Field torso-warming modalities: a comparative study using a human model.

OBJECTIVE: To compare four field-appropriate torso-warming modalities that do not require alternating-current (AC) electrical power, using a human model of nonshivering hypothermia. METHODS: Five subjects, serving as their own controls, were cooled four times in 8 degrees C water for 10-30 minutes. Shivering was inhibited by buspirone (30 mg) taken orally prior to cooling and intravenous (IV) meperidine (1.25 mg/kg) at the end of immersion. Subjects were hoisted out of the water, dried, and insulated and then underwent 120 minutes of one of the following: spontaneous warming only; a charcoal heater on the chest; two flexible hot-water bags (total 4 liters of water at 55 degrees C, replenished every 20 minutes) applied to the chest and upper back; or two chemical heating pads applied to the chest and upper back. Supplemental meperidine (maximum cumulative dose of 3.5 mg/kg) was administered as required to inhibit shivering. RESULTS: The postcooling afterdrop (i.e., the continued decrease in body core temperature during the early period of warming), compared with spontaneous warming (2.2 degrees C), was less for the chemical heating pads (1.5 degrees C) and the hot-water bags (1.6 degrees C, p < 0.05) and was 1.8 degrees C for the charcoal heater. Subsequent core rewarming rates for the hot-water bags (0.7 degree C/h) and the charcoal heater (0.6 degree C/h) tended to be higher than that for the chemical heating pads (0.2 degree C/h) and were significantly higher than that for spontaneous warming rate (0.1 degrees C/h, p < 0.05). CONCLUSION: In subjects with shivering suppressed, greater sources of external heat were effective in attenuating core temperature afterdrop, whereas sustained sources of external heat effectively established core rewarming. Depending on the scenario and available resources, we recommend the use of charcoal heaters, chemical heating pads, or hot-water bags as effective means for treating cold patients in the field or during transport to definitive care.

Thermal effects of whole head submersion in cold water on nonshivering humans.

This study isolated the effect of whole head submersion in cold water, on surface heat loss and body core cooling, when the confounding effect of shivering heat production was pharmacologically eliminated. Eight healthy male subjects were studied in 17 degrees C water under four conditions: the body was either insulated or uninsulated, with the head either above the water or completely submersed in each body-insulation subcondition. Shivering was abolished with buspirone (30 mg) and meperidine (2.5 mg/kg), and subjects breathed compressed air throughout all trials. Over the first 30 min of immersion, exposure of the head increased core cooling both in the body-insulated conditions (head out: 0.47 +/- 0.2 degrees C, head in: 0.77 +/- 0.2 degrees C; P < 0.05) and the body-exposed conditions (head out: 0.84 +/- 0.2 degrees C and head in: 1.17 +/- 0.5 degrees C; P < 0.02). Submersion of the head (7% of the body surface area) in the body-exposed conditions increased total heat loss by only 10%. In both body-exposed and body-insulated conditions, head submersion increased core cooling rate much more (average of 42%) than it increased total heat loss. This may be explained by a redistribution of blood flow in response to stimulation of thermosensitive and/or trigeminal receptors in the scalp, neck and face, where a given amount of heat loss would have a greater cooling effect on a smaller perfused body mass. In 17 degrees C water, the head does not contribute relatively more than the rest of the body to surface heat loss; however, a cold-induced reduction of perfused body mass may allow this small increase in heat loss to cause a relatively larger cooling of the body core.

Thermal effects of dorsal head immersion in cold water on nonshivering humans.

Personal floatation devices maintain either a semirecumbent flotation posture with the head and upper chest out of the water or a horizontal flotation posture with the dorsal head and whole body immersed. The contribution of dorsal head and upper chest immersion to core cooling in cold water was isolated when the confounding effect of shivering heat production was inhibited with meperidine (Demerol, 2.5 mg/kg). Six male volunteers were immersed four times for up to 60 min, or until esophageal temperature = 34 degrees C. An insulated hoodless dry suit or two different personal floatation devices were used to create four conditions: 1) body insulated, head out; 2) body insulated, dorsal head immersed; 3) body exposed, head (and upper chest) out; and 4) body exposed, dorsal head (and upper chest) immersed. When the body was insulated, dorsal head immersion did not affect core cooling rate (1.1 degrees C/h) compared with head-out conditions (0.7 degrees C/h). When the body was exposed, however, the rate of core cooling increased by 40% from 3.6 degrees C/h with the head out to 5.0 degrees C/h with the dorsal head and upper chest immersed (P < 0.01). Heat loss from the dorsal head and upper chest was approximately proportional to the extra surface area that was immersed (approximately 10%). The exaggerated core cooling during dorsal head immersion (40% increase) may result from the extra heat loss affecting a smaller thermal core due to intense thermal stimulation of the body and head and resultant peripheral vasoconstriction. Dorsal head and upper chest immersion in cold water increases the rate of core cooling and decreases potential survival time.

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.