Supplemental CO₂ improves oxygen saturation, oxygen tension_, and cerebral oxygenation in acutely hypoxic healthy subjects

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 ((Formula presented.)), arterial oxygen saturation of hemoglobin ((Formula presented.)), 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, (Formula presented.) was increased in the 3% and 5% CO₂-enriched groups by 14.9 and 9.5 mmHg, respectively. Compared to pure hypoxia, (Formula presented.) 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.