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
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.