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HYPOTHERMIA

Hypothermia is defined as a core body temperature of ≤35°C and is classified as mild (32.2°–35°C), moderate (28°–32.2°C), or severe (<28°C).

ETIOLOGY

Most cases occur during the winter in cold climates, but hypothermia may occur in mild climates and is usually multifactorial. Heat is generated in most tissues of the body and is lost by radiation, conduction, convection, evaporation, and respiration. Factors that impede heat generation and/or increase heat loss lead to hypothermia (Table 30-1).

TABLE 30-1RISK FACTORS FOR HYPOTHERMIA

CLINICAL FEATURES

Acute cold exposure causes tachycardia, increased cardiac output, peripheral vasoconstriction, and increased peripheral vascular resistance, tachypnea, increased skeletal muscle tone, shivering, and dysarthria. As body temperature drops below 32°C, cardiac conduction becomes impaired, the heart rate slows, and cardiac output decreases. Atrial fibrillation with slow ventricular response is common. Other ECG changes include Osborn (J) waves. Additional manifestations of hypothermia include volume depletion, hypotension, increased blood viscosity (which can lead to thrombosis), coagulopathy, thrombocytopenia, DIC, acid-base disturbances, and bronchospasm. CNS abnormalities are diverse and can include ataxia, amnesia, hallucinations, hyporeflexia, and (in severe hypothermia) an isoelectric EEG. Hypothermia may mask other concurrent disorders, such as an acute abdomen, drug toxicity, or spinal cord injury. Hypothermia in the ICU setting (sepsis, etc.) is a poor prognostic sign.

DIAGNOSIS

Hypothermia is confirmed by measuring the core temperature, preferably at two sites. Since oral thermometers are usually calibrated only as low as 34.4°C, the exact temperature of a pt whose initial reading is <35°C should be determined with a rectal thermocouple probe inserted to ≥15 cm and not adjacent to cold feces. Simultaneously, an esophageal probe should be placed 24 cm below the larynx.

TREATMENT Hypothermia

Cardiac monitoring and supplemental oxygen should be instituted, along with attempts to limit further heat loss. Mild hypothermia is managed by passive external rewarming and insulation. The pt should be placed in a warm environment and covered with blankets to allow endogenous heat production to restore normal body temperature. With the head also covered, the rate of rewarming is usually 0.5°–2.0°C/h. Active rewarming is necessary for moderate to severe hypothermia, cardiovascular instability, age extremes, CNS dysfunction, endocrine insufficiency, or hypothermia due to complications from systemic disorders. Active rewarming may be external (forced-air heating blankets, radiant heat sources, and hot packs) or internal (by inspiration of heated, humidified oxygen warmed to 40°–45°C, by administration of IV fluids warmed to 40°–42°C, or by peritoneal or pleural lavage with dialysate or saline warmed to 40°–45°C). The most efficient active internal rewarming techniques are extracorporeal ...

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