The body shows a rapid and remarkable response to heat stress. As the core temperature rises, afferent receptors stimulate the anterior hypothalamus, causing multiple physiologic responses that result in heat loss. The hypothalamus stimulates the autonomic nervous system, resulting in a decrease in vasomotor tone and increased blood flow to the skin. This can cause a marked increase in cutaneous blood flow, from 0.2 to 0.5 L/min in cool temperature to 7 to 8 L/min in hot temperature. Sweating is stimulated by parasympathetic fibers, with the recruitment of more sweat glands followed by an increase in sweating rate of each gland. Sweating can increase dramatically, from less than 0.5 L/day in a temperate environment to 10 to 15 L/day in a trained individual exercising in a warm environment. Sweating is an efficient means of cooling. Evaporation of 1 L of sweat consumes 600 kcal of heat. For this reason, dehydration can predispose individuals to heat illness by inhibiting their sweating.5 The tremendous peripheral vasodilation results in significant stress on the heart; cardiac output rises approximately 3 L/min for each 1°C (1.8°F) rise in core body temperature.6 Since stroke volume is reduced by peripheral vasodilation, a compensatory rise in heart rate is required in order to maintain cardiac output. Patients with preexisting cardiac disease or on medications that blunt an increased heart rate are at significant risk for heat injury.
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