Heat disorders such as heat exhaustion and heatstroke result from a failure of the body’s regulatory mechanisms to maintain a constant body temperature. Individuals at the extremes of age and those with chronic diseases are the most vulnerable. Approximately 400 people die annually in the United States from heat-related illness.¹ Fever, on the other hand, is an elevation in body temperature secondary to mediators of inflammation and involves an adjustment in the physiologic set point (see Chapter 24). Malignant hyperthermia, another thermoregulatory disorder, results from a triggering exposure in a genetically susceptible individual.

Heat disorders occur most commonly in the summer months or in tropical regions. Although elderly patients are most severely affected, healthy children are also susceptible. Young children have suboptimally developed thermoregulatory controls and are dependent on others to provide them with fluids and to keep them from unsafe environments. Only 4% of the heat-related deaths in the United States occur in children younger than 14 years.¹ Comorbid conditions such as obesity, physical disabilities that limit rapid egress from a warm environment, or cystic fibrosis increase a child’s susceptibility to heat stress.

The human body maintains a relatively constant body temperature despite wide swings in environmental temperature. Heat is acquired both endogenously (from basal metabolism, muscle activity, hormonal effects, and sympathetic stimulation) and exogenously (when environmental temperature exceeds body temperature). Heat is lost to the environment by radiation (up to 60% of losses), evaporation of sweat (22% to 25%), conduction (3%), and convection (12% to 15%).

The hypothalamus, the body’s primary thermoregulator, maintains the core body temperature within a narrow range. Increases in body temperature result in sympathetically mediated peripheral blood vessel dilation (increased radiation losses), increased sweat gland activity (higher evaporative losses), and reduced endogenous heat production. Hyperthermia occurs when the body cannot adequately dissipate excess exogenous or endogenous heat.

Malignant hyperthermia is a group of genetic myopathies associated with a defect in the calcium channels of skeletal muscles.² Most patients are asymptomatic until exposed to the triggering agent. A muscle biopsy is needed to confirm the predisposing defect. Implicated triggers include medications such as inhaled anesthetics and neuromuscular blockers as well as physical stressors (Table 172-1).

Table 172-2 describes the symptoms, clinical and laboratory findings, treatment, and prognosis of the three major types of heat-related illnesses: heat cramps, heat exhaustion, and heatstroke.

HEAT CRAMPS

Heat cramps are painful muscle spasms that typically occur several hours after vigorous exertion. They are thought to be due to repletion of water without adequate salt intake. The voluntary muscles of the calves, thighs, and shoulders are most commonly affected. Symptoms typically last only a few minutes but may recur. The affected muscles feel hard to palpation. Laboratory abnormalities may include hyponatremia with very low or undetectable urine sodium.

HEAT EXHAUSTION

Heat exhaustion occurs secondary to water or salt depletion, or both. Affected patients experience systemic complaints, including fatigue, weakness, nausea, vomiting, diarrhea, and headache. Irritability may be a prominent sign in infants and nonverbal children. The core temperature is mildly elevated (<39°C), and the patient often experiences tachycardia, orthostatic hypotension, profuse sweating, and flushed skin. Laboratory abnormalities may include hyponatremia or hypernatremia and hemo- and urinary concentration.

HEATSTROKE

Without adequate recognition and initiation of effective therapies, heat exhaustion can progress to a heatstroke, a life-threatening condition.³ Classic heatstroke, also referred to as nonexertional heatstroke, typically occurs in children with predisposing factors that put them at increased risk. These factors include children’s limited ability to increase their fluid intake (e.g. young infancy, nonverbal status), remove themselves from a hot environment (e.g. immobility), or dissipate heat (e.g. overdressing, obesity, underlying medical condition). Exertional heatstroke usually occurs in otherwise healthy individuals who participate in vigorous activities when it is hot and humid, often with inadequate efforts to maintain hydration.

Patients present with a significantly elevated core body temperature (>40°C) associated with central nervous system dysfunction. Anhidrosis is frequently but not universally observed. Neurologic symptoms include progressive lethargy, confusion, headache, delirium, seizures, and coma. On physical examination, patients are tachycardic, hypotensive, and tachypneic (hyperventilation causes a respiratory alkalosis). Laboratory abnormalities may include hyponatremia or hypernatremia, hypokalemia, hemo- and urinary concentration, acute renal failure, and elevated liver function tests.