47 HYPOTHERMIA AND FROSTBITE
Harrison’s Manual of Medicine
HYPOTHERMIA AND FROSTBITE
Hypothermia is defined as a core body temperature of £35°C, and is classified as mild (32–35°C), moderate (28–32°C), or severe (<28°C).
ETIOLOGY Most cases occur during the winter in cold climates, but hypothermia may occur in mild climates at any season and is usually multifactorial. Heat is generated in most tissues of the body and is lost by radiation, evaporation, respiration, conduction, and convection. Factors that impede heat generation and/or increase heat loss lead to hypothermia (Table 47-1).
Table 47-1 Risks Factors for Hypothermia
CLINICAL FEATURES Acute cold exposure causes tachycardia, increased cardiac output, peripheral vasoconstriction, and increased peripheral vascular resistance. 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, delayed deep tendon reflexes, and (in severe hypothermia) an isoelectric EEG.
DIAGNOSIS Hypothermia is confirmed by measuring the core body temperature, preferably at two sites. Since oral thermometers are usually calibrated only as low as 34.4°C, the exact temperature of a patient whose initial reading is <35°C should be determined with a thermometer reading down to 15°C or, ideally, with a rectal thermocouple probe inserted to ³15 cm.
Cardiac monitoring 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. Moderate to severe hypothermia requires active rewarming, which may be external (by application of heat sources such as heating blankets or immersion in warm water at 44–45°C) or internal (by inspiration of heated, humidified oxygen; 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 rewarming by hemodialysis and cardiopulmonary bypass. External rewarming may cause a fall in blood pressure by relieving peripheral vasoconstriction. Volume should be repleted with warmed isotonic solutions; lactated Ringer’s solution should be avoided because of impaired lactate metabolism in hypothermia. If sepsis is a possibility, empirical broad-spectrum antibiotics should be administered after sending blood cultures. Atrial arrhythmias usually require no specific treatment. Ventricular fibrillation is often refractory, and bretylium tosylate (10 mg/kg) is the drug of choice for its treatment. Only a single sequence of 3 defibrillation attempts (2 J/kg) should be attempted when the temperature is <30°C. Since it is sometimes difficult to distinguish profound hypothermia from death, cardiopulmonary resuscitation efforts and active internal rewarming should continue until the core temperature is >32°C or cardiovascular status has been stabilized.
Frostbite occurs when the tissue temperature drops below 0°C. Clinically, it is most practical to classify frostbite as superficial (without tissue loss) or deep (with tissue loss). Classically, frostbite is retrospectively graded like a burn (first- to fourth-degree) once the resultant pathology is demarcated over time.
CLINICAL FEATURES The initial presentation of frostbite can be deceptively benign. The symptoms always include a sensory deficit affecting light touch, pain, and temperature perception. Deep frostbitten tissue can appear waxy, mottled, yellow, or violaceous-white. Favorable presenting signs include some warmth or sensation with normal color.
A treatment protocol for frostbite is summarized in Table 47-2. Frozen tissue should be rapidly and completely thawed by immersion in circulating water at 37–40°C. Thawing should not be terminated prematurely due to pain from reperfusion; ibuprofen 400 mg [12 (mg/kg)/day] q8–12h should be given, and parenteral narcotics are often required. If cyanosis persists after rewarming, the tissue compartment pressures should be monitored carefully.
Table 47-2 Treatment for Frostbite
For a more detailed discussion, see Danzl DF: Hypothermia and Frostbite, Chap. 20, p. 107, in HPIM-15.