ESSENTIALS OF DIAGNOSIS
Spectrum of preventable heat-related illnesses: heat cramps, heat exhaustion, heat syncope, and heat stroke.
Heat stroke: hyperthermia with cerebral dysfunction in a patient with heat exposure.
Best outcome: early recognition, initiation of rapid cooling, and avoidance of shivering during cooling.
Best choice of cooling method: whichever can be instituted the fastest with the least compromise to the patient. Delays in cooling result in higher morbidity and mortality in heat stroke victims.
Heat-related illnesses are among the most commonly seen environmental emergencies presenting to emergency departments. The amount of heat retained in the body is determined by internal metabolic function and environmental conditions, including temperature and humidity. Hyperthermia results from the body’s inability to maintain normal internal temperature through heat loss. Hyperthermia results from either compromised heat dissipation mechanisms or abnormally high heat production. Heat loss occurs primarily through sweating and peripheral vasodilation. The direct transfer of heat from the skin to the surrounding air, by convection or conduction, occurs with diminishing efficiency as ambient temperature rises, especially above 37.2°C, at which point heat transfer reverses direction. At normal temperatures, evaporation accounts for approximately 20% of the body’s heat loss, but at high temperatures it becomes the major mechanism for dissipation of heat. With vigorous exertion, sweat loss can be as much as 2.5 L/h. This mechanism diminishes as humidity rises.
Heat stress can be caused by a combination of environmental heat and metabolic heat. For children under the age of 15, heat stroke is the largest cause of non-accident–related deaths in cars. This is despite substantial campaigns and preventive devices to alert drivers to check the back seat when turning off the car. Climate change may significantly contribute to the risk of heat-related conditions.
There is a spectrum of preventable heat stress conditions, ranging from mild forms, such as heat cramps, to severe forms, such as heat stroke. Risk factors include longer duration of exertion, hot environment, insufficient acclimatization, and dehydration. Additional risk factors include skin disorders or other medical conditions that inhibit sweat production or evaporation, obesity, prolonged seizures, hypotension, reduced cutaneous blood flow, (eg, vasoconstrictor drugs, beta-adrenergic blocking agents, dehydration), reduced cardiac output, the use of drugs that increase metabolism or muscle activity or impair sweating, and withdrawal syndromes. Medications that impair sweating include anticholinergics, antihistamines, phenothiazines, tricyclic antidepressants, monoamine oxidase inhibitors, and diuretics. Illicit drugs, including stimulants and some hallucinogens, and antipsychotic agents, can cause increased muscle activity and thus generate increased body heat.
Classic (nonexertional) heat-related illness may occur in any individual in a hot, relaxing environment (eg, hot bath, steam room, sunbathing, or sauna) with increased severity in individuals with the risk factors mentioned above, even despite minimal physical activity.
Heat cramping results from dilutional hyponatremia as sweat losses are replaced with water alone or sometimes from high utilization of aldosterone leading to potassium wasting. Heat exhaustion...