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New Findings: What is the central question of this study? Does the cold‐water immersion (14°C) of profoundly hyperthermic individuals induce reductions in cutaneous and limb blood flow of sufficient magnitude to impair heat loss relative to the size of the thermal gradient? What is the main finding and its importance? The temperate‐water cooling (26°C) of profoundly hyperthermic individuals was found to be rapid and reproducible. A vascular mechanism accounted for that outcome, with temperature‐dependent differences in cutaneous and limb blood flows observed during cooling. Decisions relating to cooling strategies must be based upon deep‐body temperature measurements that have response dynamics consistent with the urgency for cooling. 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 study was designed to test both the validity of that observation, and its underlying hypothesis. Eight healthy men participated in four head‐out water immersions: two when normothermic, and two after exercise‐induced, moderate‐to‐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 and 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 only 54 s faster when immersed in cold rather 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 that reduced convective heat delivery to the periphery. [ABSTRACT FROM AUTHOR]

In this experiment, hand and forearm vasomotor activity was investigated during localised, but stable heating and cooling of the face, hand and thigh, under open-loop (clamped) conditions. It was hypothesised that facial stimulation would provoke the most potent vascular changes. Nine individuals participated in two normothermic trials (mean body temperature clamp: 36.6 °C; water-perfused suit and climate chamber) and two mildly hyperthermic trials (37.9 °C). Localised heating (+5 °C) and cooling (−5 °C) stimuli were applied to equal surface areas of the face, hand and thigh (perfusion patches: 15 min), while contralateral forearm or hand blood flows (venous-occlusion plethysmography) were measured (separate trials). Thermal sensation and discomfort votes were recorded before and during each thermal stimulation. When hyperthermic, local heating induced more sensitive vascular responses, with the combined thermosensitivity of both limb segments averaging 0.011 mL·100 mL −1 ·min −1 ·mmHg −1 ·°C −1 , and 0.005 mL·100 mL −1 ·min −1 ·mmHg −1 ·°C −1 during localised cooling ( P <0.05). Inter-site comparisons among the stimulated sites yielded minimal evidence of variations in local thermal sensation, and no differences were observed for vascular conductance ( P >0.05). Therefore, regional differences in vasomotor and sensory sensitivity appeared not to exist. When combined with previous observations of sudomotor sensitivity, it seems that, during mild heating and cooling, regional representations within the somatosensory cortex may not translate into meaningful differences in thermal sensation or the central integration of thermoafferent signals. It was concluded that inter-site variations in the cutaneous thermosensitivity of these thermolytic effectors have minimal physiological significance over the ranges investigated thus far. [ABSTRACT FROM AUTHOR]