Human Cerebrovascular Response to Combined Hypoxia and Hypercapnia

Abstract

The N ₂ O technique was used in 6 human subjects to measure cerebral blood flow and metabolism during hypoxia and hypercapnia induced by the inhalation of 10% O ₂ -5% CO ₂ . Ventilation increased from 7.7 to 46.3 liters/min; Pao ₂ decreased from 88 to 62 mm Hg; Paco ₂ increased from 38 to 45 mm Hg (for each P <.01). Mean cerebral blood flow increased from 56 to 97 ml/100 g/min ( P <.01). Because cerebral O ₂ consumption was unchanged, the technique of estimating changes in cerebral flow from arterial-jugular venous O ₂ differences was used to follow changes during the first 10 min of inhalation of 10% O ₂ -5% CO ₂ in 6 additional subjects. The rapidity of cerebral vasodilatation was increased by this combination of stimuli. The enhanced respiratory response produced by breathing 10% O ₂ -5% CO ₂ appeared responsible for the more rapid cerebrovascular response and may offer some protective benefits. Comparisons of the present data with previous studies lead to the conclusion that simultaneous hypoxia and hypercapnia have additive dilator effects on the cerebral vasculature. Thus, the observed increases in cerebral flow were the sum of their individual effects.