Heat Exhaustion

Heat exhaustion is a heat illness with a core temperature up to 40 degrees Celsius, caused by water or salt depletion, producing weakness, nausea and collapse without significant central nervous system failure. It is far more common than heat stroke. It occurs in workers, athletes, military personnel and pilgrims, and in elderly people during heat waves.

Two overlapping patterns

• Water depletion: inadequate fluid intake. It leads to thirst, hypernatremia and hemoconcentration.
• Salt depletion: heavy sweating replaced by water alone. It leads to hyponatremia, muscle cramps (heat cramps), nausea and headache.

Mechanisms are volume contraction, impaired cardiovascular compensation and peripheral vasodilation. Predisposing factors: lack of acclimatization, high humidity, heavy clothing, diuretics, ACE inhibitors, alcohol, poor fitness, illness and age extremes. Untreated or with continued exposure, it progresses to heat stroke. Most patients recover within hours. Complications: syncope, injury from falls, hyponatremia, rhabdomyolysis and progression to heat stroke.

• Headache, dizziness, weakness, fatigue, nausea, vomiting and muscle cramps
• Heavy sweating, pallor, tachycardia, orthostatic hypotension and syncope
• Core temperature 37 to 40 degrees Celsius
• Mental state normal or only mildly irritable (any confusion, ataxia or seizure means heat stroke)
• Red flags: temperature over 40 degrees Celsius, altered consciousness, persistent vomiting, no improvement after 30 to 60 minutes of treatment, and dark urine

1. Core temperature (rectal) and mental state assessment, the key step to separate heat exhaustion from heat stroke.
2. Vital signs with orthostatic blood pressure and heart rate.
3. Blood tests: sodium, potassium, urea, creatinine, glucose and creatine kinase, with a urinalysis for specific gravity and myoglobin.
4. ECG, especially in older patients or those with chest pain or palpitations.
5. Exclusion of other causes: sepsis, hypoglycemia, myocardial infarction, adrenal insufficiency, gastroenteritis and medication effects.

Differentials: heat stroke, dehydration from illness, hyponatremia of other cause, diabetic ketoacidosis, adrenal crisis and cardiac syncope.

1. Rest in a cool, shaded environment, with removal of excess clothing and cooling by wet cloths and fanning. Active immersion is not required.
2. Oral rehydration in alert patients who can drink: oral rehydration solution (sodium 50 to 90 mmol/L) 500 mL over 30 minutes, then 250 mL every 15 to 20 minutes up to 2 to 3 L over 2 to 4 hours. For cramps, 0.1 to 0.2% salt solution by mouth.
3. Intravenous fluid when vomiting, unable to drink, orthostatic or after 30 to 60 minutes without improvement: 0.9% saline 1 L over 1 to 2 hours (children 20 mL/kg over 1 hour), then reassess.
4. Hyponatremia: mild cases settle with oral salt and fluid. For sodium under 125 mmol/L with seizures or confusion, give 3% saline 100 mL IV over 10 minutes, repeated up to 3 times, without raising sodium over 8 mmol/L in 24 hours.
5. Hypernatremia: replace the water deficit slowly with 0.45% saline or 5% dextrose, lowering sodium by no more than 10 to 12 mmol/L in 24 hours.
6. Nausea: ondansetron 4 mg IV or orally (children 0.15 mg/kg). Paracetamol is not helpful, and avoid NSAIDs.
7. Observation: until temperature is under 38 degrees Celsius, symptoms have cleared, the patient is passing urine and orthostatic changes have resolved (usually 1 to 4 hours). Admit older patients, those with comorbidities, abnormal electrolytes or creatinine, creatine kinase over 5000 units/L, or a poor response.
8. Discharge advice: avoid exertion and heat for 24 to 48 hours, drink 2 to 3 L of fluid a day, and review diuretics and other medicines with the prescriber.
9. Return to work or sport: after 24 to 48 hours without symptoms, with a gradual increase in exertion over 1 week.

Prevention: acclimatize over 10 to 14 days, drink 0.5 to 1 L an hour during heavy exertion in the heat with electrolytes for prolonged effort, wear light clothing, and schedule work and rest cycles. Counsel on early recognition of symptoms.