Acid-base management during hypothermic cardiopulmonary bypass does not affect cerebral metabolism but does affect blood flow and neurological outcome

Abstract

In order to compare the effects of blood-gas management on cerebral blood flow, metabolism and neurological outcome after hypothermic cardiopulmonary bypass (CPB) we have studied 65 patients undergoing aorto-coronary bypass surgery allocated randomly to either a pH-stat (temperature-corrected blood-gas management) or an alpha-stat (temperature-uncorrected blood-gas management) group. All patients were examined neurologically on the day before and the 7th day after operation. In 20 patients of the pH-stat group and in 15 patients of the alpha-stat group we measured cerebral blood flow (CBF), using the argon washin technique, and also cerebral oxygen (CMRO2) and glucose (CMRg) uptake. Measurements were performed in awake patients, after induction of anaesthesia with fentanyl, midazolam and pancuronium under normothermic conditions, during CPB at a venous blood temperature of 26 degrees C and at the end of surgery. Compared with postinduction values, hypothermia was associated with an 18% reduction in CBF and decreases in CMRO2 and CMRg of 61% and 60%, respectively, in the alpha-stat group. In the pH-stat group, CMRO2 and CMRg decreased also, by 58% and 74%, respectively, whereas CBF increased by 191%, indicating uncoupling of flow and metabolism. As there were no statistically significant differences between the metabolic variables in both groups, we conclude that acid-base management did not affect cerebral metabolism, despite its influence on blood flow. After rewarming, CBF and cerebral metabolism normalized independently of acid-base management during hypothermia. Nevertheless, neurological dysfunction occurred more often in the pH-stat group (P = 0.036).