Emergency Medicine · Environmental Emergencies
The facts most likely to be tested
Heat exhaustion is defined by a core body temperature between 37°C and 40°C (98.6°F–104°F) without significant central nervous system dysfunction.
The primary pathophysiologic mechanism is volume depletion and electrolyte imbalance resulting from excessive sweating and inadequate fluid replacement.
Patients present with profuse diaphoresis, tachycardia, orthostatic hypotension, and nausea or vomiting.
The absence of altered mental status, seizures, or coma is the critical clinical feature that differentiates heat exhaustion from heat stroke.
Initial management requires volume resuscitation with isotonic intravenous fluids and moving the patient to a cool environment.
Laboratory findings often reveal hemoconcentration, hyponatremia or hypernatremia, and elevated liver transaminases.
Failure to improve with aggressive fluid resuscitation or the development of neurologic deficits necessitates immediate transition to the management protocol for heat stroke.
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A 24-year-old male marathon runner is brought to the medical tent after collapsing near the finish line on a humid 92°F day. He is profusely diaphoretic, tachycardic at 128 bpm, and has an orthostatic blood pressure drop. His rectal temperature is 101.8°F (38.8°C). He is alert and oriented to person, place, and time, though he complains of severe nausea and dizziness.
What is the most appropriate initial management for this patient?
Aggressive volume resuscitation with isotonic intravenous fluids and cooling.
The patient's presentation of heat-related illness without altered mental status confirms heat exhaustion, which is primarily managed with fluid replacement and cooling to prevent progression to heat stroke.
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Etiology / Epidemiology
Occurs due to volume depletion and heat stress in hot environments. High risk in elderly, athletes, and laborers.
Clinical Manifestations
Profuse diaphoresis, tachycardia, and orthostatic hypotension. Mental status remains intact.
Diagnosis
Clinical diagnosis. Core body temperature < 40°C (104°F). Exclude heat stroke.
Treatment
Move to cool environment and volume resuscitation. Avoid rapid cooling if shivering occurs.
Prognosis
Excellent with rapid rehydration. Failure to treat leads to heat stroke.
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Epidemiology & Etiology
Common in individuals exposed to high ambient temperatures without adequate fluid/electrolyte replacement. Risk is exacerbated by dehydration, alcohol use, and medications like anticholinergics or diuretics. It represents a failure of the cardiovascular system to compensate for heat stress.
Pertinent Anatomy
The hypothalamus serves as the body's thermostat, regulating heat dissipation via vasodilation and sweat production. Peripheral skin vessels dilate to shunt blood to the surface for cooling, which can lead to orthostatic hypotension.
Pathophysiology
Excessive heat exposure leads to sweat-induced volume depletion and sodium loss. The body attempts to maintain cardiac output through tachycardia, but eventually, peripheral pooling limits venous return. If uncorrected, the compensatory mechanisms fail, leading to systemic hypoperfusion.
Clinical Manifestations
Patients present with profuse diaphoresis, headache, nausea, and orthostatic hypotension. Unlike heat stroke, the mental status is preserved. Altered mental status or seizures indicate progression to heat stroke.
Diagnosis
Diagnosis is clinical, supported by a core body temperature < 40°C (104°F). Rectal temperature is the gold standard for accuracy. Laboratory tests are used to rule out other causes, often showing hemoconcentration or electrolyte imbalances.
Treatment
Immediate cessation of heat exposure and oral or IV isotonic saline are the mainstays. Patients should be moved to a cool area and have excess clothing removed. Do not use antipyretics as they are ineffective and potentially hepatotoxic in heat-related illness.
Prognosis
Prognosis is excellent with prompt volume resuscitation. Patients must be monitored for electrolyte abnormalities and renal function. Failure to stabilize leads to the life-threatening complication of heat stroke.
Differential Diagnosis
Heat Stroke: Altered mental status and core temp > 40°C
Dehydration: Absence of heat exposure history
Sepsis: Fever with infectious source
Thyroid Storm: Hypermetabolic state with tachycardia and agitation
Drug Toxicity: Sympathomimetic or anticholinergic overdose