. Thermal effects of whole head submersion in cold water on nonshivering humans. J Appl Physiol 101: 669 -675, 2006. First published April 13, 2006; doi:10.1152/japplphysiol.01241.2005This 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°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 Ϯ 0.2°C, head in: 0.77 Ϯ 0.2°C; P Ͻ 0.05) and the body-exposed conditions (head out: 0.84 Ϯ 0.2°C and head in: 1.17 Ϯ 0.5°C; P Ͻ 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°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.hypothermia; heat loss; submersion; perfused body mass; thermal model; symptomless hypothermia; thermal core; cold-water near drowning MANY RECREATIONAL, COMMERCIAL, and military activities involve cold water and the possible development of accidental hypothermia. Several studies (6,11,14,21) have addressed the effect of cold-water immersion on the rate of body core cooling. The initiation and degree of hypothermia are related to many variables, including water temperature, insulation, duration of exposure, and the amount of body surface area (BSA) exposed to the water. The effect of whole head cold-water submersion on core cooling is unknown.One hypothesis predicts a substantial heat loss through the head due to the great amount of surface blood flow in the scalp and because scalp blood vessels do not vasoconstrict in response to cold as do surface vessels in other body areas (8). An alternative hypothesis predicts minimal heat loss from the head because submersion of the head and neck would only involve 7-9% more of the body surface area (20). As well, mathematical modeling predicts minimal conductive heat loss directly through the scalp and skull (27). This topic has important practical implications for conditions where th...

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.