# Heat-Related Illness: From Heat Exhaustion to Exertional Heat Stroke

## Introduction

Heat-related illness represents a spectrum of conditions ranging from mild heat cramps to life-threatening heat stroke. Heat stroke is a true medical emergency with mortality rates exceeding 50 percent if treatment is delayed. The emergency physician must recognize the continuum of heat illness, initiate aggressive cooling without delay, and understand the distinct pathophysiology and management of classic versus exertional heat stroke.

## Thermoregulation Physiology

Normal core body temperature is maintained between 36.5 and 37.5 degrees Celsius through a balance of heat production and dissipation. The four mechanisms of heat dissipation are radiation (the primary mechanism at rest), convection, conduction, and evaporation (the primary mechanism during exercise and at high ambient temperatures). The hypothalamus serves as the thermoregulatory center, integrating peripheral and central thermal inputs. When ambient temperature exceeds skin temperature, evaporation becomes the sole mechanism of heat loss, and high humidity significantly impairs evaporative cooling. Acclimatization, achieved through 7 to 14 days of gradual heat exposure, increases sweat rate, dilutes sweat sodium concentration, and expands plasma volume, allowing the body to tolerate heat more effectively.

## Spectrum of Heat-Related Illness

### Heat Cramps

Heat cramps are painful, involuntary muscle spasms occurring during or after intense exercise in heat. They are caused by electrolyte depletion (sodium and chloride losses in sweat) and dehydration. Treatment involves rest in a cool environment, oral or IV isotonic fluid replacement, and electrolyte supplementation.

### Heat Exhaustion

Heat exhaustion is the inability to continue activity in a hot environment due to cardiovascular strain. Core temperature is typically below 40 degrees Celsius (104 degrees Fahrenheit), and there is no CNS dysfunction. Symptoms include fatigue, weakness, headache, nausea, vomiting, profuse sweating, tachycardia, and orthostatic hypotension. Treatment consists of removal from heat, cooling measures, and IV normal saline. Most patients improve rapidly with supportive care, but heat exhaustion can progress to heat stroke if not recognized and treated.

### Heat Stroke

Heat stroke is defined as life-threatening hyperthermia with a core temperature above 40 degrees Celsius (104 degrees Fahrenheit) combined with altered mental status or CNS dysfunction. It exists in two forms: classic (non-exertional) and exertional.

| Feature | Classic Heat Stroke | Exertional Heat Stroke |
|---------|-------------------|----------------------|
| Population | Elderly, chronically ill, isolated | Young athletes, military, laborers |
| Onset | Hours to days | Minutes to hours |
| Skin | Hot, DRY (anhidrosis) | Hot, often still SWEATING |
| Rhabdomyolysis | Less common | Extremely common |
| DIC/AKI | Less common | More frequent |
| Mortality | Higher (delayed presentation) | Lower if cooled rapidly |
| Key cooling | Evaporative or immersion | Cold water immersion preferred |

<image>Comparison diagram illustrating the spectrum of heat-related illness from heat cramps through heat exhaustion to heat stroke, showing the progressive increases in core temperature, symptom severity, and organ dysfunction at each stage, with key differentiating features highlighted</image>

## Classic Heat Stroke

Classic heat stroke occurs in elderly, chronically ill, or socially isolated individuals during heat waves. Risk factors include extremes of age, cardiovascular disease, psychiatric illness, medications (anticholinergics, diuretics, beta-blockers, antipsychotics), lack of air conditioning, and obesity. It develops over hours to days of sustained heat exposure and classically presents with hot, dry skin (anhidrosis due to sweat gland fatigue), altered mental status, and core temperature above 40 degrees Celsius. Classic heat stroke carries a higher mortality than exertional heat stroke due to delayed presentation and underlying comorbidities.

## Exertional Heat Stroke

Exertional heat stroke occurs in young, healthy individuals such as athletes, military personnel, and laborers during intense physical activity in heat. It develops rapidly over minutes to hours during or immediately after exertion. In contrast to classic heat stroke, sweating is often still present. The key differentiator is that rhabdomyolysis is extremely common, and DIC and acute kidney injury occur more frequently. Core temperature may exceed 41 degrees Celsius (106 degrees Fahrenheit) at presentation. The prognosis is better than classic heat stroke if cooling is initiated rapidly.

## Diagnosis

Rectal temperature is the gold standard for core temperature measurement. Oral, axillary, and temporal measurements are unreliable and underestimate true core temperature. CNS dysfunction manifests as confusion, delirium, seizures, ataxia, combativeness, or coma. Laboratory findings include elevated lactate, metabolic acidosis, elevated CK (often above 10,000 IU/L in exertional heat stroke), liver transaminase elevation that peaks at 48 to 72 hours, DIC (elevated INR, thrombocytopenia, elevated D-dimer), acute kidney injury, hyperkalemia, and hypoglycemia.

## Treatment

### Rapid Cooling -- The Priority

Cooling should begin immediately in the field and continue in the ED, as every minute of delay increases morbidity and mortality. The target is to reduce core temperature to below 39 degrees Celsius (102.2 degrees Fahrenheit) within 30 minutes. Cold water immersion (CWI) is the most effective cooling method, achieving a cooling rate of 0.15 to 0.35 degrees Celsius per minute by immersing the patient in a tub of ice water (1 to 15 degrees Celsius) up to the neck. If CWI is unavailable, evaporative cooling with continuous misting of tepid water and fan-directed airflow achieves a lower cooling rate of approximately 0.05 degrees Celsius per minute. Adjunctive cooling methods include cold IV fluids (4 degrees Celsius normal saline boluses), ice packs to the neck, axillae, and groin (which have limited efficacy alone), and cooling blankets. Antipyretics such as acetaminophen and NSAIDs are ineffective because the thermoregulatory set point is not elevated -- rather, the hypothalamus is overwhelmed. Dantrolene has no proven benefit in heat stroke, unlike its role in malignant hyperthermia.

### Resuscitation

IV fluid resuscitation with isotonic crystalloid should target a urine output above 0.5 mL/kg per hour. Rhabdomyolysis should be treated aggressively with IV normal saline targeting urine output of 200 to 300 mL per hour, with serial monitoring of CK, potassium, and creatinine. Seizures should be managed with benzodiazepines, as phenytoin is ineffective for heat-related seizures. DIC is common in severe cases and should be monitored and corrected with blood products as needed. Shivering during cooling increases metabolic heat production and should be treated with benzodiazepines or magnesium.

<image>Emergency department cooling setup for heat stroke showing cold water immersion technique with a patient submerged in an ice water tub, with continuous rectal temperature monitoring, IV access for cold crystalloid infusion, and a nurse monitoring vital signs on a bedside monitor</image>

## Complications and Disposition

Hepatic failure is a feared complication, as transaminases may continue to rise for 48 to 72 hours. Peak AST and ALT above 1000 are associated with poor prognosis, and liver transplantation may be required in extreme cases. Acute kidney injury results from rhabdomyolysis, hypovolemia, and direct thermal injury and may require renal replacement therapy. ARDS can develop from capillary leak and direct thermal injury. Cardiac dysfunction may occur from myocardial injury due to hyperthermia and hemodynamic stress. Cerebellar dysfunction is the most common permanent neurological sequela. All heat stroke patients require ICU admission for ongoing monitoring and management. Exertional heat stroke survivors should avoid intense exercise for at least 3 to 4 weeks and follow a graduated return-to-play protocol under physician supervision.

## Prevention

Acclimatization protocols for athletes and military personnel should involve a gradual increase in heat exposure over 10 to 14 days. Wet bulb globe temperature (WBGT) monitoring provides evidence-based activity modification guidelines. Additional prevention measures include adequate hydration with sodium-containing fluids during prolonged exercise, avoiding peak heat hours, scheduling rest breaks in shaded or cooled areas, and community interventions for classic heat stroke such as cooling centers and wellness checks on elderly individuals during heat waves.

<image>Wet bulb globe temperature (WBGT) risk stratification chart showing activity modification guidelines at different temperature ranges with color-coded risk levels (green, yellow, red, black) and corresponding work-rest ratios and fluid intake recommendations</image>

## Clinical Pearls

Heat stroke is defined by core temperature above 40 degrees Celsius and altered mental status, and rectal temperature should always be measured for accurate assessment. Cold water immersion is the gold standard for cooling and should be initiated immediately without delay for diagnostic workup. Antipyretics are ineffective in heat stroke because the hypothalamus is overwhelmed, not reset to a higher set point. Exertional heat stroke patients commonly develop rhabdomyolysis, DIC, and multi-organ failure, making aggressive fluid resuscitation critical. Hepatic injury in heat stroke peaks at 48 to 72 hours, and serial monitoring of liver function is essential throughout the hospital course.

## References

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2. Casa DJ, DeMartini JK, Bergeron MF, et al. National Athletic Trainers' Association position statement: exertional heat illnesses. *J Athl Train*. 2015;50(9):986-1000.
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4. Lipman GS, Eifling KP, Ellis MA, et al. Wilderness Medical Society practice guidelines for the prevention and treatment of heat illness: 2019 update. *Wilderness Environ Med*. 2019;30(4S):S33-S46.