Prevention and Treatment of Heat Illness: Guidelines From the Wilderness Medical Society

Heat illness leads to 700 deaths each year in the United States. Heat stroke, the most severe form of heat illness, has a 5% mortality rate, and 16% of survivors have a poor neurologic outcome. In patients presenting with heat illness complicated by hypotension, the mortality rate increases to 33%. The Wilderness Medical Society published updated guidelines for prevention and treatment of heat illness.

Heat stroke is defined as a core temperature greater than 104°F (40°C) with central nervous system dysfunction demonstrated by encephalopathy, seizures, or coma. Exertional heat stroke results from pathologic hyperthermia during strenuous exercise, and nonexertional heat stroke results from passive exposure to high environmental temperatures.

Heat illness has several presentations that are less severe than heat stroke. Heat cramps present with involuntary large muscle contractions that are diffuse and associated with exertion in hot environments. Heat edema is a benign accumulation of fluid in dependent extremities. Heat syncope refers to a mild condition from heat exposure with transient loss of consciousness and rapid return to baseline after rest and rehydration in ambient temperatures. Exercise-associated collapse is the loss of postural tone without loss of consciousness that is not associated with hyperthermia or dehydration. Heat exhaustion is mild to moderate heat illness with symptoms including thirst, weakness, discomfort, and core temperature less than 104°F.

Heat illness occurs when metabolic heat production surpasses heat dissipation, which occurs primarily through the skin. When deficient heat dissipation results in shunting of blood away from renal and gastrointestinal systems, symptoms can include cramping, nausea, and diarrhea. Fluid loss can lead to acute kidney injury. Cooling can be impaired if the skin is kept at an elevated temperature due to environment or clothing. Heat stroke is the result of an exaggerated acute phase inflammatory response due to high temperatures.

Personal Factors

The guidelines recommend screening patients for significant preexisting medical conditions or medications that can predispose them to heat illness, including elevated body mass index. Clinicians should also consider a history of heat injury as a risk factor for recurrence. Acutely sunburned skin impairs the skin's ability to sweat for 7 days (ie, beyond the time of pain and redness).

The guidelines also recommend counseling patients to prevent heat-related illness by acclimatization to heat before prolonged exposure. Three 90-minute sessions of outdoor exercise in 1 week can reduce subsequent heat-related physiologic strain by approximately 20%. About 80% of acclimatization can be gained in 7 days of moderate exertion in heat for 1 to 2 hours per day. Gradual exposure after arriving in a hot environment, using personal protective equipment that increases heat load during training, periodic heat exposure before traveling to a hot environment, and overdressing in a temperate environment are all acceptable methods of acclimatization.