Heatstroke occurs when compensatory mechanisms for dissipating heat fail and core temperature increases substantially. Inflammatory cytokines are activated, and multiple organ dysfunction may develop. Endotoxin from gastrointestinal (GI) flora may also play a role. Organ dysfunction may occur in the central nervous system (CNS), skeletal muscle (rhabdomyolysis), liver, kidneys, lungs (acute respiratory distress syndrome), and heart. The coagulation cascade is activated, sometimes causing disseminated intravascular coagulation. Hyperkalemia and hypoglycemia may occur.
Heatstroke is sometimes divided into 2 variants, although the usefulness of this classification is controversial (see table Some Differences Between Classic and Exertional Heatstroke):
Classic heatstroke takes 2 to 3 days of exposure to develop. It occurs during summer heat waves, typically in older, sedentary people with no air-conditioning and often with limited access to fluids. It can occur rapidly in children left in a hot car, particularly with closed windows.
Exertional heatstroke occurs more abruptly and affects healthy active people (eg, athletes, military recruits, factory workers). It is the 2nd most common cause of death in young athletes. Intense exertion in a hot environment causes a sudden massive heat load that the body cannot modulate. Rhabdomyolysis is common; acute kidney injury and coagulopathy are somewhat more likely and severe. Heat exhaustion can transition to heatstroke as heat illness progresses and is characterized by impairment of mental status and neurologic function.
Some Differences Between Classic and Exertional Heatstroke
Heatstroke may occur after using certain drugs (eg, cocaine, phencyclidine [PCP], amphetamines, monoamine oxidase inhibitors) that cause a hypermetabolic state. Usually, an overdose is required, but exertion and environmental conditions can be additive.
Malignant hyperthermia can result from exposure to some anesthetics in genetically predisposed patients. Neuroleptic malignant syndrome can develop in patients taking antipsychotics. These disorders are life threatening.
(See also Overview of Heat Illness.)
Central nervous system (CNS) dysfunction, ranging from confusion or bizarre behavior to delirium, seizures, and coma, is the hallmark of heatstroke. Ataxia may be an early manifestation. Tachycardia, even when the patient is supine, and tachypnea are common. Sweating may be present or absent. Temperature is > 40° C.
Diagnosis is usually clear from a history of exertion and environmental heat. Heatstroke is differentiated from heat exhaustion by presence of the following:
When the diagnosis of heatstroke is not obvious, other disorders that can cause CNS dysfunction and hyperthermia should be considered. These disorders include the following:
Laboratory testing includes complete blood count, prothrombin time, partial thromboplastin time, electrolytes, blood urea nitrogen, creatinine, calcium, creatine kinase (CK), and hepatic profile to evaluate organ function. A urethral catheter is placed to obtain urine, which is checked for occult blood by dipstick, and to monitor output. Tests to detect myoglobin are unnecessary. If a urine sample contains no red blood cells but has a positive reaction for blood and if serum CK is elevated, myoglobinuria is likely. A urine drug screen may be helpful. Continual monitoring of core temperature, usually by rectal, esophageal, or bladder probe, is desired.
Mortality rate and morbidity are significant in heatstroke patients but vary markedly with age, underlying disorders, maximum temperature, and, most importantly, duration of hyperthermia and promptness of cooling. Without prompt and effective treatment, mortality approaches 80%. About 20% of survivors have residual brain damage, regardless of intervention. In some patients, renal insufficiency persists. Temperature may be labile for weeks.
Classic and exertional heatstroke are treated similarly. The importance of rapid recognition and effective, aggressive cooling cannot be overemphasized.
The main cooling techniques are
Cold water immersion results in the lowest morbidity and mortality rates and is the treatment of choice when available. Large cooling tanks are often used at outdoor activities such as football practices and endurance races. In more remote areas, patients may be immersed in a cool pond or stream. Immersion can be used in an emergency department if suitable equipment is available and the patient is stable enough (eg, no need for endotracheal intubation, absence of seizures). The rate of heat loss during cooling may be decreased by vasoconstriction and shivering; shivering can be decreased by giving a benzodiazepine (eg, diazepam 5 mg or lorazepam 2 to 4 mg IV, with additional doses as needed) or chlorpromazine 25 to 50 mg IV.
Evaporative cooling can also be effective but works best if the environment is dry and the patient has adequate peripheral circulation (requiring adequate cardiac output). When humidity is high or profound shock is present, cold water immersion should be used. Evaporative cooling can be accomplished by splashing or spraying tepid water over the patient while fanning. Evaporative cooling is more effective when using warm rather than cold water. Warm water maximizes skin-to-air vapor pressure gradient and minimizes vasoconstriction and shivering. Some specially designed body cooling units suspend patients naked on a net over a drainage table while finely misted water at 15° C is sprayed over the entire body from above and below. Fans are used to circulate air warmed to 45 to 48° C around the body. With this technique, most patients who have heatstroke can be cooled in < 60 minutes. In addition, ice or chemical cold packs can be applied to the neck, axillae, and groin or to hairless skin surfaces (ie, palms of hands, soles of feet, cheeks) that contain densely packed subcutaneous vessels to augment cooling, but are not adequate as the sole cooling method.
Necessary resuscitation should proceed while cooling is done. Endotracheal intubation and mechanical ventilation (sometimes with paralysis) may be needed to prevent aspiration in obtunded patients, who commonly develop vomiting and seizures. Supplemental oxygen is given because heatstroke increases metabolic demand. The patient is admitted to an intensive care unit (ICU), and IV hydration with 0.9% saline solution is begun as in heat exhaustion (see Heat Exhaustion: Treatment). Theoretically, giving 1 to 2 L of IV 0.9% saline cooled to 4° C, as used in protocols to induce hypothermia after cardiac arrest, may also help decrease core temperature. Fluid deficits range from minimal (eg, 1 to 2 L) to severe dehydration. IV fluids should be given as boluses, assessing responses and the need for additional boluses by monitoring blood pressure, urine output, and central venous pressures. Excessive amounts of IV fluids, particularly if patients develop heatstroke-induced acute kidney injury, can cause acute pulmonary edema.
Organ dysfunction and rhabdomyolysis are treated. An injectable benzodiazepine (eg, lorazepam, diazepam) may be used aggressively to prevent agitation and to treat seizures (which increase heat production).
Platelets and fresh frozen plasma may be required for severe disseminated intravascular coagulation. If myoglobinuria is present, giving enough fluids to maintain urine output of ≥ 0.5 mL/kg/hour and giving IV sodium bicarbonate to alkalinize the urine can help prevent or minimize nephrotoxicity. IV calcium salts may be necessary to treat hyperkalemic cardiotoxicity. Vasoconstrictors used to treat hypotension may reduce cutaneous blood flow and decrease heat loss. When vasoconstrictors are used in an ICU, a pulmonary artery catheter may be used to monitor filling pressures. Catecholamines (epinephrine, norepinephrine, and dopamine) may increase heat production. Hemodialysis may be required. Antipyretics (eg, acetaminophen) are of no value and may contribute to liver or kidney damage. Dantrolene is used to treat anesthetic-induced malignant hyperthermia but has no proven benefit for other causes of severe hyperthermia. Activated protein C shows promising results in animal models, but is unproven in humans.
Heatstroke differs from heat exhaustion by the failure of mechanisms to dissipate body heat, the presence of CNS dysfunction, and temperature > 40° C.
If the diagnosis of heatstroke is not obvious in febrile, obtunded patients, consider a wide variety of other disorders, such as infection, intoxication, thyroid storm, stroke, seizures (interictal), neuroleptic malignant syndrome, and serotonin syndrome.
Rapid recognition and effective, aggressive cooling is extremely important.
Use cool water immersion if feasible.
Evaporative cooling can also be effective, but requires a dry environment and adequate peripheral circulation; use tepid (not cold) water, and fanning.
Monitor patients closely (including their fluid status), and provide aggressive supportive treatment.