Hypercarbia depresses cerebral oxygen consumption during cardiopulmonary bypass

No human studies have systematically examined the relations among PaCO2, cerebral blood flow, and the cerebral metabolic rate for oxygen during hypothermic cardiopulmonary bypass. We varied PaCO2 during hypothermic (26-28 degrees C) cardiopulmonary bypass and estimated the cerebral metabolic rate for oxygen by multiplying cerebral blood flow (measured using xenon-133 clearance) by the cerebral arteriovenous difference in oxygen contents. Patients were randomly assigned to either of two methods of managing PaCO2 (uncorrected for body temperature). In group 1 (PACO2 32-48 mm Hg, n = 13) the mean +/- SD cerebral metabolic rate for oxygen was 0.40 +/- 0.11 ml O2 X 100 g-1 X min-1 at a mean +/- SD PaCO2 of 36 +/- 2.0 mm Hg and 0.40 +/- 0.14 ml O2 X 100 g-1 X min-1 at a mean +/- SD PaCO2 of 45 +/- 2 mm Hg. and 49-72 mm Hg, n = 12) the mean +/- SD cerebral metabolic rate for oxygen was 0.31 +/- 0.09 ml O2 X 100 g-1 X min-1 at a mean +/- SD PaCO2 of 55 +/- 3 mm Hg and 0.21 +/- 0.07 ml O2 X 100 g-1 X min-1 at a mean +/- SD PaCO2 of 68 +/- 2 mm Hg. Group 2 values differed significantly from those in Group 1 (p less than 0.05). In both groups, cerebral blood flow increased as PaCO2 increased. During cardiopulmonary bypass, increasing PaCO2 increases cerebral blood flow and decreases the cerebral metabolic rate for oxygen.