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

Abstract

Study objective: We study 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°C (91.4°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°C (107.6°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 passive 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°C±0.2°C to 35.2°C±0.4°C during 3 hours of active warming, a difference of 2.3°C±0.4°C. In contrast, core temperature with foil insulation only increased from 32.9°C±0.2°C to 33.8°C±0.5°C, a difference of only 0.8°C±0.4°C. The difference in the core temperature increase between the two treatments was thus 1.5°C±0.4°C (95% CI 1.2°C to 1.7°C; P <.001 between treatments). Active warming was not associated with an afterdrop, whereas the afterdrop was 0.2°C±0.2°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 prehospital setting. [Greif R, Rajek A, Laciny S, Bastanmehr H, Sessler DI. Resistive heating is more effective than metallic-foil insulation in an experimental model of accidental hypothermia: a randomized controlled trial. Ann Emerg Med. April 2000;35:337-345.]

Introduction

Accidental hypothermia is defined by core temperatures of less than 35°C (95°F). However, core temperatures above 32°C (90°F) are rarely associated with morbidity unrelated to underlying pathology and are considered to be mild hypothermia. In contrast, lower temperatures can cause morbidity and mortality, even in the absence of underlying pathology. Hypothermia causes approximately 600 deaths each year in the United States.1, 2

Patients presenting with accidental hypothermia demonstrate a range of core temperatures and physiologic states. Temperatures of less than 28°C are often associated with myocardial fibrillation. Experience has shown that defibrillation is likely to be ineffective so long as hypothermia persists.³ In most such cases, rapid internal warming, such as that provided by peritoneal dialysis and femorofemoral cardiopulmonary bypass, is required. Both methods are highly effective4, 5 but require considerable technical skills and equipment to institute. Because of these technical difficulties and associated complications, invasive rewarming methods are usually applied with some caution, if at all, to subjects with accidental hypothermia who are hemodynamically stable.

Simple and relatively safe alternatives to invasive methods in nonarrested patients include passive or active external rewarming. A single layer of passive insulation decreases cutaneous heat loss by approximately 30%,⁶ and more layers provide only slight additional benefit.⁷ This modest decrease in heat loss is unlikely to return hypothermic subjects to thermal steady state much less rewarm them. Airway heating and humidification has been proposed as an effective treatment,⁸ although simple thermodynamic calculations⁹ and clinical experience10, 11 indicate that the method is of essentially no value. Body-to-body rewarming is safe but also of limited efficacy.¹² Forced air, which is currently considered the most effective cutaneous warming system,13, 14, 15, 16, 17 is not available for field use because it requires excessive electric power.

A recently developed alternative rewarming method is electrical heating through a carbon fiber–resistive blanket. However, the efficacy and safety of carbon fiber–resistive heating may be restricted by afterdrop (paradoxical core cooling during rewarming),18, 19 although no afterdrop was observed in a previous study in which forced air was used to treat accidental hypothermia in an emergency ward.²⁰ Thermoregulatory vasoconstriction, which decreases transfer of applied heat from peripheral to central tissues, may also reduce apparent efficacy of a warming device.21, 22 Core rewarming rates in subjects with accidental hypothermia thus cannot be predicted from laboratory measurements of heat flux alone. Instead, it is necessary to test heating systems in hypothermic subjects. Fortunately, a recently developed model allows realistic tests of significant hypothermia (sufficient to block shivering) in volunteers who are only mildly hypothermic.²³ Accordingly, we used this model to test the hypothesis that carbon fiber–resistive rewarming increases core temperature faster than passive insulation without provoking excessive afterdrop.