Chapter 119. Heat Emergencies

Heat-related illnesses range from minor heat disorders, such as prickly heat and heat cramps, to life-threatening heat stroke. In heat stroke, thermal regulation breaks down, resulting in hyperthermia (temperature >40°C) and end-organ damage.

Heat edema is a self-limited, mild swelling of dependent extremities (hands and feet) that occurs in the first few days of exposure to a new hot environment. It is due to cutaneous vasodilation and pooling of interstitial fluid in dependent extremities. Treatment consists of elevation of the extremities or compressive stockings. Administration of diuretics may exacerbate volume depletion and should be avoided.

Heat rash (aka: prickly heat, lichen tropicus, miliaria rubra) is a vesiculopapular eruption that is found most commonly over clothed areas of the body. It results from inflammation and obstruction of sweat ducts. Antihistamines, low potency topical corticosteroids, or calamine lotion may provide symptomatic relief. Advise patients to wear light, loose fitting clothing.

Heat syncope results from volume depletion, peripheral vasodilation, and decreased vasomotor tone. It occurs most commonly in the elderly and poorly acclimatized individuals. Postural vital signs may or may not be demonstrable on presentation to the emergency department. Potentially serious causes of syncope (eg, cardiovascular, neurologic, infectious, endocrine, and electrolyte abnormalities) should be investigated, especially in the elderly. Treatment consists of rest and oral or IV rehydration.

Heat cramps are characterized by painful muscle spasms, especially in the calves, thighs, and shoulders during athletic events. They usually occur when individuals replace evaporative losses with free water but not with salt. Core body temperature may be normal or elevated. Treatment consists of rest and administration of oral electrolyte solution or IV normal saline.

Heat tetany is due to the effects of respiratory alkalosis that results when an individual hyperventilates in response to an intense heat stress. Patients may complain of paresthesia of the extremities, circumoral paresthesia, and carpopedal spasm. Muscle cramps are minimal or nonexistent. Treatment consists of removal from the heat source and decreasing the respiratory rate.

Clinical Features

Exertional heat stroke usually occurs after strenuous physical activity in a hot environment, whereas nonexertional heat stroke more commonly affects chronically ill or debilitated patients and persons at the extremes of age, especially during a prolonged heat wave. The cardinal features are hyperthermia (core temperature >40°C [104°F]) and altered mental status. Anhidrosis or profuse sweating may be seen. Prominent neurologic abnormalities include confusion, agitation, bizarre behavior, ataxia, seizures, obtundation, and coma. Other findings include hyperventilation, nausea, vomiting, diarrhea, muscle cramps, and oliguria.

Diagnosis and Differential

Heat stroke should be considered in the clinical context of environmental heat stress, hyperthermia, and altered mental status. The differential diagnosis includes infection (eg, sepsis, meningitis, encephalitis, malaria, typhoid, tetanus), endocrine disorders (eg, diabetic ketoacidosis, thyroid storm), neurologic disorders (eg, cerebrovascular accident, status epilepticus), and toxicologic causes (eg, anticholinergics, sympathomimetics, salicylates, serotonin syndrome, malignant hyperthermia, neuroleptic malignant syndrome, alcohol or benzodiazepine withdrawal). About 20% of heat stroke patients are hypotensive. Initial diagnostic studies are directed at detecting end-organ damage and excluding other disease processes. Respiratory alkalosis and lactic acidosis are seen in exertional heat stroke; respiratory alkalosis in nonexertional heatstroke. Early laboratory abnormalities associated with exertional heat stroke include hypoglycemia, hypophosphatemia and hypokalemia, elevated liver enzymes due to hepatocellular damage, hypercalcemia and an elevated hematocrit due to hemoconcentration, and elevated creatine phosphokinase and myoglobin from rhabdomyolysis. Laboratory abnormalities of DIC, renal failure develop with time. Obtain an ECG and CXR. Neuroimaging studies and other evaluations (eg, septic workup, toxicology screens) can be individualized as clinically indicated.

Emergency Department Care and Disposition

1. Emergent priorities are airway, breathing, circulation, rapid initiation of cooling, and supportive care. Intubate patients with significantly altered mental status, diminished gag reflex, or hypoxia. Initiate continuous monitoring of vital signs, pulse oximetry, and core temperature (rectal, bladder, or esophageal thermometer). Provide high flow oxygen and begin IV crystalloids to maintain mean arterial pressure above 80 to 90 mm Hg. Avoid volume overload. Vasopressors may be required.

2. Evaporative cooling is the most efficient and practical means of cooling hyperthermic patients in the emergency department. Place fans near the completely disrobed patient and spray the patient with tepid water. The goal is to decrease core temperature to below 39°C while avoiding hypothermia.

   1. Avoid spraying with ice water because this may cause shivering, which induces thermogenesis.

   2. Excessive shivering can be treated with short-acting benzodiazepines.

   3. Other cooling methods are listed in Table 119-1.

3. Treat seizures with benzodiazepines, such as lorazepam 1 to 2 milligrams IV or diazepam 5 milligrams IV.

4. Treat rhabdomyolysis with IV hydration. To date, no prospective control studies have shown improved outcomes from alkalinization of the urine or forced diuresis with mannitol or loop diuretics.

5. Monitor serum electrolytes every hour initially. Treat hyperkalemia with standard regimens.

6. Most heat stroke patients require admission to the ICU.