Comparison of fast versus slow rewarming following acute moderate hypothermia in rats

Background: The aim of this study was to compare the biochemical and physiological responses of fast vs. slow rewarming from moderate hypothermia in anaesthetized rats. Methods: Anaesthetized rats were surface cooled to 28degreesC, for 20 min, then rewarmed either quickly over 30 min or slowly over 120 min with monitoring of vital signs, systemic vascular resistance (SVR), cardiac output, biochemical changes and activity for 31 days. Results: At hypothermia, cardiac output decreased to 77 +/- 38 ml.min(-1) and lactate increased to 4.62 +/- 4.73 mmol.l(-1). Fast rewarming caused an abrupt increase in cardiac output (270 +/- 24 ml-min(-1)) and a sharp drop in SVR (325.6 +/- 23.3 dyne.s(-1).cm(-5)), compared with a smoother course with cardiac output (142 +/- 18 ml.min(-1), P < 0.01) and SVR (662.8 +/- 41.0 dyne.s(-1).cm(-5), p < 0.01), measured during slow rewarming. Lactate failed to return to normal values (upon returning to normothermia) (2.5 +/- 0.75 mmol.l(-1)) only in the fast rewarming group. In both groups, activity in the open field was not different from control rats. Conclusions: In rats, moderate hypothermia for 20 min does not appear to cause lasting biochemical or behavioural consequences, whether rewarming lasted over 30 or 120 min. However, there: was a greater early change in cardiac output and heart rate, due to systemic vasodilatation in the fast rewarming animals. These acute changes may have consequences in patients with compromized cardiovascular reserves.