Emergency Medicine · Environmental Emergencies
The facts most likely to be tested
Hypothermia is defined as a core body temperature of less than 35°C (95°F).
The Osborn wave (or J-wave) is a pathognomonic ECG finding characterized by a positive deflection at the J-point.
Passive external rewarming is the first-line treatment for mild hypothermia (32–35°C) in patients with intact thermoregulatory mechanisms.
Active internal rewarming, such as warmed intravenous fluids or thoracic lavage, is indicated for severe hypothermia (below 28°C) or cardiac instability.
Patients with severe hypothermia and cardiac arrest require extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass for rapid core rewarming.
The 'no one is dead until they are warm and dead' principle mandates that resuscitation efforts continue until the patient reaches a core temperature of at least 30–32°C.
Hypothermia-induced cold diuresis occurs due to peripheral vasoconstriction increasing central blood volume, which suppresses antidiuretic hormone (ADH) secretion.
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A 62-year-old homeless male is brought to the emergency department after being found unresponsive in an alleyway during a winter storm. His core body temperature is 27.5°C (81.5°F). On physical examination, he is obtunded, has bradycardia, and his skin is cold and pale. An ECG reveals a sinus rhythm with a prominent J-wave at the junction of the QRS complex and the ST segment. He suddenly develops ventricular fibrillation.
What is the most appropriate next step in the management of this patient?
Extracorporeal rewarming (e.g., cardiopulmonary bypass or ECMO)
This patient has severe hypothermia (<28°C) complicated by cardiac arrest, necessitating active internal rewarming via extracorporeal circulation to restore core temperature and cardiac stability.
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Etiology / Epidemiology
Core body temperature <35°C (95°F). Primary risk factors include extremes of age, alcohol/substance abuse, and homelessness.
Clinical Manifestations
Mild: shivering, tachycardia. Severe: Osborn (J) waves on ECG, loss of shivering below 30°C, and paradoxical undressing.
Diagnosis
Gold standard is esophageal temperature probe. ECG shows Osborn (J) waves and atrial fibrillation.
Treatment
Passive external rewarming for mild cases; active internal rewarming (warmed IV fluids, pleural/peritoneal lavage) for severe cases.
Prognosis
Mortality correlates with severity. Do not pronounce dead until warm and dead; core temp >32°C required for resuscitation efforts.
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Epidemiology & Etiology
Hypothermia occurs via heat loss exceeding production, often due to environmental exposure, sepsis, or hypothyroidism (myxedema coma). Elderly patients are at high risk due to impaired thermoregulation and decreased subcutaneous fat. Alcohol consumption causes peripheral vasodilation, accelerating heat loss.
Pertinent Anatomy
The hypothalamus acts as the body's thermostat, regulating heat production and dissipation. Peripheral thermoreceptors in the skin provide input to the central nervous system to initiate shivering and vasoconstriction.
Pathophysiology
Initial response involves sympathetic activation to increase metabolic rate and shivering. As temperature drops below 32°C, metabolic rate slows, leading to CNS depression and cardiac irritability. Below 30°C, the body loses the ability to shiver, and the risk of ventricular fibrillation increases significantly.
Clinical Manifestations
Presentation follows a continuum: mild (32-35°C) presents with shivering and tachypnea; moderate (28-32°C) shows paradoxical undressing and loss of shivering; severe (<28°C) presents with coma and ventricular fibrillation. Look for Osborn (J) waves on ECG, which are pathognomonic for hypothermic cardiac changes.
Diagnosis
Diagnosis requires a low-reading rectal thermometer or esophageal probe to measure core temperature. Standard thermometers often fail to register below 34°C. ECG is mandatory to monitor for arrhythmias and identify characteristic Osborn (J) waves.
Treatment
Management depends on severity: passive external rewarming for mild cases; active internal rewarming (warmed IV fluids, heated humidified oxygen) for moderate/severe cases. Avoid aggressive movement of the patient to prevent triggering ventricular fibrillation. In cardiac arrest, perform prolonged CPR as patients may be salvageable once warmed.
Prognosis
Complications include rhabdomyolysis, acute kidney injury, and coagulopathy. Patients must be warmed to >32°C before determining if resuscitation is futile. Monitor for rewarming shock due to peripheral vasodilation.
Differential Diagnosis
Sepsis: fever or hypothermia with infectious source
Hypoglycemia: altered mental status with low blood glucose
Myxedema Coma: hypothermia with bradycardia and history of thyroid disease
Wernicke Encephalopathy: hypothermia with ataxia and ophthalmoplegia
Drug Overdose: hypothermia with specific toxidrome findings