Heatstroke occurs when compensatory mechanisms for dissipating heat fail and core temperature increases substantially. Inflammatory cytokines are activated, and multiple organ dysfunction may develop. Organ dysfunction may occur in the central nervous system (CNS), skeletal muscle (rhabdomyolysis Rhabdomyolysis Rhabdomyolysis is a clinical syndrome involving the breakdown of skeletal muscle tissue. Symptoms and signs include muscle weakness, myalgias, and reddish-brown urine, although this triad is... read more ), liver, kidneys, lungs (acute respiratory distress syndrome), and heart. Hyperkalemia Hyperkalemia Hyperkalemia is a serum potassium concentration > 5.5 mEq/L (> 5.5 mmol/L), usually resulting from decreased renal potassium excretion or abnormal movement of potassium out of cells. There... read more and hypoglycemia Hypoglycemia Hypoglycemia, or low plasma glucose level can result in sympathetic nervous system stimulation, and central nervous system dysfunction. In patients with diabetes who take insulin or antihyperglycemic... read more may occur. The coagulation cascade is activated, sometimes causing disseminated intravascular coagulation Disseminated Intravascular Coagulation (DIC) Disseminated intravascular coagulation (DIC) involves abnormal, excessive generation of thrombin and fibrin in the circulating blood. During the process, increased platelet aggregation and coagulation... read more .
Heatstroke is sometimes divided into 2 variants, although the usefulness of this classification is controversial (see table ):
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 a 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 Acute Kidney Injury (AKI) Acute kidney injury is a rapid decrease in renal function over days to weeks, causing an accumulation of nitrogenous products in the blood (azotemia) with or without reduction in amount of urine... read more and coagulopathy are somewhat more likely and severe. Heat exhaustion Heat Exhaustion Heat exhaustion is a non–life-threatening clinical syndrome of weakness, malaise, nausea, syncope, and other nonspecific symptoms caused by heat exposure. Thermoregulation and central nervous... read more 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
Patients usually affected
Older, sedentary people
Healthy active people (eg, athletes, military recruits, factory workers)
No air-conditioning during summer heat waves
Intense exertion, particularly without acclimatization
Usually hot and dry but sometimes moist with sweat
Often moist with sweat
Heatstroke may occur after using stimulant drugs (eg, cocaine, phencyclidine [PCP], amphetamines), monoamine oxidase inhibitors, or anticholinergic medications (eg, antihistamines, antimuscarinics) that cause a hypermetabolic state or impair the ability to sweat. Usually, an overdose is required, but exertion and environmental conditions can be additive.
(See also Overview of Heat Illness Overview of Heat Illness Heat illness encompasses a number of disorders ranging in severity from muscle cramps and heat exhaustion to heatstroke (which can be a life-threatening emergency). Current estimates of heat-related... read more .)
Symptoms and Signs of Heatstroke
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 of Heatstroke
Clinical evaluation, including core temperature measurement
Laboratory testing for organ dysfunction
Diagnosis is usually clear from a history of exertion and environmental heat. Heatstroke is differentiated from heat exhaustion by presence of the following:
Temperature > 40° C
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:
Acute infection (eg, sepsis, malaria, meningitis, toxic shock syndrome)
Status epilepticus (interictal)
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.
Treatment of Heatstroke
Aggressive supportive care
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
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).
Evaporative cooling is also very effective and works best if the patient has adequate peripheral circulation (requiring adequate cardiac output). Evaporative cooling can be accomplished quickly by spraying tepid water over the patient and using a large industrial fan (often used by the janitorial department). The use of warm or tepid water maximizes the skin-to-air vapor pressure gradient and minimizes vasoconstriction and shivering. 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.
Cooling measures should be stopped once temperature reaches approximately 39° C to avoid overcooling and causing iatrogenic hypothermia.
Necessary resuscitation should proceed while cooling is done. Neuromuscular blockade with endotracheal intubation and mechanical ventilation may be needed to control shivering and prevent aspiration in obtunded patients. Supplemental oxygen is given because heatstroke increases metabolic demand. IV hydration with 0.9% saline solution should be started with 1 to 2 L of cooled 0.9% saline to 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.
Patients should be admitted to an intensive care unit and observed for multiple organ dysfunction, disseminated intravascular coagulation Disseminated Intravascular Coagulation (DIC) Disseminated intravascular coagulation (DIC) involves abnormal, excessive generation of thrombin and fibrin in the circulating blood. During the process, increased platelet aggregation and coagulation... read more , and rhabdomyolysis Treatment Rhabdomyolysis is a clinical syndrome involving the breakdown of skeletal muscle tissue. Symptoms and signs include muscle weakness, myalgias, and reddish-brown urine, although this triad is... read more . Hemodialysis Hemodialysis In hemodialysis, a patient’s blood is pumped into a dialyzer containing 2 fluid compartments configured as bundles of hollow fiber capillary tubes or as parallel, sandwiched sheets of semipermeable... read more may be required. Antipyretics have no value and can contribute to liver or kidney damage.
Prognosis for Heatstroke
Mortality 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.
Heatstroke differs from heat exhaustion by 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 of heatstroke and effective, aggressive cooling are extremely important.
Use cool water immersion or evaporative cooling to rapidly cool the patient.
Patients will require intensive care monitoring with aggressive supportive care.