Rarely, severe heat exhaustion after hard work may be complicated by rhabdomyolysis, myoglobinuria, and acute kidney injury. It is distinguished from heatstroke by the absence of brain dysfunction (eg, confusion, ataxia).
Symptoms and Signs of Heat Exhaustion
Signs and symptoms of heat exhaustion are often vague, and patients may not realize that heat is the cause. Initial signs include increased thirst and muscle cramps. 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 fatigued, are usually sweaty and tachycardic, and may have orthostatic hypotension. Mental status is intact, unlike in heatstroke. Temperature is usually normal but, if elevated, usually does not exceed 40° C.
Diagnosis of Heat Exhaustion
Clinical evaluation
Diagnosis of heat exhaustion is clinical and requires exclusion of other possible causes of a patient's symptoms (eg, hypoglycemia, acute coronary syndrome, various infections). Laboratory testing is required only if needed to exclude such disorders. Electrolyte levels should be measured when possible to exclude severe hyponatremia in patients who have had excessive free water intake or sweating.
Treatment of Heat Exhaustion
Rest
Passive cooling (shade, loosen clothing, contact with cool surface)
Oral or IV fluid and electrolyte replacement
Treatment of heat exhaustion involves stopping all exertion and removing patients to a cool environment, having them lie flat (or with legs elevated), and attempting oral rehydration with an isotonic or hypertonic) oral rehydration solution (≥ 10 g or 2 level teaspoons of salt per liter of water) (1, 2). Patients should drink approximately 1 L/hour. 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 minutes of oral rehydration, patients should be transported to an emergency department, where rehydration is usually administered 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/hour is often adequate. Older patients and patients with heart disorders may require lower rates. Rapid external cooling measures for heat exhaustion are usually not required, but in remote situations or if the core temperature is ≥ 40° C, external cooling measures including evaporative, convective, or conductive cooling may be considered. See Heatstroke: Treatment for more details.
Treatment references
1. Eifling KP, Gaudio FG, Dumke C, et al. Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Heat Illness: 2024 Update. Wilderness Environ Med. 2024;35(1_suppl):112S-127S. doi:10.1177/10806032241227924
2. Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers' Association Position Statement: Exertional Heat Illnesses [published correction appears in J Athl Train. 2017 Apr;52(4):401. doi: 10.4085/1062-6050-52.4.07.]. J Athl Train. 2015;50(9):986-1000. doi:10.4085/1062-6050-50.9.07
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; transport patients to an emergency department if these measures are unsuccessful.
