Initial heat stress on subsequent responses to cold water immersion while wearing protective clothing.

Introduction: In cold water emergency situations, helicopter aircrew will enter the water with a raised body temperature due to wearing immersion suits. Prewarming has been demonstrated to accelerate core cooling during subsequent cold water immersion (CWI) when wearing swimsuits. For this study we hypothesized that wearing an immersion suit would slow the rate of cooling in subjects who were prewarmed compared to those kept in a normothermic state.

Methods: Two different groups of male subjects (age, 24.7 +/- 4.2 yr; height, 183.1 +/- 6.5 cm; weight, 86.7 +/- 15.0 kg; body fat, 16.8 +/- 3.3%) were used to gather data under two conditions: prewarming by exercise (Warm-CWI) and baseline (Base-CWI) when wearing a dry immersion suit (2.97 Clo). In Warm-CWI, seven subjects rested for 20 min and then cycled on an ergometer cycle for 20 min before immersion in water at 5 degrees C for 140 min. In Base-CWI, six subjects were directly immersed in 5 degrees C water after resting.

Results: Tre and Tsk were significantly higher after Warm-CWI at start of CWI, resulting in faster core cooling rate, and a drop in Tre and Tsk during the first 10 min. In the long term, the overall core cooling did not differ between Warm-CWI (0.34 +/- 0.11 degrees C x h(-1)) and Base-CWI (0.31 +/- 0.05 degrees C x h(-1)).

Discussion: Wearing a dry immersion suit eliminates long-term differences in core cooling between prewarmed subjects and those kept in a normothermic state. When entering cold water with a raised Tre and Tsk, different thermal responses during the first 10 min are expected, but this does not alter long-term core cooling.