Heat stress in athletes: a hot safety topic!

Heat-related illness in athletes can have potentially catastrophic consequences. Andrew Hamilton looks at what recent research says about assessing the risks and heat stroke/illness prevention.

When it comes to life-threatening conditions associated with sport participation, sudden cardiac arrest, and concussion probably spring to mind. However, although less publicized, heat stroke and heat-related illness are equally serious. Data shows that when heatwaves occur, overall mortality increases across all segments of the population, mainly as a result of complications such as respiratory failure and myocardial infarction⁽¹⁾. 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⁽²⁾.

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 individual. These include environmental factors (temperature of the environment, humidity levels, wind chill), the age, gender and cardiovascular fitness of the individual and even the medications (see table 1) they take⁽⁴⁾. Tolerance to dehydration is more robust when individuals are physically fit and young while the ability to adapt to heat declines with age, reduced fitness levels, and the presence of disease.

The type of clothing worn plays a significant role in influencing the ability to maintain safe core temperatures, and exercise workload is also critical – the higher the workload, the greater the amount of heat needs to be dissipated to maintain stable core temperature⁽⁵⁾. In addition to hydration status, fluid replacement before, during, and after exercise is also a critical factor; as long as one adequately hydrates, the body is well equipped to maintain a stable core temperature in hot conditions^(6,7).

Screening for heat stroke risk

The principles of temperature regulation outlined above are easy enough to understand. However, what’s far trickier is to determine the risk of heat stroke for any one individual at any given time - yet this is  what sports medicine personnel, coaches and trainers need to be able to do to keep athletes safe and well in hot conditions. One approach is to use data on environmental variations in temperature, humidity, and radiation as well as the individual’s heart rate and aerobic tolerance. These variables combine in a formula to give a ‘heat stress risk’ score. One such formula incorporates both environmental factors (environmental stress index - ESI) and personal factors (personal stress index - PSI) and uses them in a formula to assess heat stress (see box 1)⁽⁸⁾.

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Box 1: Formula for calculating heat stress index⁽⁸⁾

This heat stress risk formula is as follows:

ESI = 0.63T_a– 0.03RH + 0.002SR + 0.0054 (T_a× RH) – 0.073(0.1 + SR)^–1

PSI = 5(T_ret– T_re0) × (39.5 – T_re0)^–1+ 5(HR_t– HR₀) × (180 – HR₀)^–1

In this formula:

Ta = ambient temperature

RH = relative humidity

SR = solar radiation

T_re0= initial rectal temperature

T_ret= rectal temperature after time t

HR₀= initial heart rate

HR_t– heart rate after time t

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The PSI and ESI values obtained by this formula are designed to reflect cardiovascular strain on the individual relative to the environmental stress on a scale between 0 and 5. The higher the value obtained, the higher the strain. Importantly, the use of this formula and the coordination of work-rest cycles in training have successfully been used to prevent the risk of future heat stress injuries in military recruits.

One limitation of this type of risk assessment is that it doesn’t account for certain conditions, such as cardiac or respiratory disease – for this reason, susceptible persons with severe cardiopulmonary disease need to complement this risk assessment formula with other types of risk assessment (see later). Another limitation is the need to measure rectal temperature, which is invasive, and the need for specific meteorological equipment to measure the strength of solar radiation and relative humidity.

Which athletes are at risk?

Athletes at risk for heat stress include all those whose typical workloads are high and whose events/training takes place in warm indoor or outdoor conditions (where direct heating from solar radiation is also a potential problem). In reality, this includes distance runners, football players, hockey players, triathlon competitors, weight-lifters, boxers, etc. Cyclists can be at risk in sweltering conditions, but the risk is generally lower than in runners because of the cooling effects generated by moving through the air at relatively high speeds. Athletes with learning disabilities may be at proportionately higher risk because they might find it harder to absorb the information about and put into practice the guidelines on regular drinking during exercise. These athletes will benefit from more frequent rest cycles and regular prompting from coaches to drink fluid.

The workload undertaken is also important. All other things being equal, for any given individual, the higher the workload, the greater the risk of heat stress in that individual. Across different individuals, the fitter the individual, the lower the risk of heat stress in that individual. In addition to the use of heat stress risk formulae, what other tools can and should be used before training athletes in the heat?

According to Dr. Lilly Ramphal-Naley, a US expert in preventative medicine, the use of a medical questionnaire to assess cardiopulmonary function, fitness level, prior episodes of heat stress, and medications is  desirable. Premedical screenings should include at minimum a pre-exercise assessment of blood pressure and BMI.  For those who expect to be working at higher intensities, other pre-exercise tests such as electrocardiograms (EKG), step tests, and even exercise stress tests may be warranted (see table 1).

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Physical demand level of work or activity	Medical questionnaire	BP and BMI	EKG >35 yrs	3-step test	Exercise stress test
Light	Yes	Yes	No	No	No
Medium light	Yes	Yes	No	Optional	No
Medium	Yes	Yes	Yes	Yes	No
Medium heavy	Yes	Yes	Yes	Yes	Optionals
Heavy	Yes	Yes	Yes	Yes	Yes

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Screening

Cardiac screening tests such as a stress echocardiogram are considered the gold standard for determining whether someone might be susceptible to the physiological stresses of exercising in a hot environment⁽⁹⁾. However, this kind of testing is expensive and time-consuming and given that young athletes with no history of cardiac problems are at very low risk, tests such as stress echocardiograms are better reserved for athletes over 35 who are either a) intending to participate in medium to high-intensity exercise or b) whose medical questionnaires show that they have cardiac risk factors.

Another category of athlete that deserves special mention is the diabetic. Diabetes is a known cardiac risk factor; research shows that diabetes is associated with ischemic heart disease and stroke, and also that these kinds of events are more likely to occur during episodes of heat stress⁽¹⁰⁾. Many people with diabetes (including athletes) may be oblivious to their condition, especially in the early stages; the safest option for those with athletes in their care, therefore, is to carry out routine screening for the presence of diabetes before subjecting them to heat stress.

Medical questionnaires

The implementation of a pre-exercise medical screening questionnaire to assess heat stress risk is always desirable in athletes, regardless of age or fitness. Typically, this screening should include an evaluation of general medical considerations, along with orthopedic injuries. However, some evidence suggests that athletic trainers might be better equipped to prevent heat stress illness by seeking information about any previous history of cardiovascular, respiratory, and heat illnesses^(11-13).

The intrinsic risk factors for exertional heat illnesses include a previous episode of heat illness, poor cardiovascular and physical fitness, inadequate heat acclimatization, dehydration or electrolyte imbalance, recent febrile illness, sleep deprivation, a ‘never give up’ or ‘warrior’ mentality, an unusually high level of motivation or zealousness and use of questionable drugs or herbal supplements.

Many of these factors can be identified during a routine screening; recent research supports extending the scope to include more indicators for cardiovascular, respiratory, and general medical conditions, including exertional heat illnesses^(11-15). In one study, researchers identified some key questions which clarify the intrinsic risk factors and further identify those athletes who might be at particular risk (see table 2)⁽¹⁶⁾.

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Risk Factor	How identified
History of exertional heat illness	Ask: ˜˜Have you ever experienced exertional heat illness? (Provide descriptions, if necessary.) If YES, ask: ˜˜What type and how many incidents?
Poor physical fitness	Determine body mass index or use the body-composition test.
Poor cardiovascular fitness level	Determine maximal oxygen consumption run test (12-min walk/run), use graded exercise test, or use other test with norms for comparison. Patients MUST be cleared for participation by a physician. This test should be performed before the beginning of preseason practices.
Recent febrile illness (101F [38.3C] or higher)	Ask: ˜˜In the last week, have you had any illness with a fever (101+F) or digestive problems, such as vomiting or diarrhea?
Current hydration status	Measure urine specific gravity using clinical refractometer
Insufficient heat acclimatization	Ask: ˜˜During your cardiovascular training, were you performing outdoors in hot or humid conditions?
Poor nutrition or consumption of questionable supplements or medications	Ask: ˜˜What products (including medications, drugs, herbs, or supplements) do you consume? (Use dosage or serving-size information to determine how much and how often these products are consumed.)
A ˜never give up or ˜warrior mentality	Ask: ˜˜When you practice or compete, what is your level of motivation?(NB “ responses may often be unreliable)
Sleep deprivation or exposure to heat and humidity throughout night	Ask: ˜˜How many hours do you usually sleep on a daily basis?Ask: ˜˜In the last week, how many nights did you get less than your normal amount of sleep?Ask: ˜˜In the last week, how many nights did you sleep in a non“airconditioned room?

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According to the researchers, when combined with the information gathered from a conventional screening, these questions help to determine those at particular risk of exertional heat illness, especially those who might otherwise slip through the net. Sports professionals should consider the ‘heat stress risk score’ nad make appropriate adjustments to training to ensure athletes avoid heat stress/illness.

Summary

Heat stress and heat illness during exercise can lead to serious health complications and even fatalities in athletes. Moreover, the evidence suggests that this risk may be underestimated by some healthcare practitioners. In addition to identifying the environmental heat loading, it’s crucial to assess each athlete’s vulnerability to heat stress/illness. Doing so 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.

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