Pathophysiology of heat-related illness

The pathophysiological basis of heat-related illness is the failure of the body’s thermoregulation system to dissipate sufficient heat. The thermostat in the hypothalamus in a healthy individual maintains core body temperature within a narrow range. It activates heat-gain and heat-loss mechanisms, largely via the autonomic nervous system.

In a healthy individual, most heat is lost by radiation; however, the most important adaptive mechanism for heat loss is evaporation. This makes heat-related illness most likely in environments where ambient temperature is higher than skin temperature, and evaporative capacity is limited by high humidity.

An increase in core body temperature is associated with an increase in metabolic rate and oxygen consumption. At core body temperatures higher than 40°C, there is a high risk of intracellular dysfunction, causing organ system failure and death.

The major risk factors for heat-related illness are:

  • behavioural factors, such as:
    • failing to train before exertion or not drinking enough fluid during exercise
    • working in hot or poorly ventilated environments
    • leaving children in cars
    • failing to take self-protective measures (eg due to cognitive difficulties, alcohol or drug usage)
    • struggling against physical restraint
    • wearing personal protective clothing
  • drugs—see the New South Wales Health website; examples include:
    • carbonic anhydrase inhibitors (eg acetazolamide, topiramate)Cheshire, 2016
    • anticholinergic drugs
    • amfetamines
  • medical conditions such as:
    • previous heat-related illness
    • idiopathic anhidrosis
    • cholinergic neuropathy
    • multiple system atrophy.

Children, especially the very young, are more susceptible to heat-related illness than most adults, as they are less able to control their environment and gain heat more readily due to their greater body surface area. Fortunately, proportionately greater surface area and reduced subcutaneous fat facilitate rapid cooling in children. Elderly people are also at increased risk of heat-related illness for a range of reasons (eg reduced thirst awareness, reduced sweating, impaired self-care skills)Wilson, 2011.