Hypothermia in the adult is defined by a core body temperature below 35°C (95°F). It is considered a medical emergency and is usually accidental in adults. Acute hypothermia occurs due to exposure to cold environments and impaired thermoregulation due to alcohol, drugs, trauma, extremes of age, or other comorbidities. Chronic hypothermia occurs with certain chronic illnesses, in elderly patients, or with prolonged exposure to cold weather.

The hypothalamus controls the body's response to cold by stimulating heat production through mechanisms such as shivering, increased ventilation, catecholamine release, and increased cardiac output. Once body temperature gets to 32°C, neurologic function is affected, as is the body's ability to combat cooling. Elderly patients are more prone to developing hypothermia due to comorbidities, isolation, and medications that can inhibit response to cold stress. Other risk factors include being undomiciled, having alcohol use disorder, and psychiatric illness. Regions with severe winters have increased incidence of hypothermia, but it can still occur in milder climates or in the summer. Hypothermia can be characterized as mild (35°C-32°C), moderate (32°C-28°C), or severe (below 28°C). Clinical manifestations of hypothermia vary according to severity and differ across patients. The Swiss model stages hypothermia based on physical examination outside of the hospital setting.

Swiss staging system:

• HT I – Normal mentation and shivering; generally with core temperature 32°C-35°C
• HT II – Impaired consciousness without shivering; generally with core temperature 28°C-32°C
• HT III – Unconscious; generally with core temperature 24°C-28°C
• HT IV – Not breathing; generally with core temperature 13.7°C-24°C
• HT V – Death due to irreversible hypothermia; generally with core temperature < 9°C-13.7°C

In mild cases, presentation includes pallor, shivering, tachycardia, and tachypnea, but mental status is preserved. Moderate cases can present with lethargy, hallucinations, bradycardia, arrhythmias, hypoventilation, and muscle rigidity with decreased shivering. Severe cases present with coma, hypotension, arrhythmias, apnea, and pseudo-rigor mortis. Laboratory derangements such as hyper- or hypoglycemia, thrombocytopenia, leukopenia, elevated creatine phosphokinase (CPK) secondary to rhabdomyolysis, and abnormal liver function tests may also be present.

The key to management is timely diagnosis and initial support of the patient's airway, breathing, and circulation (ABC). Patients should be removed from the hypothermic environment, and rewarming should be initiated as soon as possible. Cardiopulmonary resuscitation should be started immediately in patients with cardiopulmonary arrest. Rewarming can take several hours with the goal body temperature of 32°C-35°C.