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Hypothermia is a core temperature of <35°C (<95°F). Although most commonly seen in cold climates, it may develop without exposure to extreme environmental conditions. Hypothermia is not uncommon in temperate regions and may even develop indoors during the summer. In the U.S., an average of 700 people die of hypothermia each year. Half of those who die of hypothermia are >65 years of age.1

Body temperature may fall due to heat loss by conduction, convection, radiation, or evaporation. Conduction is the transfer of heat by direct contact down a temperature gradient, such as from a warm body to the cold environment. When immersed in water, the body loses heat rapidly, which results in a swift decline in body temperature, because the thermal conductivity of water is approximately 30 times that of air. Convection is the transfer of heat by the actual movement of heated material, such as the wind’s disrupting the layer of warm air surrounding the body. Convective heat loss increases markedly in windy conditions. Heat also may be lost by radiation to the environment (primarily from noninsulated body areas) and by evaporation of water. Evaporation of the water contained in exhaled, water-saturated air occurs over a wide range of ambient temperatures and may be prevented by inhalation of warmed, humidified air.

Opposing the loss of body heat are the mechanisms of heat conservation and gain. In general, these are controlled by the hypothalamus; thus hypothalamic dysfunction may cause impairment in temperature homeostasis. Heat is conserved by peripheral vasoconstriction and, importantly, by behavioral responses. If behavioral responses such as putting on clothing or coming indoors from a cold environment are impaired for any reason (e.g., dementia, drug intoxication, or trauma), the risk of hypothermia is increased.

Heat gain is effected by shivering and by nonshivering thermogenesis. The nonshivering component of heat production consists of an increase in metabolic rate brought about by increased output from the thyroid and adrenal glands.

The most important causes of hypothermia are listed in Table 203-1. Accidental (environmental) hypothermia can be divided into immersion and nonimmersion cold exposure. Exposure to cold environmental conditions may lead to hypothermia even in healthy individuals, especially in wind and rain, and cold swimming water. Inadequate clothing and physical exhaustion contribute to the loss of body heat. The high thermal conductivity of water leads to the rapid development of hypothermia during immersion. The rate of heat loss is determined by water temperature, and immersion in any water colder than 16°C to 21°C (60.8°F to 69.8°F) can lead to severe hypothermia.

Table 203-1 Causes of Hypothermia

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