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.
ASJC Scopus subject areas
- Pulmonary and Respiratory Medicine
- Cardiology and Cardiovascular Medicine