The effects of gas mixtures comprising supplementary 3% carbon dioxide, 35% oxygen or a combination of 3% CO2 plus 35% O2 in ambient air have been compared on arterial blood gases, peripheral and cerebral oxygenation and middle cerebral artery velocity (MCAV) at 150m and on acute exposure to 3459m in 12 healthy subjects. Breathing 3% CO2 or 35% O2 increased arterial blood oxygen at both altitudes, and the CO2/O2 combination resulted in the most marked rise. MCAV increased on ascent to 3459m, increasing further with 3% CO2 and decreasing with 35% O2 at both altitudes. The CO2/O2 combination resulted in an increase in MCAV at 150m, but not at 3549m. Cerebral regional oxygenation fell on ascent to 3459m. Breathing 3% CO2 or 35% O2 increased cerebral oxygenation at both altitudes, and the CO2/O2 combination resulted in the greatest rise at both altitudes. The combination also resulted in significant rises in cutaneous and muscle oxygenation at 3459m. The key role of carbon dioxide in oxygenation at altitude is confirmed, and the importance of this gas for tissue oxygenation is demonstrated.

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