A vascular mechanism to explain thermally mediated variations in deep-body cooling rates during the immersion of profoundly hyperthermic individuals

Abstract

Physiologically trivial time differences for cooling the intrathoracic viscera of hyperthermic individuals have been reported between cold- and temperate-water immersion treatments. One explanation for that observation is reduced convective heat delivery to the skin during cold immersion, and this experiment was designed to test both the validity of that observation, and its underlying hypothesis. Eight healthy males participated in four head-out, water immersions: two when normothermic, and two following exercise-induced, moderate-profound hyperthermia. Two water temperatures were used within each thermal state: temperate (26°C) and cold (14°C). Tissue temperatures were measured at three deep-body sites (oesophagus, auditory canal, rectum) and eight skin surfaces, with cutaneous vascular responses simultaneously evaluated from both forearms (laser-Doppler flowmetry and venous-occlusion plethysmography). During the cold immersion of normothermic individuals, oesophageal temperature decreased relative to baseline (−0.31°C over 20 min; P < 0.05), whilst rectal temperature increased (0.20°C; P < 0.05). When rendered hyperthermic, oesophageal (−0.75°C) and rectal temperatures decreased (−0.05°C) during the transition period (< 8.5 min, mostly in air at 22°C), with the former dropping to 37.5°C just 54 s faster when immersed in cold, than in temperate water (P < 0.05). Minimal cutaneous vasoconstriction occurred during either normothermic immersion, whereas pronounced constriction was evident during both immersions when subjects were hyperthermic, with the colder water eliciting a greater vascular response (P < 0.05). It was concluded that the rapid intrathoracic cooling of asymptomatic, hyperthermic individuals in temperate water was a reproducible phenomenon, with slower than expected cooling in cold water brought about by stronger cutaneous vasoconstriction reducing convective heat delivery to the periphery.

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