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Emergency medicine

Managing heat exhaustion in primary care

Heat-exhaustion

Heat-related conditions account for more than 1,000 deaths in the UK each year, a death toll set to rise as extreme weather events become the new norm.

Heat-related conditions include dehydration, cramps, heat exhaustion and heat stroke. They are all preventable and appropriate advice and timely management can help reduce morbidity and mortality.

Currently heat-related conditions account for around 1,100 premature deaths and more than 100,000 patient-days in hospital per year in the UK. As global temperatures rise, the UK government’s Climate Change Risk Assessment predicts heat-related premature mortality in the UK will rise by around 60% by the 2020s; 200% by the 2050s; and 400% by the 2080s[1].

Pathophysiology

The hypothalamus controls thermoregulation in the body through a homeostatic negative feedback system. A core temperature increase of less than 1 degree Celsius (°C) stimulates the hypothalamus to cool the body, mainly by thermal perspiration, peripheral vasodilatation and cardiac changes[2].

Physiological limitations or environmental factors may cause an inability to thermoregulate at higher temperatures, resulting in heat-related illness. On a cellular level, this results in cascade of inflammatory processes that releases cytokines and toxins, promoting endothelial changes, ischaemia and apoptosis. In extreme circumstances this can result in hyperthermia, shock, disseminated intravascular coagulation, multiple organ damage and death.

Risk factors

Certain groups of people are at greater risk of being affected by heat-related conditions. These people should be referred to their doctor or emergency services if a heat-related condition is suspected.

Environmental risk factors for heat-related conditions include being unaccustomed to exposure to heat, wearing inappropriate clothing, or not drinking enough fluids.

In addition, environmental factors may increase a patient’s risk. These include:

  • Occupational exposure, especially among those in the military or outdoor labourers;
  • Attending events where there is a lack of hydration and cooling;
  • Unaccustomed or excessive exertion, such as during sports events. Athletes should be advised to acclimatise for at least three to four days before competing in a hot environment;
  • Religious and cultural traditions eg, fasting during Ramadan;
  • Poor housing, such as being homeless or in crowded accommodation.

Age is an important risk factor. Elderly patients have an age-related decline in thermoregulation and physical functioning, which often coincides with a decreased fluid intake. The risk of heat-related conditions is increased in older patients who are socially isolated, bedbound, single or unable to self-care.

Older women are particularly at risk due to differences in physiology, renal function and reduced physical ability.

Infants are also at risk of heat-related conditions, as they are often unable to communicate their condition clearly, have a smaller body mass and blood volume and an immature thermoregulation system.

Medical conditions such as infectious diseases, pneumonia, gastroenteritis, acute renal failure, cerebrovascular disease and heart failure all impair thermoregulatory responses and increase the risk of dehydration during heat exposure.

Patients with chronic conditions may also be at increased risk of heat-related conditions and acute events (see panel 1). Diseases may also be exacerbated because of a patient’s sensitivity to the heat.

Patients with a reduced ability to self-care or lack of awareness may also fail to take appropriate preventative action. In particular, people with cardiac, pulmonary disease and mental health conditions have a higher mortality risk if they develop a heat-related condition[3].

Medical conditions increasing the risk of heat-related illness

  • Diabetes and other endocrine disorders
  • Organic mental disorders
  • Schizophrenia and delusional-type mental health conditions
  • Neurological disease such as Parkinson’s disease, Alzheimer’s disease and dementia
  • Alcohol and substance misuse
  • Cardiovascular diseases including hypertension, coronary artery disease and heart conduction disorders
  • Respiratory diseases
  • Disorders of renal system, renal failure and kidney stones
  • Obesity
  • Chronic diseases affecting sweating and electrolyte balance, such as cystic fibrosis and scleroderma

Medication may interfere with the body’s ability to cool itself through a variety of mechanisms. These include:

  • Altering central thermoregulation via physiological and behavioural responses (e.g., anticholinergics)
  • Reducing cognitive alertness (e.g., antipsychotics)
  • Changing electrolyte balance or renal function (e.g., diuretics)
  • Altering cardiac function, which can affect cooling by vasodilatation or increase the risk of dizziness or syncope (e.g., beta-blockers)
  • Inhibiting normal sweating mechanisms by blocking the parasympathetic nervous system (e.g., antihistamines, anti-Parkinsonian drugs).

Other medicines may cause side effects of vomiting and diarrhoea, which increases the risk of dehydration.

Heat exposure can also affect the pharmacological activity and toxicity of medicines, in particular digoxin or lithium. Certain medicines have reduced efficacy after exposure to higher temperatures. These must be stored and transported at the licensed temperature <25°C. These include insulins, analgesics, sedatives, antibiotics and adrenergics.

The World Health Organization’s (WHO) heat protection guidelines recommend preventative measures instead of adjusting essential medication[4]. Patients on multiple medicines for chronic conditions should be advised to avoid prolonged heat exposure and have immediate access to cool places.

Citation: The Pharmaceutical Journal DOI: 10.1211/PJ.2014.20065525

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