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 172-1Drugs That Can Cause Malignant Hyperthermia |Favorite Table|Download (.pdf) TABLE 172-1 Drugs That Can Cause Malignant Hyperthermia
|Type of Agent ||Causative Drugs |
|Inhalation anesthetic ||Halothane |
|Depolarizing muscle relaxant ||Succinylcholine |
|Other ||Phenothiazines |
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.
TABLE 172-2Descriptions of Heat Illnesses