SUPERIOR CEREBRAL PROTECTION WITH PROFOUND HYPOTHERMIA DURING CIRCULATORY ARREST

The optimal temperature for cerebral protection during hypothermic cir culatory arrest is not known. This study was undertaken to test the hy pothesis that deeper levels of cerebral hypothermia (<10-degrees-C) co nfer better protection against neurologic injury during prolonged hypo thermic circulatory arrest (''colder is better''). Twelve male dogs (2 0 to 25 kg) were placed on closed-chest cardiopulmonary bypass via fem oral artery and femoral/external jugular vein. Using surface and core cooling, tympanic membrane temperature was lowered to 18-degrees to 20 -degrees-C (deep hypothermia, n = 6) or 5-degrees to 7-degrees-C (prof ound hypothermia, n = 6). After 2 hours of hypothermic circulatory arr est, animals were rewarmed to 35-degrees to 37-degrees-C on cardiopulm onary bypass. All were mechanically ventilated and monitored in an int ensive care unit setting for 20 hours. Neurologic assessment was perfo rmed every 12 hours using a species-specific behavior scale that yield ed a neurodeficit score ranging from 0% to 100%, where 0 = normal and 100% = brain dead. After 72 hours, animals were sacrificed and examine d histologically 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%) compar ed 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 (his tologic injury score, 19.2 +/- 1.2 versus 48.3 +/- 1.5; p < 0.0001). T hese results demonstrate that profound cerebral hypothermia (5-degrees to 7-degrees-C) affords better neurologic protection than deep hypoth ermia (18-degrees to 20-degrees-C) during prolonged hypothermic circul atory arrest and suggest that special efforts to achieve profound hypo thermia may be advantageous when extended periods of circulatory arres t are anticipated.