Cerebral effects of low-flow cardiopulmonary bypass and hypothermic circulatory arrest

Although both hypothermic circulatory arrest (HCA) and low-flow cardiopulmonary bypass (CPB) are accepted techniques for the operative management of complex cardiovascular pathology, the potential for neurologic sequelae is still a concern. To assess the relative safety of these techniques, we compared cerebral hemodynamics and clinical outcome in two groups of puppies. Sixteen puppies underwent 45 minutes of either HCA or low-flow CPB (25 mL · kg^-1 · min^-1) after cooling to 13 °C. Methodology included radioactive microsphere determination of cerebral blood flow; calculation of cerebral oxygen extraction (arteriovenous oxygen content difference) and consumption; measurement of glucose consumption, and determination of cerebrovascular resistance. Measurements were obtained at baseline (37 °C, 13 °C, and 30 °C and at 2, 4, and 8 hours after HCA or low-flow CPB. No neurologic deficits were observed in any of the survivors ( 15 16). In both groups, cerebral metabolic rate of oxygen was maintained at baseline or greater levels postoperatively. Cerebrovascular resistance rose slightly in the low-flow CPB group postoperatively in contrast to a marked elevation in the HCA group. During the period of high cerebrovascular resistance after HCA, cerebral metabolic rate of oxygen was maintained by increased oxygen extraction. After low-flow CPB, oxygen extraction was not significantly different from baseline, presumably because of less severe changes in cerebrovascular resistance. Glucose metabolism followed the same trends as oxygen metabolism in both groups. These data suggest that after HCA there is a vulnerable interval, lasting as late as 8 hours postoperatively, in which cerebrovascular resistance remains high and cerebral metabolism is maintained primarily by high oxygen and glucose extraction. Any additional stress during this interval (a decrease in arterial oxygen content or perfusion pressure) could result in cerebral injury.