Hypothermia results when the body loses more heat than it produces. In humans, hypothermia is defined as a core body temperature less than 35º C (95º F).
Between 1979 and 1998, hypothermia was the reported cause of death for nearly 14,000 people in the United States. Approximately one-half of those fatalities involved individuals older than 65 years, with a male-to-female ratio of 2.5:1. (Hypothermia-related deaths—Philadelphia, 2001, and United States, 1999. Morb Mortal Wkly Rep 2003;52:86-7)
Hypothermia most commonly occurs during cold weather or following immersion in cold water, but it may occur in other settings.
Causes of Hypothermia
- Environmental exposure: Immersion, extreme cold, prolonged contact with cool surfaces
- Drug-induced: Alcohol, sedatives, phenothiazines, etc.
- Skin damage: Severe psoriasis, burns, etc.
- Metabolic: Hypothyroidism, hypoadrenalism, hypopituitarism
- Neurologic: Head trauma, acute spinal cord injury, stroke, tumor
- Neuromuscular insufficiency: Age extremes, lack of acclimatization, impaired shivering, malnutrition
Stages of Hypothermia and Their Features
- Mild: Core temperature 32.2-35º C (90-95º F). Characterized by shivering; constriction of blood vessels in skin and extremities; increased blood pressure, heart rate, and respiratory rate. If untreated, progresses to apathy, impaired judgment, irritability, difficulty walking, cold diuresis (urination) and dehydration.
- Moderate: Core temperature 28-32.2º C (82.4-90º F). Decreased heart rate, respiratory rate, blood pressure, reflexes, and level of consciousness; hallucinations; dilated pupils; extinction of shivering; atrial dysrhythmias (irregular heartbeat).
- Severe: Core temperature <28º C. Non-reactive pupils; respiration severely depressed or absent; comatose; ventricular dysrhythmias or asystole (irregular or absent pulse); EEG would show limited or absent brain activity.
Treatment of Hypothermia
When obviously lethal injuries exist or if rescuers will be endangered by evacuation efforts, comatose victims may be pronounced dead at the scene. All others conform to an old medical adage: a hypothermia victim is not dead until he or she is warm and dead.
The first priority at all stages of hypothermia is to prevent further heat loss: Remove wet clothing; wrap the victim in blankets; cover the head and extremities. Move to a warm location, if possible.
Victims of mild or early moderate hypothermia (still shivering, relatively alert) usually respond to passive warming: blankets, warm fluids to drink, and a warm environment—including sharing a sleeping bag with another individual.
When hypothermia is more profound, management is more complicated--particularly in the field.
For victims in cardiac arrest, life support should be initiated, but pulses may not be palpable even when the heart is still beating. The American Heart Association recommends palpating for pulses for at least 30-45 seconds before beginning CPR.
Most cardiac dysrhythmias will correct with warming of the victim. However, ventricular fibrillation—when it can be diagnosed—should be defibrillated with an AED or hospital-type defibrillator. Only one or two attempts should be made, however, until the victim is rewarmed.
Active external re-warming (application of heating pads or water bottles) is only effective in the presence of intact circulation and must be administered carefully to avoid burns.
The most effective treatment—active core rewarming—must be administered by trained medical personnel who can manage the complications (core temperature after drop, acidosis, etc.). Transport to a hospital should be performed as expeditiously and gently as possible.
Life support measures should be continued until a victim is warmed and stable, or until the core temperature is above 32º C (89.6º F) and no signs of life are present. (McCullough L, Arora S. Diagnosis and treatment of hypothermia. Am Fam Phys 2004;70(12):2325-32)
Many cases of hypothermia can be prevented by preparing and dressing sensibly for outdoor excursions, updating the homes of the elderly (weather stripping, insulated doors, etc.), and creating winter survival kits for cars and homes.
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