Does the rate of rewarming from profound hypothermic arrest influence the outcome in a swine model of lethal hemorrhage?

Background: Rapid induction of profound hypothermic arrest (suspended animation) can provide valuable time for the repair of complex injuries and improve survival. The optimal rate for re-warming from a state of profound hypothermia is unknown. This experiment was designed to test the impact of different warming rates on outcome in a swine model of lethal hemorrhage from complex vascular injuries. Methods: Uncontrolled lethal hemorrhage was induced in 40 swine (80-120 Ibs) by creating an iliac artery and vein injury, followed 30 minutes later (simulating transport time) by laceration of the descending thoracic aorta. Through a thoracotomy approach, a catheter was placed in the aorta and hyperkalemic organ preservation solution was infused on cardiopulmonary bypass to rapidly (2°C/min) induce profound (10°C) hypothermia. Vascular injuries were repaired during 60 minutes of hypothermic arrest. The 4 groups (n = 10/group) included normothermic controls (NC) where core temperature was maintained between 36 to 37°C, and re-warming from profound hypothermia at rates of: 0.25°C/min (slow), 0.5°C/min (medium), or 1°C/min (fast). Hyperkalemia was reversed during the hypothermic arrest period, and blood was infused for resuscitation during re-warming. After discontinuation of cardiopulmonary bypass, the animals were recovered and monitored for 6 weeks for neurologic deficits, cognitive function (learning new skills), and organ dysfunction. Detailed examination of brains was performed at 6 weeks. Results: All the normothermic animals died, whereas survival rates for slow, medium and fast re-warming from hypothermie arrest were 50, 90, and 30%, respectively (p < 0.05 slow and medium warming versus normothermic control, p < 0.05 medium versus fast re-warming). All the surviving animals were neurologically intact, displayed normal learning capacity, and had no long-term organ dysfunction. Conclusions: Rapid induction of hypothermie arrest maintains viability of brain during repair of lethal vascular injuries. Long-term survival is influenced by the rate of reversal of hypothermia.