DETERMINANTS OF CEREBRAL OXYGENATION DURING CARDIAC-SURGERY

Background Neuropsychological deficits after cardiac surgery are attri buted to the side effects of cardiopulmonary bypass (CPB). To protect the brain from ischemic damage, the influences of temperature, blood p ressure, blood gases, acid-base status, and hemodilution on cerebral o xygenation have to be elucidated and quantified. Methods Forty-one con secutive patients were investigated during cardiac surgery while on CP B. Operative management included moderate hypothermia of 26 degrees C and the alpha-stat pH management. With near-infrared spectrophotometry , changes in oxygenated hemoglobin (HbO(2), representing oxygen delive ry) and oxidized cytochrome a,a3 (CtO(2), cellular oxygenation) in bra in tissue were obtained noninvasively. In addition, venous saturation of the brain was measured via a catheter in the jugular bulb (SBJO2) T he influence of operative management parameters on cerebral oxygenatio n was calculated by univariate and multiple regression analyses. Resul ts Before and after CPB there was no significant multivariate determin ant of cerebral oxygenation. During CPB, HbO(2) depended solely on PCO 2, (P<.01; r=.89). CtO(2) was determined by pH (P<.01), esophageal tem perature (P<.01), PCO2 (P<.01), and Hb (P<.01). These parameters expla ined nearly all changes of the cytochrome measurements during CPB (r=. 99). Arterial PCO2 (P<.01) and pH (P<.01) influenced brain venous oxyg en saturation (SBJO2; r=.98). Conclusions Cerebral oxygenation is auto regulated during cardiac surgery before and after CPB. During CPB, Hb, temperature, pH, and PCO2 determine at least 85% of all changes in ce rebral oxygenation. The main causes of impaired cerebral oxygenation a re the decrease in Hb with hemodilution, vasoconstriction due to hypoc apnia, acid the leftward shift of the Hb binding curve in alkalosis an d hypothermia.