Resistive heating is more effective than metallic-foil insulation in an experimental model of accidental hypothermia: A randomized controlled trial

Study objective: We studied a resistive-heating blanket in a volunteer model of severe accidental hypothermia to evaluate differences in rates of rewarming, core temperature afterdrop, and body heat content and distribution during active and passive rewarming. Methods: Eight volunteers participated in a crossover design on 2 days. The volunteers were anesthetized and cooled to 33 degrees C (91.4 degrees F); anesthesia was subsequently discontinued, and shivering was prevented with meperidine. On one randomly assigned day, the volunteers were rewarmed passively with reflective foil (passive insulation), whereas on the other they were covered with a carbon fiber-resistive heating blanket set to 42 degrees C (107.6 degrees F; active rewarming). Trunk and head temperature and heat content were calculated from core (tympanic membrane) temperature. Peripheral (arm and leg) tissue temperature and heat content were estimated by using fourth-order regressions and integration over volume from 30 tissue and skin temperatures. Results: Core heat content increased 73+/-14 kcal (mean+SD) during 3 hours of active warming, but only 31+/-24 kcal with pas sive insulation, a difference of 41+/-20 kcal (95% confidence interval [CI] 27 to 55 kcal; P<.001). Peripheral tissue heat content increased linearly by 111+/-16 kcal during active warming but only by 38+/-31 kcal during passive warming, a difference of 74+/-34 kcal (95% CI 50 to 97; P<.001). Consequently, total body heat increased 183+/-22 kcal during active warming but only 68+/-54 kcal with passive insulation, a difference of 115+/-42 kcal (95% CI 86 to 144 kcal; P<.001). Core temperature increased from 32.9 degrees C+/-0.2 degrees C to 35.2 degrees C+/-0.4 degrees C during 3 hours of active warming, a difference of 2.3 degrees C+/-0.4 degrees C. In contrast, core temperature with foil insulation only increased from 32.9 degrees C+/-0.2 degrees C to 33.8 degrees C+/-0.5 degrees C, a difference of only 0.8 degrees C+/-0.4 degrees C. The difference in the core temperature increase between the two treatments was thus 1.5 degrees C+/-0.4 degrees C (95% CI 1.2 degrees C to 1.7 degrees C; P<.001 between treatments), Active warming was not associated with an afterdrop, whereas the afterdrop was 0.2 degrees C+/-0.2 degrees C and lasted a median of 45 minutes (interquartile range, 41 to 64 minutes) with passive insulation. Conclusion: Resistive heating more than doubles the rewarming rate compared with that produced by reflective metal foil and does so without producing an afterdrop. It is therefore likely to be useful in the out-of-hospital setting.