Hyperthermia

Basics

Description

  • Range of progressively more severe illnesses due to increasingly overwhelming heat stress
  • Not synonymous with fever, which is induced by cytokines during inflammation and regulated by the hypothalamus
  • Begins with dehydration and electrolyte abnormalities and progresses to thermoregulatory dysfunction and multisystem organ failure
  • Body temperature is maintained within a narrow range by balancing heat production with heat dissipation
  • Oxidative phosphorylation becomes uncoupled and essential enzymes cease to function above 108 °F (42 °C)

Heat Stroke

  • Core body temp >105 °F (40.5 °C)
  • Failure of thermoregulatory function leads to severe central nervous system (CNS) dysfunction and multisystem organ failure
  • Classic heat stroke (nonexertional):
    • Absorption of heat from the environment and poor heat dissipation
    • Occurs in patients with compromised thermoregulation or an inability to remove themselves from a hot environment (eg, extremes of age, debilitated)
    • Develops over days to weeks, usually during heat waves
  • Exertional heat stroke:
    • Excess heat production overwhelms heat-loss mechanisms, often despite persistent sweating
    • Younger, athletic patients with combined environmental and exertional heat stress (eg, military personnel, laborers)
    • Develops over hours

Heat Exhaustion

  • Core temp moderately elevated but usually <104 °F (40 °C)
  • Thermoregulatory function is maintained and CNS function is preserved
  • Results from exposure to environmental heat or strenuous exertion → fluid and/or salt depletion
  • Variable nonspecific symptoms including malaise, headache, fatigue, and nausea
  • Most common heat-related ED presentation
  • If left untreated, progresses to heat stroke

Etiology

  • Preexisting conditions that hinder the body’s ability to dissipate heat predispose for heat-related illness:
    • Age extremes
    • Dehydration (including gastroenteritis, inadequate fluid intake)
    • Cardiovascular disease (including CHF, CAD)
    • Obesity
    • Diabetes mellitus, hyperthyroidism, pheochromocytoma
    • Febrile illness
    • Skin diseases that hinder sweating (including psoriasis, eczema, cystic fibrosis, scleroderma)
  • Pharmacologic contributors:
    • Sympathomimetics
    • LSD, PCP, cocaine
    • MAO inhibitors, antipsychotics, anxiolytics
    • Anticholinergics
    • Antihistamines
    • β-blockers
    • Diuretics
    • Laxatives
    • Drug or alcohol withdrawal
  • Environmental factors:
    • Excessive heat/humidity
    • Prolonged exertion
    • Lack of mobility
    • Lack of air conditioning
    • Lack of acclimatization
    • Occlusive, nonporous clothing

Pediatric Considerations

Children are at increased risk of heat illness due to increased body surface area to mass ratio, higher basal metabolic rate, and lower sweat production (eg, left unattended in vehicle)

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