Cerebrovascular carbon dioxide reactivity and conductance in patients awake and under general anesthesia

Paired cerebral blood flow measurements at two levels of arterial carbon dioxide (CO2) tension were made in 36 awake patients and 85 patients under general anesthesia to measure the percent change in flow per mm Hg change in PaCO2 (CO2 reactivity). CO2 reactivity was significantly greater in the generally anesthetized (5.96) than in the awake group (3.65). In both groups, a linear correlation was found between mean percent CO2 reactivity and conductance (the reciprocal of resistance), up to a level of conductance of 0.85 in awake and 0.95 in unconscious patients. When the CO2 reactivity of 18 awake patients and 59 unconscious patients was compared with that of respective control groups, only 11 patients were found to have abnormal CO2 reactivities for their conductance levels.