Enhancement of cerebrovascular effect of CO2 by hypoxia

Internal carotid blood flow, taken as an index of cerebral blood flow (CBF); arterial pressure; and respiratory O2 and CO2 concentrations, were measured in halothane (1%)-anesthetized, paralyzed and mechanically ventilated rabbits. CBF was determined at end-tidal CO2 of 4% (normocapnia) and 8%, as inspired O2 [O2]I) was varied stepwise over the range of 6.5 to 92%. Normocapnic CBF was constant over the range of 15 to 92% [O2]I, but it increased significantly to 240% and 380% of control when [O2]I was at 10% and 6.5%, respectively. Cerebrovascular response to CO2 was constant over the range of [O2]I tested, except for a significant elevation to 180% of control at [O2]I of 10%.