Accidental hypothermia occurs when there is an unintentional drop in the body’s core temperature below 35°C (95°F). At this temperature, many of the compensatory physiologic mechanisms that conserve heat begin to fail. Primary accidental hypothermia is a result of the direct exposure of a previously healthy individual to the cold. The mortality rate is much higher for patients who develop secondary hypothermia as a complication of a serious systemic disorder or injury.
Primary accidental hypothermia is geographically and seasonally pervasive. Although most cases occur in the winter months and in colder climates, this condition is surprisingly common in warmer regions as well. Multiple variables render individuals at the extremes of age—both the elderly and neonates—particularly vulnerable to hypothermia (Table 464-1). The elderly have diminished thermal perception and are more susceptible to immobility, malnutrition, and systemic illnesses that interfere with heat generation or conservation. Dementia, psychiatric illness, and socioeconomic factors often compound these problems. Neonates have high rates of heat loss because of their increased surface-to-mass ratio and their lack of effective shivering and adaptive behavioral responses. At all ages, malnutrition can contribute to heat loss because of diminished subcutaneous fat and as a result of depleted energy stores used for thermogenesis.
TABLE 464-1Risk Factors for Hypothermia ||Download (.pdf) TABLE 464-1 Risk Factors for Hypothermia
Toxicologic and pharmacologic
Spinal cord injury
Hepatic or renal failure
Burns and exfoliative dermatologic disorders
Immobility or debilitation
Individuals whose occupations or hobbies entail extensive exposure to cold weather are at increased risk for hypothermia. Military history is replete with hypothermic tragedies. Hunters, sailors, skiers, and climbers also are at great risk of exposure, whether it involves injury, changes in weather, or lack of preparedness.
Ethanol causes vasodilation (which increases heat loss), reduces thermogenesis and gluconeogenesis, and may impair judgment or lead to obtundation. Some antipsychotics, antidepressants, anxiolytics, benzodiazepines, and other medications reduce centrally mediated vasoconstriction. Many hypothermic patients are admitted to intensive care because of drug overdose. Anesthetics can block shivering responses; these effects are compounded when patients are not insulated adequately in the operating or recovery units.
Several types of endocrine dysfunction cause hypothermia. Hypothyroidism—particularly when extreme, as in myxedema coma—reduces the metabolic rate and impairs thermogenesis and behavioral responses. Adrenal insufficiency and hypopituitarism also increase susceptibility to hypothermia. Hypoglycemia, most commonly caused by insulin or oral hypoglycemic agents, is associated with hypothermia, in part because of neuroglycopenic effects on hypothalamic function. Increased osmolality and metabolic derangements associated with uremia, diabetic ketoacidosis, and lactic acidosis can lead to altered hypothalamic thermoregulation.