Mild hypothermia during reperfusion reduces injury following ischemia of the rabbit ear

Ischemia and reperfusion causes tissue injury that can be partially prevented by mild hypothermia. In this study we postulated that hypothermic protection could occur if imposed only during reperfusion. Rabbit ears were partially amputated, the central artery occluded for 6 h followed by reperfusion at an ambient temperature of either 20 or 24 degrees 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 increased 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 days after the ears were allowed to reperfuse, Normalized myeloperoxidase content (polymorphonuclear cell accumulation) was significantly greater in the 24 degrees C ischemia-24 degrees C reperfusion group compared with the other groups. Ear edema was significantly less in the two groups exposed to 20 degrees C reperfusion compared with the 24 degrees C ischemia-24 degrees C reperfusion group. Peak ear volume was 5.0 times baseline for the 24 degrees C ischemia-24 degrees C reperfusion, 4.0 times baseline for the 20 degrees C ischemia-24 degrees C reperfusion, 3.4 times baseline for the 24 degrees C ischemia-20 degrees C reperfusion, and 3.3 times baseline for the 20 degrees C ischemia-20 degrees C group. We conclude that mild hypothermia reduces PMN accumulation and is more effective in preventing tissue injury when imposed during reperfusion compared with during ischemia.