The effect of limited rewarming and postoperative hypothermia on cognitive function in a rat cardiopulmonary bypass model.

BACKGROUND: Clinical studies have failed to demonstrate significant benefits of hypothermia for the prevention of postoperative cognitive dysfunction (POCD) after cardiopulmonary bypass (CPB). One explanation for this might be that potentially injurious cerebral hyperthermia occurs during rewarming at the end of CPB, off-setting the protective benefits of hypothermia. In this study, we investigated the relative influence of CPB temperature, rewarming strategies, and postoperative temperature in a rat CPB model.
METHODS: Four groups of male Sprague-Dawley rats were surgically prepared and subjected to 90 min of CPB. Group A was normothermic (37.5 degrees C) during and after CPB. Group B underwent hypothermic (32 degrees C) CPB, followed by rewarming to 37.5 degrees C at the end of bypass. Group C had hypothermic (32 degrees C) CPB, followed by limited rewarming to 35 degrees C. Group D had normothermic CPB with hypothermia (35 degrees C) induced only postoperatively. Groups were compared for POCD determined by the performance in the Morris water maze on postoperative days 3-9. Histologic analysis of the brains (CA1 and CA3 hippocampal regions) was also performed.
RESULTS: Hypothermia induced only during (group B versus group A) or after CPB (group D versus group A) conferred no significant POCD benefit. Hypothermia when induced during CPB and continued into the postoperative period resulted in a significant improvement in water maze performance versus all other temperature regimens (group C versus group A, P = 0.044; group C versus group B, P = 0.011; group C versus group D, P = 0.012). No histological differences among groups were demonstrated.
CONCLUSIONS: The combination of hypothermic (32 degrees C) CPB coupled with limited rewarming and prolonged postoperative hypothermia (35 degrees C) decreased POCD after CPB in rats.