Blood Pco₂ and brain oxygenation at reduced ambient pressure

Hyperventilation during breathing of 100% oxygen elevates the Po2 of alveolar gas by the same amount that it lowers its Pco 2 . Since the development of arterial hypocapnia causes cerebral vasoconstriction, brain oxygenation is drastically decreased even while arterial oxygenation is improved by hyperventilation. Administration of 30% CO 2 with oxygen at an ambient pressure equivalent to that at 39,000-ft altitude prevented alkalemia and, in spite of hyperventilation, restored cerebral venous oxygenation to a level at least equivalent to that found when pure oxygen was breathed at rest at the same altitude. The respiratory minute volume during administration of CO 2 with O 2 was greater than when O 2 alone was breathed at reduced ambient pressure. Since neither arterial Po 2 nor cerebral venous Pco 2 values differed in these two experimental situations, the respiratory stimulation may represent the quantitative demonstration in man of a respiratory effect of CO 2 mediated by arterial chemoreceptor activation and unrelated to change in the level of central chemical stimulus. Submitted on March 16, 1962