Blood Pco₂ and brain oxygenation at reduced ambient pressure

¹ Laboratories of Pharmacology, Schools of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania

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.