To compare the rewarming effectiveness of a radiofrequency coil (13.56 MHz) at a specific absorption rate of 2.5 W/kg with warm water immersion (40°C) and a mummy-type insulating sack under simulated field conditions. Four subjects were immersed in 10°C water for up to 90 min or until their rectal temperatures decreased to 35°C. After immersion, rewarming was accomplished with either radiofrequency, warm water, or insulating sack, so that each subject was rewarmed once with each method.
Warm water had significantly greater rate of increase of rectal temperature during rewarming and rectal temperatures 60 min after the start of rewarming than either radiofrequency or insulating sack and a smaller afterdrop duration than insulated sack. Insulated sack had significantly greater afterdrop duration than either warm water or radiofrequency. No significant differences in rectal temperature during rewarming, rectal temperatures 60 min after the start of rewarming, or afterdrop duration were revealed between insulated sack and radiofrequency. No significant differences between methods were found for heart rate during rewarming. No significant differences were revealed between methods for any of the subjective measures recorded. Warm water rewarming resulted in significantly higher skin temperatures during most of rewarming compared with radiofrequency and insulated sack.
The findings indicate that for mildly hypothermic subjects, active rewarming with radiofrequency at a SAR of 2.5 W/kg is less effective than warm water and roughly equivalent to passive rewarming with insulated sack.