HYPOTHERMIA DEFINITION Definitions vary, but typically a fall in core body temperature below 35°C (95°F). It may be slight (32 to 35°C), moderate (27 to 32°C), severe (20 to 27°C) or profound (< 20°C). TOXIC CAUSES Alcohols Alpha-blockers Hypoglycaemic agents Nefazoline Opioids Phenothiazines Sedatives NON-TOXIC CAUSES Central nervous system injury Environmental Sepsis CLINICAL FEATURES Clinical signs and symptoms depend on the degree of hypothermia. Progressive CNS depression is observed with decreasing core body temperature. The diagnosis is made by the measurement of a core temperature of less than 35°C (using either a low-reading thermometer or thermocouple). Lethargy usually occurs at temperatures less than 32°C, and lack of response to verbal stimuli at temperatures less than 27°C. In mild to moderate hypothermia, tachycardia, hyperventilation, and shivering are observed but at temperatures below 27°C, these thermoregulatory responses are lost, shivering ceases, and progressive bradycardia and hypoventilation are observed. In profound hypothermia (less than 20°C), the patient may appear to be dead. In severe hypothermia, the myocardium is very sensitive and prone to fibrillation. Arterial blood gases are difficult to interpret, even when corrected for temperature. ECG abnormalities (depression of cardiac conduction, Osborne waves, ST segment and T wave modification), bradycardia, hypotension and relative hypovolaemia are also seen. RELEVANT INVESTIGATIONS Arterial blood gases Clinical chemistry, including serum electrolytes, serum creatinine, blood urea, blood glucose concentration, creatine phosphokinase activity and liver function tests. Chest x-ray Electrocardiograph Toxicology screening TREATMENT Symptomatic and supportive care. Fluids given parenterally should be warmed to between 40 and 42°C. In mild to moderate cases, passive external rewarming (e.g. covering with blankets at room temperature) is sufficient. Active core rewarming is generally reserved for severely hypothermic patients unresponsive to passive external rewarming and/or with unstable cardiac rhythms (ventricular tachycardia or fibrillation, asystole). It may be undertaken by: administration of heated, humidified oxygen; gastric lavage with warm fluids; peritoneal lavage with warmed dialysate; or extracorporeal methods (haemodialysis, cardiopulmonary or femoral-femoral bypass). Standard criteria of brain death do no apply to hypothermic patients and, if required, cardiac resuscitation should be performed until the patient is warm. Standard therapy for ventricular fibrillation is often unsuccessful until hypothermia is corrected. CLINICAL COURSE AND MONITORING Arterial blood gases Core temperature (using low-reading thermometer or thermocouple) Continuous ECG Repeat analytical toxicology when indicated Routine clinical chemistry including serum electrolytes, serum creatinine, blood urea, blood glucose, creatine phosphokinase activity and liver function tests. Continue monitoring until core temperature is greater than 35°C. LONG-TERM COMPLICATIONS Hypoxic organ damage may result in long-term organ dysfunction. AUTHOR(S)/REVIEWERS Author: Dr Albert J. Nantel, Directeur, Centre de Toxicologie du Québec Québec Canada Peer Review: Cardiff 9/96: M. Burger, J. Deng, L. Fruchtengarten, L Lubomirov, T Meredith, H Persson.