When it comes to life-threatening conditions associated with sport participation, sudden cardiac arrest and concussion probably spring to mind. However, although less publicised, heat stroke and heat-related illness is equally serious. Data shows that when heatwaves occur, overall mortality is increased across all segments of the population, mainly as a result of complications such as respiratory failure and myocardial infarction. However, what is less well known is that the incidence of heat-related illness is highest among men aged 15 to 19 years and is most often associated with athletic activities.

To complicate things further, cases of heat stress tend to be handled on a case-by-case basis rather than by being addressed collectively as a public health concern. In the US, The National Institute for Occupational Safety and Health initiated heat stress criteria regarding worker heat stress safety in 1986, yet in even now, few people are aware of these criteria, of the risk factors associated with a predisposition to heat stress, or of the signs or symptoms to look for that represent risk factors for heat stress.

 

Temperature regulation

Heat stress/illness occurs when the body is no longer able to maintain its temperature equilibrium and core temperature rises sufficiently to interfere with other physiological processes. There are a large number of factors that will determine if and when heat stress will strike any particular athlete. These include environmental factors (temperature of the environment, humidity levels, wind chill), the age, gender and cardiovascular fitness of the athlete and even any medications (see table below) they take. Overall however, tolerance to dehydration is more robust when individuals are physically fit and young, while the ability to adapt to heat declines with age, poor fitness levels and the presence of disease.

Common Medications and heat stroke risk

Alcohol  :  HR decrease, plasma volume depletion

Anticholinergics   :  HR lability, abnormal sweating

Antihistamines   :   HR lability, abnormal sweating

Antihypertensives    :   HR lability

Diuretics   :   Plasma volume depletion, decreased vasodilation

Laxatives    :   HR lability, abnormal vasodilation

Tricyclic antidepressants    :   Electrolyte imbalance

The type of clothing worn plays a major role in influencing the ability to maintain safe core temperatures and adjust to extreme environmental heat. Indeed, the wrong or excessive clothing can be extremely important in determining whether athletic heat exhaustion and heat stroke mortality occur. The exercise workload is also critical – the higher the workload, the greater the amount of heat needs to be dissipated in order to maintain a stable core temperature. In addition to hydration status, fluid replacement before during and after exercise is also a critical factor; under normal conditions (ie in the absence of disease, medications and other complicating factors), the human body can maintain a stable core temperature even when stressed by extreme environmental temperatures as long as adequate hydration is continuously provided. However, while maintaining adequate hydration sounds straightforward in principle, the practice can be much more challenging. 

Hydration fundamentals

A comprehensive hydration strategy entails ensuring good hydration before training/competition, maintaining hydration as far as possible during training/competition and then ensuring that any shortfall is fully replaced as soon as possible afterwards. Fluid loss via urine and especially via sweating involves the loss of electrolyte minerals – calcium, magnesium, sodium, potassium and chloride. Although the composition varies from person to person, a litre of sweat typically contains around the following: Calcium 0.02g; Magnesium 0.05g; Sodium   1.15g; Potassium 0.23g; Chloride 1.48g.

Replacement of these minerals in an electrolyte mineral containing drink is desirable because while the amounts of these minerals lost during sweating is generally small compared to total body stores, prolonged heavy sweating can lead to significant losses (particularly of sodium), which has been associated with undesirable side effects such as cramping. Drinking lots of pure water effectively dilutes these concentrations of electrolyte minerals, which can lead to an impairment of a number of normal physiological processes. An extreme example of this is hyponatraemia, where low plasma sodium levels disrupts the dynamics of water exchange (osmotic balance) across the blood-brain barrier, causing symptoms such as confusion, seizure, coma and even death.

Another reason is that drinks containing added sodium have been shown to help the physiological drive to drink, therefore stimulating a greater voluntary intake of fluid compared to plain water. In addition, when the electrolyte minerals (in particular sodium) are present at the appropriate concentrations, the rate of fluid absorption from the small intestine appears to be enhanced, especially in conjunction with small amounts of glucose. This is especially important where the rapid uptake of fluid into the body is important, for example ingesting fluid to maintain hydration during exercise.

Drinking lots of pure water (without essential mineral present such as in hydrating sports drinks) effectively dilutes these concentrations of electrolyte minerals, which can lead to an impairment of a number of normal physiological processes…

Key questions to determine intrinsic risk of exercise-related heat illness through exertion

  1. Is there a history of exertion heat illness?
  2. Are you in poor physical fitness?
  3. Do you have a poor cardiovascular fitness level?
  4. Have you had a recent febrile illness (i.e. illnesses involving a fever, vomiting etc.)
  5. What is your current hydration status?
  6. Have you had insufficient heat acclimatisation?
  7. Do you have a poor nutritional intake or use sub-standard medications and supplements?
  8. Do you have a ‘never give up’ or ‘warrior’ mentality?
  9. Do you experience sleep deprivation and/or exposure to heat and humidity throughout the night?

According to the researchers, when combined with the information gathered from a PPE (pre-participation physical examination), these questions could help to determine those at particular risk of exertional heat illness and who might otherwise slip through the net. This in turn could allow a more accurate ‘heat stress risk score’ to be obtained, resulting in more effective adjustments to training in order to ensure athletes avoid heat stress/illness.

Summary

Heat stress and heat illness during exercise can lead to serious health complications in athletes and in the most serious cases, can be fatal. Moreover, the evidence suggests that this risk may be underestimated by some coaches, athletic trainers and healthcare practitioners. In addition to identifying the environmental heat loading, it’s extremely important to assess each athlete’s vulnerability to heat stress/illness. This requires consideration of a large number of factors, including medications, medical history, lifestyle factors, training history, cardiovascular fitness index. This information should be considered an essential part of any pre-participation physical examination to determine heat stress/illness risk and keep athletes safe.