CHAPTER 131

#### KEY POINTS

• Accidental hypothermia results from the unintentional decrease in core body temperature to lower than 35°C (95°F) and can be classified as mild (32.2-35°C, or 90-95°F), moderate (28-32.2°C, 82-90°F), or severe (<28°C, 82°F).

• Although hypothermia from environmental exposure is common, several medical conditions may also predispose to hypothermia necessitating hospitalization and admission to the intensive care unit.

• Individuals at highest risk for hypothermia include the homeless, the mentally ill, trauma victims, outdoor workers, those at the extremes of age, those with serious underlying medical conditions, and those with ethanol or drug intoxication.

• Multiple organ systems are affected by hypothermia: clinical manifestations depend on the underlying cause and core body temperature. Below 30°C (86°F), shivering ceases, level of consciousness progressively declines, and cardiac arrhythmias become more common.

• In the initial stages, wet clothing should be removed promptly, continued heat loss must be prevented, and any underlying illness should be identified and treated.

• Rewarming is the primary treatment for moderate to severe hypothermia. For patients with a core body temperature above 32°C (90°F), passive and active external rewarming and supportive therapies are preferred; for patients with lower temperatures and those with hemodynamic instability, active core rewarming using body cavity lavage or extracorporeal blood warming may be considered.

• In addition to rewarming, all patients with hypothermia need continuous monitoring of cardiac status, intensive fluid resuscitation, and circulatory support.

Hypothermia is defined as a core body temperature lower than 35°C (<95°F). The severity of hypothermia is indicated by the degree to which the core body temperature is lowered and is classified as mild, moderate, or severe. Several medical conditions may increase the risk of hypothermia. Individuals at the extremes of age are at greatest risk. Risk of death from hypothermia is related to age, preexisting illnesses, nutritional status, and alcohol and drug intoxication.1 In cases of severe hypothermia, prompt intervention with rapid rewarming is crucial and may be life saving. This chapter discusses the pathophysiology, risk factors, clinical diagnosis, and management of hypothermia. The use of hypothermia as a treatment, such as after cardiac arrest, is covered in Chap. 26.

#### REGULATION OF BODY TEMPERATURE

Body temperature is closely regulated through a balance between heat production and heat dissipation.2 The majority of endogenous heat production results from metabolic activity in the heart and liver. The skin accounts for 90% of heat loss and the lungs contribute the rest. Radiation cooling (heat loss in the form of infrared radiation) is the primary method of thermal load dissipation and accounts for approximately 60% of heat loss. Conduction (direct transfer of heat to a cooler object) and convection (heat removal by air currents) account for 10% to 15% of heat loss while evaporation from skin and the respiratory tract accounts for 25% to 30% of heat loss. Conduction is an important mechanism of heat loss in immersion accidents because thermal conduction of water is approximately 30 times that of air. Convection is important in windy conditions by removing the warm insulating layer of air around the body. The preoptic nucleus of the anterior hypothalamus is the thermal control center, which maintains body temperature at a given set value. In response to a decrease in core body temperature, the hypothalamus initiates mechanisms to conserve heat by cutaneous vasoconstriction and produce heat ...

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