MODERATE HYPOTHERMIA REDUCES CARDIOPULMONARY BYPASS-INDUCED IMPAIRMENT OF CEREBROVASCULAR RESPONSES TO PLATELET PRODUCTS

Background. The purpose of this study was to determine whether cerebra l cortical microvascular responses to platelet-derived vasoactive subs tances are altered after normothermic cardiopulmonary bypass (CPB), an d whether these alterations are modified by moderate hypothermia. Meth ods. Pigs were placed on normothermic CPB (37 degrees C) for 2 hours a nd then perfused off CPB with normothermic blood for either 15 minutes (n = 8) or 60 minutes (n = 6). Another group was placed on moderately hypothermic CPB (25 degrees C) for 2 hours and then perfused off CPB at 37 degrees C for 15 minutes (n = 6). Alpha-stat pH management was u sed. In vitro responses of isolated cortical cerebral arterioles (90 t o 170 mu m internal diameter) to platelet-derived vasoactive substance s were examined in a pressurized no-flow state with videomicroscopy. M icrovessels from noninstrumented pigs (n = 14) were used as controls f or in vitro studies. Results. Cerebrovascular resistance and internal carotid artery blood now were similar 15 minutes after CPB in both nor mothermic and hypothermic groups. However, relaxations of microvessels to adenosine 5' diphosphate or serotonin were reduced in vessels from both groups. One hour of after CPB cerebral perfusion did not change this pattern of altered vascular reactivity. Hypothermia caused a part ial but significant reduction in impairment of responses to adenosine 5' diphosphate and serotonin. Microvascular relaxation to the endothel ium-independent agent sodium nitroprusside and contraction to a thromb oxane A(2) analog were similar in all experimental groups, suggesting normal vascular smooth muscle responsiveness. Conclusions. This study demonstrates that normothermic extracorporeal circulation reduces endo thelium-dependent relaxation responses to products of platelet activat ion in the cerebral microcirculation. Moderate hypothermia attenuates the CPB-induced impairment of endothelium-dependent relaxation, but ha s no effect on baseline cerebral blood flow after rewarming.