Superior cerebral protection with profound hypothermia during circulatory arrest

The optimal temperature for cerebral protection during hypothermic circulatory arrest is not known. This study was undertaken to test the hypothesis that deeper levels of cerebral hypothermia (< 10 °C) confer better protection against neurologic injury during prolonged hypothermic circulatory arrest ("colder is better"). Twelve male dogs (20 to 25 kg) were placed on closed-chest cardiopulmonary bypass via femoral artery and femoral/external jugular vein. Using surface and core cooling, tympanic membrane temperature was lowered to 18 ° to 20 °C (deep hypothermia, n = 6) or 5 ° to 7 °C (profound hypothermia, n = 6). After 2 hours of hypothermic circulatory arrest, animals were rewarmed to 35 ° to 37 °C on cardiopulmonary bypass. All were mechanically ventilated and monitored in an intensive care unit setting for 20 hours. Neurologic assessment was performed every 12 hours using a species-specific behavior scale that yielded a neurodeficit score ranging from 0% to 100%, where 0 = normal and 100% = brain dead. After 72 hours, animals were sacrificed and examined histoiogically for neurologic injury. Histologic injury scores were assigned to each animal (range, 0 [normal] to 100 [severe injury]). At the end of the observation period, profoundly hypothermic animals had better neurologic function (neurodeficit score, 5.7% ± 4.0%) compared with deeply hypothermic animals (neurodeficit score, 41% ± 9.3%; p < 0.006). Every animal had histologic evidence of neurologic injury, but profoundly hypothermic animals had significantly less injury (histologic injury score, 19.2 ± 1.2 versus 48.3 ± 1.5; p < 0.0001). These results demonstrate that profound cerebral hypothermia (5 ° to 7 °C) affords better neurologic protection than deep hypothermia (13 ° to 20 °C) during prolonged hypothermic circulatory arrest and suggest that special efforts to achieve profound hypothermia may be advantageous when extended periods of circulatory arrest are anticipated.