A comparison of endogenous versus exogenous heating on the subsequent development of hypothermia.

This study investigated the effect of endogenous and exogenous heating on body core cooling rate, afterdrop and re-warming rate in humans during subsequent cold-water immersion. On three separate days, following 30 min baseline resting at an ambient temperature of 25°C, seven subjects were either: (1) seated upright for 15 min rest (Control); (2) exercised 15 min on a cycle ergometer at 70% VO2 max (Exercise); or, (3) immersed 16.8 +/- 6.2 min in warm water (40°C) (Warm-water immersion) to an esophageal temperature (TES) similar to that of end of exercise. Subjects were then cooled in water (7°C) to a TES of 34.5°C and rewarmed by spontaneous shivering (shivering thermogenesis) in air. There was a 1.3-fold increase in the overall body core cooling rate during Exercise as compared to Control (P < 0.01). Warm-water immersion demonstrated the largest difference in overall cooling rate above Exercise and Control with a 2.5- and 3.3-fold increase, respectively (P < 0.01). The greatest difference between trials occurred within the initial 15 min (i.e., a 2.0- and 4.9-fold increase above Control for Exercise and Warm-water immersion, respectively). There was no significant difference in the TES afterdrop (∼0.5°C) nor afterdrop duration (∼15 min). Similarly, rewarming rates were almost identical under all conditions (∼3.1°C·h-1). These data show that pre-warming can have a detrimental effect on survival time during a subsequent cold-water immersion