BackgroundMild hypothermia, a promising therapy being evaluated for various clinical situations, may suppress the formation of reactive oxygen species during reperfusion and may ameliorate microcirculatory perfusion failure (the “no‐reflow phenomenon”).MethodsIsolated rat livers underwent 30 min of perfusion, 2.5 h of ischemia, and 3 h of reperfusion. The temperature was maintained at 34 [degree sign]C (mild hypothermia, n = 5) or 38 [degree sign]C (normothermia, n = 6) for all three periods by perfusion of a modified Krebs Henseleit solution, air surface cooling, or both. A third group of livers was normothermic before and during ischemia and mildly hypothermic during reperfusion (reperfusion hypothermia, n = 6). Control livers had 3 h of perfusion at normothermia. Chemiluminescence (a measure of the generation of reactive oxygen species) and hepatic vascular resistance were monitored simultaneously to evaluate the effect of temperature on the formation of reactive oxygen species and the development of no reflow. Also measured were thiobarbituric acid reactive species and lactate dehydrogenase, as indicators of oxidative stress and cell injury.ResultsMild hypothermia decreased formation of reactive oxygen species and postischemic increases in vascular resistance. Reperfusion hypothermia also decreased postischemic increases in vascular resistance, but not as effectively as did mild hypothermia. Levels of thiobarbituric acid reactive species were lower for reperfusion hypothermia than for mild hypothermia at only 0 and 30 min of reperfusion. Lactate dehydrogenase was significant only at 0 min of reperfusion for the normothermic group. Oxygen consumption did not change.ConclusionThe prevention of hepatic vascular injury by suppression of oxidative stress may be an important protective mechanism of mild hypothermia.