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Heat Exhaustion

by John Lissoway, MD, Eric A. Weiss, MD

Heat exhaustion is a non–life-threatening clinical syndrome of weakness, malaise, nausea, syncope, and other nonspecific symptoms caused by heat exposure. Thermoregulation and CNS function are not impaired, but patients are usually dehydrated and may have mild elevations of body temperature (< 40° C). Treatment involves rest in a cool environment and replacing fluids and electrolytes.

Rarely, severe heat exhaustion after hard work may be complicated by rhabdomyolysis, myoglobinuria, and acute kidney injury. It is distinguished from heat stroke by the absence of brain dysfunction (eg, confusion, ataxia).

Symptoms and Signs

Symptoms are often vague, and patients may not realize that heat is the cause. Symptoms may include malaise, weakness, dizziness, headache, nausea, and sometimes vomiting. Syncope due to standing for long periods in the heat (heat syncope) may occur. On examination, patients appear tired, are usually sweaty and tachycardic, and may have orthostatic hypotension. Mental status is intact, unlike in heatstroke. Temperature is usually normal and, when elevated, usually does not exceed 40° C.

Diagnosis

  • Clinical evaluation

Diagnosis is clinical and requires exclusion of other possible causes (eg, hypoglycemia, acute coronary syndrome, various infections). Laboratory testing is required only if needed to rule out such disorders. Electrolyte levels should be measured to exclude severe hyponatremia in patients who have had excessive free water intake, particularly if they develop findings of brain dysfunction

Treatment

  • Oral or IV fluid and electrolyte replacement

Treatment involves stopping all exertion and removing patients to a cool environment, having them lie flat, and attempting oral rehydration with a solution of 0.1% NaCl. Patients should drink about 1L/h. If vomiting or nausea prevents oral rehydration, IV fluid and electrolyte replacement therapy, typically using 0.9% saline solution, is indicated. Also, if symptoms do not resolve after 30 to 60 min, patients should be transported to an emergency department, where rehydration is usually done IV. Rate and volume of IV rehydration are guided by age, underlying disorders, and clinical response. Replacement of 1 to 2 L at 500 mL/h is often adequate. Elderly patients and patients with heart disorders may require lower rates. External cooling measures (see Heatstroke : Treatment) are usually not required. However, if patients with heat exhaustion have a core temperature of 40° C, measures may be taken to reduce it.

Key Points

  • In heat exhaustion, symptoms tend to be nonspecific, temperature is usually < 40°C, and CNS function is not impaired.

  • Diagnose heat exhaustion clinically, testing as indicated to exclude other clinically suspected disorders.

  • Have patients rest in a cool environment and try oral rehydration, transporting patients to an emergency department if these measures are unsuccessful.

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