Pre-hospital torso-warming modalities for severe hypothermia: a comparative study using a human model.

OBJECTIVE: To compare 5 active torso-warming modalities in a human model of severe hypothermia with shivering heat production inhibited by intravenous meperidine.

METHODS: Six subjects were cooled on 6 different occasions each, in 8 degrees C water, for 30 minutes or to a core temperature of 35 degrees C. Spontaneous warming was the first torso-warming modality to be tested for every subject, and results served both as a comparative control and for determination of the meperidine dose for subsequent trials. Meperidine (1.5 mg/kg) was administered during the final 10 minutes of immersion to suppress shivering. Subjects were removed from the water, dried and insulated for 30 minutes, followed by 120 minutes of 1) forced-air warming with either a 600-W heater and commercial soft warming blanket; or 2) a 600-W heater and rigid cover; or 3) an 850-W heater and rigid cover; or 4) a charcoal heater on the chest; or 5) direct body-to-body contact with a normothermic partner. Supplemental meperidine (to a maximum cumulative dose of 3.2 mg/kg) was administered as required to inhibit shivering.

RESULTS: The initial post-cooling afterdrop was approximately 1.0 degrees C. After 30 minutes, core temperature continued to drop by 0.45 degrees C in spontaneous and body-to-body warming modalities. This post-warming afterdrop was significantly less with 600-W heater and rigid cover and the charcoal heater (0.26 degrees C) and the least with 850-W heater and rigid cover (0.17 degrees C). Core rewarming rates were highest using 850-W heater and rigid cover (1.45 degrees C/hr), with charcoal heating and 600-W rigid heater (0.7 degrees C/hr), 600-W heater and blanket (0.57 degrees C/hr) and body-to-body warming (0.52 degrees C/hr) being more effective than spontaneous warming (0.36 degrees C/hr).

CONCLUSIONS: In non-shivering subjects, external heat application was effective in attenuating core temperature afterdrop and facilitating safe core rewarming; this was more evident when heat was delivered preferentially to the chest, and dependent upon the amount of heat donated. The modalities studied appear sufficiently practical and portable for pre-hospital use and should be considered for such situations, particularly in rural or wilderness locations where anticipated transport time to the hospital exceeds 30 minutes.