Accidental hypothermia is defined as a core body temperature less than 35° C in which the body’s temperature regulation mechanism is overwhelmed by a cold exposure.1  Primary hypothermia is failure of heat production in an otherwise healthy person in the face of environmental exposure where there is no underlying disorder causing temperature regulation disruption.

Each year in the United States, approximately 700 people die from accidental primary hypothermia, and about 1500 individuals have hypothermia noted on their death certificate.2 Even with modern supportive care, the in-hospital mortality of patients with moderate or severe accidental hypothermia approaches 40%.3 However, with many patients having recovered from severe hypothermia, early recognition and prompt treatment is paramount.

Epidemiology, Etiology, and Risk Factors for Hypothermia

Accidental hypothermia is the most common form of hypothermia encountered in the emergency department.4 Emergency room encounters for hypothermia are increasing as more individuals search for outdoor activities or engage in winter sports.

While incidence of accidental hypothermia may commonly occur in regions of the country with severe winter weather, milder climates that experience rapid temperature changes due to seasonal changes, large shifts in nighttime temperatures, or those at higher altitudes also have reported cases of hypothermia.5

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States with the highest overall death rates for hypothermia are Alaska, New Mexico, North Dakota, and Montana.1,4 Cold-related deaths are substantially higher in rural counties rather than urban areas.5 In urban setting, societal factors (homelessness) and pre-existing conditions (alcoholism, drug abuse, and mental illness) contribute to a steady stream of hypothermia cases to inner-city hospitals.1

When diagnosing hypothermia, clinicians must also consider underlying etiologies of hypothermia in which the patient has a medical illness or other factor that impairs temperature regulation or predisposes them to heat loss.  Medical conditions that cause secondary hypothermia may occur alone or simultaneously with accidental hypothermia. Alcohol is a vasodilator and a diuretic, for example, and alcohol intoxication promotes peripheral heat loss that increases risk of hypothermia. Endocrine derangements should be considered in patients presenting with unexplained hypothermia who fail to rewarm with standard therapy.4

Certain medications may directly or indirectly cause hypothermia, either by impairing thermoregulatory mechanisms, decreasing awareness of cold, or clouding judgment. The most common medications that impair thermoregulation are anxiolytics, antidepressants, antipsychotics, and opioids; while those that can impair a patient’s ability to compensate for a low ambient temperature include oral antihyperglycemics, beta-blockers, alpha-adrenergic agonists, ethanol, sedative-hypnotics, and general anesthetic agents.1,4

Elderly individuals and infants are more susceptible to hypothermia. Half of recorded deaths from accidental hypothermia occurred in individuals older than 65 years, and the rate of cold-related deaths for persons aged 75 years and over was substantially higher than that of younger individuals; 15.5 deaths per million among persons aged 75 to 84 years and 39.6 deaths per million among persons aged 85 years and older.4,5 Infants (particularly preterm) are also at increased risk of heat loss due to scant subcutaneous and brown fat and muscular inactivity.6

Staging of Accidental Hypothermia

Effective diagnosis and management of hypothermia depend upon the use of a low-reading thermometer to determine core temperature; many standard thermometers read only to a minimum of 34° C (93° F) and are therefore unsuitable.1 Pulmonary artery temperature is the gold standard for central body temperature but is invasive and may precipitate arrhythmia.6 In the emergency department, core temperature is best measured using a low-reading temperature probe in the bladder or rectum, or an esophageal probe.

In patients with severe hypothermia, particularly those requiring endotracheal intubation, an esophageal probe inserted into the lower one-third of the esophagus (about 24 cm below the larynx) is preferred and provides a near approximation of cardiac temperature. However, measurements obtained with the use of a proximal esophageal probe may be falsely elevated due to ventilation with warmed gases.2 Rectal and bladder temperatures may be monitored in mild to moderate hypothermia in conscious patients but should not be used in critical patients. Rectal readings may rise following peritoneal lavage or fall if adjacent to or lodged in cold feces; rectal and bladder temperatures will significantly lag behind the core temperature during rewarming.1

The International Commission for Mountain Emergency Medicine developed a clinical staging system for accidental hypothermia intended to help rescuers in the field estimate severity of hypothermia by observing clinical signs if core body temperature cannot be readily measured.7 While the response to hypothermia varies widely among patients, the system is helpful for understanding the physiologic effects of hypothermia on the body and the expected clinical manifestations per stage (Table 1).2,6 Discrepancy between core body temperature and hypothermia stage is well documented and in many cases the Swiss system overestimates core temperature.6

TABLE 1. Swiss Staging System2

 Swiss System
Mild hypothermia Core temperature 32-35° CMild (HT I) – Normal mental status with shivering Estimated core temperature 32-35° C
Moderate hypothermia Core temperature 28-32° CModerate (HT II) – Altered mental status with or without shivering Estimated core temperature 28-32° C
Severe hypothermia Core temperature <28º CSevere (HT III) – Unconscious; vital signs present Estimated core temperature 24-28° C
Profound (HT IV) – Apparent death; vital signs absent Core temperature 13.7-24° C (resuscitation may be possible)
Death (HT V) – Death due to irreversible hypothermia Core temperature less than 9-13.7° C (resuscitation not possible)