Human cerebrovascular response to combined hypoxia and hypercapnia.

The N2O technique was used in 6 human subjects to measure cerebral blood flow and metabolism during hypoxia and hypercapnia induced by the inhalation of 10% O2-5% CO2. Ventilation increased from 7.7 to 46.3 liters/min; Pao2 decreased from 88 to 62 mm Hg; Pacoo increased from 38 to 45 mm Hg (for each P < . 0 1 ) . Mean cerebral blood flow increased from 56 to 97 ml/100 g/min ( P < .01). Because cerebral O2 consumption was unchanged, the technique of estimating changes in cerebral flow from arterial-jugular venous O2 differences was used to follow changes during the first 10 min of inhalation of 10% O2-5% CO2 in 6 additional subjects. The rapidity of cerebral vasodilatation was increased by this combination of stimuli. The enhanced respiratory response produced by breathing 10% O2-5% CO2 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. ADDITIONAL