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° 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×l00 g⁻¹×min⁻¹ at a mean±SD Paco₂ of 36±2.0 mm Hg and 0.40±0.14 ml O₂×l00 g⁻¹×min⁻¹ at a mean±SD Paco₂ of 45±2 mm Hg. In group 2 (Paco₂49-72 mm Hg, n = 12) the mean±SD cerebral metabolic rate for oxygen was 0.31 ±0.09 ml O₂×l00 g⁻¹×min⁻¹ at a mean±SD Paco₂ of 55±3 mm Hg and 0.21±0.07 ml O₂×l00 g⁻¹×min⁻¹ at a mean±SD Paco₂ of 68±2 mm Hg. Group 2 values differed significantly from those in Group 1 (p<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.