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 (CMRO2) more than alpha-stat management. In contrast, other studies have not found CMRO2 to differ between techniques. Using each animal as its own control, the authors assessed the effect of alpha-stat versus pH-stat management of CMRO2, 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 alpha-stat management for the remaining 30 min. Measurement of CBF (radiolabeled microspheres), CMRO2 (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

CMRO2 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 alpha-stat management (37 +/- 5 vs. 30 +/- 3 ml.100 g-1.min-1, respectively). The CBF response to changing PaCO2 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 CMRO2. CMRO2 was neither extraction limited nor dependent on either PaCO2, CBF, or hemoglobin oxygen affinity differences between alpha-stat and pH-stat management. Cerebral blood flow responses to changing CMRO2 depend on the "starting" conditions, with alpha-stat management appearing to better preserve CBF reactivity than pH-stat management.