Key words: Cardiopulmonary bypass; brain; hypothermia; cerebral blood flow; cerebral metabolism; brain protection C EREBRAL HYPOTHERMIA IS THE PRINCIPAL MEANS OF protecting the brain during cardiac surgery, permitting periods of reduced perfusion and/or circulatory arrest. For example, at 10-20 °C, circulatory arrest for 45-75 minutes is tolerated in animals without subsequent behavioral or histopathologic evidence of neurologic injury. 1 " 4 Nevertheless, clinically, die protective effect of hypothermia is far from complete. In children undergoing hypothermic circulatory arrest under equivalent conditions, coma, seizures, choreoathetosis, and developmental retardation are recognized as potential neurologic sequels, occurring widi a combined incidence of 10-30%. 59 Neurologic complications (stroke, neuropsychologic change) also occur with distressing frequency in adults undergoing cardiac surgery, even in the absence of circulatory arrest. 10 An improved understanding of the physiology of cerebral hypothermia during cardiopulmonary bypass and hypothermic arrest is needed to improve upon these results. This review will address current concepts and controversies regarding flow of blood to the brain and its metabolism during hypothermic cardiopulmonary bypass and their relationship to current techniques for cerebral protection.
Mechanisms of hypothermic cerebral protectionThe time between the onset of cerebral ischemia and subsequent failure of neuronal energy 11 ' 12 and terminal depolarization of membranes 13 is related to cerebral metabolic rate. By reducing this rate, hypothermia slows the rate of depletion of high energy phosphate 111214 ' 15 and the development of intracellular acidosis during cerebral ischemia. 1415 In this way, hypothermia is thought to delay or prevent terminal depolarization of membranes during periods of cerebral ischemia (such as hypotension or circulatory arrest). The suppressive effect of hypothermia upon cerebral metabolism may be somewhat greater in children than in adults. Croughwell et al, 15 studying 41 adults during cardiopulmonary bypass at 27 °C, found the cerebral metabolic rate for oxygen to be reduced to a value ofapproximately36%ofthenormothermicvalue (1.4+0.3 vs 0.5±0.2 ml # 100 gnr^min' 1 ). In this, and comparable adult studies, the ratio of the metabolic rate over a 10 °C interval, referred to as Q^, ranged from 2.4 to 2.8. 17 " 20 Greeley et al, 21 studying cerebral blood flow and metabolism in 46 children during cardiopulmonary bypass, found the metabolic rate to decrease logarithmically with decreasing temperature to values of approximately 36% and 12% of normothermic values at 28 °C and 19 °C, respectively ( Figure 1). In this study, Q 10 ranged between 3.3 and 3.6, values somewhat greater than those found in adults. Assuming the tolerable cerebral ischemic interval at 37 °C to be 3-4 minutes, a cerebral temperature of 17 °C should permit 33-65 minutes of arrest ([3-5 min] x [3.3-3.6] x [3.3-3.6]) = 33-65 min). Greeley et al suggested that this predicted duration ...