Hyperventilation during breathing of 100% oxygen elevates the Po2 of alveolar gas by the same amount that it lowers its Pco2. 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% CO2 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 CO2 with O2 was greater than when O2 alone was breathed at reduced ambient pressure. Since neither arterial Po2 nor cerebral venous Pco2 values differed in these two experimental situations, the respiratory stimulation may represent the quantitative demonstration in man of a respiratory effect of CO2 mediated by arterial chemoreceptor activation and unrelated to change in the level of central chemical stimulus.

Submitted on March 16, 1962