Heat Loss Through the Glabrous Skin Surfaces of Heavily Insulated, Heat-Stressed Individuals

Insulation reduces heat exchange between a body and the environment. Glabrous (nonhairy) skin surfaces (palms of the hands, soles of the feet, face, and ears) constitute a small percentage of total body surface area but contain specialized vascular structures that facilitate heat loss. We have previously reported that cooling the glabrous skin surfaces is effective in alleviating heat stress and that the application of local subatmospheric pressure enhances the effect. In this paper, we compare the effects of cooling multiple glabrous skin surfaces with and without vacuum on thermal recovery in heavily insulated heat-stressed individuals. Esophageal temperatures $(Tes)$ and heart rates were monitored throughout the trials. Water loss was determined from pre- and post-trial nude weights. Treadmill exercise (5.6 km/h, 9–16% slope, and 25–45 min duration) in a hot environment (⁠$41.5°C$⁠, 20–30% relative humidity) while wearing insulating pants and jackets was used to induce heat stress $(Tes≥39°C)$⁠. For postexercise recovery, the subjects donned additional insulation (a balaclava, winter gloves, and impermeable boot covers) and rested in the hot environment for 60 min. Postexercise cooling treatments included control (no cooling) or the application of a $10°C$ closed water circulating system to (a) the hand(s) with or without application of a local subatmospheric pressure, (b) the face, (c) the feet, or (d) multiple glabrous skin regions. Following exercise induction of heat stress in heavily insulated subjects, the rate of recovery of $Tes$ was $0.4±0.2°C/h(n=12)$⁠, but with application of cooling to one hand, the rate was $0.8±0.3°C/h(n=12)$⁠, and with one hand cooling with subatmospheric pressure, the rate was $1.0±0.2°C/h(n=12)$⁠. Cooling alone yielded two responses, one resembling that of cooling with subatmospheric pressure $(n=8)$ and one resembling that of no cooling $(n=4)$⁠. The effect of treating multiple surfaces was additive (no cooling, $ΔTes=−0.4±0.2°C$⁠; one hand, $−0.9±0.3°C$⁠; face, $−1.0±0.3°C$⁠; two hands, $−1.3±0.1°C$⁠; two feet, $−1.3±0.3°C$⁠; and face, feet, and hands, $−1.6±0.2°C$⁠). Cooling treatments had a similar effect on water loss and final resting heart rate. In heat-stressed resting subjects, cooling the glabrous skin regions was effective in lowering $Tes$⁠. Under this protocol, the application of local subatmospheric pressure did not significantly increase heat transfer per se but, presumably, increased the likelihood of an effect.