When is it safe to resume training after suffering heat stroke?

During the closing stages of the recent Commonwealth Games marathon, Scottish runner Callum Hawkins succumbed mercilessly under the hot Gold Coast sun. He was leading the race with only 2.5km to the finish. As much as his mind was willing to battle it out, he staggered precariously, before collapsing to the road, unable to continue. With medical care, thankfully he has since recovered enough to soldier on with his life in one piece. The question is, how much physiological damage was done, and when is it safe for Callum, and others similarly affected by exertional heat stroke (EHS), to return to regular training and competition?

Understand the risks of exertional heat stroke:

When prepared appropriately, the human body can tolerate significant environmental and metabolic heat loads very well. Athletes expecting to compete in warm/hot weather need regular heat exposure to acclimatise, with careful monitoring in place during training to adjust intensity, duration, recovery and hydration; amongst others. For ambitious athletes, strategic periodised training in artificial heat under the scrutiny of an experienced exercise physiologist is also a worthy investment - not only for accentuating the heat acclimation process, but also quantifying your specific physiological responses and adaptations. Exercise intensity, and subsequent metabolic demands, contributes significantly to internal heat generation. Therefore adjusting intensity and subsequent pace for the conditions is imperative. Whilst not privy to the exact cause/s of Callum Hawkins demise during the marathon, one can only assume that he was well prepared for competing in the heat, but may have been over zealous with an aggressive race pace, and perhaps underdone on his race hydration plan. Whether Callum was affected by other means remains to be confirmed.

That said, the following points are established contributors to impairment of thermoregulation during exercise and sport. These contributing factors may independently or collectively contribute to increased risk of succumbing to EHS:

• Inadequate training preparation for competing in the heat
• Inadequate hydration
• Recent illness
• Certain medications, including: psychotropics; ACE inhibitor;  angiotensin blocker; anticholinergics; diuretics
• Use of recreational drugs or alcohol
• Genetic predisposition to heat intolerance (gene defects)
• Sleep disturbances
• Inappropriate intensity for the conditions
• Warm / hot weather
• High humidity (even when cooler ambient temperature)

Those most at risk include children, athletes affected by spinal cord injuries (e.g. racing in wheelchairs), unfit individuals, fit but overzealous athletes.

Timeline of returning to sport after heat illness:

Following an acute episode of EHS, numerous physiological changes may occur, including temporary or permanent heat intolerance. The duration of time spent above critical thresholds of core temperature, and the time taken to receive appropriate medical attention will ultimately determine the magnitude of physiological disturbances.

Athletes are advised to comply with the following return to sport guidelines:

• Medical clearance prior to return to sport and exercise
• Athlete should be clear of cognitive, anatomical and physiological dysfunction
• Blood tests must be normal (e.g. kidney and liver function; muscle enzymes, et al)
• Following a minor EHS event, athletes are generally able to resume short duration, light physical activity after 48 hours, under the supervision of an exercise physiologist with expertise in thermoregulation
• Following a significant EHS event, athletes should avoid exercise for at least one week
• Once ready to resume sport after one week, under the guidance of an exercise physiologist, athletes should progress through a series of stages during the forthcoming weeks, including (1) light exercise in temperate conditions; (2) strenuous exercise in temperate conditions; (3) light exercise in heat; (4) strenuous exercise in heat - with the 3^rd and 4^th stages monitored with physiological equipment.

Evaluating readiness to resume competitive sport:

When an athlete has been affected by a significant EHS event, with concomitant skeletal muscle (e.g exertional rhabdomyolysis), kidney or heart (or other organ) disturbances, the individual will be advised to undertake a series of physiological tests, such as (1) running test under thermoneutral  conditions to investigate metabolic and cardiovascular responses, and (2) heat tolerance tests, which encompass a range of intensities and durations in the heat, measuring core temperature, skin temperature, cardiovascular and thermal comfort (subjective feedback) responses. The thermoneutral exercise test can be undertaken once an athlete has resumed training at a stage that includes intense exercise in temperate conditions. The heat tolerance test can be undertaken 6-8 weeks after the EHS event. If the athlete is classified as heat intolerant, the test is repeated after an additional 6-8 weeks. The results of these tests provides critical insight into the athlete’s readiness to return to competitive sport, or evidence to support appropriate modified exercise.

Heat tolerance testing at Melbourne Sports & Allied Health Clinic:

Melbourne Sports & Allied Health Clinic has the expertise and resources to provide high precision heat tolerance testing for individuals (primarily athletes, and those working in thermally challenging occupations) affected by an acute or multiple EHS episode. We also work closely with sports cardiology at the Baker Institute in Melbourne, and specialists at the Malignant Hyperthermia Diagnostic Unit at Royal Melbourne Hospital.

Most recipients of the heat tolerance tests are referred by their sports physician, GP, or cardiologist. In our experience, following physiological tests and management plans, most individuals make a full recovery from EHS. Some take longer than others to recover, depending on the magnitude of the EHS event, and the initial timeliness of medical attention. During physiological tests in thermoneutral conditions, some athletes demonstrate hypermetabolic muscle disorders (characterised by abnormal muscle oxidative metabolism; rapid lactate production; sympathetic hyperactivity; delayed rise in skin temperature) in which case we refer for skeletal muscle genetic mutation screening, and subsequent malignant hyperthermia susceptibility.

 

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