Background: Rapid induction of hypothermia has been shown to improve survival in uncontrolled hemorrhagic shock (UHS) rat studies. We hypothesized that prolonged induction of hypothermia would be equally beneficial for survival during UHS.
Methods: Light anesthesia was induced with halothane in 30 rats, and spontaneous breathing was maintained. Rectal temperature (Tr) was monitored and maintained at 38 degrees C. UHS was induced by blood withdrawal of 2.5 mL/100 g during a 15-minute period, followed by 75% tail amputation. Immediately after cutting the tail, rats were randomized into three groups of 10 rats each: Group 1, maintained at Tr 38 degrees C; group 2, passively cooled to 34 degrees C by exposure to room temperature (23 degrees C); and group 3, actively cooled to 34 degrees C by applying alcohol to the skin and under an electric fan. Next, rats were controlled at each target Tr and observed without fluid resuscitation until either death or a maximum of 240 minutes.
Results: Cooling rate was -0.09 +/- 0.01 degrees C/min in group 2 and -0.36 +/- 0.9 degrees C/min in group 3 (p < 0.01). Mean survival time was 72 +/- 21 minutes in group 1 (38 degrees C), and was nearly doubled by hypothermia to 132 +/- 62 minutes for group 2 (p < 0.01 vs. group 1) and 150 +/- 69 minutes for group 3 (p < 0.01 vs. group 1). No significant difference in survival was noted between groups 2 and 3. Additional blood loss from the tail stump did not differ significantly between groups.
Conclusion: Therapeutic mild hypothermia, induced either slowly (approximately -0.1 degrees C/min) or rapidly (approximately -0.4 degrees C/min) prolongs survival during lethal UHS in rats.