Effectiveness of three field treatments for induced mild (33.0 degrees C) hypothermia

Abstract

Three field applicable treatments for hypothermia were compared. Subjects were cooled in stirred cold water (8.0 degrees C) to a core temperature (Tco) as low as 33 degrees C and rewarmed in a random order by each of three techniques: shivering, external heat, and treadmill exercise. Tco was monitored with an esophageal thermistor probe at the level of the heart. Treatment effectiveness was determined by calculating the amount of Tco afterdrop, length of afterdrop period, rate of Tco increase, and total recovery time. Rate of Tco increase for exercise (4.9 degrees C/h) was significantly higher (P less than 0.05) than shivering (3.5 degrees C/h) but not external heat (3.7 degrees C/h). Exercise afterdrop amount and afterdrop length values (0.95 degrees C and 24 min, respectively) were significantly higher (P less than 0.05) than both shivering (0.33 degrees C, 15 min) and external heat (0.32 degrees C, 14 min). Therefore, although rate of Tco increase during recovery for exercise was faster than for shivering or external heat, as it was preceded by a greater afterdrop length and amount, total recovery time did not differ among the three treatments.

Comparison of Heat Donation Through the Head or Torso on Mild Hypothermia Rewarming

“…These rewarming methods have been studied extensively in mildly-hypothermic, vigorously shivering individuals (25,33). These methods increase the T skin , reduce the integrated thermal signal for shivering, and thereby suppress the shivering heat production.…”

“…Shivering can prevent (43) or attenuate (64) the rate of core cooling in a mildly hypothermic, vigorously shivering individual. If well insulated from the external environment, this individual can rewarm at a significantly high rate of 3-4˚C/hr by shivering thermogenesis only (25). However, shivering is inhibited in case of a severely hypothermic victim, resulting in a greater T c afterdrop and a reduced rate of rewarming (26).…”

“…Vigorous shivering prevents (43) or attenuates (64) the rate of core cooling during cold exposure. As well, a mildly hypothermic, vigorously shivering individual rewarms at a rate of 3-4˚C/hr (25). Inhibition of shivering results in a greater post-cooling fall in the core temperature (T c; afterdrop) and a reduction in the rate of rewarming (26).…”

“…As shivering heat production can effectively rewarm a mildly hypothermic individual, application of external heat is not necessary but may still be beneficial as it helps conserve energy stores and decreases work of the heart. If there is no external heat source available, then shivering should be maximized by drying the patient and providing a vapour barrier and maximal insulation (25). However, external heat application is necessary when the shivering response is absent, as in the case of severe hypothermia (24).…”

“…Therefore, it is likely that the amount of heat donated by these external heat sources is approximately similar to the amount of shivering heat production that is inhibited (25,29,30).…”