a series of IE exercises and to examine whether or not trauma memories were activated. A secondary aim was to explore the relationships among AS, PTSD symptom severity, and IE responses. Data were collected from 23 people with PTSD who completed measures of PTSD symptoms and AS and a standardized battery of 10 IE exercises. Elevated anxiety and strong arousal responses were frequently elicited by the exercises, and trauma memories were also frequently triggered. AS and IE-triggered trauma memories significantly predicted IE-induced peak anxiety. The implications of the findings are discussed in terms of how IE might exert its therapeutic effects in the treatment of PTSD.

Title: Response to repeated CO2 in individuals with elevated anxiety sensitivity: replication with 20% CO2

Authors: Beck JG, Wolf MS.

Journal: J Behav Ther Exp Psychiatry. 2001 Mar;32(1):1-16. doi: 10.1016/s0005-7916(01)00018-0. PMID: 11729942.

Link to PubMed: Response to repeated CO2 in individuals with elevated anxiety sensitivity: replication with 20% CO2

Abstract: The present report replicates and extends previous work examining response patterns to repeated presentation of CO2. In previous studies, two distinct response patterns to repeated presentation of 35% CO2 were noted, representing habituation and nonhabituation of anxiety. In this report, 21 participants who had never experienced a panic attack but who reported high levels of anxiety sensitivity were presented with 12 trials of 20% CO2, followed by a trial involving inhalation of room air (to examine dishabituation) and two more trials of 20% CO2. Results indicated that 67% of the sample reported habituation of anxiety. Reductions in anxiety across inhalations were paralleled by changes in tidal volume, perceived panic symptom severity, and feelings of panic. Notable dishabituation was observed in the nonhabituation sample. Results are discussed in the light of basic learning processes underlying the treatment of Panic Disorder (PD).

Title: How does interoceptive exposure for panic disorder work? An uncontrolled case study

Authors: Beck JG, Shipherd JC, Zebb BJ.

Journal: Beck JG, Shipherd JC, Zebb BJ.

Link to PubMed: How does interoceptive exposure for panic disorder work? An uncontrolled case study

Abstract: To examine the influence of interoceptive exposure (IE) when used alone in the treatment of Panic Disorder (PD), 17 PD patients were presented with six IE sessions, using 35% CO2 as the exposure medium. The data indicate that IE alone is effective in reducing panic, panic-related fears, and general anxiety. However, the positive effects of IE do not appear to extend to agoraphobia, related fears, or depressed mood. Two distinct within-session patterns of fear response to IE were noted, one indicating habituation and the other indicating a lack of fear reduction. Although both patterns were associated with reductions in panic and anxiety following IE, the Habituators appeared to have a more positive outcome, which occurred more rapidly. These data suggest that IE may operate via two different pathways. Implications for understanding fear reduction are discussed, along with directions for future study.

Title: Repeated exposure to interoceptive cues: does habituation of fear occur in panic disorder patients? A preliminary report

Authors: Beck JG, Shipherd JC.

Journal: Behav Res Ther. 1997 Jun;35(6):551-7. doi: 10.1016/s0005-7967(97)00010-7. PMID: 9159979.

Link to PubMed: Repeated exposure to interoceptive cues: does habituation of fear occur in panic disorder patients? A preliminary report

Abstract: In order to explore the mechanism of action of interoceptive exposure, 12 Panic Disorder (PD) patients were presented with two sessions of repeated CO2 inhalation. Two distinct patterns of responding were noted. The first pattern was described as habituation of fear (n = 6). These patients showed decrements in pre- and post-inhalation anxiety during both sessions (with more rapid decline during session 2), as well as spontaneous recovery of fear at the onset of session 2. The second pattern indicated fear sensitization. These patients showed relatively low levels of anticipatory anxiety preceding CO2 inhalation during both sessions but reported robust increases in fear following gas inhalation. The extent of this increase was slightly less during session 2 relative to session I and did not appear to be mediated by cardiovascular arousal, as both groups showed rapid HR habituation during both sessions. Results are discussed in light of current theories of PD and its treatment.

Title: Fearful responding to repeated CO2 inhalation: a preliminary investigation

Authors: Beck JG, Shipherd JC, Zebb BJ.

Journal: Behav Res Ther. 1996 Aug;34(8):609-20. doi: 10.1016/0005-7967(96)00039-3. PMID: 8870287.

Link to PubMed: Fearful responding to repeated CO2 inhalation: a preliminary investigation

Abstract: In an effort to explore factors which maintain fear of physical sensations, repeated administration of 35% CO2 was used with college students scoring high and low on the Anxiety Sensitivity Index. Half of each group was administered 12 CO2 trials, while the other half received 9 CO2 trials, followed by a dishabituation trial (Trial 10) and 2 more CO2 administrations (Trials 11 and 12). Measures included subjective anxiety, heart rate, skin conductance, and number of panic symptoms reported. Results indicated a nonsignificant trend for the High ASI group to show increased pre-inhalation anxiety across trials, while the Low ASI group showed a rapid reduction in pre-inhalation anxiety. Post-inhalation skin conductance mirrored this pattern, although rapid reduction in post-inhalation heart rate was observed. Overall, the High ASI participants showed a notable lack of fear reduction across trials. Results are discussed in light of sensitization as a factor contributing to anticipatory anxiety, with implications for understanding the etiology and maintenance of Panic Disorder.

Title: Response patterns to repeated CO2 inhalation in individuals with high anxiety sensitivity

Authors: Beck JG, Shipherd JC, Read J.

Journal: Behav Res Ther. 1999 Nov;37(11):1073-89. doi: 10.1016/s0005-7967(98)00196-x. PMID: 10500321.

Link to PubMed: Response patterns to repeated CO2 inhalation in individuals with high anxiety sensitivity

Abstract: The present report extends previous work which has documented two distinct response patterns to repeated presentation of interoceptive cues (using CO2 inhalation) in PD patients [Beck, J. G. & Shipherd, J. C. (1997). Repeated exposure to interoceptive cues: does habituation of fear occur in panic disorder patients? Behaviour Research and Therapy, 35, 551-557]. We were interested in determining if these two patterns of fear habituation and sensitization would be noted in panic-naive individuals who reported high levels of Anxiety Sensitivity. A second aspect of this report examined whether attention to bodily sensations versus to neutral material would impact fear habituation and sensitization. Participants included 43 panic-naive individuals who scored at least 1 standard deviation above norms on the Anxiety Sensitivity Index. Results indicated that 37% of the sample reported habituation of fear, 47% reported fear sensitization and 16% demonstrated relatively stable fear levels across 12 inhalations of CO2 during session 1. The attentional manipulation did not exert a pronounced influence on anxiety, panic symptom severity, skin conductance, or heart rate in either Habituators or Sensitizers during session 2. These results are discussed in light of their relevance in understanding fundamental psychopathological processes underlying Panic Disorder.

Title: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Authors: Blechert J, Wilhelm FH, Meuret AE, Wilhelm EM, Roth WT.

Journal: Biol Psychol. 2010 Apr;84(1):104-11. doi: 10.1016/j.biopsycho.2010.01.002. Epub 2010 Jan 12. PMID: 20064582; PMCID: PMC2891596.

Link to full text: Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Abstract: Inhalation of carbon dioxide (CO₂) enriched air triggers anxiety in panic disorder (PD) patients, which is often interpreted as a sign of biological vulnerability. However, most studies have not measured respiration in these tasks. We compared patients with PD (n=20) and social phobia (SP, n=19) to healthy controls (n=18) during eight inhalations of 20% CO₂, preceded and followed by two inhalations of room air, while continuously measuring subjective anxiety and dyspnea as well as autonomic and respiratory variables. PD patients showed increased reactivity and delayed recovery during CO₂ inhalations for most measures. Unlike both other groups, the PD group's tidal volume responses did not habituate across CO₂ inhalations. However, PD patients did not differ from SP patients on most other measures, supporting a continuum model of CO₂ sensitivity across anxiety disorders. Both patient groups showed continued reactivity during the last air inhalations, which is unlikely to be due to a biological sensitivity.

Keywords: anxiety disorder; biological challenge; carbon dioxide; habituation; psychophysiology; sensitization.

Title: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Authors: Forsyth JP, Lejuez CW, Finlay C.

Journal: J Behav Ther Exp Psychiatry. 2000 Jun;31(2):103-21. doi: 10.1016/s0005-7916(00)00014-8. PMID: 11132115.

Link to PubMed: Anxiogenic effects of repeated administrations of 20% CO2-enriched air: stability within sessions and habituation across time

Abstract: Increasingly carbon dioxide-enriched air is being used as an aversive unconditioned stimulus in laboratory examinations of anxiety. Yet, little is known about the stability of the autonomic and subjective effects of this stimulus across repeated inhalations and sessions. We examined whether repeated administrations of high concentrations of CO2-enriched air produced either habituation, stability, or sensitization across several autonomic and self-report indices within one session (Experiment 1) and then several sessions (Experiment 2) of exposure. Results suggest that non-clinical participants do not habituate to CO2 within sessions, but do show habituation on cardiac and subjective report of anxiety across sessions. Individual difference factors such as anxiety sensitivity and suffocation fear seem to moderate some of these effects, including self-reported distress and anxiety in response to the challenge. These results support the use of CO2 as a panicogenic aversive stimulus in laboratory models of fear onset and in clinical settings for interoceptive exposure treatments of panic.

Title: The effect of successful treatment on the emotional and physiological response to carbon dioxide inhalation in patients with panic disorder

Authors: Gorman JM, Martinez J, Coplan JD, Kent J, Kleber M.

Journal: Biol Psychiatry. 2004 Dec 1;56(11):862-7. doi: 10.1016/j.biopsych.2004.08.016. PMID: 15576063.

Link to PubMed: The effect of successful treatment on the emotional and physiological response to carbon dioxide inhalation in patients with panic disorder

Abstract: Background: A number of studies have shown that patients with panic disorder are more likely to have panic attacks during carbon dioxide inhalation than are normal comparison subjects. Some studies have shown that antipanic medications can reduce the anxiogenic response to carbon dioxide, but none have shown if this is the case for cognitive behavioral therapy or if successful treatment reduces the respiratory physiologic response to carbon dioxide.

Methods: Twenty-five patients with panic disorder and 13 normal comparison subjects underwent baseline testing with 5% and 7% carbon dioxide inhalation. The patients were then retested after at least 12 weeks of treatment with either antipanic medication or cognitive behavioral therapy. Comparison subjects were retested after a similar interval.

Results: Successful treatment resulted in lower panic rates, and reduced anxiogenic response. Treatment had no effect, however, on the respiratory physiologic response.

Conclusions: There is dissociation in treatment response between the subjective and objective responses to carbon dioxide inhalation in panic disorder patients, with the former but not the latter showing positive change. We hypothesize that the strengthening of higher cortical control over subcortical fear-related structures, whether via medication or cognitive behavioral therapy treatment, results in less anxiety and fear in response to provoked symptoms reminiscent of naturally occurring panic.

Title: Effect of Antipanic Treatment on Response to Carbon Dioxide

Authors: Gorman JM, Browne ST, Papp LA, Martinez J, Welkowitz L, Coplan JD, Goetz RR, Kent J, Klein DF.

Journal: Biol Psychiatry. 1997 Dec 1;42(11):982-91. doi: 10.1016/s0006-3223(97)00160-1. PMID: 9386849.

Link to full text: Effect of Antipanic Treatment on Response to Carbon Dioxide

Abstract: Background: Disordered breathing among patients with panic disorder, including hyperventilation during attacks and increased anxiogenic response to carbon dioxide (CO2) inhalation, is well established. We wished to assess whether there is a change in the physiological response to CO2 after patients have undergone antipanic therapy with either tricyclic antidepressants or cognitive behavioral therapy (CBT).

Methods: Twenty-nine patients with panic disorder underwent baseline CO2 sensitivity testing using the traditional Read rebreathing method and then received either antidepressant treatment (n = 21) or CBT (n = 8). After completing treatment, CO2 testing was repeated. A comparison sample of 14 normal volunteers also had two CO2 sensitivity tests, separated by an average of 21.6 (SD = 8.8) weeks.

Results: Using a liberal standard, in which all CO2 sensitivity tests whose correlations between minute ventilation and end-tidal CO2 were at least .75 were used, patients, but not controls, demonstrated a significant reduction in CO2 sensitivity between the first and second test. Using a more conservative .90 correlation standard reduced the sample size available and resulted in trend reduction in patients but no significant change in controls. There was a suggestion that the change was most pronounced in treatment responders, although the number of patient nonresponders is extremely small in this sample.

Conclusions: These data indicate that treatment reduces CO2 sensitivity in patients with panic disorder. We speculate that manipulation of the serotonergic and noradrenergic neurotransmission systems, both known to play a role in the control of respiration, may have a specific effect in reducing respiratory hyperactivity in panic disorder.

Title: CO2 inhalation in the treatment of panic attacks

Authors: Griez E, van den Hout MA.

Journal: Behav Res Ther. 1986;24(2):145-50. doi: 10.1016/0005-7967(86)90085-9. PMID: 3083806.

Link to PubMed: CO2 inhalation in the treatment of panic attacks

Abstract: If panic disorder is conceptualized as a fear of interoceptive sensations, it can be treated by exposure to the autonomic symptoms that are induced by CO2 inhalation. In a cross-over study of 14 patients, propranolol was used as a control treatment. CO2 produced a significant reduction of fear of autonomic panic sensations, state and trait anxiety, agoraphobic anxiety and avoidance, and of the frequency of panicking. Propranolol resulted in improvements of state and of agoraphobic anxiety. Except for Trait Anxiety, CO2 was superior on all measures, most clearly on the Fear of Autonomic Sensations. At the 6-month follow-up, 3 patients required further treatment. Except for State Anxiety, the improvements were, on average, maintained. The clinical and theoretical implications are discussed.

Title: The relationship between the effect of lactate infusion on anxiety states, and their amelioration by carbon dioxide inhalation

Authors: Haslam MT.

Journal: Br J Psychiatry. 1974 Jul;125(0):88-90. doi: 10.1192/bjp.125.1.88. PMID: 4855036.

Link to PubMed: The relationship between the effect of lactate infusion on anxiety states, and their amelioration by carbon dioxide inhalation

Abstract: Anxiety is perhaps one of the commonest and most disabling symptoms in medicine, yet its satisfactory alleviation has been elusive. Work by Kluver and Bucy (1939) suggested that the amygdala generated anxiety, the function of the hippocampus being to decrease it. The hypothalamus appears to organize the level of arousal through the amygdala.

Title: Augmented respiratory relief. A new use of CO2 therapy in the treatment of phobic conditions: a preliminary report on two cases

Authors: Orwin A.

Journal: Br J Psychiatry. 1973 Feb;122(567):171-3. doi: 10.1192/bjp.122.2.171. PMID: 4714831.

Link to PubMed: Augmented respiratory relief. A new use of CO2 therapy in the treatment of phobic conditions: a preliminary report on two cases

Abstract: Respiratory relief (R.R.) appeared to be a simple and rapid treatment for specific phobias and for the phobic components of more complex neurotic conditions (Orwin, 1971). The basic hypothesis was that the R.R., satisfying an intense need to breathe, if simultaneous with the presentation of a phobic stimulus, would inhibit the degree of anxiety previously provoked and that this inhibition would persist. The need to breathe was induced by voluntary restriction up to maximum voluntary respiratory arrest (M.V.R.A.). As in conventional desensitization (Wolpe, 1958), a hierarchy was drawn up and the patients were desensitized using R.R. instead of relaxation. Carbon dioxide-oxygen mixtures were introduced because CO2 is known to stimulate respiration, and it was expected that by intensifying the need to breathe the speed and effectiveness of treatment would be increased by the augmented respiratory relief (A.R.R.).

Title: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Authors: Perna G, Bertani A, Caldirola D, Gabriele A, Cocchi S, Bellodi L.

Journal: J Clin Psychopharmacol. 2002 Jun;22(3):300-8. doi: 10.1097/00004714-200206000-00011. PMID: 12006901.

Link to PubMed: Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Abstract: Carbon dioxide (CO2) inhalation induces acute anxiety and panic attacks in patients with Panic Disorder (PD). Anti-panic drugs decrease CO2 reactivity after the first days of treatment; however, the clinical meaning of this finding has not yet been established. This study investigated the effects of treatment with tricyclic antidepressants and selective serotonin re-uptake inhibitors (SSRIs) on CO2 reactivity and compared the relationships between 35% CO2 hyperreactivity modulation and short-term clinical outcome. One hundred twenty-three patients with PD with or without agoraphobia who were hyperreactive to CO2 were randomly assigned to treatment groups with imipramine, clomipramine, paroxetine, sertraline, or fluvoxamine. A double-blind, randomized design was applied. Each patient received the 35% CO2 challenge on days 0, 7, and 30. The severity of clinical symptomatology was measured on days 0 and 30. Decreased hyperreactivity to 35% CO2 in all five treatment groups was already evident after the first week. The decrease in CO2 reactivity at the end of treatment was proportional to the degree of clinical improvement. Multiple regression analyses showed that the decrease in CO2 reactivity after the first week was a significant predictor for good clinical outcome after one month. The results of this study confirm evidence that psychoactive drugs effective in the treatment of PD decrease CO2 hyperreactivity. They also suggest that precocious modulation of CO2 reactivity might fairly reliably predict short-term clinical outcome in patients with "respiratory" PD.

Title: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Authors: Schmidt NB, Trakowski JH, Staab JP.

Journal: J Abnorm Psychol. 1997 Nov;106(4):630-8. doi: 10.1037//0021-843x.106.4.630. PMID: 9358693.

Link to PubMed: Extinction of panicogenic effects of a 35% CO2 challenge in patients with panic disorder

Abstract: Inhalations of high concentrations of carbon dioxide (CO2) reliably produce panic attacks in patients with panic disorder. The present study evaluated whether cognitive-behavioral treatment (CBT) for panic disorder would extinguish CO2-induced panic and whether changes in panic and arousal-related cognitions were associated with the induction of panic. Patients with panic disorder (N = 54) were assigned to 1 of 3 experimental conditions: CBT with respiratory training (CBT-R), CBT without respiratory training (CBT), or delayed treatment. Participants received 5 repeated vital-capacity inhalations of 35% CO2/65% O2 prior to and following either 12 treatment sessions or a 12-week waiting period. During pretreatment assessments, 74% of patients experienced a panic attack during at least 1 inhalation. At posttreatment, only 20% of treated participants (CBT-R = 19%, CBT = 22%), compared with 64% of untreated participants, panicked. Forty-four percent of treated participants, compared with 0% of untreated participants, reported no anxiety during all posttreatment inhalations. Anxiety sensitivity as well as panic appraisals regarding the likelihood of panic and self-efficacy with coping with panic were significantly related to fearful responding to the CO2 challenge.

Title: The effects of inhaling a 35 per cent CO2-65 per cent O2 mixture upon anxiety level in neurotic patients

Authors: Slater SL, Leavy A.

Journal: Behav Res Ther. 1966 Nov;4(4):309-16. doi: 10.1016/0005-7967(66)90027-1. PMID: 5978686.

Link to PubMed: The effects of inhaling a 35 per cent CO2-65 per cent O2 mixture upon anxiety level in neurotic patients

Abstract: Wolpe has reported the use of a 35 per cent CO2-65 per cent O2 mixture to relax patients who cannot otherwise be relaxed sufficiently to allow desensitization. In this investigation, the anxiety-reducing effects of single inhalations of a 35 per cent CO2-65 per cent O2 were studied on twelve patients, all of whom had been diagnosed by their doctors as having a high level of anxiety. On the basis of the subjects' ratings, this mixture was more effective than was an equal volume of air which was breathed through the same apparatus as was the experimental mixture. Evidence is offered which suggests that the decrement in anxiety which followed administration of the 35 per cent CO2-65 per cent O2 mixture was due to some effect of the gas other than the hyperventilation which it induces.

Title: Mechanism of change in cognitive-behavioral treatment of panic disorder: evidence for the fear of fear mediational hypothesis

Authors: Smits JA, Powers MB, Cho Y, Telch MJ.

Journal: J Consult Clin Psychol. 2004 Aug;72(4):646-52. doi: 10.1037/0022-006X.72.4.646. PMID: 15301649.

Link to PubMed: Mechanism of change in cognitive-behavioral treatment of panic disorder: evidence for the fear of fear mediational hypothesis

Abstract: Numerous clinical trials have demonstrated the efficacy of cognitive-behavioral treatment (CBT) for panic disorder. However, studies investigating the mechanisms responsible for improvement with CBT are lacking. The authors used regression analyses outlined by R. M. Baron and D. A. Kenny (1986) to test whether a reduction in fear of fear (FOF) underlies improvement resulting from CBT. Pre- and posttreatment measures were collected from 90 CBT-treated patients and 40 wait-list control participants. Overall, treatment accounted for 31% of the variance in symptom reduction. The potency of FOF as a mediator varied as a function of symptom facet, as full mediation was observed for the change in global disability, whereas the effects of CBT on agoraphobia, anxiety, and panic frequency were partially accounted for by reductions in FOF. Clinical implications and future research directions are discussed.

Title: Reduction of CO2-induced anxiety in patients with panic attacks after repeated CO2 exposure

Authors: van den Hout MA, van der Molen GM, Griez E, Lousberg H, Nansen A.

Journal: Am J Psychiatry. 1987 Jun;144(6):788-91. doi: 10.1176/ajp.144.6.788. PMID: 3109265.

Link to PubMed: Reduction of CO2-induced anxiety in patients with panic attacks after repeated CO2 exposure

Abstract: The authors compared the subjective reaction of 13 panic patients and eight control subjects to a 35% CO2 challenge, a treatment known to produce physical symptoms comparable to those of natural or lactate-induced panic, and to placebo treatment (inhalation of air). They found that patients had higher placebo scores than control subjects, patients tended to get highly anxious on CO2 and control subjects did not, and CO2-induced subjective anxiety in patients decreased as the number of CO2-induced exposures to interoceptive anxiety symptoms increased. The data support a behavioral account of the effects of anxiogenics.

Title: Carbon dioxide inhalation treatments of neurotic anxiety. An overview

Authors: Wolpe J.

Journal: J Nerv Ment Dis. 1987 Mar;175(3):129-33. doi: 10.1097/00005053-198703000-00001. PMID: 3102688.

Link to PubMed: Carbon dioxide inhalation treatments of neurotic anxiety. An overview

Abstract: A lucky chance more than 30 years ago revealed the remarkable efficacy of single inhalations of high concentrations of carbon dioxide in eliminating or markedly reducing free-floating anxiety. The reduction of anxiety lasts for days, weeks, or longer--well beyond the persistence of carbon dioxide in the body. The effects are explicable on the hypothesis that free-floating anxiety is anxiety conditioned to continuously present sources of stimulation, such as background noise or the awareness of space or time, and that the anxiety response habit is weakened when the anxiety is inhibited by the competition of responses that carbon dioxide induces. More recently, it has become apparent that inhalations of carbon dioxide, applied in a different manner, are effective in overcoming maladaptive anxiety responses to specific stimuli, e.g., social stimuli. The substance is also proving to be a valuable resource in the treatment of the common variety of panic attacks.

Title: Comparison of the panicogenic effect of cholecystokinin 30-33 and carbon dioxide in panic disorder

Authors: Bradwejn J, Koszycki D.

Journal: Prog Neuropsychopharmacol Biol Psychiatry. 1991;15(2):237-9. doi: 10.1016/0278-5846(91)90086-g. PMID: 1908105.

Link to PubMed: Comparison of the panicogenic effect of cholecystokinin 30-33 and carbon dioxide in panic disorder

Abstract: 1. Twenty-two patients who met DSM-III-R criteria for panic disorder received either cholecystokinin 30-33 (25 micrograms i.v.) or 33% carbon dioxide. 2. The principal outcome measures of the study included the number and sum intensity of panic symptoms and the incidence of panic attacks. 3. The incidence of panic attacks tended (P = .07) to be higher with cholecystokinin 30-33 than with carbon dioxide. Nevertheless, patients who panicked within each group did not differ significantly with regard to the number and sum intensity of symptoms or symptom profile. 4. That cholecystokinin 30-33 and 35% carbon dioxide induced panic attacks which were qualitatively and quantitatively similar suggest that these agents might act on distinct systems that have a final common target or a final common mechanism of action.

Title: Restoring Acid-sensing ion channel-1a in the amygdala of knock-out mice rescues fear memory but not unconditioned fear responses

Authors: Coryell MW, Wunsch AM, Haenfler JM, Allen JE, McBride JL, Davidson BL, Wemmie JA.

Journal: J Neurosci. 2008 Dec 17;28(51):13738-41. doi: 10.1523/JNEUROSCI.3907-08.2008. PMID: 19091964; PMCID: PMC2651157.

Link to full text: Restoring Acid-sensing ion channel-1a in the amygdala of knock-out mice rescues fear memory but not unconditioned fear responses

Abstract: Acid-sensing ion channel-1a (ASIC1a) contributes to multiple fear behaviors, however the site of ASIC1a action in behavior is not known. To explore a specific location of ASIC1a action, we expressed ASIC1a in the basolateral amygdala of ASIC1a-/- mice using viral vector-mediated gene transfer. This rescued context-dependent fear memory, but not the freezing deficit during training or the unconditioned fear response to predator odor. These data pinpoint the basolateral amygdala as the site where ASIC1a contributes to fear memory. They also discriminate fear memory from fear expressed during training and from unconditioned fear. Furthermore, this work illustrates a strategy for identifying discrete brain regions where specific genes contribute to complex behaviors.

Title: Baseline respiratory parameters in panic disorder: a meta-analysis

Authors: Grassi M, Caldirola D, Vanni G, Guerriero G, Piccinni M, Valchera A, Perna G.

Journal: J Affect Disord. 2013 Apr 5;146(2):158-73. doi: 10.1016/j.jad.2012.08.034. Epub 2012 Oct 27. PMID: 23107756.

Link to PubMed: Baseline respiratory parameters in panic disorder: a meta-analysis

Abstract: Background: The presence of abnormalities in baseline respiratory function of subjects with panic disorder (PD) is expected according to PD respiratory theories. We aimed to meta-analyze results from studies comparing baseline respiratory and hematic parameters related to respiration between subjects with PD and controls.

Methods: A literature research in bibliographic databases was performed. Fixed-effects models were applied for all parameters while random-effects models only when suitable (at least 10 independent studies). Several moderator analyses and publication bias diagnostics were performed.

Results: We found significantly higher mean minute ventilation and lower et-pCO(2) in subjects with PD than controls. Moreover we also found evidences of reduced HCO(3)(-) and PO(4)(-) hematic concentrations, higher indexes of respiratory variability/irregularity and higher rate of sighs and apneas. Evidence of heterogeneity was partly explained by moderator analyses. No relevant publication bias was found.

Limitations: Several shortcomings affected the included studies, such as over-inclusive recruitment criteria, samples unbalanced for socio-demographic characteristics, lack of statistical details and small number of studies available for several parameters.

Discussion: Our results support the idea of abnormalities in respiratory function of subjects with PD. Compared to controls, they showed baseline hyperventilation; the results from hematic parameters suggest that hyperventilation may be chronic and not simply caused by their high anxiety levels during respiratory assessment. Evidences of higher variability and irregularity in respiratory patterns of subjects with PD were also found. It is unclear to what extent the higher rate of sighs and apneas may explain the other baseline respiratory abnormalities found in PD.

Title: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Authors: Johnson PL, Fitz SD, Hollis JH, Moratalla R, Lightman SL, Shekhar A, Lowry CA.

Journal: J Psychopharmacol. 2011 Jan;25(1):26-36. doi: 10.1177/0269881109353464. Epub 2010 Jan 15. PMID: 20080924.

Link to PubMed: Induction of c-Fos in 'panic/defence'-related brain circuits following brief hypercarbic gas exposure

Abstract: Inspiration of air containing high concentrations of carbon dioxide (CO(2); hypercarbic gas exposure) mobilizes respiratory, sympathetic and hypothalamic-pituitary-adrenal axis responses and increases anxiety-like behaviour in rats and humans. Meanwhile the same stimulus induces panic attacks in the majority of panic disorder patients. However, little is known about the neural circuits that regulate these acute effects. In order to determine the effects of acute hypercarbic gas exposure on forebrain and brainstem circuits, conscious adult male rats were placed in flow cages and exposed to either atmospheric air or increasing environmental CO(2) concentrations (from baseline concentrations up to 20% CO(2)) during a 5 min period. The presence of immunoreactivity for the protein product of the immediate-early gene c-fos was used as a measure of functional cellular responses. Exposing rats to hypercarbic gas increased anxiety-related behaviour and increased numbers of c-Fos-immunoreactive cells in subcortical regions of the brain involved in: (1) the initiation of fear- or anxiety-associated behavioural responses (i.e. the dorsomedial hypothalamus, perifornical nucleus and dorsolateral and ventrolateral periaqueductal gray); (2) mobilization of the hypothalamic-pituitary-adrenal axis (i.e. the dorsomedial hypothalamus, perifornical nucleus and paraventricular hypothalamic nucleus); and (3) initiation of stress-related sympathetic responses (i.e. the dorsomedial hypothalamus, dorsolateral periaqueductal grey and rostroventrolateral medulla). These findings have implications for understanding how the brain senses changes in environmental CO(2) concentrations and the neural mechanisms underlying the subsequent adaptive changes in stress-related physiology and behaviour.

Title: Neural pathways underlying lactate-induced panic

Authors: Johnson PL, Truitt WA, Fitz SD, Lowry CA, Shekhar A.

Journal: Neuropsychopharmacology. 2008 Aug;33(9):2093-107. doi: 10.1038/sj.npp.1301621. Epub 2007 Dec 5. PMID: 18059441; PMCID: PMC3065200.

Link to full text: Neural pathways underlying lactate-induced panic

Abstract: Panic disorder is a severe anxiety disorder characterized by susceptibility to induction of panic attacks by subthreshold interoceptive stimuli such as 0.5 M sodium lactate infusions. Although studied for four decades, the mechanism of lactate sensitivity in panic disorder has not been understood. The dorsomedial hypothalamus/perifornical region (DMH/PeF) coordinates rapid mobilization of behavioral, autonomic, respiratory and endocrine responses to stress, and rats with disrupted GABA inhibition in the DMH/PeF exhibit panic-like responses to lactate, similar to panic disorder patients. Utilizing a variety of anatomical and pharmacological methods, we provide evidence that lactate, via osmosensitive periventricular pathways, activates neurons in the compromised DMH/PeF, which relays this signal to forebrain limbic structures such as the bed nucleus of the stria terminalis to mediate anxiety responses, and specific brainstem sympathetic and parasympathetic pathways to mediate the respiratory and cardiovascular components of the panic-like response. Acutely restoring local GABAergic tone in the DMH/PeF blocked lactate-induced panic-like responses. Autonomic panic-like responses appear to be a result of DMH/PeF-mediated mobilization of sympathetic responses (verified with atenolol) and resetting of the parasympathetically mediated baroreflex. Based on our findings, DMH/PeF efferent targets such as the C1 adrenergic neurons, paraventricular hypothalamus, and the central amygdala are implicated in sympathetic mobilization; the nucleus of the solitary tract is implicated in baroreflex resetting; and the parabrachial nucleus is implicated in respiratory responses. These results elucidate neural circuits underlying lactate-induced panic-like responses and the involvement of both sympathetic and parasympathetic systems.

Title: A key role for orexin in panic anxiety

Authors: Johnson PL, Truitt W, Fitz SD, Minick PE, Dietrich A, Sanghani S, Träskman-Bendz L, Goddard AW, Brundin L, Shekhar A.

Journal: Nat Med. 2010 Jan;16(1):111-5. doi: 10.1038/nm.2075. Epub 2009 Dec 27. PMID: 20037593; PMCID: PMC2832844.

Link to full text: A key role for orexin in panic anxiety

Abstract: Panic disorder is a severe anxiety disorder with recurrent, debilitating panic attacks. In individuals with panic disorder there is evidence of decreased central gamma-aminobutyric acid (GABA) activity as well as marked increases in autonomic and respiratory responses after intravenous infusions of hypertonic sodium lactate. In a rat model of panic disorder, chronic inhibition of GABA synthesis in the dorsomedial-perifornical hypothalamus of rats produces anxiety-like states and a similar vulnerability to sodium lactate-induced cardioexcitatory responses. The dorsomedial-perifornical hypothalamus is enriched in neurons containing orexin (ORX, also known as hypocretin), which have a crucial role in arousal, vigilance and central autonomic mobilization, all of which are key components of panic. Here we show that activation of ORX-synthesizing neurons is necessary for developing a panic-prone state in the rat panic model, and either silencing of the hypothalamic gene encoding ORX (Hcrt) with RNAi or systemic ORX-1 receptor antagonists blocks the panic responses. Moreover, we show that human subjects with panic anxiety have elevated levels of ORX in the cerebrospinal fluid compared to subjects without panic anxiety. Taken together, our results suggest that the ORX system may be involved in the pathophysiology of panic anxiety and that ORX antagonists constitute a potential new treatment strategy for panic disorder.

Title: Respiratory Feedback for Treating Panic Disorder

Authors: Meuret AE, Wilhelm FH, Roth WT.

Journal: J Clin Psychol. 2004 Feb;60(2):197-207. doi: 10.1002/jclp.10245. PMID: 14724927.

Link to PubMed: Respiratory Feedback for Treating Panic Disorder

Abstract: Panic disorder patients often complain of shortness of breath or other respiratory complaints, which has been used as evidence for both hyperventilation and false suffocation alarm theories of panic. Training patients to change their breathing patterns is a common intervention, but breathing rarely has been measured objectively in assessing the patient or monitoring therapy results. We report a new breathing training method that makes use of respiratory biofeedback to teach individuals to modify four respiratory characteristics: increased ventilation (Respiratory Rate x Tidal Volume), breath-to-breath irregularity in rate and depth, and chest breathing. As illustrated by a composite case, feedback of respiratory rate and end-tidal pCO2 can facilitate voluntary control of respiration and reduce symptoms. Respiratory monitoring may provide relevant diagnostic, prognostic, and outcome information.

Title: Changes in respiration mediate changes in fear of bodily sensations in panic disorder

Authors: Meuret AE, Rosenfield D, Hofmann SG, Suvak MK, Roth WT.

Journal: J Psychiatr Res. 2009 Mar;43(6):634-41. doi: 10.1016/j.jpsychires.2008.08.003. Epub 2008 Oct 5. PMID: 18835608; PMCID: PMC3327292.

Link to full text: Changes in respiration mediate changes in fear of bodily sensations in panic disorder

Abstract: The purpose of the study was to examine whether changes in pCO(2) mediate changes in fear of bodily sensation (as indexed by anxiety sensitivity) in a bio-behavioral treatment for panic disorder that targets changes in end-tidal pCO(2). Thirty-five panic patients underwent 4 weeks of capnometry-assisted breathing training targeting respiratory dysregulation. Longitudinal mediation analyses of the changes in fear of bodily symptoms over time demonstrated that pCO(2), but not respiration rate, was a partial mediator of the changes in anxiety sensitivity. Results were supported by cross lag panel analyses, which indicated that earlier pCO(2) levels predicted later levels of anxiety sensitivity, but not vice versa. PCO(2) changes also led to changes in respiration rate, questioning the importance of respiration rate in breathing training. The results provide little support for changes in fear of bodily sensations leading to changes in respiration, but rather suggest that breathing training targeting pCO(2) reduced fear of bodily sensations in panic disorder.

Title: Feedback of end-tidal pCO2 as a therapeutic approach for panic disorder

Authors: Meuret AE, Wilhelm FH, Ritz T, Roth WT.

Journal: J Psychiatr Res. 2008 Jun;42(7):560-8. doi: 10.1016/j.jpsychires.2007.06.005. Epub 2007 Aug 3. PMID: 17681544; PMCID: PMC2890048.

Link to full text: Feedback of end-tidal pCO2 as a therapeutic approach for panic disorder

Abstract: Background: Given growing evidence that respiratory dysregulation is a central feature of panic disorder (PD) interventions for panic that specifically target respiratory functions could prove clinically useful and scientifically informative. We tested the effectiveness of a new, brief, capnometry-assisted breathing therapy (BRT) on clinical and respiratory measures in PD.

Methods: Thirty-seven participants with PD with or without agoraphobia were randomly assigned to BRT or to a delayed-treatment control group. Clinical status, respiration rate, and end-tidal pCO(2) were assessed at baseline, post-treatment, 2-month and 12-month follow-up. Respiratory measures were also assessed during homework exercises using a portable capnometer as a feedback device.

Results: Significant improvements (in PD severity, agoraphobic avoidance, anxiety sensitivity, disability, and respiratory measures) were seen in treated, but not untreated patients, with moderate to large effect sizes. Improvements were maintained at follow-up. Treatment compliance was high for session attendance and homework exercises; dropouts were few.

Conclusions: The data provide preliminary evidence that raising end-tidal pCO(2) by means of capnometry feedback is therapeutically beneficial for panic patients. Replication and extension will be needed to verify this new treatment's efficacy and determine its mechanisms.

Title: Does fear reactivity during exposure predict panic symptom reduction?

Authors: Meuret AE, Seidel A, Rosenfield B, Hofmann SG, Rosenfield D.

Journal: J Consult Clin Psychol. 2012 Oct;80(5):773-85. doi: 10.1037/a0028032. Epub 2012 Apr 9. PMID: 22486408; PMCID: PMC3404244.

Link to full text: Does fear reactivity during exposure predict panic symptom reduction?

Abstract: Objective: Fear reactivity during exposure is a commonly used indicator of learning and overall therapy outcome. The objective of this study was to assess the predictive value of fear reactivity during exposure using multimodal indicators and an advanced analytical design. We also investigated the degree to which treatment condition (cognitive training vs. respiratory skill training) moderated fear reactivity and therapeutic outcome.

Method: Thirty-four patients with panic disorder and agoraphobia completed a total of 123 in-vivo exposure sessions, comprising 3 weekly sessions and a 4th session 2 months following therapy completion. Sessions varied in length and phobic stimuli. Cardiorespiratory physiology (heart rate, carbon dioxide partial pressure [PCO2], respiration rate) and experiential symptoms (panic symptoms and anxiety) were assessed repeatedly throughout exposure sessions, in addition to weekly assessments of panic cognitions, avoidance, and functioning.

Results: Panic symptomatology decreased substantially in both treatment conditions during therapy and follow-up. Significant cardiorespiratory and experiential reactivity was observed during all exposures, characterized by activation followed by reduction. Greater within-session activation of anxiety and panic symptoms was inversely related to improvement in panic symptoms severity, but neither physiological activation nor within- or between-session reduction of either physiological or experiential variables was predictive of outcome. No moderating effects of treatment condition were found.

Conclusions: Fear activation and reduction during exposure are weak predictors of corrective learning and fear extinction. Clinical implications for exposure therapy and directions for future research are discussed.

Title: Do unexpected panic attacks occur spontaneously?

Authors: Meuret AE, Rosenfield D, Wilhelm FH, Zhou E, Conrad A, Ritz T, Roth WT.

Journal: Biol Psychiatry. 2011 Nov 15;70(10):985-91. doi: 10.1016/j.biopsych.2011.05.027. Epub 2011 Jul 23. PMID: 21783179; PMCID: PMC3327298.

Link to full text: Do unexpected panic attacks occur spontaneously?

Abstract: Background: Spontaneous or unexpected panic attacks, per definition, occur "out of the blue," in the absence of cues or triggers. Accordingly, physiological arousal or instability should occur at the onset of, or during, the attack, but not preceding it. To test this hypothesis, we examined if points of significant autonomic changes preceded the onset of spontaneous panic attacks.

Methods: Forty-three panic disorder patients underwent repeated 24-hour ambulatory monitoring. Thirteen natural panic attacks were recorded during 1960 hours of monitoring. Minute-by-minute epochs beginning 60 minutes before and continuing to 10 minutes after the onset of individual attacks were examined for respiration, heart rate, and skin conductance level. Measures were controlled for physical activity and vocalization and compared with time matched control periods within the same person.

Results: Significant patterns of instability across a number of autonomic and respiratory variables were detected as early as 47 minutes before panic onset. The final minutes before onset were dominated by respiratory changes, with significant decreases in tidal volume followed by abrupt carbon dioxide partial pressure increases. Panic attack onset was characterized by heart rate and tidal volume increases and a drop in carbon dioxide partial pressure. Symptom report was consistent with these changes. Skin conductance levels were generally elevated in the hour before, and during, the attacks. Changes in the matched control periods were largely absent.

Conclusions: Significant autonomic irregularities preceded the onset of attacks that were reported as abrupt and unexpected. The findings invite reconsideration of the current diagnostic distinction between uncued and cued panic attacks.

Title: Brain glucose and lactate levels during ventilator-induced hypo- and hypercapnia

Authors: van Hulst RA, Lameris TW, Haitsma JJ, Klein J, Lachmann B.

Journal: Clin Physiol Funct Imaging. 2004 Jul;24(4):243-8. doi: 10.1111/j.1475-097X.2004.00557.x. PMID: 15233840.

Link to PubMed: Brain glucose and lactate levels during ventilator-induced hypo- and hypercapnia

Abstract: Objective: Levels of glucose and lactate were measured in the brain by means of microdialysis in order to evaluate the effects of ventilator-induced hypocapnia and hypercapnia on brain metabolism in healthy non-brain-traumatized animals.

Design and setting: Prospective animal study in a university laboratory.

Subjects: Eight adult Landrace/Yorkshire pigs.

Interventions: The microdialysis probe was inserted in the brain along with a multiparameter sensor and intracranial pressure (ICP) probe. The animals were ventilated in a pressure-controlled mode according to the open lung concept with an inspired oxygen fraction of 0.4/1.0. Starting at normoventilation (PaCO(2) +/-40 mmHg) two steps of both hypercapnia (PCO(2) +/- 70 and 100 mmHg) and hypocapnia (PaCO(2) +/- 20 and 30 mmHg) were performed. Under these conditions, brain glucose and lactate levels as well as brain oxygen (PbrO(2)), brain carbon dioxide (PbrCO(2)), brain pH (brpH), brain temperature and ICP were measured.

Results: At hypercapnia (PaCO(2) = 102.7 mmHg) there were no significant changes in brain glucose and lactate but there was a significant increase in PbrCO(2), PbrO(2) and ICP. In contrast, at hypocapnia (PCO(2) = 19.8 mmHg) there was a significant increase in brain lactate and a significant decrease in both brain glucose and PbrCO(2).

Conclusions: Hypocapnia decreases brain glucose and increases brain lactate concentration, indicating anaerobic metabolism, whereas hypercapnia has no influence on levels of brain glucose and brain lactate.

Title: Voluntary hyperventilation in the treatment of panic disorder--functions of hyperventilation, their implications for breathing training, and recommendations for standardization

Authors: Meuret AE, Ritz T, Wilhelm FH, Roth WT.

Journal: Clin Psychol Rev. 2005 May;25(3):285-306. doi: 10.1016/j.cpr.2005.01.002. PMID: 15792851.

Link to PubMed: Voluntary hyperventilation in the treatment of panic disorder--functions of hyperventilation, their implications for breathing training, and recommendations for standardization

Abstract: Hyperventilation has numerous theoretical and empirical links to anxiety and panic. Voluntary hyperventilation (VH) tests have been applied experimentally to understand psychological and physiological mechanisms that produce and maintain anxiety, and therapeutically in the treatment of anxiety disorders. From the theoretical perspective of hyperventilation theories of anxiety, VH is useful diagnostically to the clinician and educationally to the patient. From the theoretical perspective of cognitive-behavior therapy, VH is a way to expose patients with panic disorder to sensations associated with panic and to activate catastrophic cognitions that need restructuring. Here we review panic disorder treatment studies using breathing training that have included VH. We differentiate the roles of VH in diagnosis, education about symptoms, training of breathing strategies, interoceptive exposure, and outcome measurement--discussing methodological issues specific to these roles and VH test reliability and validity. We propose how VH procedures might be standardized in future studies.

Title: Carbon dioxide hypersensitivity, hyperventilation, and panic disorder

Authors: Papp LA, Klein DF, Gorman JM.

Journal: Am J Psychiatry. 1993 Aug;150(8):1149-57. doi: 10.1176/ajp.150.8.1149. PMID: 8392296.

Link to PubMed: Carbon dioxide hypersensitivity, hyperventilation, and panic disorder

Abstract: Objective: The purpose of this article is to offer a comprehensive, data-based explanation of the relationship between hyperventilation and panic disorder linking CO2 hypersensitivity, cognitive/behavioral factors, and the respiratory effects of antipanic pharmacologic and psychological treatments.

Method: The authors conducted a computerized search of MEDLINE for relevant articles.

Results: Some panic patients have a chronic, subtle respiratory disturbance. Acute hyperventilation is neither necessary nor sufficient for panic to occur. Respiratory abnormalities in panic patients may adaptively aim at coping with a hypersensitive CO2 chemoreceptor system. Pharmacologic panicogens also stimulate the respiratory system, causing hyperventilation. Triggering this hypersensitive respiratory control mechanism may incite panic. Antipanic medications may reset the receptor threshold. Misattribution and catastrophic interpretation of somatic symptoms or the sense of loss of control may contribute to panic symptoms. Behavioral interventions such as desensitization or breathing retraining may block the full-blown attack. Cognitive strategies through cognitive control of respiration may supplement and accentuate these interventions.

Conclusions: Panic disorder may be due to an inherently unstable autonomic nervous system, coupled with cognitive distress.

Title: Respiratory muscle tension as symptom generator in individuals with high anxiety sensitivity

Authors: Ritz T, Meuret AE, Bhaskara L, Petersen S.

Journal: Psychosom Med. 2013 Feb;75(2):187-95. doi: 10.1097/PSY.0b013e31827d1072. Epub 2013 Jan 16. PMID: 23324873.

Link to PubMed: Respiratory muscle tension as symptom generator in individuals with high anxiety sensitivity

Abstract: Objective: Anxiety and panic are associated with the experience of a range of bodily symptoms, in particular unpleasant breathing sensations (dyspnea). Respiratory theories of panic disorder have focused on disturbances in blood gas regulation, but respiratory muscle tension as a source of dyspnea has not been considered. We therefore examined the potential of intercostal muscle tension to elicit dyspnea in individuals with high anxiety sensitivity, a risk factor for developing panic disorder.

Methods: Individuals high and low in anxiety sensitivity (total N=62) completed four tasks: electromyogram biofeedback for tensing intercostal muscle, electromyogram biofeedback for tensing leg muscles, paced breathing at three different speeds, and a fine motor task. Global dyspnea, individual respiratory sensations, nonrespiratory sensations, and discomfort were assessed after each task, whereas respiratory pattern (respiratory inductance plethysmography) and end-tidal carbon dioxide (capnography) were measured continuously.

Results: In individuals with high compared to low anxiety sensitivity, intercostal muscle tension elicited a particularly strong report of obstruction (M=5.1, SD=3.6 versus M=2.5, SD=3.0), air hunger (M=1.9, SD=2.1 versus M=0.4, SD=0.8), hyperventilation symptoms (M=0.6, SD=0.6 versus M=0.1, SD=0.1), and discomfort (M=5.1, SD=3.2 versus M=2.2, SD=2.1) (all p values<.05). This effect was not explained by site-unspecific muscle tension, voluntary manipulation of respiration, or sustained task-related attention. Nonrespiratory control sensations were not significantly affected by tasks (F<1), and respiratory variables did not reflect any specific responding of high-Anxiety Sensitivity Index participants to intercostal muscle tension.

Conclusions: Respiratory muscle tension may contribute to the respiratory sensations experienced by panic-prone individuals. Theories and treatments for panic disorder should consider this potential source of symptoms.

Title: Autonomic function in panic disorder: cardiorespiratory and plasma catecholamine responsivity to multiple challenges of the autonomic nervous system

Authors: Stein MB, Asmundson GJ.

Journal: Biol Psychiatry. 1994 Oct 15;36(8):548-58. doi: 10.1016/0006-3223(94)90619-x. PMID: 7827218.

Link to PubMed: Autonomic function in panic disorder: cardiorespiratory and plasma catecholamine responsivity to multiple challenges of the autonomic nervous system

Abstract: Panic disorder has been widely hypothesized to be associated with dysfunction of the autonomic nervous system. In this study, 24 patients with panic disorder and 26 healthy control subjects took part in a broad battery of autonomic function tests, each designed to stress the autonomic nervous system in a particular fashion. Testing consisted of postural challenge, isometric exercise, cold pressor, and Valsalva maneuver. Dependent measures included heart rate, vagal tone, blood pressure, respiratory frequency, end-tidal CO2 levels, and plasma norepinephrine and epinephrine levels. The testing procedures reliably produced changes in autonomic output in the expected directions, but patients with panic disorder were not found to differ from healthy controls in their cardiorespiratory or plasma catecholaminergic responses. This pattern of normal autonomic responsivity in the patients with panic disorder was evident across multiple test conditions with varying autonomic demand characteristics, thereby supporting the integrity of autonomic regulatory systems in this illness. These data run counter to a simple notion of autonomic dysfunction in panic disorder.

Title: The bed nucleus of the stria terminalis is critical for anxiety-related behavior evoked by CO2 and acidosis

Authors: Taugher RJ, Lu Y, Wang Y, Kreple CJ, Ghobbeh A, Fan R, Sowers LP, Wemmie JA.

Journal: J Neurosci. 2014 Jul 30;34(31):10247-55. doi: 10.1523/JNEUROSCI.1680-14.2014. PMID: 25080586; PMCID: PMC4115136.

Link to full text: The bed nucleus of the stria terminalis is critical for anxiety-related behavior evoked by CO2 and acidosis

Abstract: Carbon dioxide (CO2) inhalation lowers brain pH and induces anxiety, fear, and panic responses in humans. In mice, CO2 produces freezing and avoidance behavior that has been suggested to depend on the amygdala. However, a recent study in humans with bilateral amygdala lesions revealed that CO2 can trigger fear and panic even in the absence of amygdalae, suggesting the importance of extra-amygdalar brain structures. Because the bed nucleus of the stria terminalis (BNST) contributes to fear- and anxiety-related behaviors and expresses acid-sensing ion channel-1A (ASIC1A), we hypothesized that the BNST plays an important role in CO2-evoked fear-related behaviors in mice. We found that BNST lesions decreased both CO2-evoked freezing and CO2-conditioned place avoidance. In addition, we found that CO2 inhalation caused BNST acidosis and that acidosis was sufficient to depolarize BNST neurons and induce freezing behavior; both responses depended on ASIC1A. Finally, disrupting Asic1a specifically in the BNST reduced CO2-evoked freezing, whereas virus-vector-mediated expression of ASIC1A in the BNST of Asic1a(-/-) and Asic1a(+/+) mice increased CO2-evoked freezing. Together, these findings identify the BNST as an extra-amygdalar fear circuit structure important in CO2-evoked fear-related behavior.

Keywords: ASIC1A; CO2; anxiety; bed nucleus of the stria terminalis; pH; panic.

Title: Acid-base dysregulation and chemosensory mechanisms in panic disorder: a translational update

Authors: Vollmer LL, Strawn JR, Sah R.

Journal: Transl Psychiatry. 2015 May 26;5(5):e572. doi: 10.1038/tp.2015.67. PMID: 26080089; PMCID: PMC4471296.

Link to full text: Acid-base dysregulation and chemosensory mechanisms in panic disorder: a translational update

Abstract: Panic disorder (PD), a complex anxiety disorder characterized by recurrent panic attacks, represents a poorly understood psychiatric condition which is associated with significant morbidity and an increased risk of suicide attempts and completed suicide. Recently however, neuroimaging and panic provocation challenge studies have provided insights into the pathoetiology of panic phenomena and have begun to elucidate potential neural mechanisms that may underlie panic attacks. In this regard, accumulating evidence suggests that acidosis may be a contributing factor in induction of panic. Challenge studies in patients with PD reveal that panic attacks may be reliably provoked by agents that lead to acid-base dysbalance such as CO2 inhalation and sodium lactate infusion. Chemosensory mechanisms that translate pH into panic-relevant fear, autonomic, and respiratory responses are therefore of high relevance to the understanding of panic pathophysiology. Herein, we provide a current update on clinical and preclinical studies supporting how acid-base imbalance and diverse chemosensory mechanisms may be associated with PD and discuss future implications of these findings.

Title: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Authors: Ziemann AE, Allen JE, Dahdaleh NS, Drebot II, Coryell MW, Wunsch AM, Lynch CM, Faraci FM, Howard MA 3rd, Welsh MJ, Wemmie JA.

Journal: Cell. 2009 Nov 25;139(5):1012-21. doi: 10.1016/j.cell.2009.10.029. PMID: 19945383; PMCID: PMC2808123.

Link to full text: The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior

Abstract: The amygdala processes and directs inputs and outputs that are key to fear behavior. However, whether it directly senses fear-evoking stimuli is unknown. Because the amygdala expresses acid-sensing ion channel-1a (ASIC1a), and ASIC1a is required for normal fear responses, we hypothesized that the amygdala might detect a reduced pH. We found that inhaled CO(2) reduced brain pH and evoked fear behavior in mice. Eliminating or inhibiting ASIC1a markedly impaired this activity, and localized ASIC1a expression in the amygdala rescued the CO(2)-induced fear deficit of ASIC1a null animals. Buffering pH attenuated fear behavior, whereas directly reducing pH with amygdala microinjections reproduced the effect of CO(2). These data identify the amygdala as an important chemosensor that detects hypercarbia and acidosis and initiates behavioral responses. They also give a molecular explanation for how rising CO(2) concentrations elicit intense fear and provide a foundation for dissecting the bases of anxiety and panic disorders.

Title: Carbon dioxide for neurogenic orthostatic hypotension in adults: a novel therapy

Authors: Baker JR, Ranada SI, Incognito AV, Sheldon RS, Morillo CA, Wilson RJA, Phillips AA, Raj SR.

Journal: Eur Heart J. 2024 Oct 7:ehae653. doi: 10.1093/eurheartj/ehae653. Epub ahead of print. PMID: 39374345.

Link to PubMed: Carbon dioxide for neurogenic orthostatic hypotension in adults: a novel therapy

Abstract: Neurogenic orthostatic hypotension (nOH) is a hallmark feature of autonomic nervous system failure. On standing, people with nOH experience large reductions in blood pressure (≥20/10 mmHg) and debilitating symptoms (e.g. light-headedness, blurred vision, and syncope). Current blood pressure therapies have limited efficacy and potentially serious side effects, including supine hypertension, creating a strong clinical need for new therapeutic approaches. Previous studies examining the effects of hypocapnia [i.e. low arterial carbon dioxide (CO2)] on blood pressure have shown that hyperventilation-induced hypocapnia decreases supine blood pressure in nOH patients and can be readily prevented with exogenous CO2. Whether exogenous CO2 can prevent OH in these patients is unknown. In this proof-of-concept study, we tested the hypothesis that increased inspired CO2 increases standing blood pressure in patients with nOH.

Title: Hyperventilation syndrome after general anesthesia: Our experience

Authors: Bansal T, Hooda S.

Journal: J Anaesthesiol Clin Pharmacol. 2016 Oct-Dec;32(4):536-537. doi: 10.4103/0970-9185.168192. PMID: 28096595; PMCID: PMC5187629.

Link to full text: Hyperventilation syndrome after general anesthesia: Our experience

Abstract: We read with interest the article published in J Anaesthesiol Clin Pharmacol 2015;31:284-5. Tomioka et al. have reported two cases of hyperventilation syndrome after general anesthesia. These authors have described that hyperventilation syndrome reflects the enhancement of sympathoadrenal tone. The first case was managed with midazolam. The second case was managed with beta-adrenergic blocker, propranolol hydrochloride. Hyperventilation syndrome often occurs under stressful conditions and has been reported before, during or after anesthesia and surgery. Hyperventilation is associated with multiple somatic symptoms due to hypocalcemia induced by respiratory alkalosis. We wish to share our experience of a 30 years old full term parturient presenting in labor in a highly anxious state with symptoms and signs of hypocalcemia in the form of circumoral numbness and severe carpopedal spasm. The patient was managed successfully with reassurance and infusion of 10 ml of 10% calcium gluconate over 10 min. The blood sample taken for ionised serum calcium before administering calcium gluconate showed a low level of 0.7 mmol/L (normal range 1–1.5 mmol/L). We did not use anxiolytic or beta blocker for fear of any untoward effect on the fetus as the patient was in labor.

Title: Humans without a sense of smell breathe differently

Authors: Gorodisky, L., Honigstein, D., Weissbrod, A. et al.

Journal: Nat Commun 15, 8809 (2024). https://doi.org/10.1038/s41467-024-52650-6

Link to full text: Humans without a sense of smell breathe differently

Abstract: Olfaction may play a restricted role in human behavior, yet paradoxically, its absence in anosmia is associated with diverse deleterious outcomes, culminating in reduced life expectancy. The mammalian nose serves two purposes: olfaction and breathing. Because respiratory patterns are impacted by odors, we hypothesized that nasal respiratory airflow may be altered in anosmia. We apply a wearable device that precisely logs nasal airflow for 24-hour-long sessions in participants with isolated congenital anosmia and controls. We observe significantly altered patterns of respiratory nasal airflow in anosmia in wake and in sleep. These differences allow classification of anosmia at 83% accuracy using the respiratory trace alone. Patterns of respiratory airflow have pronounced impact on health, emotion and cognition. We therefore suggest that a portion of the deleterious outcomes associated with anosmia may be attributed to altered patterns of respiratory nasal airflow rather than a direct result of lost odor perception per se.

Title: Dismantling the Component-Specific Effects of Yogic Breathing: Feasibility of a Fully Remote Three-Arm RCT with Virtual Laboratory Visits and Wearable Physiology

Authors: Ma Y, Yang H, Vazquez M, Buraks O, Haack M, Mullington JM, Goldstein MR.

Journal: Int J Environ Res Public Health. 2023 Feb 11;20(4):3180. doi: 10.3390/ijerph20043180. PMID: 36833875; PMCID: PMC9958552.

Link to full text: Dismantling the Component-Specific Effects of Yogic Breathing: Feasibility of a Fully Remote Three-Arm RCT with Virtual Laboratory Visits and Wearable Physiology

Abstract: Despite the growing research base examining the benefits and physiological mechanisms of slow-paced breathing (SPB), mindfulness (M), and their combination (as yogic breathing, SPB + M), no studies have directly compared these in a ”dismantling” framework. To address this gap, we conducted a fully remote three-armed feasibility study with wearable devices and video-based laboratory visits. Eighteen healthy participants (age 18–30 years, 12 female) were randomized to one of three 8-week interventions: slow-paced breathing (SPB, N = 5), mindfulness (M, N = 6), or yogic breathing (SPB + M, N = 7). The participants began a 24-h heart rate recording with a chest-worn device prior to the first virtual laboratory visit, consisting of a 60-min intervention-specific training with guided practice and experimental stress induction using a Stroop test. The participants were then instructed to repeat their assigned intervention practice daily with a guided audio, while concurrently recording their heart rate data and completing a detailed practice log. The feasibility was determined using the rates of overall study completion (100%), daily practice adherence (73%), and the rate of fully analyzable data from virtual laboratory visits (92%). These results demonstrate feasibility for conducting larger trial studies with a similar fully remote framework, enhancing the ecological validity and sample size that could be possible with such research designs.

Title: Increased net water loss by oral compared to nasal expiration in healthy subjects

Authors: Svensson S, Olin AC, Hellgren J.

Journal: Rhinology. 2006 Mar;44(1):74-7. PMID: 16550955.

Link to full text: Increased net water loss by oral compared to nasal expiration in healthy subjects

Abstract: Aim of the study: To compare the difference in respiratory water loss during expiration through the nose and through the mouth, in healthy subjects. Methods: The study included 19 healthy, non-smoking volunteers without any present history of non-infectious rhinitis, presenting with symptoms of rhinitis, asthma or previous nasal surgery. Nasal and oral expiratory breath condensates were collected using a breath condenser during tidal respiration at indoor resting conditions. During the nasal breath condensate sampling, the subjects were breathing into a transparent face mask covering the nose and the mouth with the mouth closed. During the oral breath condensate sampling, the subjects inhaled through the nose and exhaled through a mouthpiece connected to the condenser. The airflow during the sampling was assessed with a dry-spirometer connected to the condenser. Sampling was stopped after 100 litres of expired air for each breathing mode. Nasal sampling was done before and after decongestion of the nasal mucosa with oxymetazoline, 0.5 mg/ml. The effect on the nasal mucosa was assessed with acoustic rhinometry. Results: The mean loss of expired water was 42% less by nasal expiration before decongestion than by oral expiration (1.9 x 10(-3) g/L min compared to 2.7 x 10(-3) g/L min, p < 0.001). The mean expiratory minute ventilation was 9.0 L/min by nasal respiration and 9.8 L/min by oral respiration. Decongestion of the nasal mucosa showed a mean increase of the cross-sectional area at 4 cm from the nostril (1.44 to 1.67 cm2, p = 0.0024), but there was no effect on the net water loss (1.9 x 10(-3) g/Lmin vs 1.9 x 10(-3) g/Lmin). Conclusion: This study showed that the net water loss increased by 42% when the breathing mode was switched from nasal to oral expiration during tidal breathing in healthy subjects. Increased water and energy loss by oral breathing could be a contributing factor to the symptoms seen in patients suffering from nasal obstruction.

Title: Levels of end-tidal carbon dioxide are low despite normal respiratory rate in individuals with long COVID

Authors: Wood J, Tabacof L, Tosto-Mancuso J, McCarthy D, Kontorovich A, Putrino D.

Journal: J Breath Res. 2021 Dec 8;16(1). doi: 10.1088/1752-7163/ac3c18. PMID: 34808607.

Link to full text: Levels of end-tidal carbon dioxide are low despite normal respiratory rate in individuals with long COVID

Abstract: ETCO2 and respiratory rate (RR) were measured in patients attending an outpatient clinic using an EMMA capnograph (Masimo Corporation, Irvine, CA). Approval for the collection and publication of this data was provided by the relevant Institutional Review Board (STUDY-20-001537). All patients were previously reviewed by a physician and diagnosed with confirmed (via PCR and/or antibody test) or probable (according to World Health Organization and Center for Disease Control and Prevention guidelines [6, 7]) past COVID-19 infection. The criteria for diagnosis of long COVID was the presence of persistent symptoms for at least 12 weeks after initial infection [2]. The capnograph was calibrated and a healthy control was used to check device readings at the commencement of each clinic. At the time of assessment, patients were seated and at rest, and had not performed other assessments or procedures immediately prior. Patients were instructed to breathe orally through the capnograph adapter at their resting rate and depth for a minimum of 15 s, and until the ETCO2 level was stable for a further period of at least 5 s. Data were analyzed using Stata (StataCorp LLC, TX), with descriptive statistics and Pearson correlation coefficient provided.

Title: Effects of decortication and carotid sinus nerve section on ventilation of the rat

Authors: Maskrey M, Megirian D, Nicol SC.

Journal: Respir Physiol. 1981 Mar;43(3):263-73. doi: 10.1016/0034-5687(81)90108-0. PMID: 6792671.

Link to full text: Effects of decortication and carotid sinus nerve section on ventilation of the rat

Abstract: The effect on ventilation of exposure to hypoxic, hypercapnic and hypoxic/hypercapnic gas mixtures was studied before and after functional decortication of intact rats and rats in which the carotid chemoreceptors had been disconnected. Unanaesthetized rats responded to both hypoxia and hypercapnia with an increase in minute ventilation (V) through increases in both frequency (f) and tidal volume (VT). Decortication led to a greater V response to CO2. This was through an effect on f, rather than VT. Carotid sinus nerve section (CSNS) caused a lessening in the V response to gas mixtures, f and VT being equally affected. Decortication, following CSNS, increased the V response but this time through increased VT rather than f. This effect on VT was not specific to any particular gas mixture. It is concluded that the carotid body chemoreceptors, together with the bulbopontine rate controller, influence the response to CO2. It is further suggested that this integration takes place in the reticular formation and is normally under some degree of inhibition from the cerebral cortex.

Title: CO2, brainstem chemoreceptors and breathing

Authors: Nattie E.

Journal: Prog Neurobiol. 1999 Nov;59(4):299-331. doi: 10.1016/s0301-0082(99)00008-8. PMID: 10501632.

Link to full text: CO2, brainstem chemoreceptors and breathing

Abstract: The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.

Title: Ventilatory response of decorticate and decerebrate cats to hypoxia and CO2

Authors: Tenney SM, Ou LC.

Journal: Respir Physiol. 1977 Feb;29(1):81-92. doi: 10.1016/0034-5687(77)90119-0. PMID: 847311.

Link to PubMed: Ventilatory response of decorticate and decerebrate cats to hypoxia and CO2

Abstract: The steady state ventilatory response of normal, fully awake cats was studied under graded hypoxia (at PAO2 = 110, 55, 45 torr) with PACO2 controlled throughout at the resting, normoxic level and at +5 torr. Subsequently, either a mid-collicular decerebration or a decortication was performed, and the ventilatory studies were repeated. Respiratory frequency, tidal volume, and ventilation in the decerebrate state responded to hypoxia and hypercapnia in a manner indistinguishable from the control. The decorticate cats, however, exhibited an exaggerated response to hypoxia, principally the result of increased frequency. The negative hypoxic, hypercapnic interaction, characteristic of awake cats, was demonstrable in both the decerebrate and decorticate animals. The findings are interpreted as revealing coupled descending influences on the medullary respiratory centers in hypoxia--one that is facilitatory and originates in the diencephalon, and the other, inhibitory, from the cerebrum. The significance of this suprapontine system in normal hypoxic ventilatory control is discussed.

Title: Air hunger' from increased Pco2 persists after complete neuromuscular block in humans

Authors: Banzett RB, Lansing RW, Brown R, Topulos GP, Yager D, Steele SM, Londoño B, Loring SH, Reid MB, Adams L, et al.

Journal: Respir Physiol. 1990 Jul;81(1):1-17. doi: 10.1016/0034-5687(90)90065-7. PMID: 2120757.

Link to PubMed: Air hunger' from increased Pco2 persists after complete neuromuscular block in humans

Abstract: The tolerance of totally curarized subjects for prolonged breath hold is viewed by many as evidence that respiratory muscle contraction is essential to generate the sensation of breathlessness. Although conflicting evidence exists, none of it was obtained during total neuromuscular block. We completely paralyzed four normal, unsedated subjects with vecuronium (a non-depolarizing neuromuscular blocker). Subjects were mechanically ventilated with hyperoxic gas mixtures at fixed rate and tidal volume. End-expiratory PCO2 (PETCO2) was varied surreptitiously by changing inspired PCO2. Subjects rated their respiratory discomfort or 'air hunger' every 45 sec. At low PETCO2 (median 35 Torr) they felt little or no air hunger. When PETCO2 was raised (median 44 Torr) all subjects reported severe air hunger. They had reported the same degree of air hunger at essentially the same PETCO2 before paralysis. When questioned afterwards all subjects said the sensation could be described by the terms 'air hunger', 'urge to breathe', and 'shortness of breath', and that is was like breath holding. They reported no fundamental difference in the sensation before and after paralysis. We conclude that respiratory muscle contraction is not important in the genesis of air hunger evoked by hypercapnia.

Title: Accuracy of respiratory symptom perception in different affective contexts

Authors: Bogaerts K, Notebaert K, Van Diest I, Devriese S, De Peuter S, Van den Bergh O.

Journal: J Psychosom Res. 2005 Jun;58(6):537-43. doi: 10.1016/j.jpsychores.2004.12.005. PMID: 16125521.

Link to PubMed: Accuracy of respiratory symptom perception in different affective contexts

Abstract: Objective: The accuracy of respiratory symptom perception was investigated in different affective contexts in participants (N=48) scoring high or low for negative affectivity (NA). Methods: Within-subject correlations were calculated between two subjective ratings and their respective physiological referent (the rating of "deeper breathing" and respiratory volume, and rating of "faster breathing" and breathing frequency) across nine consecutive breathing trials. Three different air mixtures were used: room air, air enriched with 7.5% CO(2), and with 10% CO(2). For half the participants, the trials were framed in a pleasant context, created by adding a pleasant odour to the air mixture in addition to information announcing pleasant feelings as a result of breathing the air mixtures. The other half received the trials in a distressing context: A foul smelling odour was added and the information announced unpleasant feelings. Results: High-NA persons were overall less accurate than were low-NA persons in the perception of respiratory volume. For breathing frequency, high-NA persons were significantly less accurate in the distressing condition than in the pleasant one, whereas for low-NA persons, the information frame did not matter. Conclusion: The study shows that the accuracy of respiratory symptom reports is reduced in high-NA persons, especially in a distressing context.

Title: A prospective examination of distress tolerance and early smoking lapse in adult self-quitters

Authors: Brown RA, Lejuez CW, Strong DR, Kahler CW, Zvolensky MJ, Carpenter LL, Niaura R, Price LH.

Journal: Nicotine Tob Res. 2009 May;11(5):493-502. doi: 10.1093/ntr/ntp041. Epub 2009 Apr 16. PMID: 19372572; PMCID: PMC2671465.

Link to full text: A prospective examination of distress tolerance and early smoking lapse in adult self-quitters

Abstract: Introduction: A significant percentage of smokers attempting cessation lapse to smoking within a matter of days, and current models of relapse devote insufficient attention to such early smoking lapse. Studies attempting to relate severity of nicotine withdrawal symptoms to short-term smoking cessation outcomes have yielded equivocal results. How one reacts to the discomfort of nicotine withdrawal and quitting smoking (i.e., distress tolerance) may be a more promising avenue of investigation with important treatment implications. Methods: The present investigation examined distress tolerance and early smoking lapse using a prospective design. Participants were 81 adult daily smokers recruited through newspaper advertisements targeted at smokers planning to quit smoking without assistance (i.e., no pharmacotherapy or psychosocial treatment; 42 males and 39 females; mean age = 42.6 years, SD = 12.20). Results: As hypothesized, both greater breath-holding duration and carbon dioxide-enriched air persistence were associated with a significantly lower risk of smoking lapse following an unaided quit attempt. These effects were above and beyond the risk associated with levels of nicotine dependence, education, and history of major depressive disorder, suggesting that distress tolerance and task persistence may operate independently of risk factors such as nicotine dependence and depressive history. In contrast to expectation, persistence on the Paced Auditory Serial Addition Test (a psychological challenge task) was not a significant predictor of earlier smoking lapse. Discussion: These results are discussed in relation to refining theoretical models of the role of distress tolerance in early smoking lapse and the utility of such models in the development of specialized treatment approaches for smoking cessation.

Title: Distress Tolerance and Duration of Past Smoking Cessation Attempts

Authors: Brown RA, Lejuez CW, Kahler CW, Strong DR.

Journal: J Abnorm Psychol. 2002 Feb;111(1):180-5. PMID: 11866171.

Link to PubMed: Distress Tolerance and Duration of Past Smoking Cessation Attempts

Abstract: The present study tested the hypothesis that limited ability to tolerate physical and psychological distress is associated with early relapse from smoking cessation. Specifically, the authors exposed 16 current smokers who had failed to sustain any previous quit attempt for more than 24 hr (immediate relapsers) and 16 smokers with at least 1 sustained quit attempt of 3 months or longer (delayed relapsers) to psychological (mental arithmetic) and physical (carbon dioxide inhalation-breath holding) stressors. Relative to delayed relapsers, immediate relapsers were characterized by higher baseline levels of affective vulnerability, by greater levels of dysphoria and urge to smoke after 12 hr of nicotine deprivation, and by less task persistence on the stressors, suggesting that these may be risk factors for early lapse in the context of quitting smoking.

Title: Rebreathing to cope with hyperventilation: experimental tests of the paper bag method

Authors: van den Hout MA, Boek C, van der Molen GM, Jansen A, Griez E.

Journal: J Behav Med. 1988 Jun;11(3):303-10. doi: 10.1007/BF00844435. PMID: 3139884.

Link to PubMed: Rebreathing to cope with hyperventilation: experimental tests of the paper bag method

Abstract: To explore if and how the common rebreathing (paper bag) approach to hyperventilation works, two experiments were carried out. In the first experiment, 12 normal volunteers, aware of the common physiological rebreathing rationale, were twice asked to overbreath intensely and then either to rebreath or to restart normal ventilation. Alveolar CO2 increased more quickly and physical symptoms disappeared earlier in the rebreathing condition. The second experiment had a similar design; however, this time the subjects were led to believe that, after both hyperventilation provocation tests, they were rebreathing in a semiclosed tube system. On one of the occasions, the tube system was, in fact, open. The CO2 restoration again differed in the two conditions. In this second experiment, the moment of symptom disappearance was not significantly earlier in the rebreathing condition. The combined results of the studies suggest that expectation and suggestion play a role in the effects of rebreathing approaches on hyperventilation.

Title: Ventilatory response to CO2. II. Studies in neurotic psychiatric patients and practitioners of transcendental meditation

Authors: Singh BS.

Journal: Psychosom Med. 1984 Jul-Aug;46(4):347-62. doi: 10.1097/00006842-198407000-00004. PMID: 6435149.

Link to PubMed: Ventilatory response to CO2. II. Studies in neurotic psychiatric patients and practitioners of transcendental meditation

Abstract: The prominence of respiratory symptoms in patients with neurotic psychiatric disorders is noted and the literature on the control of respiration is reviewed to attempt to explain this finding. A previous study demonstrated a positive correlation between the ventilatory response to CO2 (S) and neurotic personality traits in a group of normal subjects. This study attempted to follow up this finding and hypothesized that a group of neurotically disturbed patients would have a higher S value and a group of individuals who practiced a calming technique such as transcendental meditation (TM) would have a lower S value than normal subjects. The second hypothesis was confirmed, but not the first, in that the neurotically disturbed patients had the lowest mean values for S of the three groups, rather than the highest. Particular characteristics of the sample of psychiatric patients cast doubt, however, on the validity of this finding. Three additional findings of this study were that anxious, depressive, and hyperventilating subject groups were no different from one another in terms of S values; that very experienced TM practitioners (sidhas) could significantly lower their ventilatory response to CO2 in the meditating state as compared to the nonmeditating alert state; and that the S value did not increase in two male subjects with endogenous depression after successful treatment with electroconvulsive therapy.

Title: Pattern of breathing and ventilatory response to CO2 in subjects practicing hatha-yoga

Authors: Stănescu DC, Nemery B, Veriter C, Maréchal C.

Journal: J Appl Physiol Respir Environ Exerc Physiol. 1981 Dec;51(6):1625-9. doi: 10.1152/jappl.1981.51.6.1625. PMID: 6798002.

Link to PubMed: Pattern of breathing and ventilatory response to CO2 in subjects practicing hatha-yoga

Abstract: WE studied eight Belgian subjects well advanced in the practice of hatha-yoga and compared them with eight sex-, age-, and height-matched control subjects. Practice of yoga (range 4-12 yr) involves control of posture and manipulation of breathing, including slow near-vital capacity maneuvers accompanied by apnea at end inspiration and end expiration. Average values for the yoga and the control group (in parentheses) are as follows: ventilation (VE) 5.53 1 X min-1 (7.07); tidal volume (VT), 1.03 liters (0.56); rate of breathing, 5.5 min-1 (13.4); end-tidal PCO2, 39.0 Torr (35.3). All differences are significant (P less than 0.05). Ventilatory response to CO2 (rebreathing technique) was significantly lower in the yoga group (P less than 0.01). The regression relating VE to VT during rebreathing of CO2 was VE = 8.1 (VT - 0.23) for the yoga group and VE = 15.8 (VT - 0.16) for the control group (P less than 0.005). We attribute these changes to chronic manipulation of respiration.

Title: Effect of transcendental meditation on breathing and respiratory control

Authors: Wolkove N, Kreisman H, Darragh D, Cohen C, Frank H.

Journal: J Appl Physiol Respir Environ Exerc Physiol. 1984 Mar;56(3):607-12. doi: 10.1152/jappl.1984.56.3.607. PMID: 6423589.

Link to PubMed: Effect of transcendental meditation on breathing and respiratory control

Abstract: We studied the effect of transcendental meditation (TM) on breathing using 16 experienced meditators and 16 control subjects. In controls, there was no significant difference in minute ventilation (VE), respiratory pattern, or hypercapnic response, whether breathing with eyes open-awake (CA), or with eyes closed-relaxing (CR). In meditators, VE decreased significantly during quiet breathing from 14.0 +/- 0.7 1/min with eyes open-awake (MA) to 12.4 +/- 0.6 1/min during meditation (MM) (P less than 0.02). The change in VE during meditation was due to a decrease in tidal volume (VT) resulting from a shortened inspiratory time (TI). Meditation was associated with a decreased response to progressive hypercapnia from 3.7 +/- 0.4 to 2.5 +/- 0.21 X min-1 X Torr-1 during MA and MM trials, respectively (P less than 0.01). During meditation VT was smaller at a given alveolar PCO2 than during MA studies because of a decrease in mean inspiratory flow rate (VT/TI). These observations suggest that an alteration in wakefulness, more subtle than sleep or the unconscious state, can significantly affect the chemical and neural regulation of breathing.

Title: Does a change in end-tidal carbon dioxide level predict high altitude mountain sickness?

Authors: Thundiyil JG, Williams AT, Little I, Stutsman M, Ladde JG, Papa L.

Journal: Heliyon. 2023 May 5;9(5):e16000. doi: 10.1016/j.heliyon.2023.e16000. PMID: 37215892; PMCID: PMC10192757.

Link to full text: Does a change in end-tidal carbon dioxide level predict high altitude mountain sickness?

Abstract: Background It is postulated that lack of hypoxic ventilatory response is a predictor for AMS. End-tidal carbon dioxide (ETCO2) is an accurate, noninvasive surrogate measure of ventilation. Objectives We sought to determine if changes in baseline ETCO2 predicts the development of AMS. Methods This prospective cohort study took place in three separate high-altitude hiking treks. Subjects included a convenience sample of hikers. Predictor variable was change in ETCO2 levels and outcome variable was AMS. Measurements of ETCO2 levels were obtained at the base and repeated daily at various elevations and the summit of each hike. Concurrently, hikers were scored for AMS by a trained investigator. We utilized correlation coefficients and developed a linear regression model for analysis. Results 21 subjects in 3 separate hikes participated: 10 ascended to 19,341 ft over 7 days, 6 ascended to 8900 ft in 1 day, and 4 ascended to 11,006 ft in 1 day. Mean age was 40 years, 67% were males, mean daily elevation gain was 2150 ft, and 5 hikers developed AMS. The correlation coefficients for ETCO2 and development of AMS were −0.46 (95%CI -0.33 to −0.57), and −0.77 (95%CI -0.71 to −0.83) for ETCO2 and altitude. ETCO2 predicted the development of symptoms better than the elevation with AUCs of 0.90 (95%CI 0.81–0.99) versus 0.64 (95%CI 0.45–0.83). An ETCO2 measurement of ≤22 mmHg was 100% sensitive and 60% specific for predicting AMS. Conclusions ETCO2 was strongly correlated with altitude and moderately correlated with AMS and it was a better predictor than altitude.

Title: Effect of carbon dioxide in acute mountain sickness: a rediscovery

Authors: Harvey TC, Raichle ME, Winterborn MH, Jensen J, Lassen NA, Richardson NV, Bradwell AR.

Journal: Lancet. 1988 Sep 17;2(8612):639-41. doi: 10.1016/s0140-6736(88)90465-5. Erratum in: Lancet 1988 Oct 1;2(8614):808. PMID: 2901515.

Link to PubMed: Effect of carbon dioxide in acute mountain sickness: a rediscovery

Abstract: The effect of adding CO2 to inhaled air in six subjects with acute mountain sickness was investigated during a medical expedition to 5400 m.3% CO2 in ambient air increased ventilation and resulted in a rise in PaO2 of between 24% and 40%. There was a 9-28% increase in PaCO2 and a reduction of the respiratory alkalosis normally seen at high altitude. Symptoms of acute mountain sickness were rapidly relieved. In three subjects cerebral blood flow increased by 17-39%, so that oxygen delivery to the brain would have been considerably improved. This study confirms earlier suggestions of the beneficial effect of CO2 inhalation at high altitude.

Title: Supplemental CO2 improves oxygen saturation, oxygen tension, and cerebral oxygenation in acutely hypoxic healthy subjects

Authors: Stepanek J, Dunn RA, Pradhan GN, Cevette MJ.

Journal: Physiol Rep. 2020 Jul;8(14):e14513. doi: 10.14814/phy2.14513. PMID: 32725791; PMCID: PMC7387889.

Link to full text: Supplemental CO2 improves oxygen saturation, oxygen tension, and cerebral oxygenation in acutely hypoxic healthy subjects

Abstract: Oxygen is viewed in medicine as the sole determinant of tissue oxygenation, though carbon dioxide homeostasis is equally important and clinically often ignored. The aims of this study were as follows: (a) to examine the effects of different acute hypoxic conditions on partial pressure of arterial oxygen ( PaO2), arterial oxygen saturation of hemoglobin (SaO2), and regional cerebral saturation of hemoglobin (rSO2 ); and (b) to evaluate supplemental CO2 as a tool to improve oxygenation in acutely hypoxic individuals. We hypothesized that exposure to gas mixtures with added CO2 would improve oxygenation in hypoxic human subjects. Twenty healthy subjects were exposed to 5-min intervals of two gas mixtures: hypoxic gas mixture containing 8% oxygen, and a CO2-enriched mixture containing 8% oxygen plus either 3% or 5% CO2. Ten subjects received the 3% CO2 -enriched mixture, and the remaining 10 subjects received the 5% CO2 -enriched mixture. The order of exposure was randomized. Blood gases, pulse oximetry, end-tidal CO2, and cerebral oximetry were measured. Compared to the purely hypoxic gas group, PaO2 was increased in the 3% and 5% CO2-enriched groups by 14.9 and 9.5 mmHg, respectively. Compared to pure hypoxia, SaO2 was increased in the 3% and 5% CO2-enriched groups by 16.8% and 12.9%, respectively. Both CO2-enriched gas groups had significantly higher end-exposure rSO2 and recovered to baseline rSO2 within 1 min, compared to the pure hypoxic gas group, which returned to baseline in 5 min. These results suggest that in acutely hypoxic subjects, CO2 supplementation improves blood oxygen saturation and oxygen tension as well as cerebral oxygenation measures.

Title: Rapid elimination of CO through the lungs: coming full circle 100 years on

Authors: Fisher JA, Iscoe S, Fedorko L, Duffin J.

Journal: Exp Physiol. 2011 Dec;96(12):1262-9. doi: 10.1113/expphysiol.2011.059428. Epub 2011 Oct 3. PMID: 21967899; PMCID: PMC3274699.

Link to full text: Rapid elimination of CO through the lungs: coming full circle 100 years on

Abstract: At the start of the 20th century, CO poisoning was treated by administering a combination of CO2 and O2 (carbogen) to stimulate ventilation. This treatment was reported to be highly effective, even reversing the deep coma of severe CO poisoning before patients arrived at the hospital. The efficacy of carbogen in treating CO poisoning was initially attributed to the absorption of CO2; however, it was eventually realized that the increase in pulmonary ventilation was the predominant factor accelerating clearance of CO from the blood. The inhaled CO2 in the carbogen stimulated ventilation but prevented hypocapnia and the resulting reductions in cerebral blood flow. By then, however, carbogen treatment for CO poisoning had been abandoned in favour of hyperbaric O2. Now, a half-century later, there is accumulating evidence that hyperbaric O2 is not efficacious, most probably because of delays in initiating treatment. We now also know that increases in pulmonary ventilation with O2-enriched gas can clear CO from the blood as fast, or very nearly as fast, as hyperbaric O2. Compared with hyperbaric O2, the technology for accelerating pulmonary clearance of CO with hyperoxic gas is not only portable and inexpensive, but also may be far more effective because treatment can be initiated sooner. In addition, the technology can be distributed more widely, especially in developing countries where the prevalence of CO poisoning is highest. Finally, early pulmonary CO clearance does not delay or preclude any other treatment, including subsequent treatment with hyperbaric O2.

Title: A randomized controlled trial of theophylline versus CO2 inhalation for treating apnea of prematurity

Authors: Al-Saif S, Alvaro R, Manfreda J, Kwiatkowski K, Cates D, Qurashi M, Rigatto H.

Journal: J Pediatr. 2008 Oct;153(4):513-8. doi: 10.1016/j.jpeds.2008.04.025. Epub 2008 Jun 4. PMID: 18534618.

Link to PubMed: A randomized controlled trial of theophylline versus CO2 inhalation for treating apnea of prematurity

Abstract: Objective: To determine whether inhalation of 0.8% CO(2) in preterm infants decreases the duration and rate of apnea as effectively as or better than theophylline with fewer adverse side effects. Study design: A prospective, randomized, control study of 42 preterm infants of gestational age 27 to 32 weeks assigned to receive inhaled CO(2) (n = 21) or theophylline (n = 21). The study group had a mean (+/- standard error of the mean) birth weight of 1437 +/- 57 g, gestational age of 29.4 +/- 0.3 weeks, and postnatal age of 43 +/- 4 days. After a control period, 0.8% CO(2) or theophylline was given for 2 hours, followed by a recovery period. Results: In the CO(2) group, apneic time and rate decreased significantly, from 9.4 +/- 1.6 seconds/minute and 94 +/- 15 apneic episodes/hour to 3.0 +/- 0.5 seconds/minute and 34 +/- 5 apneic episodes/hour. In the theophylline group, apneic time and rate decreased significantly, from 8 +/- 1 seconds/minute and 80 +/- 8 apneic episodes/hour to 2.5 +/- 0.4 seconds/minute and 28 +/- 3 apneic episodes/hour. Cerebral blood flow velocity (CBFV) decreased only during theophylline administration. Conclusions: Our findings suggest that inhaled low (0.8%) CO(2) concentrations in preterm infants is at least as effective as theophylline in decreasing the duration and number of apneic episodes, has fewer side effects, and causes no changes in CBFV. We speculate that CO(2) may be a better treatment for apnea of prematurity than methylxanthines.

Title: A Modeling Study on Inspired CO2 Rebreathing Device for Sleep Apnea Treatment by Means of CFD Analysis and Experiment

Authors: Shokoueinejad, M., Pazouki, A., Levin, J. et al.

Journal: Journal of Medical and Biological Engineering. 37. 288-297. 10.1007/s40846-017-0223-7.

Link to Springer: A Modeling Study on Inspired CO2 Rebreathing Device for Sleep Apnea Treatment by Means of CFD Analysis and Experiment

Abstract: We present the device design, simulation, and measurement results of a therapy device that potentially prevents sleep apnea by slightly increasing inspired CO2 through added dead space (DS). The rationale for treatment of sleep apnea with CO2 manipulation is based on two recently reported premises: (i) preventing transient reductions in PaCO2 will prevent the patient from reaching their apneic threshold, thereby preventing “central” apnea and instabilities in respiratory motor output; and (ii) raising PaCO2 and end-tidal CO2, even by a minimal amount, provides a strong recruitment of upper airway dilator muscles, thereby preventing airway obstruction. We have also provided the simulation results, obtained from solving the Navier–Stokes (NS) equations within the device volume. Therein, the NS equations are coupled with a convection–diffusion equation that represents the transport of CO2 in the device, thus enabling the transient simulation of CO2 propagation. Using this procedure, a prototype of variable volume dead space reservoir device was designed. Volumetric factors influencing carbon dioxide increases in the added reservoir (open-ended DS) were investigated. The maximum/minimum amount of CO2 concentration were obtained for the maximum/minimum device volume; 3.4 and 2.4 mol/m3 for the DS volumes of 1.2 and 0.5 × 10−3 m3, respectively. In all case studies, the CO2 buildup reached a plateau after approximately 20 breathing cycles. The experimental measurement results are in agreement with the simulation and numerical results obtained using the proposed simplified modeling technique, with a maximum relative error of 3.5%.

Title: Oxidative Stress in the Lung - The Essential Paradox

Authors: Rogers LK, Cismowski MJ.

Journal: Curr Opin Toxicol. 2018 Feb;7:37-43. doi: 10.1016/j.cotox.2017.09.001. Epub 2017 Sep 19. PMID: 29308441; PMCID: PMC5754020.

Link to full text: Oxidative Stress in the Lung - The Essential Paradox

Abstract: As eukaryotic life evolved, so too did the need for a source of energy that meets the requirements of complex organisms. Oxygen provides this vast potential energy source, but the same chemical reactivity which provides this potential also can have detrimental effects. The lung evolved as an organ that can efficiently promote gas exchange for the entire organism but as such, the lung is highly susceptible to its external environment. Oxygen can be transformed through both enzymatic and non-enzymatic processes into reactive oxygen species (ROS) and reactive nitrogen species (RNS), which can lead to protein, lipid, and DNA damage. Under normal conditions ROS/RNS concentrations are minimized through the activity of antioxidants located both intracellularly and in the epithelial lining fluid of the lung. Oxidative stress in the lung results when the antioxidant capacity is overwhelmed or depleted through external exposures, such as altered oxygen tension or air pollution, or internally. Internal sources of oxidative stress include systemic disease and the activation of resident cells and inflammatory cells recruited in response to an exposure or systemic response. Pulmonary responses to oxidative stress include activation of oxidases, lipid peroxidation, increases in nitric oxide, and autophagy. These internal and external exposures with the subsequent pulmonary responses contribute to development of diseases directly linked to oxidative stress. These include asthma, COPD, and lung cancers. While the vulnerability of the lung to oxidative stress is acknowledged, few effective preventative strategies or therapeutics are currently available.

Title: Oxidative stress and free radicals in COPD--implications and relevance for treatment

Authors: Domej W, Oettl K, Renner W.

Journal: Int J Chron Obstruct Pulmon Dis. 2014 Oct 17;9:1207-24. doi: 10.2147/COPD.S51226. PMID: 25378921; PMCID: PMC4207545.

Link to full text: Oxidative stress and free radicals in COPD--implications and relevance for treatment

Abstract: Oxidative stress occurs when free radicals and other reactive species overwhelm the availability of antioxidants. Reactive oxygen species (ROS), reactive nitrogen species, and their counterpart antioxidant agents are essential for physiological signaling and host defense, as well as for the evolution and persistence of inflammation. When their normal steady state is disturbed, imbalances between oxidants and antioxidants may provoke pathological reactions causing a range of nonrespiratory and respiratory diseases, particularly chronic obstructive pulmonary disease (COPD). In the respiratory system, ROS may be either exogenous from more or less inhalative gaseous or particulate agents such as air pollutants, cigarette smoke, ambient high-altitude hypoxia, and some occupational dusts, or endogenously generated in the context of defense mechanisms against such infectious pathogens as bacteria, viruses, or fungi. ROS may also damage body tissues depending on the amount and duration of exposure and may further act as triggers for enzymatically generated ROS released from respiratory, immune, and inflammatory cells. This paper focuses on the general relevance of free radicals for the development and progression of both COPD and pulmonary emphysema as well as novel perspectives on therapeutic options. Unfortunately, current treatment options do not suffice to prevent chronic airway inflammation and are not yet able to substantially alter the course of COPD. Effective therapeutic antioxidant measures are urgently needed to control and mitigate local as well as systemic oxygen bursts in COPD and other respiratory diseases. In addition to current therapeutic prospects and aspects of genomic medicine, trending research topics in COPD are presented. Keywords: antioxidants; chronic obstructive pulmonary disease; reactive nitrogen species; reactive oxygen species.

Title: Reactive oxygen species and mitochondria: A nexus of cellular homeostasis

Authors: Dan Dunn J, Alvarez LA, Zhang X, Soldati T.

Journal: Redox Biol. 2015 Dec;6:472-485. doi: 10.1016/j.redox.2015.09.005. Epub 2015 Sep 10. PMID: 26432659; PMCID: PMC4596921.

Link to full text: Reactive oxygen species and mitochondria: A nexus of cellular homeostasis

Abstract: Reactive oxygen species (ROS) are integral components of multiple cellular pathways even though excessive or inappropriately localized ROS damage cells. ROS function as anti-microbial effector molecules and as signaling molecules that regulate such processes as NF-kB transcriptional activity, the production of DNA-based neutrophil extracellular traps (NETs), and autophagy. The main sources of cellular ROS are mitochondria and NADPH oxidases (NOXs). In contrast to NOX-generated ROS, ROS produced in the mitochondria (mtROS) were initially considered to be unwanted by-products of oxidative metabolism. Increasing evidence indicates that mtROS have been incorporated into signaling pathways including those regulating immune responses and autophagy. As metabolic hubs, mitochondria facilitate crosstalk between the metabolic state of the cell with these pathways. Mitochondria and ROS are thus a nexus of multiple pathways that determine the response of cells to disruptions in cellular homeostasis such as infection, sterile damage, and metabolic imbalance. In this review, we discuss the roles of mitochondria in the generation of ROS-derived anti-microbial effectors, the interplay of mitochondria and ROS with autophagy and the formation of DNA extracellular traps, and activation of the NLRP3 inflammasome by ROS and mitochondria. Keywords: Autophagy; Immunity; Inflammasome; Mitochondria; Neutrophil extracellular traps; Reactive oxygen species.

Title: Reactive Oxygen Species in Metabolic and Inflammatory Signaling

Authors: Forrester SJ, Kikuchi DS, Hernandes MS, Xu Q, Griendling KK.

Journal: Circ Res. 2018 Mar 16;122(6):877-902. doi: 10.1161/CIRCRESAHA.117.311401. PMID: 29700084; PMCID: PMC5926825.

Link to full text: Reactive Oxygen Species in Metabolic and Inflammatory Signaling

Abstract: Reactive oxygen species (ROS) are well known for their role in mediating both physiological and pathophysiological signal transduction. Enzymes and subcellular compartments that typically produce ROS are associated with metabolic regulation, and diseases associated with metabolic dysfunction may be influenced by changes in redox balance. In this review, we summarize the current literature surrounding ROS and their role in metabolic and inflammatory regulation, focusing on ROS signal transduction and its relationship to disease progression. In particular, we examine ROS production in compartments such as the cytoplasm, mitochondria, peroxisome, and endoplasmic reticulum and discuss how ROS influence metabolic processes such as proteasome function, autophagy, and general inflammatory signaling. We also summarize and highlight the role of ROS in the regulation metabolic/inflammatory diseases including atherosclerosis, diabetes mellitus, and stroke. In order to develop therapies that target oxidative signaling, it is vital to understand the balance ROS signaling plays in both physiology and pathophysiology, and how manipulation of this balance and the identity of the ROS may influence cellular and tissue homeostasis. An increased understanding of specific sources of ROS production and an appreciation for how ROS influence cellular metabolism may help guide us in the effort to treat cardiovascular diseases. Keywords: cardiovascular diseases; inflammation; metabolism; oxidative stress; signal transduction.

Title: The Effects of Hyperbaric Oxygenation on Oxidative Stress, Inflammation and Angiogenesis

Authors: De Wolde SD, Hulskes RH, Weenink RP, Hollmann MW, Van Hulst RA.

Journal: Biomolecules. 2021 Aug 14;11(8):1210. doi: 10.3390/biom11081210. PMID: 34439876; PMCID: PMC8394403.

Link to full text: The Effects of Hyperbaric Oxygenation on Oxidative Stress, Inflammation and Angiogenesis

Abstract: Hyperbaric oxygen therapy (HBOT) is commonly used as treatment in several diseases, such as non-healing chronic wounds, late radiation injuries and carbon monoxide poisoning. Ongoing research into HBOT has shown that preconditioning for surgery is a potential new treatment application, which may reduce complication rates and hospital stay. In this review, the effect of HBOT on oxidative stress, inflammation and angiogenesis is investigated to better understand the potential mechanisms underlying preconditioning for surgery using HBOT. A systematic search was conducted to retrieve studies measuring markers of oxidative stress, inflammation, or angiogenesis in humans. Analysis of the included studies showed that HBOT-induced oxidative stress reduces the concentrations of pro-inflammatory acute phase proteins, interleukins and cytokines and increases growth factors and other pro-angiogenesis cytokines. Several articles only noted this surge after the first HBOT session or for a short duration after each session. The anti-inflammatory status following HBOT may be mediated by hyperoxia interfering with NF-κB and IκBα. Further research into the effect of HBOT on inflammation and angiogenesis is needed to determine the implications of these findings for clinical practice. Keywords: angiogenesis; hyperbaric oxygen therapy; hyperbaric oxygenation; inflammation; neovascularization; oxidative stress.

Title: The Role of Reactive-Oxygen-Species in Microbial Persistence and Inflammation

Authors: Spooner R, Yilmaz O.

Journal: Int J Mol Sci. 2011 Jan 13;12(1):334-52. doi: 10.3390/ijms12010334. PMID: 21339989; PMCID: PMC3039955.

Link to full text: The Role of Reactive-Oxygen-Species in Microbial Persistence and Inflammation

Abstract: The mechanisms of chronic infections caused by opportunistic pathogens are of keen interest to both researchers and health professionals globally. Typically, chronic infectious disease can be characterized by an elevation in immune response, a process that can often lead to further destruction. Reactive-Oxygen-Species (ROS) have been strongly implicated in the aforementioned detrimental response by host that results in self-damage. Unlike excessive ROS production resulting in robust cellular death typically induced by acute infection or inflammation, lower levels of ROS produced by host cells are increasingly recognized to play a critical physiological role for regulating a variety of homeostatic cellular functions including growth, apoptosis, immune response, and microbial colonization. Sources of cellular ROS stimulation can include "danger-signal-molecules" such as extracellular ATP (eATP) released by stressed, infected, or dying cells. Particularly, eATP-P2X(7) receptor mediated ROS production has been lately found to be a key modulator for controlling chronic infection and inflammation. There is growing evidence that persistent microbes can alter host cell ROS production and modulate eATP-induced ROS for maintaining long-term carriage. Though these processes have yet to be fully understood, exploring potential positive traits of these "injurious" molecules could illuminate how opportunistic pathogens maintain persistence through physiological regulation of ROS signaling. Keywords: NADPH oxidase; NLRX1; P2X7 receptor; Porphyromonas gingivalis; epithelium; extracellular ATP; microbial persistence; opportunistic pathogens; reactive-oxygen-species.

Title: Humming, Nitric Oxide and Paranasal Sinus Ventilation

Authors: Maniscalco, M.

Journal: Karolinska Institutet (Sweden), 2006.

Link to full text: Humming, Nitric Oxide and Paranasal Sinus Ventilation

Abstract: The paranasal sinuses are air-filled bony cavities surrounding the nose. They communicate with the nose via the sinus ostia through which fluid and gases pass in both directions. A proper ventilation is crucial for sinus integrity and blockage of the ostia is a major risk factor for development of sinusitis. In this thesis we have explored an entirely new approach to monitor sinus ventilation- the nasal humming test. We show in human studies in vivo and in a sinus/nasal model that the oscillating airflow generated during humming produce a dramatic increase in sinus ventilation. Interestingly, this increased gas exchange can be readily monitored on-line by simultaneously measuring the levels of the gas nitric oxide (NO) in nasally exhaled air. The sinuses constitute a major natural reservoir of NO and when gas-exchange increases during humming NO escapes rapidly into the nasal cavity thereby creating a highly reproducible peak in exhaled NO. When exploring the different factors that determine the humming peak in NO we found that sinus ostium size was the most important but the humming frequency also influenced the sinus NO release. In patients with severe nasal polyposis and completely blocked sinus ostia the humming peak in NO was abolished. Moreover, in patients with allergic rhinitis, absence of a NO peak was associated with endoscopic signs suggestive of ostial obstruction. In the last study we went on to study if an oscillating airflow could be used not only to wash a gas out from the sinuses but also to enhance passage of substances into the sinuses. Indeed, we found evidence of an increased intra-sinus drug deposition by adding a sounding airflow to an aerosol. In conclusion, the ventilation of the paranasal sinuses increases greatly when a person is humming; a finding that could have both diagnostic and therapeutic implications. Measurements of nasal NO during humming may represent a test of sinus ostialfunction. In addition, aerosol in combination with a sounding airflow could possibly be useful to increase the delivery of drugs into the paranasal sinuses.

Title: Exhaled nasal nitric oxide during humming: potential clinical tool in sinonasal disease?

Authors: Maniscalco M, Pelaia G, Sofia M.

Journal: Biomark Med. 2013 Apr;7(2):261-6. doi: 10.2217/bmm.13.11. PMID: 23547821.

Link to PubMed: Exhaled nasal nitric oxide during humming: potential clinical tool in sinonasal disease?

Abstract: The use of nasal nitric oxide (nNO) in sinonasal disease has recently been advocated as a potential tool to explore upper inflammatory airway disease. However, it is currently hampered by some factors including the wide range of measurement methods, the presence of various confounding factors and the heterogeneity of the study population. The contribution of nasal airway and paranasal sinuses communicating with the nose through the ostia represents the main confounding factor. There is accumulating evidence that nasal humming (which is the production of a tone without opening the lips or forming words) during nNO measurement increases nNO levels due to a rapid gas exchange in the paranasal sinuses. The aim of this review is to discuss the basic concepts and clinical applications of nNO assessment during humming, which represents a simple and noninvasive method to approach sinonasal disease.

Title: Strong humming for one hour daily to terminate chronic rhinosinusitis in four days: a case report and hypothesis for action by stimulation of endogenous nasal nitric oxide production

Authors: Eby GA.

Journal: Med Hypotheses. 2006;66(4):851-4. doi: 10.1016/j.mehy.2005.11.035. Epub 2006 Jan 10. PMID: 16406689.

Link to PubMed: Strong humming for one hour daily to terminate chronic rhinosinusitis in four days: a case report and hypothesis for action by stimulation of endogenous nasal nitric oxide production

Abstract: Rhinosinusitis is an inflammation or infection of the nose and air pockets (sinuses) above, below and between the eyes which connect with the back of the nose through tiny openings (ostia). Rhinosinusitis can be caused by bacteria, viruses, fungi (molds) and possibly by allergies. Chronic rhinosinusitis (CRS) is an immune disorder caused by fungi. The immune response produced by eosinophils causes the fungi to be attacked, which leads to damage of the sinus membranes, resulting in full-blown rhinosinusitis symptoms. Gaseous nitric oxide (NO) is naturally released in the human respiratory tract. The major part of NO found in exhaled air originates in the nasal airways, although significant production of NO also takes place in the paranasal sinuses. Proper ventilation is essential for maintenance of sinus integrity, and blockage of the ostium is a central event in pathogenesis of sinusitis. Concentrations of NO in the healthy sinuses are high. Nasal NO is known to be increased 15- to 20-fold by humming compared with quiet exhalation. NO is known to be broadly antifungal, antiviral and antibacterial. This case report shows that a subject hummed strongly at a low pitch ( approximately 130 Hz) for 1h (18 hums per minute) at bedtime the first night, and hummed 60-120 times 4 times a day for the following 4 days as treatment for severe CRS. The humming technique was described as being one that maximally increased intranasal vibrations, but less than that required to produce dizziness. The morning after the first 1-h humming session, the subject awoke with a clear nose and found himself breathing easily through his nose for the first time in over 1 month. During the following 4 days, CRS symptoms slightly reoccurred, but with much less intensity each day. By humming 60-120 times four times per day (with a session at bedtime), CRS symptoms were essentially eliminated in 4 days. Coincidentally, the subject's cardiac arrhythmias (PACs) were greatly lessened. It is hypothesized that strong, prolonged humming increased endogenous nasal NO production, thus eliminating CRS by antifungal means.

Title: The human ventilatory response to stress: rate or depth?

Authors: Tipton MJ, Harper A, Paton JFR, Costello JT.

Journal: J Physiol. 2017 Sep 1;595(17):5729-5752. doi: 10.1113/JP274596. Epub 2017 Jul 27. PMID: 28650070; PMCID: PMC5577533.

Link to full text: The human ventilatory response to stress: rate or depth?

Abstract: Many stressors cause an increase in ventilation in humans. This is predominantly reported as an increase in minute ventilation (V̇E). But, the same V̇E can be achieved by a wide variety of changes in the depth (tidal volume, VT ) and number of breaths (respiratory frequency, ƒR ). This review investigates the impact of stressors including: cold, heat, hypoxia, pain and panic on the contributions of ƒR and VT to V̇E to see if they differ with different stressors. Where possible we also consider the potential mechanisms that underpin the responses identified, and propose mechanisms by which differences in ƒR and VT are mediated. Our aim being to consider if there is an overall differential control of ƒR and VT that applies in a wide range of conditions. We consider moderating factors, including exercise, sex, intensity and duration of stimuli. For the stressors reviewed, as the stress becomes extreme V̇E generally becomes increased more by ƒR than VT . We also present some tentative evidence that the pattern of ƒR and VT could provide some useful diagnostic information for a variety of clinical conditions. In The Physiological Society's year of 'Making Sense of Stress', this review has wide-ranging implications that are not limited to one discipline, but are integrative and relevant for physiology, psychophysiology, neuroscience and pathophysiology.

Title: The physiological effects of slow breathing in the healthy human

Authors: Russo MA, Santarelli DM, O'Rourke D.

Journal: Breathe (Sheff). 2017 Dec;13(4):298-309. doi: 10.1183/20734735.009817. PMID: 29209423; PMCID: PMC5709795.

Link to full text: The physiological effects of slow breathing in the healthy human

Abstract: Slow breathing practices have been adopted in the modern world across the globe due to their claimed health benefits. This has piqued the interest of researchers and clinicians who have initiated investigations into the physiological (and psychological) effects of slow breathing techniques and attempted to uncover the underlying mechanisms. The aim of this article is to provide a comprehensive overview of normal respiratory physiology and the documented physiological effects of slow breathing techniques according to research in healthy humans. The review focuses on the physiological implications to the respiratory, cardiovascular, cardiorespiratory and autonomic nervous systems, with particular focus on diaphragm activity, ventilation efficiency, haemodynamics, heart rate variability, cardiorespiratory coupling, respiratory sinus arrhythmia and sympathovagal balance. The review ends with a brief discussion of the potential clinical implications of slow breathing techniques. This is a topic that warrants further research, understanding and discussion.

Title: Slow breathing for reducing stress: The effect of extending exhale

Authors: Birdee G, Nelson K, Wallston K, Nian H, Diedrich A, Paranjape S, Abraham R, Gamboa A.

Journal: Complement Ther Med. 2023 May;73:102937. doi: 10.1016/j.ctim.2023.102937. Epub 2023 Mar 5. PMID: 36871835; PMCID: PMC10395759.

Link to full text: Slow breathing for reducing stress: The effect of extending exhale

Abstract: Introduction: Slow breathing techniques are commonly used to reduce stress. While it is believed by mind-body practitioners that extending the exhale time relative to inhale increases relaxation, this has not been demonstrated.

Methods: We conducted a 12-week randomized, single-blinded trial among 100 participants to compare if yoga-based slow breathing with an exhale greater inhale versus an exhale equals inhale produces measurable differences in physiological and psychological stress among healthy adults.

Results: Participants mean individual instruction attendance was 10.7 ± 1.5 sessions out of 12 offered sessions. The mean weekly home practice was 4.8 ± 1.2 practices per week. There was no statistical difference between treatment groups for frequency of class attendance, home practice, or achieved slow breathing respiratory rate. Participants demonstrated fidelity to assigned breath ratios with home practice as measured by remote biometric assessments through smart garments (HEXOSKIN). Regular slow breathing practice for 12 weeks significantly reduced psychological stress as measured by PROMIS Anxiety (-4.85 S.D. ± 5.53, confidence interval [-5.60, -3.00], but not physiological stress as measured by heart rate variability. Group comparisons showed small effect size differences (d = 0.2) with further reductions in psychological stress and physiological stress from baseline to 12 weeks for exhale greater than inhale versus exhale equals inhale, however these differences were not statistically significant.

Conclusion: While slow breathing significantly reduces psychological stress, breath ratios do not have a significant differential effect on stress reduction among healthy adults.

Title: Respiratory alkalosis may impair the production of vitamin D and lead to significant morbidity, including the fibromyalgia syndrome

Authors: Lewis JM, Fontrier TH, Coley JL.

Journal: Med Hypotheses. 2017 May;102:99-101. doi: 10.1016/j.mehy.2017.03.013. Epub 2017 Mar 8. PMID: 28478843.

Link to PubMed: Respiratory alkalosis may impair the production of vitamin D and lead to significant morbidity, including the fibromyalgia syndrome

Abstract: Hyperventilation caused by physical and/or psychological stress may lead to significant respiratory alkalosis and an elevated systemic pH. The alkalotic pH may in turn suppress the normal renal release of phosphate into the urine, thereby interrupting the endogenous production of 1,25-dihydroxyvitamin D (calcitriol). This could cause a shortfall in its normal production, leading to a variety of adverse consequences. It might partially explain the pathogenesis of acute mountain sickness, a treatable disease characterized by severe hyperventilation secondary to the hypoxia of high altitude exposure. Milder degrees of hyperventilation due to different forms of stress may produce other conditions which share characteristics with acute mountain sickness. One of these may be the fibromyalgia syndrome, a chronic painful disorder for which no satisfactory treatment exists. Should fibromyalgia and acute mountain sickness have a common etiology, may they also share a common form of treatment? Evidence is presented to support this hypothesis.

Keywords: Acute mountain sickness; Fibromyalgia; Respiratory alkalosis; Vitamin D.

Title: Hyperventilation with Maintenance of Isocapnia. An "Old New" Method in Carbon Monoxide Intoxication

Authors: Sein Anand J, Schetz D, Waldman W, Wiśniewski M.

Journal: PLoS One. 2017 Jan 20;12(1):e0170621. doi: 10.1371/journal.pone.0170621. PMID: 28107437; PMCID: PMC5249161.

Link to full text: Hyperventilation with Maintenance of Isocapnia. An "Old New" Method in Carbon Monoxide Intoxication

Abstract: Introduction: Exposure to carbon monoxide (CO) is among the most common causes of acute and chronic poisonings worldwide. The crucial point of treatment of such acute poisonings is to eliminate CO from the body as fast as possible. There are currently two approaches to the management of the CO intoxication: hyperbaric oxygen therapy (HOT) and normobaric oxygen therapy (NOT). HOT is highly effective and capable of achieving the CO elimination half-time (T½) as low as 15 minutes. Unfortunately this method is expensive and not always readily available. The elimination of CO with the use of NOT (T½~70 min) is slower, but treatment can be started even on the site of the exposure and continued while the patient is transported to a hospital. The aim of the study was to evaluate the effectiveness of a method using therapeutic hyperventilation with maintenance of isocapnia (IH) in the elimination of CO in volunteers exposed to CO and to compare selected gasometric and respiratory parameters during IH with the values obtained during hyperventilation with pure oxygen ("non-isocapnic hyperventilation"-NIH).

Material and methods: The study involved 13 healthy, chronically-smoking volunteers. Each of them participated in two independent hyperventilation tests: IH and NIH. The levels of carboxyhemoglobin (COHb) and selected gasometric, cardiac and respiratory parameters were measured at 0, 10 and 20 minutes during both tests. Among 13 volunteers (8 women and 5 men) the initial COHb level was 5.0±1.5% (mean±SD) before the IH tests and 5.1±1.9% before the NIH tests (p>0.05). After 20 minutes of the procedures the mean COHb level was 2.9±0.9% for IH and 3.6±1.2% for NIH (p<0.01). The T½ of COHb was 29.6±12.2 min and 47.3±19.2 min respectively (p<0.01). After 10 minutes of NIH respiratory alkalosis was noted in 11 participants (84.6%). Such problem was not seen during the IH procedures. No serious adverse effects were recorded during either IH or NIH. Mild symptoms such as: dyspnea, headache and paresthesias were reported by 6 volunteers (46%) during both IH and NIH tests. It is worth noting that paresthesias were only reported during NIH, by 2 participants (15.4%).

Conclusions: The elimination T½ of CO during IH was comparable with the values reported during HOT, and lower than can be achieved with NOT or NIH. No serious adverse effects were reported during IH procedures. Further studies, especially direct comparisons with NOT and HOT, are necessary to evaluate the effectiveness of IH in the treatment of acute CO poisoning.

Title: Isocapnic hyperpnea accelerates carbon monoxide elimination

Authors: Fisher JA, Rucker J, Sommer LZ, Vesely A, Lavine E, Greenwald Y, Volgyesi G, Fedorko L, Iscoe S.

Journal: Am J Respir Crit Care Med. 1999 Apr;159(4 Pt 1):1289-92. doi: 10.1164/ajrccm.159.4.9804040. PMID: 10194179.

Link to full text: Isocapnic hyperpnea accelerates carbon monoxide elimination

Abstract: A major impediment to the use of hyperpnea in the treatment of CO poisoning is the development of hypocapnia or discomfort of CO2 inhalation. We examined the effect of nonrebreathing isocapnic hyperpnea on the rate of decrease of carboxyhemoglobin levels (COHb) in five pentobarbital-anesthetized ventilated dogs first exposed to CO and then ventilated with room air at normocapnia (control). They were then ventilated with 100% O2 at control ventilation, and at six times control ventilation without hypocapnia ("isocapnic hyperpnea") for at least 42 min at each ventilator setting. We measured blood gases and COHb. At control ventilation, the half-time for elimination of COHb (t1/2) was 212 +/- 17 min (mean +/- SD) on room air and 42 +/- 3 min on 100% O2. The t1/2 decreased to 18 +/- 2 min (p < 0.0005) during isocapnic hyperpnea. In two similarly prepared dogs treated with hyperbaric O2, the t1/2 were 20 and 28 min. We conclude that isocapnic hyperpnea more than doubles the rate of COHb elimination induced by normal ventilation with 100% O2. Isocapnic hyperpnea could improve the efficacy of the standard treatment of CO poisoning, 100% O2 at atmospheric or increased pressures.

Title: Accelerated ethanol elimination via the lungs

Authors: Klostranec JM, Vucevic D, Crawley AP, Venkatraghavan L, Sobczyk O, Duffin J, Sam K, Holmes R, Fedorko L, Mikulis DJ, Fisher JA.

Journal: Sci Rep. 2020 Nov 12;10(1):19249. doi: 10.1038/s41598-020-76233-9. PMID: 33184355; PMCID: PMC7665168.

Link to full text: Accelerated ethanol elimination via the lungs

Abstract: Ethanol poisoning is endemic the world over. Morbidity and mortality depend on blood ethanol levels which in turn depend on the balance between its rates of absorption and clearance. Clearance of ethanol is mostly at a constant rate via enzymatic metabolism. We hypothesized that isocapnic hyperpnea (IH), previously shown to be effective in acceleration of clearance of vapour anesthetics and carbon monoxide, would also accelerate the clearance of ethanol. In this proof-of-concept pilot study, five healthy male subjects were brought to a mildly elevated blood ethanol concentration (~ 0.1%) and ethanol clearance monitored during normal ventilation and IH on different days. IH increased elimination rate of ethanol in proportion to blood levels, increasing the elimination rate more than three-fold. Increased veno-arterial ethanol concentration differences during IH verified the efficacy of ethanol clearance via the lung. These data indicate that IH is a nonpharmacologic means to accelerate the elimination of ethanol by superimposing first order elimination kinetics on underlying zero order liver metabolism. Such kinetics may prove useful in treating acute severe ethanol intoxication.

Title: The treatment of acute alcoholism: with ten per cent carbon dioxide and ninety per cent oxygen inhalation

Authors: Robinson LJ, Selesnick S.

Journal: JAMA. 1935;105(22):1734–1738. doi:10.1001/jama.1935.02760480004002

Link to full text: The treatment of acute alcoholism: with ten per cent carbon dioxide and ninety per cent oxygen inhalation

Abstract: For many years the Fifth Medical Service of the Boston City Hospital has been in charge of almost all medical alcoholic patients admitted to this hospital. It is estimated that approximately 700 alcoholic patients are admitted to this service for treatment each year. The opportunity has been present, therefore, to study alcoholism in all its phases.
The acute alcoholic patient1 generally can be given a good prognosis as to recovery. For this reason, even when medical aid is sought for him, he receives little active therapy. In the paralytic stage of acute alcoholic poisoning, however, emergency treatment may be life -saving. Patients in this stage present a picture best described by Sollmann, who states that the symptoms are those of beginning medullary paralysis. Since carbon dioxide increased the respiratory excretion of alcohol, and since the oxidation of alcohol might be increased through the administration of oxygen, it is conceivable that the inhalation of a combination of 10 per cent carbon dioxide and 90 per cent oxygen mixture might accelerate recovery from acute alcoholic coma, especially of the paralytic state with respiratory depression and cyanosis. If it did no more than keep such a patient from dying of respiratory depression, it would, by keeping him alive, enable him to oxidize the alcohol himself. CONCLUSIONS: Acute alcoholic coma with dangerous respiratory depression, paralysis and cyanosis is a medical emergency. Death may be definitely prevented and recovery accelerated by inhalation of a mixture of 10 per cent carbon dioxide and 90 per cent oxygen for a length of time sufficient to reestablish and maintain normal respiration and color even after the inhalation is suspended.

Title: Carbon Dioixde in Anaesthesia

Authors: Wilson S.R.

Journal: BJA: British Journal of Anaesthesia, Volume 3, Issue 3, January 1926, Pages 112–118, https://doi.org/10.1093/bja/3.3.112

Link to full text: Carbon Dioixde in Anaesthesia

Abstract: All general anaesthetics are toxic agents in varying degrees and prone to produce undesirable after-effects on the patient. It is therefore advantageous to effect their elimination the moment their work is completed. Henderson and Haggard have investigated the rate of excretion of ether after general anaesthesia and provided us with accurate data on this subject. They found that half the total ether in the body was eliminated in the first half hour after the administration is stopped. Half the remainder is excreted during the next one to two hours, but complete elimination does not occur until one to two days. If it is possible to accelerate this process undesirable after-effects can be greatly reduced or completely avoided.
Now, if 5–6 per cent CO₂ be added to the oxygen pulmonary ventilation is increased two, three or four times. The writer has tested these experimental observations in actual clinical practice during the last two years with most gratifying results, both to himself and the patients concerned.

Title: Isocapnic hyperventilation provides early extubation after head and neck surgery: A prospective randomized trial.

Authors: Hallén K, Jildenstål P, Stenqvist O, Oras J, Ricksten SE, Lindgren S.

Journal: Acta Anaesthesiol Scand. 2018 Sep;62(8):1064-1071. doi: 10.1111/aas.13133. Epub 2018 Apr 19. PMID: 29671866.

Link to PubMed: Isocapnic hyperventilation provides early extubation after head and neck surgery: A prospective randomized trial.

Abstract: Background: Isocapnic hyperventilation (IHV) shortens recovery time after inhalation anaesthesia by increasing ventilation while maintaining a normal airway carbon dioxide (CO2)-level. One way of performing IHV is to infuse CO2 to the inspiratory limb of a breathing circuit during mechanical hyperventilation (HV). In a prospective randomized study, we compared this IHV technique to a standard emergence procedure (control).

Methods: Thirty-one adult ASA I-III patients undergoing long-duration (>3 hours) sevoflurane anaesthesia for major head and neck surgery were included and randomized to IHV-treatment (n = 16) or control (n = 15). IHV was performed at minute ventilation 13.6 ± 4.3 L/min and CO2 delivery, dosed according to a nomogram tested in a pilot study. Time to extubation and eye-opening was recorded. Inspired (FICO2) and expired (FETCO2) CO2 and arterial CO2 levels (PaCO2) were monitored. Cognition was tested preoperatively and at 20, 40 and 60 minutes after surgery.

Results: Time from turning off the vapourizer to extubation was 13.7 ± 2.5 minutes in the IHV group and 27.4 ± 6.5 minutes in controls (P < .001). Two minutes after extubation, PaCO2 was 6.2 ± 0.5 and 6.2 ± 0.6 kPa in the IHV and control group respectively. In 69% (IHV) vs 53% (controls), post-operative cognition returned to pre-operative values within 40 minutes after surgery (NS). Incidences of pain and nausea/vomiting did not differ between groups.

Conclusions: In this randomized trial comparing an IHV method with a standard weaning procedure, time to extubation was reduced with 50% in the IHV group. The described IHV method can be used to decrease emergence time from inhalation anaesthesia.

Title: Isocapnic hyperventilation shortens washout time for sevoflurane - an experimental in vivo study

Authors: Hallén K, Stenqvist O, Ricksten SE, Lindgren S.

Journal: Acta Anaesthesiol Scand. 2016 Oct;60(9):1261-9. doi: 10.1111/aas.12761. Epub 2016 Jul 10. PMID: 27396945.

Link to PubMed: Isocapnic hyperventilation shortens washout time for sevoflurane - an experimental in vivo study

Abstract: Background: Isocapnic hyperventilation (IHV) is a method that fastens weaning from inhalation anaesthesia by increasing airway concentration of carbon dioxide (CO2 ) during hyperventilation (HV). In an animal model, we evaluated a technique of adding CO2 directly to the breathing circuit of a standard anaesthesia apparatus.

Methods: Eight anaesthetised pigs weighing 28 ± 2 kg were intubated and mechanically ventilated. From a baseline ventilation of 5 l/min, HV was achieved by doubling minute volume and fresh gas flow. Respiratory rate was increased from 15 to 22/min. The CO2 absorber was disconnected and CO2 was delivered (DCO2 ) to the inspiratory limb of a standard breathing circuit via a mixing box. Time required to decrease end-tidal sevoflurane concentration from 2.7% to 0.2% was defined as washout time. Respiration and haemodynamics were monitored by blood gas analysis, spirometry, electric impedance tomography and pulse contour analysis.

Results: A DCO2 of 261 ± 19 ml/min was necessary to achieve isocapnia during HV. The corresponding FICO2 -level remained stable at 3.1 ± 0.3%. During IHV, washout of sevoflurane was three times faster, 433 ± 135 s vs. 1387 ± 204 s (P < 0.001). Arterial CO2 tension and end-tidal CO2 , was 5.2 ± 0.4 kPa and 5.6 ± 0.4%, respectively, before IHV and 5.1 ± 0.3 kPa and 5.7 ± 0.3%, respectively, during IHV.

Conclusions: In this experimental in vivo model of isocapnic hyperventilation, the washout time of sevoflurane anaesthesia was one-third compared to normal ventilation. The method for isocapnic hyperventilation described can potentially be transferred to a clinical setting with the intention to decrease emergence time from inhalation anaesthesia.

Title: A simple method for isocapnic hyperventilation evaluated in a lung model

Authors: Hallén K, Stenqvist O, Ricksten SE, Lindgren S.

Journal: Acta Anaesthesiol Scand. 2016 May;60(5):597-606. doi: 10.1111/aas.12674. Epub 2015 Dec 21. PMID: 26688296.

Link to PubMed: A simple method for isocapnic hyperventilation evaluated in a lung model

Abstract: Background: Isocapnic hyperventilation (IHV) has the potential to increase the elimination rate of anaesthetic gases and has been shown to shorten time to wake-up and post-operative recovery time after inhalation anaesthesia. In this bench test, we describe a technique to achieve isocapnia during hyperventilation (HV) by adding carbon dioxide (CO2) directly to the breathing circuit of a standard anaesthesia apparatus with standard monitoring equipment.

Methods: Into a mechanical lung model, carbon dioxide was added to simulate a CO2 production (V(CO2)) of 175, 200 and 225 ml/min. Dead space (V(D)) volume could be set at 44, 92 and 134 ml. From baseline ventilation (BLV), HV was achieved by doubling the minute ventilation and fresh gas flow for each level of V(CO2), and dead space. During HV, CO2 was delivered (D(CO2)) by a precision flow meter via a mixing box to the inspiratory limb of the anaesthesia circuit to achieve isocapnia.

Results: During HV, the alveolar ventilation increased by 113 ± 6%. Tidal volume increased by 20 ± 0.1% during IHV irrespective of V(D) and V(CO2) level. D(CO2) varied between 147 ± 8 and 325 ± 13 ml/min. Low V(CO2) and large V(D) demanded a greater D(CO2) administration to achieve isocapnia. The FICO2 level during IHV varied between 2.3% and 3.3%.

Conclusion: It is possible to maintain isocapnia during HV by delivering carbon dioxide through a standard anaesthesia circuit equipped with modern monitoring capacities. From alveolar ventilation, CO2 production and dead space, the amount of carbon dioxide that is needed to achieve IHV can be estimated.

Title: Evaluation of a method for isocapnic hyperventilation: a clinical pilot trial

Authors: Hallén K, Jildenstål P, Stenqvist O, Ricksten SE, Lindgren S.

Journal: Acta Anaesthesiol Scand. 2018 Feb;62(2):186-195. doi: 10.1111/aas.13008. Epub 2017 Oct 16. PMID: 29034967.

Link to PubMed: Evaluation of a method for isocapnic hyperventilation: a clinical pilot trial

Abstract: Background: Isocapnic hyperventilation (IHV) is a method that shortens time to extubation after inhalation anaesthesia using hyperventilation (HV) without lowering airway CO2 . In a clinical trial on patients undergoing long-duration sevoflurane anaesthesia for major ear-nose-throat (ENT) surgery, we evaluated the utility of a technique for CO2 delivery (DCO2 ) to the inspiratory limb of a closed breathing circuit, during HV, to achieve isocapnia.

Methods: Fifteen adult ASA 1-3 patients were included. After end of surgery, mechanical HV was started by doubling baseline minute ventilation. Simultaneously, CO2 was delivered and dosed using a nomogram developed in a previous experimental study. Time to extubation and eye opening was recorded. Inspired (FICO2 ) and expired (FETCO2 ) CO2 and arterial CO2 levels were monitored during IHV. Cognition was tested pre-operatively and at 20, 40 and 60 min after surgery.

Results: A DCO2 of 285 ± 45 ml/min provided stable isocapnia during HV (13.5 ± 4.1 l/min). The corresponding FICO2 level was 3.0 ± 0.3%. Time from turning off the vaporizer (1.3 ± 0.1 MACage) to extubation (0.2 ± 0.1 MACage) was 11.3 ± 1.8 min after 342 ± 131 min of anaesthesia. PaCO2 and FETCO2 remained at normal levels during and after IHV. In 85% of the patients, post-operative cognition returned to pre-operative values within 60 min.

Conclusions: In this cohort of patients, a DCO2 nomogram for IHV was validated. The patients were safely extubated shortly after discontinuing long-term sevoflurane anaesthesia. Perioperatively, there were no adverse effects on arterial blood gases or post-operative cognition. This technique for IHV can potentially be used to decrease emergence time from inhalation anaesthesia.

Title: Topical cutaneous application of carbon dioxide via a hydrogel for improved fracture repair: results of phase I clinical safety trial

Authors: Niikura T, Iwakura T, Omori T, Lee SY, Sakai Y, Akisue T, Oe K, Fukui T, Matsushita T, Matsumoto T, Kuroda R.

Journal: BMC Musculoskelet Disord. 2019 Nov 25;20(1):563. doi: 10.1186/s12891-019-2911-7. PMID: 31766994; PMCID: PMC6878668.

Link to full text: Topical cutaneous application of carbon dioxide via a hydrogel for improved fracture repair: results of phase I clinical safety trial

Abstract: Background: Clinicians have very limited options to improve fracture repair. Therefore, it is critical to develop a new clinically available therapeutic option to assist fracture repair biologically. We previously reported that the topical cutaneous application of carbon dioxide (CO2) via a CO2 absorption-enhancing hydrogel accelerates fracture repair in rats by increasing blood flow and angiogenesis and promoting endochondral ossification. The aim of this study was to assess the safety and efficacy of CO2 therapy in patients with fractures.

Methods: Patients with fractures of the femur and tibia were prospectively enrolled into this study with ethical approval and informed consent. The CO2 absorption-enhancing hydrogel was applied to the fractured lower limbs of patients, and then 100% CO2 was administered daily into a sealed space for 20 min over 4 weeks postoperatively. Safety was assessed based on vital signs, blood parameters, adverse events, and arterial and expired gas analyses. As the efficacy outcome, blood flow at the level of the fracture site and at a site 5 cm from the fracture in the affected limb was measured using a laser Doppler blood flow meter.

Results: Nineteen patients were subjected to complete analysis. No adverse events were observed. Arterial and expired gas analyses revealed no adverse systemic effects including hypercapnia. The mean ratio of blood flow 20 min after CO2 therapy compared with the pre-treatment level increased by approximately 2-fold in a time-dependent manner.

Conclusions: The findings of the present study revealed that CO2 therapy is safe to apply to human patients and that it can enhance blood flow in the fractured limbs.

Title: Transcutaneous carbon dioxide application accelerates muscle injury repair in rat models

Authors: Akahane S, Sakai Y, Ueha T, Nishimoto H, Inoue M, Niikura T, Kuroda R.

Journal: Int Orthop. 2017 May;41(5):1007-1015. doi: 10.1007/s00264-017-3417-2. Epub 2017 Feb 16. PMID: 28210805.

Link to PubMed: Transcutaneous carbon dioxide application accelerates muscle injury repair in rat models

Abstract: Purpose: Skeletal muscle injuries are commonly observed in sports and traumatology medicine. Previously, we demonstrated that transcutaneous application of carbon dioxide (CO2) to lower limbs increased the number of muscle mitochondria and promoted muscle endurance. Therefore, we aimed to investigate whether transcutaneous CO2 application could enhance recovery from muscle injury.

Methods: Tibialis anterior muscle damage was induced in 27 Sprague Dawley rats via intramuscular injection of bupivacaine. After muscle injury, rats were randomly assigned to transcutaneous CO2-treated or -untreated groups. From each group, three rats were sacrificed at weeks one, two, four and six. At each time point, histology and immunofluorescence analyses were performed, and changes in muscle weight, muscle weight/body weight ratio, muscle fibre circumference, gene expression levels and capillary density were measured.

Results: Injured muscle fibres were completely repaired at week six in the CO2-treated group but only partially repaired in the untreated group. The repair of basement and plasma membranes did not differ significantly between groups. However, expression levels of genes and proteins related to muscle protein synthesis were significantly higher in the CO2-treated group and significantly more capillaries four weeks after injury.

Conclusion: Transcutaneous CO2 application can accelerate recovery after muscle injury in rats.

Title: Transcutaneous carbon dioxide application suppresses bone destruction caused by breast cancer metastasis

Authors: Takemori T, Kawamoto T, Ueha T, Toda M, Morishita M, Kamata E, Fukase N, Hara H, Fujiwara S, Niikura T, Kuroda R, Akisue T.

Journal: Oncology Reports, 40, 2079-2087. https://doi.org/10.3892/or.2018.6608

Link to full text: Transcutaneous carbon dioxide application suppresses bone destruction caused by breast cancer metastasis

Abstract: Hypoxia plays a significant role in cancer progression, including metastatic bone tumors. We previously reported that transcutaneous carbon dioxide (CO2) application could decrease tumor progression through the improvement of intratumor hypoxia. Therefore, we hypothesized that decreased hypoxia using transcutaneous CO2 could suppress progressive bone destruction in cancer metastasis. In the present study, we examined the effects of transcutaneous CO2 application on metastatic bone destruction using an animal model. The human breast cancer cell line MDA-MB-231 was cultured in vitro under three different oxygen conditions, and the effect of altered oxygen conditions on the expression of osteoclast-differentiation and osteolytic factors was assessed. An in vivo bone metastatic model of human breast cancer was created by intramedullary implantation of MDA-MB-231 cells into the tibia of nude mice, and treatment with 100% CO2 or a control was performed twice weekly for two weeks. Bone volume of the treated tibia was evaluated by micro-computed tomography (µCT), and following treatment, histological evaluation was performed by hematoxylin and eosin staining and immunohistochemical staining for hypoxia-inducible factor (HIF)-1α, osteoclast-differentiation and osteolytic factors, and tartrate-resistant acid phosphatase (TRAP) staining for osteoclast activity. In vitro experiments revealed that the mRNA expression of RANKL, PTHrP and IL-8 was significantly increased under hypoxic conditions and was subsequently reduced by reoxygenation. In vivo results by µCT revealed that bone destruction was suppressed by transcutaneous CO2, and that the expression of osteoclast-differentiation and osteolytic factors, as well as HIF-1α, was decreased in CO2-treated tumor tissues. In addition, multinucleated TRAP-positive osteoclasts were significantly decreased in CO2-treated tumor tissues. Hypoxic conditions promoted bone destruction in breast cancer metastasis, and reversal of hypoxia by transcutaneous CO2 application significantly inhibited metastatic bone destruction along with decreased osteoclast activity. The findings in this study strongly indicated that transcutaneous CO2 application could be a novel therapeutic strategy for treating metastatic bone destruction.

Title: Effects of the duration of transcutaneous CO2 application on the facilitatory effect in rat fracture repair

Authors: Oda T, Iwakura T, Fukui T, Oe K, Mifune Y, Hayashi S, Matsumoto T, Matsushita T, Kawamoto T, Sakai Y, Akisue T, Kuroda R, Niikura T.

Journal: J Orthop Sci. 2020 Sep;25(5):886-891. doi: 10.1016/j.jos.2019.09.017. Epub 2019 Oct 18. PMID: 31635930.

Link to full text: Effects of the duration of transcutaneous CO2 application on the facilitatory effect in rat fracture repair

Abstract: Background: Carbon dioxide therapy has been reported to be effective in treating certain cardiac diseases and skin problems. Although a previous study suggested that transcutaneous carbon dioxide application accelerated fracture repair in association with promotion of angiogenesis, blood flow, and endochondral ossification, the influence of the duration of carbon dioxide application on fracture repair is unknown. The aim of this study was to investigate the effect of the duration of transcutaneous carbon dioxide application on rat fracture repair.

Methods: A closed femoral shaft fracture was created in each rat. Animals were randomly divided into four groups: the control group; 1w-CO2 group, postoperative carbon dioxide treatment for 1 week; 2w-CO2 group, postoperative carbon dioxide treatment for 2 weeks; 3w-CO2 group, postoperative carbon dioxide treatment for 3 weeks. Transcutaneous carbon dioxide application was performed five times a week in the carbon dioxide groups. Sham treatment, where the carbon dioxide was replaced with air, was performed for the control group. Radiographic, histological, and biomechanical assessments were performed at 3 weeks after fracture.

Results: The fracture union rate was significantly higher in the 3w-CO2 group than in the control group (p < 0.05). Histological assessment revealed promotion of endochondral ossification in the 3w-CO2 group than in the control group. In the biomechanical assessment, all evaluation items related to bone strength were significantly higher in the 3w-CO2 group than in the control group (p < 0.05).

Conclusions: The present study, conducted using an animal model, demonstrated that continuous carbon dioxide application throughout the process of fracture repair was effective in enhancing fracture healing.

Title: Carbon Dioxide Therapy in the Treatment of Localized Adiposities: Clinical Study and Histopathological Correlations

Authors: Brandi C, D'Aniello C, Grimaldi L, Bosi B, Dei I, Lattarulo P, Alessandrini C.

Journal: Aesthetic Plast Surg. 2001 May-Jun;25(3):170-4. doi: 10.1007/s002660010116. PMID: 11426306.

Link to full text: Carbon Dioxide Therapy in the Treatment of Localized Adiposities: Clinical Study and Histopathological Correlations

Abstract: The authors report their experience using carbon dioxide (CO2) therapy for the treatment of 48 female patients presenting adipose accumulations, located on the thighs, knees, and/or abdomen; a Carbomed Programmable Automatic Carbon Dioxide Therapy apparatus was used. In light of the effects of CO2 on the microcirculation recently described in the literature, we expected this gas, which we administered subcutaneously, to positively affect the physiological oxidative lipolytic process. The aim of our study was to evaluate the effect of this therapy on localized adiposities. As such, we describe the method we used and report the results observed in the areas treated (in terms of reduction in maximum circumference) as well as side effects. Furthermore, we assessed the effect of subcutaneous administration of CO2 on the microcirculation by showing changes in the Laser Doppler signal and in the concentration of transcutaneous oxygen tension (tcPO2). Pre- and posttreatment biopsies of tissues were performed in seven patients in order to study the changes induced by the use of CO2 on both adipose and connective tissues. All data obtained were statistically analyzed; values of P < 0.05 were considered significant.

Title: An overview of the role of carboxytherapy in dermatology

Authors: Bagherani N, Smoller BR, Tavoosidana G, Ghanadan A, Wollina U, Lotti T.

Journal: J Cosmet Dermatol. 2023 Sep;22(9):2399-2407. doi: 10.1111/jocd.15741. Epub 2023 Mar 31. PMID: 36999460.

Link to full text: An overview of the role of carboxytherapy in dermatology

Abstract: Background: Carboxytherapy is defined as intradermal and/or subcutaneous microinjections of sterile purified carbon dioxide into different parts of the body for therapeutic aims. The vasodilatation effect and intradermal collagen reorganization associated with carboxytherapy have advantages for aesthetic dermatology and cosmetology.

Objective: In the current article, we have reviewed some of the most important indications of this modality in dermatology and aesthetic dermatology.

Method: Our review is a narrative one which has gathered some of the most important indications of carboxytherapy in dermatology and cosmetology.

Results: Carboxytherapy has successfully been applied for some dermatologic and cosmetic conditions among which skin aging, cellulite, localized fat deposits, striae distensae, infraorbital hyperpigmentation, scar, lymphedema, androgenetic alopecia, alopecia areata, psoriasis, morphea, and vitiligo are the most important.

Conclusion: Carboxytherapy can be considered as a safe, minimally-invasive modality used for rejuvenation, restoration, and recondition of the skin.

Title: Carboxytherapy: effects on microcirculation and its use in the treatment of severe lymphedema. A review

Authors: Varlaro V., Manzo G., Mugnaini F., Bisacci C., Fiorucci P., De Rango P., Bisacci R.

Journal: Acta Phlebologica 2007 August;8(2):79-91

Link to full text: Carboxytherapy: effects on microcirculation and its use in the treatment of severe lymphedema. A review

Abstract: Carboxytherapy refers to the administration of CO2 for therapeutic purposes. It has been shown that, because of the interaction between CO2 and regulating factors of tissue perfusion, Carboxytherapy acts on the microcirculation at the level of metarterioles, arterioles and precapillary shpincteres by increasing tissue flow velocity and consequently, by improving lymphatic drainage. Analysis of literature data shows a wide range of today applications for this treatment involving either phlebology or non-phlebology fields. Specifically, the positive effect on the increase of lymphatic drainage has more recently made Carboxytherapy useful for treatment of lymphatic stasis. Basic hemodynamic, histologyc and biochemical principles that explain the effects on microcirculation bed and lymphatic drainage are here analyzed to show how Carboxytherapy can be useful in the treatment of diseases such as severe lymphedema.

Title: The Influence of Carboxytherapy on Scar Reduction

Authors: Stolecka-Warzecha A, Chmielewski Ł, Deda A, Śmich A, Lebiedowska A, Wilczyński S.

Journal: Clin Cosmet Investig Dermatol. 2022 Dec 23;15:2855-2872. doi: 10.2147/CCID.S389380. PMID: 36582848; PMCID: PMC9793937.

Link to full text: The Influence of Carboxytherapy on Scar Reduction

Abstract: Introduction
Although it is not a new method, a carboxytherapy, which is based on intradermal or subcutaneous administration of controlled doses of CO2, is gaining more and more recognition among aesthetic medicine doctors, dermatologists and cosmetologists around the world. The consequence of applying carbon dioxide directly into tissues is associated with an immediate expansion of blood vessels, improvement of local blood supply, and, thus, tissue metabolism. It does also support natural regenerative processes. Oxygen and growth factors released from blood, within the area which undergoes treatment, stimulate fibroblasts to produce collagen and formation of new blood vessels also known as neovascularization. In addition to biochemical mechanisms, CO2 injection into the dermis or subcutaneous tissue leads to a mechanical effect exerted by pressure and flow of CO2 which is injected. It is of particular importance in scar treatments.

Methods
Twelve mature scars were subjected to the carboxytherapy which was performed in people aged 23–45 years. A small amount of heated, medical CO2 was injected till the moment a scar turns white. The applied flow rate equalled 100 mL/min (cc/min). Before and after a series of four treatments, the level of hydration, elasticity and colour of the skin were measured. A structured-light 3D scanner was used to determine an exact morphology of the examined scars. The 3D scanning device is seen as a sensitive and precise method of qualitative and quantitative assessment of a morphology of scars.

Results
The results of the measurements performed showed a reduction in the surface of the analyzed changes, as well as proved the ability of CO2 to rebuild collagen fibres. The decrease in value of parameters, which have been obtained thanks to the kutometric examination, indicates softening and loosening of connective tissue. It does confirm the effectiveness of carboxytherapy.

Title: Effect of leg immersion in mild warm carbonated water on skin and muscle blood flow

Authors: Ogoh S, Washio T, Suzuki K, Ikeda K, Hori T, Olesen ND, Muraoka Y.

Journal: Physiol Rep. 2018 Sep;6(18):e13859. doi: 10.14814/phy2.13859. PMID: 30221833; PMCID: PMC6139710.

Link to full text: Effect of leg immersion in mild warm carbonated water on skin and muscle blood flow

Abstract: Leg immersion in carbonated water improves endothelial-mediated vasodilator function and decreases arterial stiffness but the mechanism underlying this effect remains poorly defined. We hypothesized that carbonated water immersion increases muscle blood flow. To test this hypothesis, 10 men (age 21 ± 0 years; mean ± SD) underwent lower leg immersion in tap or carbonated water at 38°C. We evaluated gastrocnemius muscle oxyhemoglobin concentration and tissue oxygenation index using near-infrared spectroscopy, skin blood flow by laser Doppler flowmetry, and popliteal artery (PA) blood flow by duplex ultrasound. Immersion in carbonated, but not tap water elevated PA (from 38 ± 14 to 83 ± 31 mL/min; P < 0.001) and skin blood flow (by 779 ± 312%, P < 0.001). In contrast, lower leg immersion elevated oxyhemoglobin concentration and tissue oxygenation index with no effect of carbonation (P = 0.529 and P = 0.495). In addition, the change in PA blood flow in response to immersion in carbonated water correlated with those of skin blood flow (P = 0.005) but not oxyhemoglobin concentration (P = 0.765) and tissue oxygenation index (P = 0.136) while no relations was found for tap water immersion. These findings indicate that water carbonation has minimal effect on muscle blood flow. Furthermore, PA blood flow increases in response to lower leg immersion in carbonated water likely due to a large increase in skin blood flow.

Keywords: Carbonated water; Doppler ultrasound; near-infrared spectroscopy; popliteal artery.

Title: Carbonate ion-enriched hot spring water promotes skin wound healing in nude rats

Authors: Liang J, Kang D, Wang Y, Yu Y, Fan J, Takashi E.

Journal: PLoS One. 2015 Feb 11;10(2):e0117106. doi: 10.1371/journal.pone.0117106. PMID: 25671581; PMCID: PMC4324962.

Link to full text: Carbonate ion-enriched hot spring water promotes skin wound healing in nude rats

Abstract: Hot spring or hot spa bathing (Onsen) is a traditional therapy for the treatment of certain ailments. There is a common belief that hot spring bathing has therapeutic effects for wound healing, yet the underlying molecular mechanisms remain unclear. To examine this hypothesis, we investigated the effects of Nagano hot spring water (rich in carbonate ion, 42°C) on the healing process of the skin using a nude rat skin wound model. We found that hot spring bathing led to an enhanced healing speed compared to both the unbathed and hot-water (42°C) control groups. Histologically, the hot spring water group showed increased vessel density and reduced inflammatory cells in the granulation tissue of the wound area. Real-time RT-PCR analysis along with zymography revealed that the wound area of the hot spring water group exhibited a higher expression of matrix metalloproteinases-2 and -9 compared to the two other control groups. Furthermore, we found that the enhanced wound healing process induced by the carbonate ion-enriched hot spring water was mediated by thermal insulation and moisture maintenance. Our results provide the evidence that carbonate ion-enriched hot spring water is beneficial for the treatment of skin wounds.

Title: Carbon dioxide and MAPK signalling: towards therapy for inflammation

Authors: Gałgańska H, Jarmuszkiewicz W, Gałgański Ł.

Journal: Cell Commun Signal. 2023 Oct 10;21(1):280. doi: 10.1186/s12964-023-01306-x. PMID: 37817178; PMCID: PMC10566067.

Link to full text: Carbon dioxide and MAPK signalling: towards therapy for inflammation

Abstract: Inflammation, although necessary to fight infections, becomes a threat when it exceeds the capability of the immune system to control it. In addition, inflammation is a cause and/or symptom of many different disorders, including metabolic, neurodegenerative, autoimmune and cardiovascular diseases. Comorbidities and advanced age are typical predictors of more severe cases of seasonal viral infection, with COVID-19 a clear example. The primary importance of mitogen-activated protein kinases (MAPKs) in the course of COVID-19 is evident in the mechanisms by which cells are infected with SARS-CoV-2; the cytokine storm that profoundly worsens a patient's condition; the pathogenesis of diseases, such as diabetes, obesity, and hypertension, that contribute to a worsened prognosis; and post-COVID-19 complications, such as brain fog and thrombosis. An increasing number of reports have revealed that MAPKs are regulated by carbon dioxide (CO2); hence, we reviewed the literature to identify associations between CO2 and MAPKs and possible therapeutic benefits resulting from the elevation of CO2 levels. CO2 regulates key processes leading to and resulting from inflammation, and the therapeutic effects of CO2 (or bicarbonate, HCO3-) have been documented in all of the abovementioned comorbidities and complications of COVID-19 in which MAPKs play roles. The overlapping MAPK and CO2 signalling pathways in the contexts of allergy, apoptosis and cell survival, pulmonary oedema (alveolar fluid resorption), and mechanical ventilation-induced responses in lungs and related to mitochondria are also discussed. Video Abstract.

Keywords: ERK1/2; Hypercapnia; Hypercarbia; Hypocapnia; JNK; p38.

Title: The inflammatory reflex

Authors: Tracey KJ.

Journal: Nature. 2002 Dec 19-26;420(6917):853-9. doi: 10.1038/nature01321. PMID: 12490958.

Link to PubMed: The inflammatory reflex

Abstract: Inflammation is a local, protective response to microbial invasion or injury. It must be fine-tuned and regulated precisely, because deficiencies or excesses of the inflammatory response cause morbidity and shorten lifespan. The discovery that cholinergic neurons inhibit acute inflammation has qualitatively expanded our understanding of how the nervous system modulates immune responses. The nervous system reflexively regulates the inflammatory response in real time, just as it controls heart rate and other vital functions. The opportunity now exists to apply this insight to the treatment of inflammation through selective and reversible 'hard-wired' neural systems.

Title: Effects of repeated carbon dioxide-rich water bathing on core temperature, cutaneous blood flow and thermal sensation

Authors: Nishimura N, Sugenoya J, Matsumoto T, Kato M, Sakakibara H, Nishiyama T, Inukai Y, Okagawa T, Ogata A.

Journal: Eur J Appl Physiol. 2002 Aug;87(4-5):337-42. doi: 10.1007/s00421-002-0626-0. Epub 2002 Jun 7. PMID: 12172871.

Link to full text: Effects of repeated carbon dioxide-rich water bathing on core temperature, cutaneous blood flow and thermal sensation

Abstract: We examined the effects of repeated artificial CO(2) (1,000 ppm) bathing on tympanic temperature (T(ty)), cutaneous blood flow, and thermal sensation in six healthy males. Each subject was immersed in CO(2)-rich water at a temperature of 34 degrees C up to the level of the diaphragm for 20 min. The CO(2)-rich water was prepared using a multi-layered composite hollow-fiber membrane. The CO(2) bathing was performed consecutively for 5 days. As a control study, subjects bathed in fresh water at 34 degrees C under the same conditions. T(ty) was significantly lowered during CO(2) bathing (P < 0.05). Cutaneous blood flow in the immersed skin (right forearm) was significantly increased during CO(2) bathing compared with that during fresh-water bathing (P < 0.05), whereas cutaneous blood flow in the non-immersed skin (chest) was not different between CO(2) and fresh-water bathing. Subjects reported a "warm" sensation during the CO(2) bathing, whereas they reported a "neutral" sensation during the fresh-water bathing. The effects of the repeated CO(2) bathing were not obvious for core temperature and cutaneous blood flow, but the thermal sensation score during the CO(2) bathing was reduced sequentially by repeated CO(2) bathing (P < 0.05). These thermal effects of CO(2) bathing could be ascribed largely to the direct action of CO(2) on vascular smooth muscles and to the activity of thermoreceptors in the skin. Serial CO(2) bathing may influence the activity of thermoreceptors in the skin.

Title: Effect of carbon dioxide-enriched water and fresh water on the cutaneous microcirculation and oxygen tension in the skin of the foot

Authors: Hartmann BR, Bassenge E, Pittler M.

Journal: Angiology. 1997 Apr;48(4):337-43. doi: 10.1177/000331979704800406. PMID: 9112881.

Link to full text: Effect of carbon dioxide-enriched water and fresh water on the cutaneous microcirculation and oxygen tension in the skin of the foot

Abstract: The effects of immersion of the lower leg and foot in fresh water and in carbon dioxide (CO2)-enriched water (1200 mg CO2 per kg water; succinate+sodium bicarbonate: Actibath, KAO Tokyo) on cutaneous circulation, vasomotion and oxygen tension (PO2) were measured by laser Doppler flowmetry and transcutaneous oximetry. On the first of two consecutive days patients were randomly assigned to have the lower extremities immersed in either fresh water or CO2-enriched water under standardized conditions (temperature, 34 degrees C; depth, 35 cm; immersion time, twenty minutes) with concurrent measurement. On the second day patients were switched to the other bath type. For both sets of measurements probes were attached symmetrically to the dorsum of each foot. Included in the study were 18 patients with mild, bilateral, peripheral, occlusive arterial disease (intermittent claudication, femoral or iliac type). During immersion in CO2-enriched water the Doppler laser signal and vasomotion amplitude rose by 300%, while PO2 increased by 10%. These increases were still apparent during the latter part of the measurement period, following withdrawal of the limbs from the bath, while patients were seated and supine. During immersion in fresh water and thereafter the Doppler laser signal was unchanged and the PO2 increase was considerably less marked. The authors were thus able to demonstrate vasodilation and increased oxygen utilization (Bohr effect) resulting from topical CO2 application, and hence, that the use of topical CO2 has an objective basis.

Title: The Effectiveness of Dry Carbon Dioxide Baths in Menopausal Syndrome: a Randomized Clinical Study

Authors: Anzhela N. Chekhoeva, Georgy E. Zangionov, Alina B. Bugulova, Alan S. Tsogoev, Olga O. Borisevich, Natalya V. Kotenko.

Journal: Вестник восстановительной медицины. 2024;23(4):55-61. doi:10.38025/2078-1962-2024-23-4-55-61

Link to full text: The Effectiveness of Dry Carbon Dioxide Baths in Menopausal Syndrome: a Randomized Clinical Study

Abstract:
INTRODUCTION: Climacteric syndrome is characterized by significant disorders in the nervous, musculoskeletal, urinary and cardiovascular systems in women of the perimenopausal and postmenopausal periods. The main reason is a sharp decrease in estrogen levels. Menopausal hormone therapy is often used for treatment, but due to contraindications and complications, it does not always completely solve the problem. This has sparked interest in alternative treatments such as dry carbon dioxide baths.

AIM: To study the effectiveness of using dry carbon dioxide baths in women with menopausal syndrome and evaluate their impact on the neurovegetative, metabolic and psycho-emotional state of these patients.

MATERIALS AND METHODS: A prospective randomized study was conducted involving 70 patients with moderate climacteric syndrome aged from 48 to 60 years. Using a simple randomization method, the women were divided into 2 groups: 35 patients of the main group received 10 procedures of dry carbon dioxide baths with a CO2 concentration of 15–20 %, temperature 28–32 °C, lasting 15–20 minutes, 35 patients of the control group did not receive any treatment. To assess the effectiveness of treatment, a modified Kupperman-Uvarova menopausal index, an assessment of the effect of “hot flashes” on daily life on the HFRDIS scale, as well as daily blood pressure monitoring (SMAD) were used.

RESULTS AND DISCUSSION: After treatment, patients in the main group observed a significantly significant (p < 0.05) improvement in the patients’ condition in the form of a decrease in integral indicators of the modified menopausal index and an assessment of the impact of hot flashes on daily activity on the HFRDIS scale by 28.2 and 43.1 %, respectively, as well as in the form of a decrease in average daily systolic blood pressure by 13.7 %.

CONCLUSION: The use of dry carbon dioxide baths is recommended for use in women with moderate climacteric syndrome, since this method significantly improves the quality of life of patients and is an effective non-drug that can reduce the drug load on the body of a woman suffering from neurovegetative disorders

Title: Asthma at 8 years of age in children born by caesarean section

Authors: Roduit C, Scholtens S, de Jongste JC, Wijga AH, Gerritsen J, Postma DS, Brunekreef B, Hoekstra MO, Aalberse R, Smit HA.

Journal: Thorax. 2009 Feb;64(2):107-13. doi: 10.1136/thx.2008.100875. Epub 2008 Dec 3. PMID: 19052046.

Link to PubMed: Asthma at 8 years of age in children born by caesarean section

Abstract: Background: Caesarean section might be a risk factor for asthma because of delayed microbial colonisation, but the association remains controversial. A study was undertaken to investigate prospectively whether children born by caesarean section are more at risk of having asthma in childhood and sensitisation at the age of 8 years, taking into account the allergic status of the parents. Methods: 2917 children who participated in a birth cohort study were followed for 8 years. The definition of asthma included wheeze, dyspnoea and prescription of inhaled steroids. In a subgroup (n = 1454), serum IgE antibodies for inhalant and food allergens were measured at 8 years. Results: In the total study population, 12.4% (n = 362) of the children had asthma at the age of 8 years. Caesarean section, with a total prevalence of 8.5%, was associated with an increased risk of asthma (OR 1.79; 95% CI 1.27 to 2.51). This association was stronger among predisposed children (with two allergic parents: OR 2.91; 95% CI 1.20 to 7.05; with only one: OR 1.86; 95% CI 1.12 to 3.09) than in children with non-allergic parents (OR 1.36; 95% CI 0.77 to 2.42). The association between caesarean section and sensitisation at the age of 8 years was significant only in children of non-allergic parents (OR 2.14; 95% CI 1.16 to 3.98). Conclusions: Children born by caesarean section have a higher risk of asthma than those born by vaginal delivery, particularly children of allergic parents. Caesarean section increases the risk for sensitisation to common allergens in children with non-allergic parents only.

Title: Physiologic Changes in the Airway and the Respiratory System Affecting Management in Pregnancy

Authors: Izakson A, Cohen Y, Landau R.

Journal: Principles and practice of maternal critical care (2020): 271-283.

Link to Springer: Physiologic Changes in the Airway and the Respiratory System Affecting Management in Pregnancy

Abstract: Pregnancy is associated with significant anatomical and physiological changes in the airway and respiratory system. Reduction of functional residual capacity in parallel with increased oxygen consumption shortens the time available for airway manipulation before hypoxia becomes significant. Hormone-induced changes in respiratory drive cause a reduction of normal partial pressure of arterial carbon dioxide values during pregnancy, associated with compensatory metabolic acidosis. A thorough understanding of these changes is key for anesthesiologists and critical care providers managing obstetric patients with compromised oxygenation requiring airway manipulation and mechanical ventilation.

Title: Maternal carbon dioxide level during labor and its possible effect on fetal cerebral oxygenation: mini review

Authors: Tomimatsu T, Kakigano A, Mimura K, Kanayama T, Koyama S, Fujita S, Taniguchi Y, Kanagawa T, Kimura T.

Journal: J Obstet Gynaecol Res. 2013 Jan;39(1):1-6. doi: 10.1111/j.1447-0756.2012.01944.x. Epub 2012 Jul 6. PMID: 22765270.

Link to PubMed: Maternal carbon dioxide level during labor and its possible effect on fetal cerebral oxygenation: mini review

Abstract: During pregnancy, and especially during labor, the maternal carbon dioxide level declines considerably. Maternal carbon dioxide levels show a close relation with fetal carbon dioxide levels. The latter affects fetal cerebral oxygenation by regulating cerebral blood flow and shifting the oxyhemoglobin dissociation curve. In addition, maternal hypocapnia appears to impair placental oxygen transfer. Thus, maternal hyperventilation may interfere with optimal fetal cerebral oxygenation. Here, we provide a brief overview of the literature relevant to this issue.

Title: The effect of hyperventilation on maternal placental blood flow in pregnant rabbits

Authors: Leduc B.

Journal: J Physiol. 1972 Sep;225(2):339-48. doi: 10.1113/jphysiol.1972.sp009943. PMID: 4561482; PMCID: PMC1331109.

Link to full text: The effect of hyperventilation on maternal placental blood flow in pregnant rabbits

Abstract: 1. In anaesthetized pregnant rabbits near term, cardiac output and its distribution were measured by injection of isotope-labelled microspheres. Hypocapnia (mean arterial P(CO) (2) 18 mm Hg), induced by intermittent positive pressure hyperventilation, caused a 43% reduction in maternal placental blood flow, attributed mainly to vasoconstriction. Myometrial flow was not significantly changed.2. Moderate hypercapnia (mean arterial P(CO) (2) 46 mm Hg) caused no change in placental flow, compared with observations made while breathing air spontaneously (P(CO) (2) 31 mm Hg).3. Intravenous infusions of adrenaline or noradrenaline 1 mug/kg. min caused maternal placental vasoconstriction.4. During the especially warm summer of 1969, there was a mean 46% reduction in maternal placental blood flow in pregnant rabbits near term, breathing room air spontaneously with normal blood gas values and rectal temperatures. This was associated with an increase in the number of runts and dead foetuses.

Title: The effects of pregnancy on nasal physiology

Authors: Demir UL, Demir BC, Oztosun E, Uyaniklar OO, Ocakoglu G.

Journal: Int Forum Allergy Rhinol. 2015 Feb;5(2):162-6. doi: 10.1002/alr.21438. Epub 2014 Oct 27. PMID: 25348597.

Link to PubMed: The effects of pregnancy on nasal physiology

Abstract: Background: Nasal congestion that is not present before pregnancy represents a distinct clinical entity called pregnancy rhinitis. The aim of this study is to evaluate the clinical characteristics of nasal physiology over the course of pregnancy. Methods: The study was conducted with 85 pregnant women and 26 nonpregnant controls. We measured nasal airway patency objectively via acoustic rhinometry (ARM) and anterior rhinomanometry (RMM) and subjectively via the Nasal Obstruction Symptom Evaluation (NOSE) scale in each trimester and compared the results to those of the controls. Results: The NOSE scores of control and pregnant women showed no difference (p = 0.866). Minimal cross-sectional area (MCA1; minimal cross sectional area at nasal valve and MCA2; minimal cross sectional area at the level where the head of inferior turbinate is placed) decreased significantly between the first and third trimesters: first trimester 0.37 cm(2), third trimester 0.31 cm(2). There was no difference between each trimester with regard to total nasal resistance. The correlation analysis between the NOSE score and both total volume and MCA1 in all patients showed no significance (r = -0.10, p = 0.318; r = -0.04, p = 0.654, respectively). Conclusion: Pregnancy affects nasal physiology adversely and impairs nasal breathing in some women. However, based on the findings of this study, we concluded that this clinical entity may not be considered as a disease without complementary symptoms despite the presence of objective changes in nasal parameters. Keywords: acoustic rhinometry; nasal symptoms; pregnancy; rhinitis; rhinomanometry.

Title: Effects of human pregnancy on the ventilatory chemoreflex response to carbon dioxide

Authors: Jensen D, Wolfe LA, Slatkovska L, Webb KA, Davies GA, O'Donnell DE.

Journal: Am J Physiol Regul Integr Comp Physiol. 2005 May;288(5):R1369-75. doi: 10.1152/ajpregu.00862.2004. Epub 2005 Jan 27. PMID: 15677521.

Link to full text: Effects of human pregnancy on the ventilatory chemoreflex response to carbon dioxide

Abstract: This study examined the effects of human pregnancy on the central chemoreflex control of breathing. Subjects were two groups (n=11) of pregnant subjects (PG, gestational age, 36.5+/-0.4 wk) and nonpregnant control subjects (CG), equated for mean age, body height, prepregnant body mass, parity, and aerobic fitness. All subjects performed a hyperoxic CO2 rebreathing procedure, which includes prior hyperventilation and maintenance of iso-oxia. Resting blood gases and plasma progesterone and estradiol concentrations were measured. During rebreathing trials, end-tidal Pco2 increased, whereas end-tidal Po2 was maintained at a constant hyperoxic level. The point at which ventilation (Ve) began to rise as end-tidal Pco2 increased was identified as the central chemoreflex ventilatory recruitment threshold for CO2 (VRTco2). Ve levels below (basal Ve) and above (central chemoreflex sensitivity) the VRTco2 were determined. The VRTco2 was significantly lower in the PG vs. CG (40.5+/-0.8 vs. 45.8+/-1.6 Torr), and both basal Ve (14.8+/-1.1 vs. 9.3+/-1.6 l/min) and central chemoreflex sensitivity (5.07+/-0.74 vs. 3.16+/-0.29 l.min-1.Torr-1) were significantly higher in the PG vs. CG. Pooled data from the two groups showed significant correlations for resting arterial Pco2 with basal Ve, central chemoreflex sensitivity, and the VRTco2. The VRTco2 was also correlated with progesterone and estradiol concentrations. These data support the hypothesis that pregnancy decreases the threshold and increases the sensitivity of the central chemoreflex response to CO2. These changes may be due to the effects of gestational hormones on chemoreflex and/or nonchemoreflex drives to breathe.

Title: Maternal hyperventilation and the fetus

Authors: Huch R.

Journal: J Perinat Med. 1986;14(1):3-17. doi: 10.1515/jpme.1986.14.1.3. PMID: 3517286.

Link to PubMed: Maternal hyperventilation and the fetus

Abstract: Pregnant women experience hyperventilation during pregnancy for several reasons: It occurs regularly and spontaneously during pregnancy, it occurs because of the type of ventilation practiced during the actual hours of labor and delivery, and sometimes it is induced by the anesthesiologic technique during obstetrical operations. This often results in excessive hyperventilation and has significant effects on blood gases, the cardiovascular and neuro-psychometrical systems of the female organism.

Title: Phasic menstrual cycle effects on the control of breathing in healthy women

Authors: Slatkovska L, Jensen D, Davies GA, Wolfe LA.

Journal: Respir Physiol Neurobiol. 2006 Dec;154(3):379-88. doi: 10.1016/j.resp.2006.01.011. Epub 2006 Mar 15. PMID: 16542884.

Link to PubMed: Phasic menstrual cycle effects on the control of breathing in healthy women

Abstract: This study examined the effects of menstrual cycle phase on ventilatory control. Fourteen eumenorrheic women were studied in the early follicular (FP; 1-6 days) and mid-luteal (LP; 20-24 days) phase of the menstrual cycle. Blood for the determination of arterial PCO(2) (PaCO(2)) , plasma strong ion difference ([SID]), progesterone ([P(4)]), and 17beta-estradiol ([E(2)]) concentrations were obtained at rest. Subjects performed a CO(2) rebreathing procedure that included prior hyperventilation and maintenance of iso-oxia to evaluate central and peripheral chemoreflex, and nonchemoreflex drives to breathe. Resting PaCO(2) and [SID] were lower; minute ventilation (V (E)), [P(4)] and [E(2)] were higher in the LP versus FP. Within the LP, significant correlations were observed for PaCO(2) with [P(4)], [E(2)] and [SID]. Menstrual cycle phase had no effect on the threshold or sensitivity of the central and/or peripheral ventilatory chemoreflex response to CO(2). Both (V (E)) and the ventilatory response to hypocapnia (representing nonchemoreflex drives to breathe) were approximately 1L/min greater in the LP versus FP accounting for the reduction in PaCO(2) . These data support the hypothesis that phasic menstrual cycle changes in PaCO(2) may be due, at least in part, to the stimulatory effects of [P(4)], [E(2)] and [SID] on ventilatory drive.

Title: Nasal congestion during pregnancy

Authors: Ellegård E, Karlsson G.

Journal: Clin Otolaryngol Allied Sci. 1999 Aug;24(4):307-11. doi: 10.1046/j.1365-2273.1999.00264.x. PMID: 10472465.

Link to PubMed: Nasal congestion during pregnancy

Abstract: We define pregnancy rhinitis as nasal congestion in the last 6 or more weeks of pregnancy without other signs of respiratory tract infection and with no known allergic cause, disappearing completely within 2 weeks after delivery. In order to describe physiological variations of nasal obstruction during pregnancy, subjective scores and nasal as well as oral peak expiratory flow values were recorded daily in 23 pregnancies until 1 month after delivery. Scores were higher during early and late pregnancy than in the month after delivery. Objectively registered blockage increased during pregnancy in eight women only. Unexpectedly nine women showed declining blockage. Five of 23 women had pregnancy rhinitis.

Title: Nasal Congestion and Its Management in Pregnancy Rhinitis

Authors: Poerbonegoro N.L.

Journal: Indonesian Journal of Obstetrics and Gynecology (2019): 320-328.

Link to full text: Nasal Congestion and Its Management in Pregnancy Rhinitis

Abstract: Background: Pregnancy rhinitis occurs approximately in one-fifth of pregnancies, at almost any gestational week. The incidence rate of pregnancy rhinitis reaches up to 40%, with prevalence as high as 17%. Pathomechanism is still unclear, but it is suspected that estrogen and placental growth hormone (PGH) play roles in the development of disease. Objective: To elaborate the pathomechanism of pregnancy rhinitis and the proper management of rhinitis symptoms, particularly nasal obstruction. Methods: Literature review. Conclusion: Pregnancy rhinitis, manifested as nasal congestion, is considered a phenomenon and may become a serious condition. Persistent nasal congestion acts as a potential risk factor in affecting fetal growth and development through gradual hypoxia process. This condition can lead to various complications such as maternal hypertension, preeclampsia, impaired fetal growth, and low APGAR scores. Indepth knowledge of pathomechanism is essential as guidance to accurate treatment including conservative and pharmaca therapies, which will lead to optimal outcome for both mother and baby.

Title: Nasal lavage in pregnant women with seasonal allergic rhinitis: a randomized study

Authors: Garavello W, Somigliana E, Acaia B, Gaini L, Pignataro L, Gaini RM.

Journal: Int Arch Allergy Immunol. 2010;151(2):137-41. doi: 10.1159/000236003. Epub 2009 Sep 15. PMID: 19752567.

Link to PubMed: Nasal lavage in pregnant women with seasonal allergic rhinitis: a randomized study

Abstract: Background: Nasal rinsing appears particularly suitable in the management of pregnant women with seasonal allergic rhinitis since no deleterious effects on the fetus are to be expected. However, to date, no studies have specifically investigated this option. Methods: Pregnant women with seasonal allergic rhinitis were randomized to intranasal lavage with hypertonic saline solution 3 times daily (n = 22) versus no local therapy (n = 23) during a 6-week period corresponding to the pollen season. Patients were invited to keep a daily record of rhinitis symptoms (rhinorrea, obstruction, nasal itching and sneezing), to record consumption of oral antihistamine and to undergo rhinomanometry. Results: The rhinitis score was similar at study entry but a statistically significant improvement in this score was observed in the study group during all subsequent weeks (p < 0.001 for weeks 2-6). The mean number of daily antihistamines use per patient per week was significantly reduced at weeks 2, 3 and 6 (p < 0.001, p < 0.001 and p = 0.001, respectively). Baseline rhinomanometry performed at week 1 showed similar nasal resistance in the study and control groups. In contrast, a statistically significant difference emerged in the 2 following evaluations. At week 3, nasal resistance in the study and control groups was 0.96 +/- 0.44 and 1.38 +/- 0.52 Pa/ml/s, respectively (p = 0.006). At week 6, it was 0.94 +/- 0.38 and 1.35 +/- 0.60 Pa/ml/s, respectively (p = 0.006). No adverse effect was reported in the active group. Conclusions: Nasal rinsing is a safe and effective treatment option in pregnant women with seasonal allergic rhinitis.

Title: Oxygen and oxidative stress in the perinatal period

Authors: Torres-Cuevas I, Parra-Llorca A, Sánchez-Illana A, Nuñez-Ramiro A, Kuligowski J, Cháfer-Pericás C, Cernada M, Escobar J, Vento M.

Journal: Redox Biol. 2017 Aug;12:674-681. doi: 10.1016/j.redox.2017.03.011. Epub 2017 Mar 12. PMID: 28395175; PMCID: PMC5388914.

Link to full text: Oxygen and oxidative stress in the perinatal period

Abstract: Fetal life evolves in a hypoxic environment. Changes in the oxygen content in utero caused by conditions such as pre-eclampsia or type I diabetes or by oxygen supplementation to the mother lead to increased free radical production and correlate with perinatal outcomes. In the fetal-to-neonatal transition asphyxia is characterized by intermittent periods of hypoxia ischemia that may evolve to hypoxic ischemic encephalopathy associated with neurocognitive, motor, and neurosensorial impairment. Free radicals generated upon reoxygenation may notably increase brain damage. Hence, clinical trials have shown that the use of 100% oxygen given with positive pressure in the airways of the newborn infant during resuscitation causes more oxidative stress than using air, and increases mortality. Preterm infants are endowed with an immature lung and antioxidant system. Clinical stabilization of preterm infants after birth frequently requires positive pressure ventilation with a gas admixture that contains oxygen to achieve a normal heart rate and arterial oxygen saturation. In randomized controlled trials the use high oxygen concentrations (90% to 100%) has caused more oxidative stress and clinical complications that the use of lower oxygen concentrations (30-60%). A correlation between the amount of oxygen received during resuscitation and the level of biomarkers of oxidative stress and clinical outcomes was established. Thus, based on clinical outcomes and analytical results of oxidative stress biomarkers relevant changes were introduced in the resuscitation policies. However, it should be underscored that analysis of oxidative stress biomarkers in biofluids has only been used in experimental and clinical research but not in clinical routine. The complexity of the technical procedures, lack of automation, and cost of these determinations have hindered the routine use of biomarkers in the clinical setting. Overcoming these technical and economical difficulties constitutes a challenge for the immediate future since accurate evaluation of oxidative stress would contribute to improve the quality of care of our neonatal patients. Keywords: Biomarkers; High-risk pregnancy; Ischemia-reperfusion; Newborn; Oxidative stress; Oxygen.

Title: Respiratory physiology of pregnancy: Physiology masterclass

Authors: LoMauro A, Aliverti A.

Journal: Breathe (Sheff). 2015 Dec;11(4):297-301. doi: 10.1183/20734735.008615. PMID: 27066123; PMCID: PMC4818213.

Link to full text: Respiratory physiology of pregnancy: Physiology masterclass

Abstract: During healthy pregnancy, pulmonary function, ventilatory pattern and gas exchange are affected through both biochemical and mechanical pathways, as summarised in figure 1. During pregnancy, the physiological alteration of hormonal patterns is the main cause of ventilatory changes in respiratory function.

Title: Respiratory and Acid-Base changes during pregnancy

Authors: Prowse C.M, Gaensler E.A.

Journal: Anesthesiology. 1965 Jul-Aug;26:381-92. doi: 10.1097/00000542-196507000-00003. PMID: 14313450.

Link to PubMed: Respiratory and Acid-Base changes during pregnancy

Abstract: Much of our knowledge of respiratory and acid-base changes in pregnancy stems from studies made more than ten years ago and even that material is limited and incomplete. This is surprising when one considers that pregnancy constitutes one of the most severe states of physiologic adaptations. Incongruously, the effect upon lung function of a number of exceedingly rare diseases has been studied exhaustively from every aspect, and by the most modern techniques, whereas the effect of pregnancy, where we are confronted with an ever-growing and inexhaustible supply of clinical material, has been explored so incompletely. Actually, more attention has been paid to states of pathologic stress upon pregnancy such as heart disease, tuberculosis and following chest surgery than to the physiologic norm.

Title: Respiratory physiologic changes in pregnancy

Authors: Wise RA, Polito AJ, Krishnan V.

Journal: Immunol Allergy Clin North Am. 2006 Feb;26(1):1-12. doi: 10.1016/j.iac.2005.10.004. PMID: 16443140.

Link to PubMed: Respiratory physiologic changes in pregnancy

Abstract: In summary, the major physiologic changes that occur in pregnancy are the increased minute ventilation, which is caused by increased respiratory center sensitivity and drive; a compensated respiratory alkalosis; and a low expiratory reserve volume. The vital capacity and measures of forced expiration are well preserved. Patients who have many lung diseases tolerate pregnancy well, with the exception of those who have pulmonary hypertension or chronic respiratory insufficiency from parenchymal or neuromuscular disease.

Title: Sleep-disordered breathing in pregnancy

Authors: Balserak BI.

Journal: Am J Respir Crit Care Med. 2014 Aug 15;190(4):P1-2. doi: 10.1164/rccm.1904P1. PMID: 25127314.

Link to PubMed: Sleep-disordered breathing in pregnancy

Abstract: Sleep disordered breathing (SDB) is very common during pregnancy, and is most likely explained by hormonal, physiological and physical changes. Maternal obesity, one of the major risk factors for SDB, together with physiological changes in pregnancy may predispose women to develop SDB. SDB has been associated with poor maternal and fetal outcomes. Thus, early identification, diagnosis and treatment of SDB are important in pregnancy. This article reviews the pregnancyrelated changes affecting the severity of SDB, the epidemiology and the risk factors of SDB in pregnancy, the association of SDB with adverse pregnancy outcomes, and screening and management options specific for this population

Title: The effect on the fetus of maternal hyperventilation during labour

Authors: Saling E, Ligdas P.

Journal: J Obstet Gynaecol Br Commonw. 1969 Oct;76(10):877-80. doi: 10.1111/j.1471-0528.1969.tb15724.x. PMID: 5386830.

Link to PubMed: The effect on the fetus of maternal hyperventilation during labour

Abstract: ONLY a few years ago it was difficult for midwives and obstetricians to imagine that an increase in the frequency and depth of respiration of women in labour might be too great and constitute a hazard for the fetus. It is still the practice in many places to encourage as much as possible deep breathing during labour so that "a good oxygen supply is ensured for the baby", although such a policy was proved wrong many years ago. It is also suggested that the majority of women in labour are tense and cannot relax because they are afraid of pain, and that, therefore, they do not breathe properly. Whilst tension is not rare during labour, the assumption that this is always associated with an inadequate breathing technique is erroneous. Hasselbach (1912) showed that pregnant women have a decreased blood carbon dioxide tension (Pco2) during pregnancy, and later studies (Sjosted, 1962) proved that this also applied to women in labour. An increase in blood progesterone levels is thought to account for hyperventilation.

Title: Blood flow and oxygenation of tumors in mice. I. Effects of breathing gases containing carbon dioxide at atmospheric pressure

Authors: Kruuv JA, Inch WR, McCredie JA.

Journal: Cancer. 1967 Jan;20(1):51-9. doi: 10.1002/1097-0142(1967)20:1<51::aid-cncr2820200108>3.0.co;2-d. PMID: 6016872.

Link to PubMed: Blood flow and oxygenation of tumors in mice. I. Effects of breathing gases containing carbon dioxide at atmospheric pressure

Abstract: Inhalation of 5% carbon dioxide (CO,) and 95% oxygen (0,)or 10% CO, and90% 0, at one a t m pressure increased blood flow of the C3HBA isotransplanta n d C3H isoimplant in C3H mice compared with animals breathing 100% 0,or air. Oxygen concentration, measured b y a platinum microelectrode, was im-proved in anoxic areas of the tumor when the animals inhaled the gases con-taining CO, compared with those breathing 100% 0, or air; the relative increasewas less when the electrode was placed in a well-vascularized area. Rectal tem-perature, heart a n d respiration rates were decreased in animals breathing thegases containing CO, compared with air; rectal temperature was increased b yinhalation of 100% 0,. The results suggest that inhalation of a gas mixturecontaining CO, a t atmospheric pressure during radiotherapy or chemotherapymay be of value i n treating cancer in humans

Title: Transcutaneous application of carbon dioxide (CO2) induces mitochondrial apoptosis in human malignant fibrous histiocytoma in vivo

Authors: Onishi Y, Kawamoto T, Ueha T, Kishimoto K, Hara H, Fukase N, Toda M, Harada R, Minoda M, Sakai Y, Miwa M, Kurosaka M, Akisue T.

Journal: PLoS One. 2012;7(11):e49189. doi: 10.1371/journal.pone.0049189. Epub 2012 Nov 15. PMID: 23166610; PMCID: PMC3499556.

Link to full text: Transcutaneous application of carbon dioxide (CO2) induces mitochondrial apoptosis in human malignant fibrous histiocytoma in vivo

Abstract: Mitochondria play an essential role in cellular energy metabolism and apoptosis. Previous studies have demonstrated that decreased mitochondrial biogenesis is associated with cancer progression. In mitochondrial biogenesis, peroxisome proliferator-activated receptor gamma coactivator-1 alpha (PGC-1α) regulates the activities of multiple nuclear receptors and transcription factors involved in mitochondrial proliferation. Previously, we showed that overexpression of PGC-1α leads to mitochondrial proliferation and induces apoptosis in human malignant fibrous histiocytoma (MFH) cells in vitro. We also demonstrated that transcutaneous application of carbon dioxide (CO(2)) to rat skeletal muscle induces PGC-1α expression and causes an increase in mitochondrial proliferation. In this study, we utilized a murine model of human MFH to determine the effect of transcutaneous CO(2)) exposure on PGC-1α expression, mitochondrial proliferation and cellular apoptosis. PGC-1α expression was evaluated by quantitative real-time PCR, while mitochondrial proliferation was assessed by immunofluorescence staining and the relative copy number of mitochondrial DNA (mtDNA) was assessed by real-time PCR. Immunofluorescence staining and DNA fragmentation assays were used to examine mitochondrial apoptosis. We also evaluated the expression of mitochondrial apoptosis related proteins, such as caspases, cytochorome c and Bax, by immunoblot analysis. We show that transcutaneous application of CO(2)) induces PGC-1α expression, and increases mitochondrial proliferation and apoptosis of tumor cells, significantly reducing tumor volume. Proteins involved in the mitochondrial apoptotic cascade, including caspase 3 and caspase 9, were elevated in CO(2)) treated tumors compared to control. We also observed an enrichment of cytochrome c in the cytoplasmic fraction and Bax protein in the mitochondrial fraction of CO(2)) treated tumors, highlighting the involvement of mitochondria in apoptosis. These data indicate that transcutaneous application of CO(2)) may represent a novel therapeutic tool in the treatment of human MFH.

Title: Transcutaneous carbon dioxide application suppresses bone destruction caused by breast cancer metastasis

Authors: Takemori T, Kawamoto T, Ueha T, Toda M, Morishita M, Kamata E, Fukase N, Hara H, Fujiwara S, Niikura T, Kuroda R, Akisue T.

Journal: Oncol Rep. 2018 Oct;40(4):2079-2087. doi: 10.3892/or.2018.6608. Epub 2018 Jul 30. PMID: 30066936.

Link to full text: Transcutaneous carbon dioxide application suppresses bone destruction caused by breast cancer metastasis

Abstract: Hypoxia plays a significant role in cancer progression, including metastatic bone tumors. We previously reported that transcutaneous carbon dioxide (CO2) application could decrease tumor progression through the improvement of intratumor hypoxia. Therefore, we hypothesized that decreased hypoxia using transcutaneous CO2 could suppress progressive bone destruction in cancer metastasis. In the present study, we examined the effects of transcutaneous CO2 application on metastatic bone destruction using an animal model. The human breast cancer cell line MDA-MB-231 was cultured in vitro under three different oxygen conditions, and the effect of altered oxygen conditions on the expression of osteoclast-differentiation and osteolytic factors was assessed. An in vivo bone metastatic model of human breast cancer was created by intramedullary implantation of MDA-MB-231 cells into the tibia of nude mice, and treatment with 100% CO2 or a control was performed twice weekly for two weeks. Bone volume of the treated tibia was evaluated by micro-computed tomography (µCT), and following treatment, histological evaluation was performed by hematoxylin and eosin staining and immunohistochemical staining for hypoxia-inducible factor (HIF)-1α, osteoclast-differentiation and osteolytic factors, and tartrate-resistant acid phosphatase (TRAP) staining for osteoclast activity. In vitro experiments revealed that the mRNA expression of RANKL, PTHrP and IL-8 was significantly increased under hypoxic conditions and was subsequently reduced by reoxygenation. In vivo results by µCT revealed that bone destruction was suppressed by transcutaneous CO2, and that the expression of osteoclast-differentiation and osteolytic factors, as well as HIF-1α, was decreased in CO2-treated tumor tissues. In addition, multinucleated TRAP-positive osteoclasts were significantly decreased in CO2-treated tumor tissues. Hypoxic conditions promoted bone destruction in breast cancer metastasis, and reversal of hypoxia by transcutaneous CO2 application significantly inhibited metastatic bone destruction along with decreased osteoclast activity. The findings in this study strongly indicated that transcutaneous CO2 application could be a novel therapeutic strategy for treating metastatic bone destruction.

Title: Optimization of antitumor treatment conditions for transcutaneous CO2 application: An in vivo study

Authors: Ueha T, Kawamoto T, Onishi Y, Harada R, Minoda M, Toda M, Hara H, Fukase N, Kurosaka M, Kuroda R, Akisue T, Sakai Y.

Journal: Oncol Rep. 2017 Jun;37(6):3688-3694. doi: 10.3892/or.2017.5591. Epub 2017 Apr 20. PMID: 28440513.

Link to full text: Optimization of antitumor treatment conditions for transcutaneous CO2 application: An in vivo study

Abstract: Carbon dioxide (CO2) therapy can be applied to treat a variety of disorders. We previously found that transcutaneous application of CO2 with a hydrogel decreased the tumor volume of several types of tumors and induced apoptosis via the mitochondrial pathway. However, only one condition of treatment intensity has been tested. For widespread application in clinical antitumor therapy, the conditions must be optimized. In the present study, we investigated the relationship between the duration, frequency, and treatment interval of transcutaneous CO2 application and antitumor effects in murine xenograft models. Murine xenograft models of three types of human tumors (breast cancer, osteosarcoma, and malignant fibrous histiocytoma/undifferentiated pleomorphic sarcoma) were used to assess the antitumor effects of transcutaneous CO2 application of varying durations, frequencies, and treatment intervals. In all human tumor xenografts, apoptosis was significantly induced by CO2 treatment for ≥10 min, and a significant decrease in tumor volume was observed with CO2 treatments of >5 min. The effect on tumor volume was not dependent on the frequency of CO2 application, i.e., twice or five times per week. However, treatment using 3- and 4-day intervals was more effective at decreasing tumor volume than treatment using 2- and 5-day intervals. The optimal conditions of transcutaneous CO2 application to obtain the best antitumor effect in various tumors were as follows: greater than 10 min per application, twice per week, with 3- and 4-day intervals, and application to the site of the tumor. The results suggest that this novel transcutaneous CO2 application might be useful to treat primary tumors, while mitigating some side effects, and therefore could be safe for clinical trials.

Title: Transcutaneous carbon dioxide induces mitochondrial apoptosis and suppresses metastasis of oral squamous cell carcinoma in vivo

Authors: Takeda D, Hasegawa T, Ueha T, Imai Y, Sakakibara A, Minoda M, Kawamoto T, Minamikawa T, Shibuya Y, Akisue T, Sakai Y, Kurosaka M, Komori T.

Journal: PLoS One. 2014 Jul 2;9(7):e100530. doi: 10.1371/journal.pone.0100530. PMID: 24988190; PMCID: PMC4079455.

Link to full text: Transcutaneous carbon dioxide induces mitochondrial apoptosis and suppresses metastasis of oral squamous cell carcinoma in vivo

Abstract: Squamous cell carcinoma (SCC) is the main histological type of oral cancer. Its growth rate and incidence of metastasis to regional lymph nodes is influenced by various factors, including hypoxic conditions. We have previously reported that transcutaneous CO2 induces mitochondrial apoptosis and decreases lung metastasis by reoxygenating sarcoma cells. However, previous studies have not determined the sequential mechanism by which transcutaneous CO2 suppresses growth of epithelial tumors, including SCCs. Moreover, there is no report that transcutaneous CO2 suppresses lymphogenous metastasis using human cell lines xenografts. In this study, we examined the effects of transcutaneous CO2 on cancer apoptosis and lymphogenous metastasis using human SCC xenografts. Our results showed that transcutaneous CO2 affects expressions of PGC-1α and TFAM and protein levels of cleavage products of caspase-3, caspase-9 and PARP, which relatives mitochondrial apoptosis. They also showed that transcutaneous CO2 significantly inhibits SCC tumor growth and affects expressions of HIF-1α, VEGF, MMP-2 and MMP-9, which play essential roles in tumor angiogenesis, invasion and metastasis. In conclusion, transcutaneous CO2 suppressed tumor growth, increased mitochondrial apoptosis and decreased the number of lymph node metastasis in human SCC by decreasing intra-tumoral hypoxia and suppressing metastatic potential with no observable effect in vivo. Our findings indicate that transcutaneous CO2 could be a novel therapeutic tool for treating human SCC.

Title: Carbogen breathing increases prostate cancer oxygenation: a translational MRI study in murine xenografts and humans

Authors: Alonzi R, Padhani AR, Maxwell RJ, Taylor NJ, Stirling JJ, Wilson JI, d'Arcy JA, Collins DJ, Saunders MI, Hoskin PJ.

Journal: Br J Cancer. 2009 Feb 24;100(4):644-8. doi: 10.1038/sj.bjc.6604903. Epub 2009 Feb 3. PMID: 19190629; PMCID: PMC2653742.

Link to full text: Carbogen breathing increases prostate cancer oxygenation: a translational MRI study in murine xenografts and humans

Abstract: Hypoxia has been associated with poor local tumour control and relapse in many cancer sites, including carcinoma of the prostate. This translational study tests whether breathing carbogen gas improves the oxygenation of human prostate carcinoma xenografts in mice and in human patients with prostate cancer. A total of 23 DU145 tumour-bearing mice, 17 PC3 tumour-bearing mice and 17 human patients with prostate cancer were investigated. Intrinsic susceptibility-weighted MRI was performed before and during a period of carbogen gas breathing. Quantitative R(2)* pixel maps were produced for each tumour and at each time point and changes in R(2)* induced by carbogen were determined. There was a mean reduction in R(2)* of 6.4% (P=0.003) for DU145 xenografts and 5.8% (P=0.007) for PC3 xenografts. In all, 14 human subjects were evaluable; 64% had reductions in tumour R(2)* during carbogen inhalation with a mean reduction of 21.6% (P=0.0005). Decreases in prostate tumour R(2)* in both animal models and human patients as a result of carbogen inhalation suggests the presence of significant hypoxia. The finding that carbogen gas breathing improves prostate tumour oxygenation provides a rationale for testing the radiosensitising effects of combining carbogen gas breathing with radiotherapy in prostate cancer patients.

Title: CO2 bubbling-based 'Nanobomb' System for Targetedly Suppressing Panc-1 Pancreatic Tumor via Low Intensity Ultrasound-activated Inertial Cavitation

Authors: Zhang K, Xu H, Chen H, Jia X, Zheng S, Cai X, Wang R, Mou J, Zheng Y, Shi J.

Journal: Theranostics. 2015 Sep 12;5(11):1291-302. doi: 10.7150/thno.12691. PMID: 26379793; PMCID: PMC4568455.

Link to full text: CO2 bubbling-based 'Nanobomb' System for Targetedly Suppressing Panc-1 Pancreatic Tumor via Low Intensity Ultrasound-activated Inertial Cavitation

Abstract: Noninvasive and targeted physical treatment is still desirable especially for those cancerous patients. Herein, we develop a new physical treatment protocol by employing CO2 bubbling-based 'nanobomb' system consisting of low-intensity ultrasound (1.0 W/cm(2)) and a well-constructed pH/temperature dual-responsive CO2 release system. Depending on the temperature elevation caused by exogenous low-intensity therapeutic ultrasound irradiation and the low pH caused by the endogenous acidic-environment around/within tumor, dual-responsive CO2 release system can quickly release CO2 bubbles, and afterwards, the generated CO2 bubbles waves will timely explode before dissolution due to triggering by therapeutic ultrasound waves. Related bio-effects (e.g., cavitation, mechanical, shock waves, etc) caused by CO2 bubbles' explosion effectively induce instant necrosis of panc-1 cells and blood vessel destruction within panc-1 tumor, and consequently inhibit the growth of panc-1 solid tumor, simultaneously minimizing the side effects to normal organs. This new physiotherapy employing CO2 bubbling-based 'nanobomb' system promises significant potentials in targetedly suppressing tumors, especially for those highly deadly cancers. Keywords: Nanobomb; dual-responsive CO2 release; inertial cavitation; low intensity ultrasound; panc-1 pancreatic cancer.

Title: Intra-arterially infused carbon dioxide-saturated solution for sensitizing the anticancer effect of cisplatin in a rabbit VX2 liver tumor model

Authors: Katayama N, Sugimoto K, Okada T, Ueha T, Sakai Y, Akiyoshi H, Mie K, Ueshima E, Sofue K, Koide Y, Tani R, Gentsu T, Yamaguchi M.

Journal: Int J Oncol. 2017 Aug;51(2):695-701. doi: 10.3892/ijo.2017.4056. Epub 2017 Jun 26. PMID: 28656217.

Link to full text: Intra-arterially infused carbon dioxide-saturated solution for sensitizing the anticancer effect of cisplatin in a rabbit VX2 liver tumor model

Abstract: The present study aimed to evaluate the efficacy of an intra-arterially infused carbon dioxide (CO2)-saturated solution in sensitizing the anticancer effect of cisplatin in a rabbit VX2 liver tumor model. Forty VX2 liver tumor-bearing Japanese white rabbits were randomly divided into four groups and infused via the proper hepatic artery with a saline solution (control group), CO2-saturated solution (CO2 group), cisplatin solution (cisplatin group), or CO2-saturated solution and cisplatin solution (combined group). The tumor volume (TV) and the relative tumor volume (RTV), RTV = (TV on day 3 or 7)/(TV on day 0) x 100, were calculated using contrast-enhanced computed tomography. Hypoxia-inducible factor-1α (HIF‑1α) and carbonic anhydrase IX (CA IX) staining were used to evaluate cellular hypoxia. Cleaved caspase-3 and cleaved caspase-9 were analyzed to assess tumor apoptosis. The mean RTV on days 3 and 7 were 202.6±23.7 and 429.2±94.8%, respectively, in the control group; 172.2±38.1 and 376.5±61.1% in the CO2 group; 156.1±15.1 and 269.6±45.2% in the cisplatin group; and 118.3±28.1 and 210.3±55.1% in the combined group. RTV was significantly lower in the CO2 group than in the control group (day 3; P<0.05), and in the combined group than in the cisplatin group (days 3 and 7; P<0.05). HIF-1α and CA IX suppression, and increased cleaved caspase-3 and cleaved caspase-9 expression, were detected in the CO2 and combined groups, compared with the other two groups. An intra-arterially infused CO2-saturated solution inhibits liver VX2 tumor growth and sensitizes the anticancer effect of cisplatin.

Title: Transcutaneous Carbon Dioxide Decreases Immunosuppressive Factors in Squamous Cell Carcinoma In Vivo

Authors: Yatagai N, Hasegawa T, Amano R, Saito I, Arimoto S, Takeda D, Kakei Y, Akashi M.

Journal: Biomed Res Int. 2021 Jul 2;2021:5568428. doi: 10.1155/2021/5568428. PMID: 34307656; PMCID: PMC8270696.

Link to full text: Transcutaneous Carbon Dioxide Decreases Immunosuppressive Factors in Squamous Cell Carcinoma In Vivo

Abstract: Introduction: In recent years, the tumour immunosuppressive mechanism has attracted attention as a cause of tumour chemoresistance. Although chemoresistance and immunosuppression of tumours have been reported to be associated with a hypoxic environment, effective treatments to improve hypoxia in tumours have not yet been established. We have previously applied carbon dioxide (CO2) to squamous cell carcinoma and have shown that improvement in local oxygenation has an antitumour effect. However, the effects of local CO2 administration on tumour immunosuppression, chemoresistance, and combination with chemotherapy are unknown. In this study, we investigated the effects of local CO2 administration on squamous cell carcinoma and the effects of combined use with chemotherapy, focusing on the effects on tumour immunosuppressive factors. Methods: Human oral squamous cell carcinoma (HSC-3) was transplanted subcutaneously into the back of a nude mouse, and CO2 and cisplatin were administered. After administration twice a week for a total of 4 times, tumours were collected and the expression of tumour immunosuppressive factors (PD-L1, PD-L2, and galectin-9) was evaluated using real-time polymerase chain reaction and immunostaining. Results: Compared with the control group, a significant decrease in the mRNA expression of PD-L1 was observed in both, CO2-treated and combination groups. Similarly, the expression of PD-L2 and galectin-9 decreased in the CO2-treated and combination groups. Furthermore, immunostaining also showed a significant decrease in the protein expression of tumour immunosuppressive factors in the CO2-treated and combination groups. Conclusion: It was confirmed that the tumour immunosuppressive factors decreased due to local CO2 administration to the mouse model. CO2 administration has the potential to improve the hypoxic environment in tumours, and combined use with chemotherapy may also improve tumour immunosuppression.

Title: Reoxygenation using a novel CO2 therapy decreases the metastatic potential of osteosarcoma cells

Authors: Harada R, Kawamoto T, Ueha T, Minoda M, Toda M, Onishi Y, Fukase N, Hara H, Sakai Y, Miwa M, Kuroda R, Kurosaka M, Akisue T.

Journal: Exp Cell Res. 2013 Aug 1;319(13):1988-1997. doi: 10.1016/j.yexcr.2013.05.019. Epub 2013 May 29. PMID: 23727023.

Link to PubMed: Reoxygenation using a novel CO2 therapy decreases the metastatic potential of osteosarcoma cells

Abstract: Osteosarcoma is the most common primary solid malignant bone tumor. Despite substantial improvements in surgery and chemotherapy, metastasis remains a major cause of fatal outcomes, and the molecular mechanisms of metastasis are still poorly understood. Hypoxia, which is common in malignant tumors including osteosarcoma, increases expressions of hypoxia inducible factor (HIF)-1α, matrix metalloproteinase (MMP)-2 and MMP-9, and can induce invasiveness. As we previously showed a novel transcutaneous CO2 application to decrease HIF-1α expression and induce apoptosis in malignant fibrous histiocytoma, we hypothesize that transcutaneous CO2 application could suppress metastatic potential of osteosarcoma by improving hypoxic conditions. Here, we examined the effects of transcutaneous CO2 application on apoptosis, and development of pulmonary metastasis using a highly metastatic osteosarcoma cell line, LM8. Transcutaneous CO2 application significantly decreased tumor growth and pulmonary metastasis in LM8 cells. Apoptotic activity increased, and intratumoral hypoxia was improved with decreased expressions of HIF-1α, MMP-2 and MMP-9, significantly, in the CO2-treated tumors. In conclusion, we found that transcutaneous CO2 application can induce tumor cell apoptosis and might suppress pulmonary metastasis by improvement of hypoxic conditions with decreased expressions of HIF-1α and MMPs in highly metastatic osteosarcoma cell. These findings strongly indicate that this novel transcutaneous CO2 therapy could be a therapeutic breakthrough for osteosarcoma patients.

Keywords: Hypoxia; Matrix metalloproteinase (MMP); Metastasis; Osteosarcoma; Transcutaneous CO(2) application.

Title: Inhalation of low (0.5%–1.5%) CO2 as a potential treatment for apnea of prematurity

Authors: Al-Aif S, Alvaro R, Manfreda J, Kwiatkowski K, Cates D, Rigatto H.

Journal: Semin Perinatol. 2001 Apr;25(2):100-6. doi: 10.1053/sper.2001.23199. PMID: 11339662.

Link to PubMed: Inhalation of low (0.5%–1.5%) CO2 as a potential treatment for apnea of prematurity

Abstract: Apnea of prematurity is common and none of the treatments being used are fully effective and free of significant adverse side effects. We hypothesized that low concentrations of CO2 (< or = 1.5%) may reduce apnea without causing discomfort from an increase in ventilation. We studied 10 preterm infants at a gestational age of 32+/-1 wk (mean +/- SEM) and birthweight 1.8+/-0.2 kg. After a control period of 1 hour, concentrations of CO2 were given (0.5%, 1%, and 1.5%) for 1 hour each, followed by a recovery period of 1 hour. Apnea number significantly decreased from 2.0+/-0.3 apneas/min during control to 1.0+/-0.1 apneas/min (0.5% CO2; P < .05), 1.1+/-0.2 (1% CO2; P < .05), and to 0.7+/-0.2 (1.5% CO2; P < .01). The apnea time significantly decreased from 14.2+/-2.5 s/min during control to 5.2+/-0.8 (0.5% CO2; P < .01), 5.8+/-0.7 (1% CO2; P < .01), and to 3.7+/-0.9 (1.5% CO2; P < .01). Minute ventilation significantly increased with CO2 without evidence of respiratory discomfort. TcPCO2 did not change and TcPO2 increased slightly. These findings suggest that inhalation of low concentrations of CO2 in preterm infants with apnea 1) decreases the number and time of apneas, 2) improves oxygenation, 3) increases ventilation, and 4) is effective even in such low concentrations as 0.5%. We speculate that inhalation of CO2 (< 1%) is more effective and safer than methylxanthines for the treatment of apnea of prematurity.

Title: Carbon Dioxide in Sleep Medicine: The Next Frontier for Measurement, Manipulation, and Research

Authors: Thomas RJ.

Journal: J Clin Sleep Med. 2014 May 15;10(5):523-6. doi: 10.5664/jcsm.3702. PMID: 24910554; PMCID: PMC4046364.

Link to full text: Carbon Dioxide in Sleep Medicine: The Next Frontier for Measurement, Manipulation, and Research

Abstract: Carbon dioxide (CO2) is the most important regulator of respiration and blood pH. Papers are published with scant new information every so often that largely focus on clinical descriptions of hypercapnic individuals, often obese. While there is intense basic science interest in CO2 sensitive neurons in the brainstem, including the Phox2b/neurokinin-1 receptor (NK1R)-expressing neurons in the pre-Bötzinger complex (pre-BötzC),1 and exposing the carotid bodies to hypocapnia induces periodic breathing,2,3 the science and industry of sleep-breathing medicine has generally neglected CO2. Much of the information on chronic exposure to elevated but low levels of CO2 comes from submarine research, targeting sustained ambient concentrations in the low single digits.4–8 Acute exposure to high concentrations of CO2 results in extreme dyspnea and death; the gas has anesthetic properties and can induce a reversible isoelectric EEG.9 Demonstrating slowing of EEG rhythms with hypercapnia10 and power loss in classic oscillatory bands11 (besides increased slow wave sleep associated with respiratory failure12) is nonspecific and not informative on pathobiological mechanisms. Increased inhaled CO2 does suppress cerebral metabolic rate of oxygen13 and reduces resting state functional connectivity.14 While sympathetic drive is reliably increased acutely by hypercapnia, acclimatization mechanisms at the cellular and neural circuit levels, which can be remarkably potent enabling life at otherwise lethal CO2 levels, remain to be elucidated. We are not sure if there is a true CO2 sensor equivalent to hypoxia-inducible factors vs. simply pH mediated changes, or if there are profound direct effects of CO2 on inflammatory responses, the metabolome, the transcriptome, or epigenetic regulation. Widespread systemic dysregulation is plausible, and some data is suggestive. Hypercapnia can induce mitochondrial dysfunction through increased levels on microRNA-183, which decreases expression of isocitrate dehydrogenase.15 Hypoxia-hypercapnia cycles are neurotoxic.16 Obesity hypoventilation (vs. obese controls) was reported to show an increase in the pro-atherosclerotic RANTES chemokine, a decrease in the anti-inflammatory adipokine adiponectin, and impaired endothelial function.17

Title: Allergic rhinitis-induced nasal congestion: its impact on sleep quality

Authors: Storms W.

Journal: Prim Care Respir J. 2008 Mar;17(1):7-18. doi: 10.3132/pcrj.2008.00001. PMID: 18253678; PMCID: PMC6619860.

Link to full text: Allergic rhinitis-induced nasal congestion: its impact on sleep quality

Abstract: Allergic rhinitis (AR) is an extremely common health problem affecting 20 to 40 million Americans and between 10-25% of the world's population. Patients with AR suffer from both nasal symptoms (congestion, rhinorrhea, itching, and sneezing) and ocular symptoms (itching, redness, and tearing). The negative impact on sleep quality and quantity, and consequently on various aspects of the patient's life, is an under-recognised and under-treated component of AR morbidity. Nasal congestion, which is one of the most bothersome and prevalent symptoms of AR, is thought to be the leading symptom responsible for rhinitis-related sleep problems. In addition to reducing clinical symptoms, pharmacologic therapies for AR that specifically reduce inflammatory cells and mediators - and therefore nasal congestion and other symptoms - should also improve sleep quality and overall quality of life (QOL). Intranasal corticosteroids (INS) are the current mainstay of therapy for AR. Results of a number of clinical trials demonstrate that INS effectively reduce nasal congestion and ocular symptoms, improve sleep quality, and decrease daytime somnolence. Intranasal corticosteroids have also proved to be effective in reducing symptoms of acute rhinosinusitis and nasal polyposis, both of which also negatively impact on sleep quality. Intranasal corticosteroids are considered safe due to their low systemic bioavailability.

Title: Alternative approaches to treatment of Central Sleep Apnea

Authors: Thomas RJ.

Journal: Sleep Med Clin. 2014 Mar 1;9(1):87-104. doi: 10.1016/j.jsmc.2013.10.008. PMID: 24772053; PMCID: PMC3998090.

Link to full text: Alternative approaches to treatment of Central Sleep Apnea

Abstract: Divergent approaches to treatment of hypocapnic central sleep apnea syndromes reflect the difficulties in taming a hyperactive respiratory chemoreflex. As both sleep fragmentation and a narrow CO2 reserve or increased loop gain drive the disease, sedatives (to induce longer periods of stable non-rapid eye movement (NREM) sleep and reduce the destabilizing effects of arousals in NREM sleep) and CO2-based stabilization approaches are logical. Adaptive ventilation reduces mean hyperventilation yet can induce ventilator-patient dyssynchrony, while enhanced expiratory rebreathing space (EERS, dead space during positive pressure therapy) and CO2 manipulation directly stabilize respiratory control by moving CO2 above the apnea threshold. Carbonic anhydrase inhibition can provide further adjunctive benefits. Provent and Winx may be less likely to trigger central apneas or periodic breathing in those with a narrow CO2 reserve. An oral appliance can meaningfully reduce positive pressure requirements and thus enable treatment of complex apnea. Novel pharmacological approaches may target mediators of carotid body glomus cell excitation, such as the balance between gas neurotransmitters. In complex apnea patients, single mode therapy is not always successful, and multi-modality therapy might need to be considered. Phenotyping of sleep apnea beyond conventional scoring approaches is the key to optimal management.

Keywords: Winx; acetazolamide Provent; carbon dioxide; central apnea; multimodal complex; oxygen rebreathing; periodic breathing.

Title: Better quality of life when nasal breathing of snoring men is improved at night

Authors: Löth S, Petruson B, Wirén L, Wilhelmsen L.

Journal: Arch Otolaryngol Head Neck Surg. 1999 Jan;125(1):64-7. doi: 10.1001/archotol.125.1.64. PMID: 9932590.

Link to PubMed: Better quality of life when nasal breathing of snoring men is improved at night

Abstract: Objective: To evaluate whether improved nasal breathing changes the quality of life in snoring men and improves the female sleeping partners' well-being in the morning.

Design: During 1 month, 42 heavily snoring men slept with a nostril dilator. Before and after 1 month, the snorers rated their daytime tiredness and completed the Nottingham Health Profile questionnaire. Female sleeping partners rated the snoring, the quality of their sleep, and their sense of well-being in the morning. A population sample was used for comparison.

Setting: The Central Hospital, Skövde, Sweden.

Results: The snorers' quality of life before the study was significantly worse (P<.001) than that of the comparison population and improved significantly (P = .001). The men were significantly (P<.001) less tired during the day when their nasal airflow was increased. Female sleeping partners had significantly (P = .005) better sleep and an improved sense of well-being in the morning during the test period. Both were correlated with a significant reduction in the snoring (P<.001).

Conclusions: When nasal breathing of snoring men was improved at night, their quality of life was significantly improved. The female sleeping partners had a reduction in sleep disturbance that correlated well with an improvement in their own sleep and feelings of well-being in the morning.

Title: Blowing past the apneic threshold

Authors: Johal A, Lenet A, Sigua NL.

Journal: J Clin Sleep Med. 2022 Oct 1;18(10):2515-2518. doi: 10.5664/jcsm.10162. PMID: 35859324; PMCID: PMC9516585.

Link to full text: Blowing past the apneic threshold

Abstract: An 81-year-old man with severe obstructive sleep apnea, hypertension, and obesity (body mass index 32 kg/m2) presented to the sleep clinic for an initial evaluation. He has been on auto-titrating bilevel positive airway pressure (BPAP) with 2 L/min nocturnal oxygen for more than 10 years and has been adherent to therapy. He has no known cardiopulmonary disorder and was not on opiates, benzodiazepines, or other respiratory depressants. He denied tobacco, drug, or alcohol use. His initial sleep study, the indication for nocturnal oxygen supplementation, and serum bicarbonate level were not available for review.

Title: Central sleep apnea: pathophysiologic classification

Authors: Javaheri S, Badr MS.

Journal: Sleep. 2023 Mar 9;46(3):zsac113. doi: 10.1093/sleep/zsac113. PMID: 35551411; PMCID: PMC9995798.

Link to full text: Central sleep apnea: pathophysiologic classification

Abstract: Central sleep apnea is not a single disorder; it can present as an isolated disorder or as a part of other clinical syndromes. In some conditions, such as heart failure, central apneic events are due to transient inhibition of ventilatory motor output during sleep, owing to the overlapping influences of sleep and hypocapnia. Specifically, the sleep state is associated with removal of wakefulness drive to breathe; thus, rendering ventilatory motor output dependent on the metabolic ventilatory control system, principally PaCO2. Accordingly, central apnea occurs when PaCO2 is reduced below the "apneic threshold". Our understanding of the pathophysiology of central sleep apnea has evolved appreciably over the past decade; accordingly, in disorders such as heart failure, central apnea is viewed as a form of breathing instability, manifesting as recurrent cycles of apnea/hypopnea, alternating with hyperpnea. In other words, ventilatory control operates as a negative-feedback closed-loop system to maintain homeostasis of blood gas tensions within a relatively narrow physiologic range, principally PaCO2. Therefore, many authors have adopted the engineering concept of "loop gain" (LG) as a measure of ventilatory instability and susceptibility to central apnea. Increased LG promotes breathing instabilities in a number of medical disorders. In some other conditions, such as with use of opioids, central apnea occurs due to inhibition of rhythm generation within the brainstem. This review will address the pathogenesis, pathophysiologic classification, and the multitude of clinical conditions that are associated with central apnea, and highlight areas of uncertainty.

Keywords: Adaptive-Servo Ventilation (ASV); apneic threshold; bi-level positive pressure therapy (BPAP); central apnea; continuous positive pressure therapy (CPAP); controller gain; hypocapnia; loop gain; plant gain.

Title: CO2 homeostasis during periodic breathing in obstructive sleep apnea

Authors: Berger KI, Ayappa I, Sorkin IB, Norman RG, Rapoport DM, Goldring RM.

Journal: J Appl Physiol (1985). 2000 Jan;88(1):257-64. doi: 10.1152/jappl.2000.88.1.257. PMID: 10642388.

Link to full text: CO2 homeostasis during periodic breathing in obstructive sleep apnea

Abstract: The contribution of apnea to chronic hypercapnia in obstructive sleep apnea (OSA) has not been clarified. Using a model (D. M. Rapoport, R. G. Norman, and R. M. Goldring. J. Appl. Physiol. 75: 2302-2309, 1993), we previously illustrated failure of CO2 homeostasis during periodic breathing resulting from temporal dissociation between ventilation and perfusion ("temporal V/Q mismatch"). This study measures acute kinetics of CO2 during periodic breathing and addresses interapnea ventilatory compensation for maintenance of CO2 homeostasis in 11 patients with OSA during daytime sleep (37-171 min). Ventilation and expiratory CO2 and O2 fractions were measured on a breath-by-breath basis by means of a tight-fitting full facemask. Calculations included CO2 excretion, metabolic CO2 production, and CO2 balance (metabolic CO2 production - exhaled CO2). CO2 balance was tabulated for each apnea/hypopnea event-interevent cycle and as a cumulative value during sleep. Cumulative CO2 balance varied (-3,570 to +1,388 ml). Positive cumulative CO2 balance occurred in the absence of overall hypoventilation during sleep. For each cycle, positive CO2 balance occurred despite increased interevent ventilation to rates as high as 45 l/min. This failure of CO2 homeostasis was dependent on the event-to-interevent duration ratio. The results demonstrate that 1) periodic breathing provides a mechanism for acute hypercapnia in OSA, 2) acute hypercapnia during periodic breathing may occur without a decrease in average minute ventilation, supporting the presence of temporal V/Q mismatch, as predicted from our model, and 3) compensation for CO2 accumulation during apnea/hypopnea may be limited by the duration of the interevent interval. The relationship of this acute hypercapnia to sustained chronic hypercapnia in OSA remains to be further explored.

Title: Effect of Breathe Right nasal strip on snoring

Authors: Ulfberg J, Fenton G.

Journal: Rhinology. 1997 Jun;35(2):50-2. PMID: 9299650.

Link to PubMed: Effect of Breathe Right nasal strip on snoring

Abstract: Snoring is a significant problem both for the patient and for the bedpartner. It is well known that nasal stuffiness can contribute to snoring, and sleep quality may deteriorate because of the snoring. Nasal dilation can reduce snoring and improve sleep. Thirty-five habitual snorers (18 female, 17 male) and their bedpartners participated in an open label study. The patients were diagnosed as heavy snorers after they underwent overnight polysomnography showing that their apnoea indexes were below 5, thus sleep apnoea patients were not included in the study. The participants and their partners filled out evaluations concerning snoring intensity, mouth dryness and Epworth Sleepiness Scale prior to and after using Breath Right nasal strips for 14 consecutive nights. The Breathe Right external nasal dilator is a simple, nonpharmaceutical method to decrease nasal airway resistance and thus potentially reduce or eliminate snoring. After using the strips there were statistically significant decreases in snoring (p < 0.001) as graded by the bed partner, and in mouth dryness (p = 0.025) and in the Epworth Sleepiness Scale scores (p = 0.001), as graded by the patient. The results of this study indicate that Breathe Right nasal strips may be used to reduce snoring, mouth dryness and sleepiness in patients presenting with symptoms of snoring.

Title: Effect of hypoxia on the hypopnoeic and apnoeic threshold for CO(2) in sleeping humans

Authors: Xie A, Skatrud JB, Dempsey JA.

Journal: J Physiol. 2001 Aug 15;535(Pt 1):269-78. doi: 10.1111/j.1469-7793.2001.00269.x. PMID: 11507176; PMCID: PMC2278764.

Link to full text: Effect of hypoxia on the hypopnoeic and apnoeic threshold for CO(2) in sleeping humans

Abstract: 1. Rhythmic breathing during sleep requires that P(CO2) be maintained above a sensitive hypocapnic apnoeic threshold. Hypoxia causes periodic breathing during sleep that can be prevented or eliminated with supplemental CO(2). The purpose of this study was to determine the effect of hypoxia in changing the difference between the eupnoeic P(CO2) and the P(CO2) required to produce hypopnoea or apnoea (hypopnoea/apnoeic threshold) in sleeping humans. 2. The effect of hypoxia on eupnoeic end-tidal partial pressure of CO(2) (P(ET,CO2)) and hypopnoea/apnoeic threshold P(ET,CO2) was examined in seven healthy, sleeping human subjects. A bilevel pressure support ventilator in a spontaneous mode was used to reduce P(ET,CO2) in small decrements by increasing the inspiratory pressure level by 2 cmH2O every 2 min until hypopnoea (failure to trigger the ventilator) or apnoea (no breathing effort) occurred. Multiple trials were performed during both normoxia and hypoxia (arterial O(2) saturation, S(a,O2) = 80 %) in a random order. The hypopnoea/apnoeic threshold was determined by averaging P(ET,CO2) of the last three breaths prior to each hypopnoea or apnoea. 3. Hypopnoeas and apnoeas were induced in all subjects during both normoxia and hypoxia. Hypoxia reduced the eupnoeic P(ET,CO2) compared to normoxia (42.4 +/- 1.3 vs. 45.0 +/- 1.1 mmHg, P < 0.001). However, no change was observed in either the hypopnoeic threshold P(ET,CO2) (42.1 +/- 1.4 vs. 43.0 +/- 1.2 mmHg, P > 0.05) or the apnoeic threshold P(ET,CO2) (41.3 +/- 1.2 vs. 41.6 +/- 1.0 mmHg, P > 0.05). Thus, the difference in P(ET,CO2) between the eupnoeic and threshold levels was much smaller during hypoxia than during normoxia (-0.2 +/- 0.2 vs. -2.0 +/- 0.3 mmHg, P < 0.01 for the hypopnoea threshold and -1.1 +/- 0.2 vs. -3.4 +/- 0.3 mmHg, P < 0.01 for the apnoeic threshold). We concluded that hypoxia causes a narrowing of the difference between the baseline P(ET,CO2) and the hypopnoea/apnoeic threshold P(ET,CO2), which could increase the likelihood of ventilatory instability.

Title: Effect of nasal dilation on snoring and apneas during different stages of sleep

Authors: Hoffstein V, Mateika S, Metes A.

Journal: Sleep. 1993 Jun;16(4):360-5. doi: 10.1093/sleep/16.4.360. PMID: 8141871

Link to PubMed: Effect of nasal dilation on snoring and apneas during different stages of sleep

Abstract: This study was designed to test the hypothesis that nasal dilation reduces snoring. To achieve this we performed nocturnal polysomnography, including measurement of snoring, in 15 patients without nasal pathology before and after insertion of a nasal dilator (NOZOVENT). Snoring was quantified for each sleep stage by recording the number of snores per minute of sleep, number of snores per minute of snoring time and nocturnal sound intensities (maximum, average and minimum). We found that nasal dilation had no effect on the number of apneas, hypopneas or oxygen saturation. Snoring parameters were unaffected by NOZOVENT during stages I, II and REM sleep, but were all significantly reduced during slow wave sleep. We conclude that dilation of the anterior nares in patients without nasal pathology has a relatively weak effect on snoring, and routine use of nasal dilating appliances is not recommended for treatment of snoring.

Title: Hypocapnia is associated with increased upper airway expiratory resistance during sleep

Authors: Sankri-Tarbichi AG, Richardson NN, Chowdhuri S, Rowley JA, Safwan Badr M.

Journal: Respir Physiol Neurobiol. 2011 Jul 31;177(2):108-13. doi: 10.1016/j.resp.2011.04.004. Epub 2011 Apr 13. PMID: 21513820; PMCID: PMC3113471.

Link to full text: Hypocapnia is associated with increased upper airway expiratory resistance during sleep

Abstract: We hypothesized that hypocapnia is responsible for increased expiratory resistance during NREM sleep. Hypocapnia was induced by hypoxic hyperventilation in 21 subjects (aged 29.4 ± 7.8 yrs, 10 women, BMI 24.4 ± 4.3 kg/m(2)). Isocapnic hypoxia was induced in 12 subjects of whom, 6 underwent hypocapnic hypoxia in the same night. Upper airway resistance (R(UA)) was measured at the linear pressure-flow relationship during inspiration and expiration. Inspiratory flow limitation (IFL) was defined as the dissociation in pressure-flow relationship. (1) Expiratory R(UA) increased during hypocapnic but not isocapnic hypoxia relative to control (11.0 ± 5.6 vs. 8.2 ± 3.6 cm H(2)O/L/s; p < 0.05, and 11.45.0 vs. 10.94.4 cm H(2)O/L/s; p = NS, respectively). (2) No gender difference was found in R(UA) (p = NS). (3) Increased expiratory R(UA) correlated with the IFL change during hypocapnic but not isocapnic hypoxia. (4) No changes were noted in inspiratory R(UA) or IFL. Expiratory R(UA) increased during hypocapnia and was associated with IFL, indicating upper airway narrowing. Gender does not influence the upper airway response to hypocapnic hypoxia.

Title: Improved nasal breathing reduces snoring and morning tiredness. A 6-month follow-up study

Authors: Löth S, Petruson B.

Journal: Arch Otolaryngol Head Neck Surg. 1996 Dec;122(12):1337-40. doi: 10.1001/archotol.1996.01890240045010. PMID: 8956746.

Link to PubMed: Improved nasal breathing reduces snoring and morning tiredness. A 6-month follow-up study

Abstract: Background: Dilation of the nasal valve region can increase the ability to breathe through the nose and reduce the negative intrathoracic pressure required for inspiration. Vibrations of the palate and soft tissues of the throat, which generate snoring sounds, can be prevented when patients inhale less heavily.

Objectives: To evaluate the effect a nostril dilator has on patient snoring and tiredness in the morning and to determine how many patients would continue to use the device for half a year.

Design: For 6 months, 42 men who were heavy snorers graded their average tiredness in the morning and the patient's sleeping partner graded the average snoring using a visual analog scale from 0 to 100.

Setting: All patients were examined at the Department of Otorhinolaryngology, Central Hospital, in Skövde, Sweden.

Results: When the nostril dilator was used there was a significant decrease in snoring after both 1- and 6-month reports. There was a significant correlation between diminution of snoring and less tiredness in patients in the morning. The compliance was good since 60% of patients continued to use the device during the 6-month test period.

Conclusions: This study illustrates the benefits that reducing nasal airway resistance during sleep has on snoring and morning tiredness in patients.

Title: Influence of longterm CPAP therapy on CO2 drive in patients with obstructive sleep apnea

Authors: Verbraecken J, Willemen M, De Cock W, Wittesaele W, Govaert K, Van de Heyning P, De Backer W.

Journal: Respir Physiol. 2000 Oct;123(1-2):121-30. doi: 10.1016/s0034-5687(00)00140-7. PMID: 10996193.

Link to PubMed: Influence of longterm CPAP therapy on CO2 drive in patients with obstructive sleep apnea

Abstract: According to recent publications, the values of the hypercapnic ventilatory responses (HCVR) in normocapnic obstructive sleep apnea (OSA) patients are highly variable, but they are usually within the normal range. In our study, CPAP therapy during 1 month did not seem to influence the HCVR [Verbraecken, J., De Backer, W., Willemen, M., De Cock, W., Wittesaele, W., Van de Heyning, P., 1995. Respir. Physiol. 101, 279-287]. It is, however, not well studied whether long term (1 year) CPAP therapy can influence HCVR in normocapnic patients. Therefore, we evaluated the effect of 1 year CPAP therapy on CO(2) drive in 20 OSA patients. The slope of HCVR (SHCVR) changed from 2.35+/-0.21 L min(-1) mmHg(-1) (control) to 1.66+/-0.16 L min(-1) mm Hg(-1) (P=0.04), but the mean within subject coefficient of variation in repeated measurements of SHCVR in treated and untreated OSA patients was not statistically different. Pa(O(2)) increased from 72+/-2 mm Hg to 80+/-2 (1 year) mm Hg. We conclude that CPAP therapy improves daytime gas exchange in normocapnic OSA and may possibly decrease CO(2) drive (slope) after a treatment period of 1 year.

Title: Low-concentration carbon dioxide is an effective adjunct to positive airway pressure in the treatment of refractory mixed central and obstructive sleep-disordered breathing

Authors: Thomas RJ, Daly RW, Weiss JW.

Journal: Sleep. 2005 Jan;28(1):69-77. doi: 10.1093/sleep/28.1.69. PMID: 15700722.

Link to PubMed: Low-concentration carbon dioxide is an effective adjunct to positive airway pressure in the treatment of refractory mixed central and obstructive sleep-disordered breathing

Abstract: Objectives: To assess the efficacy of added carbon dioxide as adjunctive therapy to positive airway pressure-refractory mixed obstructive and central sleep-disordered breathing, using a prototype device-the positive airway pressure gas modulator.

Design: Open-label evaluation of low concentrations of carbon dioxide added to a positive airway pressure circuit.

Setting: Physician-attended polysomnographic titration in a free-standing sleep laboratory with end-tidal and transcutaneous carbon-dioxide monitoring.

Patients: Six adult men (age 54 +/- 5.7 years) with severe poorly controlled mixed sleep-disordered breathing in the absence of renal or heart failure.

Interventions: Flow-independent addition of incremental concentrations of carbon dioxide during sleep.

Measurements and results: The respiratory disturbance index before treatment was 66 +/- 14.5 events per hour of sleep, with a nocturnal desaturation low of 84.6% +/- 10.1%. Residual respiratory disturbance index on best treatment was 43 +/- 9 events per hour of sleep. There was an immediate (<1 minute) response to the addition of 0.5% to 1% carbon dioxide, and minimal changes were required to be made across the night. There was no discomfort, shortness of breath, palpitations, headache, or significant increase in respiratory or heart rate. The residual respiratory disturbance index on carbon dioxide, scored irrespective of desaturations, was in the normal range (< 5 / hour of sleep). Two subjects had a second night at the concentration of carbon dioxide determined to be efficacious, with no required concentration change. No adverse effects on overall sleep architecture were noted.

Conclusions: Low concentrations of carbon dioxide added to conventional positive airway pressure effectively control severe treatment-resistant mixed obstructive and central sleep-disordered breathing.

Title: Objective assessment of nasal obstruction in snoring and obstructive sleep apnea patients: experience of a Police Authority Hospital

Authors: Kalam I.

Journal: Ann Saudi Med. 2002 May-Jul;22(3-4):158-62. doi: 10.5144/0256-4947.2002.158. PMID: 17159386.

Link to full text: Objective assessment of nasal obstruction in snoring and obstructive sleep apnea patients: experience of a Police Authority Hospital

Abstract: Background: The role of nasal airflow resistance in the pathogenesis of obstructive sleep apnea (OSA) syndrome remains contentious. The aim of this study was to investigate the changes in apnea index in response to nasal surgery, as guided by acoustic rhinometry.

Patients and methods: Forty-five patients were referred for complaints of snoring and nasal obstruction. The patients were divided into two groups according to the severity of nasal obstruction: group 1 (nasal obstruction was the predominant symptom) included 21 patients, and group 2 (snoring was the predominant symptom) included 24 patients. In group 1, nasal surgery was performed as a first surgical procedure and was followed after 10-12 weeks by palatal surgery. In group 2, palatal surgery was performed as the primary procedure and was followed after 10-12 weeks by nasal surgery.

Results: The results showed a clear relationship between the correction of nasal obstruction and the severity of OSA, as indicated by measuring the apnea index (AI). However, in none of the group 1 cases was nasal surgery alone capable of reducing the AI by 50%. In group 2, palatal surgery effectively reduced AI, and when followed by nasal surgery, the reduction in AI was again statistically significant.

Conclusion: This indicates that nasal obstruction could be considered a contributing factor to the severity of OSA, but not a causative factor in the production of OSA. It also indicates that nasal surgery and palatal surgery combined are effective in improving snoring and OSA in properly selected patients.

Title: A review of preventing central sleep apnea by inspired CO2

Authors: Mulchrone A, Shokoueinejad M, Webster J.

Journal: Physiol Meas. 2016 May;37(5):R36-45. doi: 10.1088/0967-3334/37/5/R36. Epub 2016 Apr 19. PMID: 27093535.

Link to PubMed: A review of preventing central sleep apnea by inspired CO2

Abstract: Although almost completely unknown half a century ago, sleep disorders are gaining recognition as major issues to public health due to their growing prevalence and dire societal consequences. Despite being linked to several infamous catastrophic events such as Chernobyl, it is estimated that 90% of sufferers fail to get diagnosed and receive treatment, and a significant portion of the ones that do are often non-compliant due to the side effects of current treatments. This article presents a review of the current standard treatment for central sleep apnea, and investigates the advantages and possible consequences of using inspired carbon dioxide (CO2) as an alternative treatment option.

Title: Effects of inhaled CO2 and added dead space on idiopathic central sleep apnea

Authors: Xie A, Rankin F, Rutherford R, Bradley TD.

Journal: J Appl Physiol (1985). 1997 Mar;82(3):918-26. doi: 10.1152/jappl.1997.82.3.918. PMID: 9074983.

Link to full text: Effects of inhaled CO2 and added dead space on idiopathic central sleep apnea

Abstract: We hypothesized that reductions in arterial PCO2 (PaCO2) below the apnea threshold play a key role in the pathogenesis of idiopathic central sleep apnea syndrome (ICSAS). If so, we reasoned that raising PaCO2 would abolish apneas in these patients. Accordingly, patients with ICSAS were studied overnight on four occasions during which the fraction of end-tidal CO2 and transcutaneous PCO2 were measured: during room air breathing (N1), alternating room air and CO2 breathing (N2), CO2 breathing all night (N3), and addition of dead space via a face mask all night (N4). Central apneas were invariably preceded by reductions in fraction of end-tidal CO2. Both administration of a CO2-enriched gas mixture and addition of dead space induced 1- to 3-Torr increases in transcutaneous PCO2, which virtually eliminated apneas and hypopneas; they decreased from 43.7 +/- 7.3 apneas and hypopneas/h on N1 to 5.8 +/- 0.9 apneas and hypopneas/h during N3 (P < 0.005), from 43.8 +/- 6.9 apneas and hypopneas/h during room air breathing to 5.9 +/- 2.5 apneas and hypopneas/h of sleep during CO2 inhalation during N2 (P < 0.01), and to 11.6% of the room air level while the patients were breathing through added dead space during N4 (P < 0.005). Because raising PaCO2 through two different means virtually eliminated central sleep apneas, we conclude that central apneas during sleep in ICSA are due to reductions in PaCO2 below the apnea threshold.

Title: Effect of CO2 inhalation on central sleep apnea and arousals from sleep

Authors: Szollosi I, Jones M, Morrell MJ, Helfet K, Coats AJ, Simonds AK.

Journal: Respiration. 2004 Sep-Oct;71(5):493-8. doi: 10.1159/000080634. PMID: 15467327.

Link to PubMed: Effect of CO2 inhalation on central sleep apnea and arousals from sleep

Abstract: Background: CO(2) inhalation reduces central sleep apnea (CSA) in patients with congestive heart failure (CHF) and idiopathic CSA. CO(2) is also a stimulus for cortical arousal, which has been linked to increased sympathetic nerve activity (SNA) and increased mortality in CHF patients with CSA.

Objective: We have tested the hypothesis that during sleep, inhalation of CO(2) sufficient to reduce the apnea-hypopnea index (AHI) would not reduce the arousal index (AroI).

Methods: In 10 male patients with CSA (7 with CHF and 3 with idiopathic CSA), the inspired CO(2) concentration was increased to raise the sleeping end-tidal CO(2) by 2-4 mm Hg during established stage 2 sleep. Each intervention was maintained for a 10-min period. Sleep stage was monitored with electroencephalograms, electrooculograms, submental electromyogram, airflow with pneumotachometer and respiratory effort and blood gases with oxygen saturation and end-tidal CO(2). During periods of air and CO(2) breathing, AHI and AroI were compared with paired t tests; patients acted as their own controls.

Results: Inhalation of CO(2) produced a significant reduction in AHI (mean +/- SEM) from 74.4 +/- 12.4 events/h during air breathing to 25.8 +/- 7.8 events/h with CO(2) inhalation (p = 0.002). However, the AroI was not significantly different between the two conditions, air 67.8 +/- 12.3 events/h and CO(2) inhalation 52.8 +/- 12.4 events/h (p = 0.264).

Conclusion: CO(2) inhalation reverses CSA but not arousals from sleep. Our findings highlight the need for treatment options that reduce both respiratory events and decrease arousals from sleep, with their associated SNA sequelae.

Title: Carbon dioxide in sleep medicine: the next frontier for measurement, manipulation, and research

Authors: Thomas RJ.

Journal: J Clin Sleep Med. 2014 May 15;10(5):523-6. doi: 10.5664/jcsm.3702. PMID: 24910554; PMCID: PMC4046364.

Link to full text: Carbon dioxide in sleep medicine: the next frontier for measurement, manipulation, and research

Abstract: Carbon dioxide (CO2) is the most important regulator of respiration and blood pH. Papers are published with scant new information every so often that largely focus on clinical descriptions of hypercapnic individuals, often obese. While there is intense basic science interest in CO2 sensitive neurons in the brainstem, including the Phox2b/neurokinin-1 receptor (NK1R)-expressing neurons in the pre-Bötzinger complex (pre-BötzC),1 and exposing the carotid bodies to hypocapnia induces periodic breathing,2,3 the science and industry of sleep-breathing medicine has generally neglected CO2.

Title: Obstructive sleep apnoea and breathing retraining

Authors: Birch M.

Journal: Aust Nurs J. 2004 Aug;12(2):27-9. PMID: 19160562.

Link to PubMed: Obstructive sleep apnoea and breathing retraining

Abstract: Obstructive sleep apnoea (OSA) is a sleep disorder where repeated upper airway obstruction during sleep leads to a decrease in blood oxygen saturation and disrupted sleep. Current treatment options include oral appliances, surgery, and/or the use of a continuous positive airway pressure (CPAP) machine. However, breathing retraining with the Buteyko Institute Method (BIM) is a safe, effective and convenient approach to OSA that could help eliminate the need for surgery or CPAP. This paper examines the role of breathing retraining in OSA and presents a case study to illustrate its effectiveness.

Title: Pathogenesis of upper airway occlusion during sleep

Authors: Remmers JE, deGroot WJ, Sauerland EK, Anch AM.

Journal: J Appl Physiol Respir Environ Exerc Physiol. 1978 Jun;44(6):931-8. doi: 10.1152/jappl.1978.44.6.931. PMID: 670014.

Link to full text: Pathogenesis of upper airway occlusion during sleep

Abstract: Ten patients with daytime somnolence and obesity were found to have periodic airway occlusion (AO) during nocturnal sleep. The cyclical ventilatory pattern consisted of a series of regular inspiratory efforts against an occluded airway (occlusive phase) alternating with a period of regular breathing (ventilatory phase). Significant periods of central respiratory apnea were observed only in one case. The effects of pharyngeal intubation and pharyngeal pressure recordings showed that the locus of airway closure lay in the oropharynx. The genioglossal electromyogram (EMG) consistently revealed periodicity: low level activity at the onset of occlusion and prominent discharge at the instant of pharyngeal opening. In one case, this activity was closely related to pharyngeal patency, whereas in other cases there was considerable overlap between EMG values recorded during occluded and ventilatory phases. In these cases, the relationship of genioglossal discharge to pharyngeal pressure correlated with the presence or absence of pharyngeal occlusion. We speculate that genioglossal force acts to open the oropharynx and that negative pharyngeal pressure promotes pharyngeal closure. The results are consistent with the idea that, once the pharvnx has collapsed, relative recruitment of genioglossal and inspiratory muscle act.ivity is such that the latter influence outstrips the former, so that pharyngeal transmural pressure increases more than genioglossal force. Pharyngeal opening occurs coincident with arousal and preferential activation of the genioglossus muscle of the tongue.

Title: Rethinking O2, CO2 and breathing during wakefulness and sleep

Authors: Dempsey JA, Gibbons TD.

Journal: J Physiol. 2024 Nov;602(21):5571-5585. doi: 10.1113/JP284551. Epub 2023 Sep 26. PMID: 37750243.

Link to PubMed: Rethinking O2, CO2 and breathing during wakefulness and sleep

Abstract: We have examined the importance of three long-standing questions concerning chemoreceptor influences on cardiorespiratory function which are currently experiencing a resurgence of study among physiologists and clinical investigators. Firstly, while carotid chemoreceptors (CB) are required for hypoxic stimulation of breathing, use of an isolated, extracorporeally perfused CB preparation in unanaesthetized animals with maintained tonic input from the CB, reveals that extra-CB hypoxaemia also provides dose-dependent ventilatory stimulation sufficient to account for 40-50% of the total ventilatory response to steady-state hypoxaemia. Extra-CB hyperoxia also provides a dose- and time-dependent hyperventilation. Extra-CB sites of O2-driven ventilatory stimulation identified to date include the medulla, kidney and spinal cord. Secondly, using the isolated or denervated CB preparation in awake animals and humans has demonstrated a hyperadditive effect of CB sensory input on central CO2 sensitivity, so that tonic CB activity accounts for as much as 35-40% of the normal, air-breathing eupnoeic drive to breathe. Thirdly, we argue for a key role for CO2 chemoreception and the neural drive to breathe in the pathogenesis of upper airway obstruction during sleep (OSA), based on the following evidence: (1) removal of the wakefulness drive to breathe enhances the effects of transient CO2 changes on breathing instability; (2) oscillations in respiratory motor output precipitate pharyngeal obstruction in sleeping subjects with compliant, collapsible airways; and (3) in the majority of patients in a large OSA cohort, a reduced neural drive to breathe accompanied reductions in both airflow and pharyngeal airway muscle dilator activity, precipitating airway obstruction.

Keywords: extra‐carotid chemoreceptor O2 sensing; obstructive sleep apnoea; peripheral‐central chemoreceptor interdependence.

Title: Respiratory alkalosis may impair the production of vitamin D and lead to significant morbidity, including the fibromyalgia syndrome

Authors: Lewis JM, Fontrier TH, Coley JL.

Journal: Med Hypotheses. 2017 May;102:99-101. doi: 10.1016/j.mehy.2017.03.013. Epub 2017 Mar 8. PMID: 28478843.

Link to PubMed: Respiratory alkalosis may impair the production of vitamin D and lead to significant morbidity, including the fibromyalgia syndrome

Abstract: Hyperventilation caused by physical and/or psychological stress may lead to significant respiratory alkalosis and an elevated systemic pH. The alkalotic pH may in turn suppress the normal renal release of phosphate into the urine, thereby interrupting the endogenous production of 1,25-dihydroxyvitamin D (calcitriol). This could cause a shortfall in its normal production, leading to a variety of adverse consequences. It might partially explain the pathogenesis of acute mountain sickness, a treatable disease characterized by severe hyperventilation secondary to the hypoxia of high altitude exposure. Milder degrees of hyperventilation due to different forms of stress may produce other conditions which share characteristics with acute mountain sickness. One of these may be the fibromyalgia syndrome, a chronic painful disorder for which no satisfactory treatment exists. Should fibromyalgia and acute mountain sickness have a common etiology, may they also share a common form of treatment? Evidence is presented to support this hypothesis.

Title: Breathing evaluation and retraining as an adjunct to manual therapy

Authors: McLaughlin L, Goldsmith CH, Coleman K.

Journal: Man Ther. 2011 Feb;16(1):51-2. doi: 10.1016/j.math.2010.08.006. Epub 2010 Oct 8. PMID: 20933458.

Link to PubMed: Breathing evaluation and retraining as an adjunct to manual therapy

Abstract: Back and neck pain are extremely common reasons for patients seeking manual therapy treatment. Epidemiological evidence supports a link between breathing difficulties and back pain. Since trunk muscles perform both postural and breathing functions, it is theorized that disruption in one function can negatively impact the other. Altered breathing mechanics can change respiratory chemistry and therefore pH causing smooth muscle constriction, altered electrolyte balance and decreased tissue oxygenation. These changes can profoundly impact any body system. Increased excitability in the muscular and nervous systems may be most relevant to a manual therapist. Respiratory function can be tested via capnography which measures CO₂ at the end of exhale known as End Tidal CO₂ (ETCO₂). ETCO₂ closely reflects arterial CO₂ in people with normal cardiopulmonary function. A case series of twenty nine outpatients with neck or back pain who had plateaued with manual therapy and exercise were identified all of whom were found to have low ETCO₂. Breathing retraining improved ETCO₂, pain and function in all patients with 93% achieving at least a clinically important change in either pain or function. Screening for breathing dysfunction using capnography may improve patient outcomes in those patients where manual therapy, exercise and education do not provide full resolution of symptoms.

Title: Hypocapnia in women with fibromyalgia

Authors: Jonsson K, Pikwer A, Olsson EMG, Peterson M.

Journal: Scand J Pain. 2024 Jun 21;24(1). doi: 10.1515/sjpain-2024-0003. PMID: 38907689.

Link to full text: Hypocapnia in women with fibromyalgia

Abstract: Objectives: The purpose of this study was to investigate whether people with fibromyalgia (FM) have dysfunctional breathing by examining acid-base balance and comparing it with healthy controls. Methods: Thirty-six women diagnosed with FM and 36 healthy controls matched for age and gender participated in this cross-sectional study. To evaluate acid-base balance, arterial blood was sampled from the radial artery. Carbon dioxide, oxygen, bicarbonate, base excess, pH and lactate were analysed for between-group differences. Blood gas analyses were performed stepwise on each individual to detect acid-base disturbance, which was categorized as primary respiratory and possible compensation indicating chronicity. A three-step approach was employed to evaluate pH, carbon dioxide and bicarbonate in this order. Results: Women with FM had significantly lower carbon dioxide pressure (p = 0.013) and higher lactate (p = 0.038) compared to healthy controls at the group level. There were no significant differences in oxygen pressure, bicarbonate, pH and base excess. Employing a three-step acid-base analysis, 11 individuals in the FM group had a possible renally compensated mild chronic hyperventilation, compared to only 4 among the healthy controls (p = 0.042). Conclusions: In this study, we could identify a subgroup of individuals with FM who may be characterized as mild chronic hyperventilators. The results might point to a plausible dysfunctional breathing in some women with FM.

Title: The role of carbon dioxide therapy in the treatment of chronic wounds

Authors: Brandi C, Grimaldi L, Nisi G, Brafa A, Campa A, Calabrò M, Campana M, D'Aniello C.

Journal: In Vivo. 2010 Mar-Apr;24(2):223-6. PMID: 20363999.

Link to full text: The role of carbon dioxide therapy in the treatment of chronic wounds

Abstract: A wound is defined as chronic when it does not heal according to the normal repair times and mechanisms. This particular condition may be principally due to local hypoxia. Carbon dioxide (CO(2)) therapy refers to the transcutaneous or subcutaneous administration of CO(2) for therapeutical effects on both microcirculation and tissue oxygenation. In this study, we report the clinical and instrumental results of the application of CO(2) in the therapy of chronic wounds. The study included 70 patients affected by chronic ulcers. The patients were selected by aetiology and wound extension and equally divided into two homogeneous groups. In group A, CO(2) therapy was used in addition to the routine methods of treatment for such lesions (surgical and/or chemical debridement, advanced dressings according to the features of each lesion). In group B, patients were treated using routine methods alone. Both groups underwent to instrumental (laser doppler flow, measurement of TcPO(2)), clinical and photographic evaluation. In the group that underwent subcutaneous treatment with CO(2) therapy, the results highlighted a significant increase in tissue oxygenation values, which was confirmed by greater progress of the lesions in terms of both healing and reduction of the injured area. Considering the safety, efficacy and reliability of this method, even if further studies are necessary, we believe that it is useful to include subcutaneous carbon dioxide therapy in the treatment of wounds involving hypoxia-related damage.

Title: Carboxytherapy: Controls the inflammation and enhances the production of fibronectin on wound healing under venous insufficiency

Authors: Brochado TMM, de Carvalho Schweich L, Di Pietro Simões N, Oliveira RJ, Antoniolli-Silva ACMB.

Journal: Int Wound J. 2019 Apr;16(2):316-324. doi: 10.1111/iwj.13031. Epub 2018 Nov 22. PMID: 30467979; PMCID: PMC7949331.

Link to full text: Carboxytherapy: Controls the inflammation and enhances the production of fibronectin on wound healing under venous insufficiency

Abstract: To examine the influence of carboxytherapy on wound healing under venous insufficiency, full‐thickness excisional wounds were created on Wistar rats. We used three groups with 32 rats each: Group (I): daily cleaning with 0.9% saline solution; Group Sulfadiazine (II): 1% silver sulfadiazine; and Carboxytherapy (III): subcutaneous application of 0.3 mL of carbon dioxide. The predetermined periods of analysis were the 3rd, 7th, 14th, and 30th day. The slides were stained with haematoxylin and eosin and Picrosirius red and submitted for immunohistochemistry. Groups II and III presented a statistically significant decrease in relation to the presence of neutrophilic and lymphocytic infiltrates. The presence of collagen significant increased in groups II and III. However, group III presented better organisation. Only group I maintained the neovascularisation until the 30th day. The new epithelium statistically significantly increased in groups II and III. On immunohistochemistry, regarding fibronectin expression, only group III demonstrated a statistically significant increase since the beginning of the healing process. Thus, the use of carboxytherapy promotes the formation of a tissue better structured and that may be an important resource for the treatment of wounds under venous insufficiency, especially those of recurrent re‐openings.

Title: Synergistic effect of pressurized carbon dioxide and sodium hypochlorite on the inactivation of Enterococcus sp. in seawater

Authors: Dang TT, Imai T, Le TV, Nguyen DK, Higuchi T, Kanno A, Yamamoto K, Sekine M.

Journal: Water Res. 2016 Dec 1;106:204-213. doi: 10.1016/j.watres.2016.10.003. Epub 2016 Oct 3. PMID: 27721172.

Link to PubMed: Synergistic effect of pressurized carbon dioxide and sodium hypochlorite on the inactivation of Enterococcus sp. in seawater

Abstract: This study investigated the effect of combined treatments using pressurized carbon dioxide (PCD) and sodium hypochlorite (NaOCl) on the inactivation of Enterococcus sp. in artificial seawater. Bacterial inactivation was conducted in a liquid-film-forming apparatus with various pressure conditions, CO2 supply rates, and chlorine dosages. Combined PCD/chlorine treatments resulted in greater disinfection efficiency than those for the two individual treatments. Synergy values were correlated with pressure and CO2 concentrations (p < 0.001). Combination of 0.9 MPa PCD (various CO2 supply rates: 25% CO2 + 75% N2, 50% CO2 + 50% N2, and 100% CO2) and chlorine (0.20 mg L-1) yielded average synergy values of 4.9, 5.2, and 4.4 log, respectively, within 3 min. Combined treatment with PCD (100% CO2, 0.3 MPa, and 20 °C) and chlorine (0.20-0.22 mg L-1) achieved an average synergy value of 4.6 log and complete inactivation (5.2-5.5 log reductions) of Enterococcus sp. within 4 min. In contrast, when the two individual treatments (PCD and chlorine) were used, only 3.7 and 1.8-2.3 log reductions, respectively, were achieved after 25 min. These findings suggest that the combined PCD/chlorine treatment has synergistic benefits and provides a promising method for the disinfection of ballast water.

Title: Antimicrobial Effect of Pressurized Carbon Dioxide on Staphylococcus aureus in Broth and Milk

Authors: Osman Erkmen

Journal: LWT - Food Science and Technology, Volume 30, Issue 8, 1997, Pages 826-829, ISSN 0023-6438, https://doi.org/10.1006/fstl.1997.0277.

Link to ScienceDirect: Antimicrobial Effect of Pressurized Carbon Dioxide on Staphylococcus aureus in Broth and Milk

Abstract: Studies were carried out to assess the use of high pressure CO2 treatment to inhibit Staphylococcus aureus in broth and raw cow's milk at 25 °C. Two phases in the survival curves were observed. The early stage was characterized by a slow rate of decrease in number of S. aureus; this rate increased sharply at the later stage. Staphylococcus aureus suspended in broth was completely inactivated after CO2 treatment at 7 MPa for 100 min and 8 MPa for 60 min. The sterilization effects of CO2 on both S. aureus and aerobic bacteria were observed at 14.6 MPa for 5 h and 9 MPa for 2 h in whole and skim milk, respectively. Staphylococcus aureus and aerobic bacteria were more difficult to inactivate when they were suspended in whole milk that may have protected the cell from penetration by CO2. Reduction rates of S. aureus and aerobic bacteria were sensitive to pressure, exposure time and the suspending medium.

Title: Humidified Warmed CO2 Treatment Therapy Strategies Can Save Lives With Mitigation and Suppression of SARS-CoV-2 Infection: An Evidence Review

Authors: El-Betany AMM, Behiry EM, Gumbleton M, Harding KG.

Journal: Front Med (Lausanne). 2020 Dec 11;7:594295. doi: 10.3389/fmed.2020.594295. PMID: 33425942; PMCID: PMC7793941.

Link to full text: Humidified Warmed CO2 Treatment Therapy Strategies Can Save Lives With Mitigation and Suppression of SARS-CoV-2 Infection: An Evidence Review

Abstract: The coronavirus disease (COVID-19) outbreak has presented enormous challenges for healthcare, societal, and economic systems worldwide. There is an urgent global need for a universal vaccine to cover all SARS-CoV-2 mutant strains to stop the current COVID-19 pandemic and the threat of an inevitable second wave of coronavirus. Carbon dioxide is safe and superior antimicrobial, which suggests it should be effective against coronaviruses and mutants thereof. Depending on the therapeutic regime, CO2 could also ameliorate other COVID-19 symptoms as it has also been reported to have antioxidant, anti-inflammation, anti-cytokine effects, and to stimulate the human immune system. Moreover, CO2 has beneficial effects on respiratory physiology, cardiovascular health, and human nervous systems. This article reviews the rationale of early treatment by inhaling safe doses of warmed humidified CO2 gas, either alone or as a carrier gas to deliver other inhaled drugs may help save lives by suppressing SARS-CoV-2 infections and excessive inflammatory responses. We suggest testing this somewhat counter-intuitive, but low tech and safe intervention for its suitability as a preventive measure and treatment against COVID-19. Overall, development and evaluation of this therapy now may provide a safe and economical tool for use not only during the current pandemic but also for any future outbreaks of respiratory diseases and related conditions.

Title: Virus and bacteria inactivation by CO2 bubbles in solution

Authors: Garrido Sanchis, A., Pashley, R. & Ninham, B.

Journal: npj Clean Water 2, 5 (2019). https://doi.org/10.1038/s41545-018-0027-5

Link to full text: Virus and bacteria inactivation by CO2 bubbles in solution

Abstract: The availability of clean water is a major problem facing the world. In particular, the cost and destruction caused by viruses in water remains an unresolved challenge and poses a major limitation on the use of recycled water. Here, we develop an environmentally friendly technology for sterilising water. The technology bubbles heated un-pressurised carbon dioxide or exhaust gases through wastewater in a bubble column, effectively destroying both bacteria and viruses. The process is extremely cost effective, with no concerning by-products, and has already been successfully scaled-up industrially.

Title: Carbon Dioxide as a Microbial Toxicity Enhancer of Some Antibacterial Agents: A New Potential Water Purification Tool

Authors: Varsik Martirosyan, Karlen Hovnanyan, Sinerik Ayrapetyan

Journal: International Scholarly Research Notices, 22 April 2012, https://doi.org/10.5402/2012/906761

Link to full text: Carbon Dioxide as a Microbial Toxicity Enhancer of Some Antibacterial Agents: A New Potential Water Purification Tool

Abstract: The aim of current paper was to investigate the possibility of increasing the toxicity of calcium hypochlorite (Ca(ClO)2) and hydrogen peroxide (H2O2) on Escherichia coli K-12 by preliminary enrichment of culture media by carbon dioxide (CO2). For this purpose, the microbes sensitivity to H2O2 or/and Ca(ClO)2 at normal and CO2-enriched medium was studied by spectrophotometric, radioisotopic, and electronmicroscopic methods. Ten-minute preincubation in CO2-enriched medium enhanced the toxic effect of both H2O2 or/and Ca(ClO)2 on bacteria as a result of induced growth inhibition, compared to no-CO2 enriched group. Additionally, changes in cell morphology and proliferation were observed. It was demonstrated that the preliminary incubation of microbes in CO2-enriched culture media in no supercritical concentration elevate the toxic effect of H2O2 or/and Ca(ClO)2 on microbes. This can serve as a novel, effective, inexpensive, and environmentally friendly approach for water purification from bacteria, further improving the protection of the environment and human health.

Title: Carbon dioxide inhibits the growth rate of Staphylococcus aureus at body temperature

Authors: Persson M, Svenarud P, Flock JI, van der Linden J.

Journal: Surg Endosc. 2005 Jan;19(1):91-4. doi: 10.1007/s00464-003-9334-z. Epub 2004 Nov 11. PMID: 15529188.

Link to PubMed: Carbon dioxide inhibits the growth rate of Staphylococcus aureus at body temperature

Abstract:
Background: Since the 1930s, carbon dioxide (CO(2)) has been combined with cold storage for the preservation of food. However, its use for the prevention of surgical wound infection was long considered to be impractical. Now CO(2) is widely used during laparoscopic procedures, and a method has been developed to create a CO(2) atmosphere in an open wound. The aim of this study was to investigate the effect of CO(2) on the growth of Staphylococcus aureus at body temperature.

Methods: First, S. aureus inoculated on blood agar were exposed to pure CO(2) (100%), standard anaerobic gas (5% CO(2), 10% hydrogen, 85% nitrogen), or air at 37 degrees C for a period of 24 h; then a viable count of the bacteria was made. Second, S. aureus inoculated in brain-heart infusion broth and kept at 37 degrees C were exposed to CO(2) or air for 0, 2, 4, 6, and 8 h; then the optical density of the bacteria was measured.

Results: After 24 h, the number of S. aureus on blood agar was about 100 times lower in CO(2) than in anaerobic gas (p = 0.001) and about 1,000 times lower than in air (p = 0.001). Also, in broth, there were fewer bacteria with CO(2) than with air (p < 0.01). After 2 h, the number of bacteria was increased with air (p < 0.001) but not with CO(2) (p = 0.13). After 8 h, the optical density had increased from zero to 1.2 with air but it had increased only to 0.01 with CO(2) (p = 0.001).

Conclusion: Pure CO(2) significantly decreased the growth rate of S. aureus at body temperature. The inhibitory effect of CO(2) increased exponentially with time. Its bacteriostatic effect may help to explain the low infection rates in patients who undergo laparoscopic procedures.