ObjectiveTo test the hypotheses that during lethal uncontrolled hemorrhagic shock (UHS) in rats compared with normothermia and room air breathing: a) mild hypothermia would prolong survival time as well as moderate hypothermia; b) oxygen breathing would prolong survival further; and c) hypothermia and oxygen would mitigate visceral ischemia (dysoxia) during UHS.DesignProspective, randomized, controlled laboratory animal study.SettingAnimal research facility.SubjectsMale Sprague-Dawley rats.InterventionFifty-four rats were lightly anesthetized with halothane during spontaneous breathing. UHS was induced by blood withdrawal of 3 mL/100 g over 15 mins, followed by 75% tail amputation with topical application of heparin. Five minutes after tail cut, rats were randomly divided into nine groups (6 rats each) with three rectal temperature levels (38[degree sign]C [100.4[degree sign]F; normothermia] vs. 34[degree sign]C [93.2[degree sign]F; mild hypothermia] vs. 30[degree sign]C [86[degree sign]F; moderate hypothermia]) by surface cooling; each with 3 FIO2levels (0.25 vs. 0.5 vs. 1.0). Rats were observed without fluid resuscitation until death (apnea and pulselessness). Visceral ischemia was monitored by observing liver and gut surface PCO2.Measurements and Main ResultsMean survival time, which was 51 mins in the control group with normothermia and FIO2of 0.25, was more than doubled with hypothermia, to 119 mins in the combined mild hypothermia groups (p < .05) and to 132 mins in the combined moderate hypothermia groups (p < .05; NS for moderate vs. mild hypothermia). FIO2had no statistically significant effect on survival time. Increases in visceral surface PCO2correlated with hypotension (r2= .22 for intestine and .40 for liver). Transiently, increased FIO2, not hypothermia, mitigated visceral ischemia.ConclusionsBoth mild and moderate hypothermia prolonged survival time during untreated, lethal UHS in rats. Increased FIO2had no effect on survival. The effects of hypothermia and increased FIO2during UHS on viscera, the ability to be resuscitated, and outcome should be explored further. (Crit Care Med 1999; 27:1557-1564)