Comparative effectiveness of hypothermia rewarming techniques: radio frequency energy vs. warm water

The purpose of this study was to compare the rewarming effectiveness of a radio frequency coil (13.56 MHz) at a specific absorption rate (SAR) of 2.5 W/kg (RF) with warm water immersion (40 degrees C) (WW) and an insulated mummy-type insulating sack (IS) under simulated field conditions. Four male subjects, ages 24-35, were immersed in 10 degrees C water for up to 90 min or until their rectal temperatures (Tre) decreased to 35 degrees C. Each subject had 3 trials in which they were immersed. After each immersion, rewarming was accomplished with either RF, WW, or IS, so that each subject was rewarmed once with each method. Comparisons of the 3 rewarming methods were based on the rate of increase of Tre during rewarming (Tre/t), Tre 60 min after the start of rewarming (Tre60), the time-interval measured from extraction from the water to the end of afterdrop (tad), and the magnitude of any observed Tre afterdrop (Tad). WW had significantly greater Tre/t and Tre60 than either RF or IS (P < 0.03) and a smaller tad than IS (P < 0.05). IS had significantly greater Tad than either WW or RF (P < 0.05). No significant differences in Tre/t, Tre60, or tad were observed between IS and RF. The results of this study indicate that for mildly hypothermic individuals, active rewarming with RF at a SAR of 2.5 W/kg is less effective than WW and roughly equivalent to passive rewarming with IS.