"Accidental" hypothermia may be divided into immersion and nonimmersion cold exposure. Exposure to cold environmental conditions may lead to hypothermia even in healthy subjects, especially in wind and rain. Inadequate clothing and physical exhaustion contribute to the loss of body heat. The high thermal conductivity of water leads to the rapid development of immersion hypothermia. Though the rate of heat loss is determined by water temperature, immersion in any water less than 16 to 21°C (60.8-69.8°F) may lead to hypothermia.
Metabolic causes of hypothermia include various hypoendocrine states (hypothyroidism, hypoadrenalism, hypopituitarism), which lead to a decrease in metabolic rate. Hypoglycemia also may lead to hypothermia; the probable mechanism is hypothalamic dysfunction secondary to glucopenia.
Other causes of hypothalamic and CNS dysfunction (e.g., head trauma, tumor, stroke) may interfere with mechanisms of temperature regulation. Wernicke's disease may involve the hypothalamus; this is a rare but important cause of hypothermia, since it is potentially reversible with parenteral thiamine.
In the United States, the majority of hypothermic patients are intoxicated with ethanol or other drugs. Ethanol is a vasodilator, and because of its anesthetic and CNS depressant effects, intoxicated subjects neither feel the cold nor respond to it appropriately. Other drugs commonly implicated in the development of hypothermia include sedative-hypnotics, phenothiazines, and occasionally insulin.
Sepsis may alter the hypothalamic temperature set point and is a well-known cause of hypothermia. Subnormal body temperature is a poor prognostic factor in patients with bacteremia.2
Severe dermal disease may impair the skin's thermoregulatory functions. Significant burns or severe exfoliative dermatitis may prevent cutaneous vasoconstriction and increase transcutaneous water loss, predisposing to the development of hypothermia.
Hypothermia may develop in anyone with an acute incapacitating illness. Thus patients with severe infections, diabetic ketoacidosis, immobilizing injuries, and various other conditions may have impaired thermoregulatory function, including altered behavioral responses.
Hypothermia also may be induced by resuscitation with room-temperature fluid or cold blood. This is a particular risk in patients undergoing massive volume replacement, such as trauma patients.
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