Etiology

Heat illness occurs when thermal loads overwhelm the body's thermoregulatory responses and homeostasis is altered. Extremes of temperature and humidity make heat dissipation less efficient and can lead to heat illness. Heat illness includes a continuum from relatively benign heat exhaustion to heat stroke, which frequently leads to significant morbidity and mortality.

Physical effort generates intrinsic heat, and can rapidly lead to heat stroke when combined with environmental factors. Children, older adults, and chronically ill patients are at increased risk by virtue of a range of physiologic limitations that can also favor progression to heat stroke. The increased risk of heat illness in children is likely associated with excessive exertion, insufficient recovery, and the use of uniforms and equipment that retain heat.[8] Other intrinsic factors, including chronic volume depletion, inability to increase cardiovascular output, and normal deficiencies in heat shock protein responses associated with aging and lack of acclimatization, can all inhibit the body's ability to respond to heat challenges.[1]

Pathophysiology

The term thermal maximum refers to the magnitude and duration of heat the body's cells can encounter before becoming damaged. Studies have established a human thermal maximum of a core body temperature of approximately 107.6°F (42°C) for 45 minutes to 8 hours.[9] Cellular destruction occurs more rapidly at higher temperatures. Thermal stresses initiate a systemic inflammatory cascade, and gastrointestinal permeability increases, which may release endotoxins into the circulation.[10][11] These mechanisms induce an encephalopathic response, which leads to profound central nervous system disturbance.

Classification

Clinical classification[1]

Classic heat stroke

  • Sedentary, often older or debilitated people under conditions of high heat stress

  • May be of insidious onset (over several days) and can present with minimally elevated core temperatures.

Exertional heat stroke

  • Active, often young people, under conditions that produce heat stress through either exertion alone or a combination of environmental thermal loading combined with physical activity

  • Rapid onset (within hours) and frequently associated with high core temperatures.

Heat exhaustion

  • Milder form of heat illness

  • Core temperatures are elevated to a smaller magnitude than in heat stroke.

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