Background: Recent clinical studies have demonstrated that hypothermia to 32 degrees to 34 degrees C provides significant clinical benefit when induced after resuscitation from cardiac arrest. However, cooling during the postresuscitation period was slow, requiring 4 to 8 hours to achieve target temperatures after return of spontaneous circulation (ROSC). Whether more rapid cooling would further improve survival remains unclear. We sought to determine whether cooling during cardiac arrest before ROSC (ie, "intra-arrest" hypothermia) has survival benefit over more delayed post-ROSC cooling, using a murine cardiac arrest model.
Methods and results: A model of potassium-induced cardiac arrest was established in C57BL/6 mice. After 8 minutes of untreated cardiac arrest, resuscitation was attempted with chest compression, ventilation, and intravenous fluid. Mice were randomized to 3 treatment groups (n=10 each): an intra-arrest hypothermia group, in which mice were cooled to 30 degrees C just before attempted resuscitation, and then rewarmed after 1 hour; a post-ROSC hypothermia group, in which mice were kept at 37 degrees C for 20 minutes after successful ROSC and then were cooled to 30 degrees C for 1 hour; and a normothermic control group, in which mice were kept at 37 degrees C. The intra-arrest hypothermia group demonstrated better 72-hour survival than delayed hypothermia and normothermia groups (6/10 versus 1/10 and 1/10 survivors, respectively, P<0.05), with similar differences seen at 6-hour survival and on neurological scoring.
Conclusions: Timing of hypothermia is a crucial determinant of survival in the murine arrest model. Early intra-arrest cooling appears to be significantly better than delayed post-ROSC cooling or normothermic resuscitation.