HYPOTHERMIC ACID-BASE MANAGEMENT DOES NOT AFFECT CEREBRAL METABOLIC-RATE FOR OXYGEN AT 27-DEGREES-C - A STUDY DURING CARDIOPULMONARY BYPASS IN RABBITS

Background. It has been contended that, during cardiopulmonary bypass at 27-degrees-C, pH-stat management decreases cerebral metabolic rate for oxygen (CMR(O2)) more than alpha-stat management. In contrast, other studies have not found CMR(O2) to differ between techniques. Using each animal as its own control, the authors assessed the effect of alpha-stat versus pH-stat management on CMR(O2), cerebral blood flow (CBF), and brain oxygen extraction during cardiopulmonary bypass at 27-degrees-C. Methods: Fourteen New Zealand White rabbits, anesthetized with fentanyl and diazepam, underwent cardiopulmonary bypass at 27-degrees-C (membrane oxygenator, centrifugal pump, and bifemoral arterial perfusion). Group 1 animals (n = 7) had alpha-stat management for the initial 65-70 min of bypass, and were then changed to pH-stat management for the remaining 30 min of bypass. Group 2 animals (n = 7) had pH-stat management for the initial 65-70 min of bypass, and were then changed to a-stat management for the remaining 30 min. Measurement of CBF (radiolabeled microspheres), CMR(O2) (CBF x brain arterial-venous oxygen content difference), brain temperature, systemic hemodynamics, and arterial blood gases were made in each animal under both alpha-stat and pH-stat conditions. Results. CMR(O2) did not differ between alpha-stat and pH-stat conditions (1.4 +/- 0.3 ml - 100 g-1. min-1; median +/- quartile deviation), and was independent of order of determination. Changes in CBF between alpha-stat and pH-stat conditions were associated with proportional opposite changes in cerebral oxygen extraction. Cerebral blood flow was significantly greater with pH-stat management than with a-stat management (37 +/- 5 vs. 30 +/- 3 ml . 100 g-1. min-1, respectively). The CBF response to changing Pa(CO2) was significantly greater when going from alpha-stat to pH-stat conditions (group 1) than in the reverse order (group 2). Conclusions. During cardiopulmonary bypass at 27-degrees-C, hypothermic acid-base management has no measurable effect on CMR(O2). CMR(O2) was neither extraction limited nor dependent on either Pa(CO2), CBF, or hemoglobin oxygen affinity differences between alpha-stat and pH-stat management. Cerebral blood flow responses to changing CMR(O2) depend on the ''starting'' conditions, with alpha-stat management appearing to better preserve CBF reactivity than pH-stat management.