Carbon dioxide improves brain oxygenation
Oxygen is your brain's most important nutrient. If you want to become smarter and make wiser decisions, be able to focus and concentrate better, and become less anxious and worried, then you need to oxygenate your brain effectively.
Although our brain and nervous system only represent two percent of our body weight, they use a full 20 percent of the oxygen we consume. This means that our brain is the organ that suffers the most when we have inefficient breathing habits.
Lower levels of carbon dioxide (CO2) constrict the blood vessels and therefore less blood is delivered to the brain. That is why hyperventilation is used in brain surgery to shrink the brain and give room for surgical instruments, as well as reduce the risk of bleeding. Even though it may be a good thing in brain surgery, a smaller brain is hardly what any of us want in our daily lives.
In their pioneering work from 1946, Kety and Schmidt >> demonstrated that cerebral blood flow is directly related to the carbon dioxide pressure in the blood. “CO2 inhaled in concentrations of 5-7% produced an increase in cerebral blood flow averaging 75%, while oxygen inhaled in concentrations of 85-100% was associated with a reduction in cerebral blood flow of 13%.”
A more recent study from 2010 by Feng Xu et al. >> found a similar effect. During the study, inhalation of 5% carbon dioxide led to a significant increase in the blood flow to the brain by 54.5%.
A study from 2020 by Jan Stepanek et al. >> found that supplemental carbon dioxide improves oxygenation in individuals experiencing acute hypoxia (low levels of oxygen). Traditionally, oxygen is viewed as the primary determinant of tissue oxygenation, but this research highlights the importance of maintaining optimum levels of CO2. The authors explored whether the addition of CO2 to a low oxygen environment could enhance arterial oxygen levels, oxygen saturation, and brain oxygenation.
The study involved 20 healthy subjects who were exposed to different gas mixtures, including an 8% oxygen mixture and oxygen mixed with either 3% or 5% CO2. Subjects underwent 5-minute intervals of exposure to the different gas mixtures, with randomized orders to ensure unbiased results. Blood gases, oxygen saturation, and brain oxygenation were measured throughout the exposure period.
The findings indicated that the addition of CO2 improved oxygenation compared to pure hypoxia. Specifically, the arterial partial pressure of oxygen (PaO2) and oxygen saturation (SaO2) were higher in both the 3% and 5% CO2-enriched groups compared to the hypoxia-only group. Additionally, brain oxygen saturation (rSO2) increased significantly in the CO2-supplemented groups, particularly with the 5% CO2 mixture.
The study suggests that CO2 supplementation enhances oxygenation by improving the efficiency of oxygen delivery to tissues, particularly in hypoxic conditions. CO2 appears to reduce the negative effects of hypocapnia (low levels of CO2), such as impaired oxygen delivery and vasoconstriction, which commonly occur during hypoxia.
In conclusion, the results highlight the potential benefit of using supplemental CO2 to improve oxygenation in individuals exposed to hypoxic environments, such as those encountered at high altitudes or in aviation.
Scientific references
Title: The effects of altered arterial tensions of carbon dioxide and oxygen on cerebral blood flow and cerebral oxygen consumption of normal young men
Authors: Seymour S. Kety and Carl F. Schmidt
Journal: The Journal of Clinical Investigation
Link to full text: Access here >>
Abstract: A method for measuring quantitatively the volume of cerebral blood flow in man by inhalation of nitrous oxide found its first application in a study of the cerebral circulatory effects of low CO2 tension achieved by hyperventilation; of high CO2 tension, and of high and low 02 tensions obtained by inhalation of appropriate gas mixtures. Only the first part of this study, the effects of active and passive hyperventilation, has been published in detail. The purpose of the present paper is to present the remainder of these findings and to derive from them, together with those of the hyperventilation experiments, evidence bearing on the intrinsic control of the human cerebral circulation as revealed by quantitative measurements.
Title: The influence of carbon dioxide on brain activity and metabolism in conscious humans
Authors: Feng Xu et al.
Journal: Journal of Cerebral Blood Flow & Metabolism (2011)
Link to full text: Access here >>
Abstract: A better understanding of carbon dioxide (CO2) effect on brain activity may have a profound impact on clinical studies using CO2 manipulation to assess cerebrovascular reserve and on the use of hypercapnia as a means to calibrate functional magnetic resonance imaging (fMRI) signal. This study investigates how an increase in blood CO2, via inhalation of 5% CO2, may alter brain activity in humans. Dynamic measurement of brain metabolism revealed that mild hypercapnia resulted in a suppression of cerebral metabolic rate of oxygen (CMRO2) by 13.4%±2.3% (N= 14) and, furthermore, the CMRO2 change was proportional to the subject’s end-tidal CO2 (Et-CO2) change.
When using functional connectivity MRI (fcMRI) to assess the changes in resting-state neural activity, it was found that hypercapnia resulted in a reduction in all fcMRI indices assessed including cluster volume, cross-correlation coefficient, and amplitude of the fcMRI signal in the default-mode network (DMN). The extent of the reduction was more pronounced than similar indices obtained in visual evoked fMRI, suggesting a selective suppression effect on resting-state neural activity. Scalp electroencephalogram (EEG) studies comparing hypercapnia with normocapnia conditions showed a relative increase in low frequency power in the EEG spectra, suggesting that the brain is entering a low arousal state on CO2 inhalation.
Title: Supplemental CO2 improves oxygen saturation, oxygen tension, and cerebral oxygenation in acutely hypoxic healthy subjects
Authors: Jan Stepanek et al.
Journal: Physiological Reports (2020)
Link to full text: Access here >>
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.
