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Hyperthermia is a state in which body core temperature rises as a result of heat generation and absorption exceeding heat loss. Given that body temperature reflects the balance between heat gain and heat loss, hyperthermia is found most frequently under conditions such as exercise in which heat production is increased or in subjects such as infants or patients with dehydration who lack the ability to increase their heat loss in response to environmental heat. Thus individuals who engage in athletic activities and infants are at particular risk for heat-related illness. Heat stress has been recognized as a cause of illness for more than 2000 years. It is responsible for increases in population death rates during hot times of the year in areas exposed to particularly high environmental temperatures,1 and increases morbidity related to other diseases. Heatstroke, the most serious form of heat-related injury, is the second most common cause of athletics-related death after head injury.

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It is important to differentiate fever, where the body temperature set point is elevated as a result of the influence of pyrogenic cytokines (see Chapters 198 and 227), from hyperthermia, where the temperature is elevated above normal because the heat-dissipating homeostatic mechanisms activated at temperatures above the set point are overwhelmed. In febrile patients, homeostatic mechanisms including vasoconstriction and shivering are activated to raise body temperature by increasing heat production and decreasing heat dissipation until a new set point is reached whereas in hyperthermic patients, homeostatic mechanisms attempt to lower body temperature.

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Metabolic activity leads to heat production. If no heat were lost to the environment, resting heat production would raise body temperature by approximately 1°C/hr. This increase would be even greater during exercise, when heat production may increase as much as 4-fold. Heat may also be absorbed from the environment by radiation from the sun or the ground, or by convection when the air temperature exceeds body temperature.

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Heat production and absorption are balanced by heat loss via four mechanisms. Conduction carries heat between a body and a contacting surface along a temperature gradient. This is the primary mechanism of cooling during ice or water immersion. Convection transfers heat from the body surface to or from a gas or fluid circulating around the body. With conduction, heat transfer stops when the contacting surface temperature reaches body temperature, whereas with convection, circulation of fresh gas or fluid around the body preserves the temperature gradient between the body and the circulating gas or fluid thus increasing the efficiency of heat transfer. Radiation transfers heat from a warmer to a colder body via electromagnetic waves. Evaporation removes heat by promoting a phase transition from liquid to a gas. Of these four mechanisms, radiation is the principal mechanism of heat elimination in temperate environments. Heat loss by convection and radiation are increased by cutaneous vasodilation, a neurally mediated response to hyperthermia. As either heat production or environmental temperature increases, evaporation of sweat becomes the ...

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