MILD HYPOTHERMIA DURING REPERFUSION REDUCES INJURY FOLLOWING ISCHEMIAOF THE RABBIT EAR

Ischemia and reperfusion causes tissue injury that can be partially pr evented by mild hypothermia. In this study we postulated that hypother mic protection could occur if imposed only during reperfusion. Rabbit ears were partially amputated, the central artery occluded for 6 h fol lowed by reperfusion at an ambient temperature of either 20 or 24 degr ees C resulting in ischemic ear temperatures of 22.5 vs, 24.7 degrees C. Ear temperature of rabbits remaining in the 24 degrees C room incre ased with reperfusion to 32.4 degrees C whereas those moved to the 20 degrees C room increased to 30.0 degrees C by 2 h of reperfusion. Ear volume was used as a measure of tissue edema and was measured for 7 da ys after the ears were allowed to reperfuse, Normalized myeloperoxidas e content (polymorphonuclear cell accumulation) was significantly grea ter in the 24 degrees C ischemia-24 degrees C reperfusion group compar ed with the other groups. Ear edema was significantly less in the two groups exposed to 20 degrees C reperfusion compared with the 24 degree s C ischemia-24 degrees C reperfusion group. Peak ear volume was 5.0 t imes baseline for the 24 degrees C ischemia-24 degrees C reperfusion, 4.0 times baseline for the 20 degrees C ischemia-24 degrees C reperfus ion, 3.4 times baseline for the 24 degrees C ischemia-20 degrees C rep erfusion, and 3.3 times baseline for the 20 degrees C ischemia-20 degr ees C group. We conclude that mild hypothermia reduces PMN accumulatio n and is more effective in preventing tissue injury when imposed durin g reperfusion compared with during ischemia.