There are several forms of heat injury which are usually caused by exercise participation in extreme heat or a lack of fluid intake (dehydration).
The three main forms of heat injury are:
- Heat cramps
- Heat exhaustion (exercise-associated collapse)
- Heat stroke
The human body is well adapted to cope with the strains of exercise. Our methods of cooling the body, mainly through sweating, are far superior to most other mammals.
When exercising in conditions where the environmental temperature exceeds the body temperature, sweating is the primary method for cooling the body. This works through the evaporation of sweat, from the skins surface.
In dry heat conditions this method works well, however as humidity (lots of moisture in the air) increases, the rate of evaporation is much lower as the air is already saturated with water vapor. This greatly decreases the beneficial effects of sweat production. Exercise in these warm and humid conditions should be avoided if possible.
As well as high environmental temperature, heat is also produced by the body whilst exercising due to an increase in metabolic rate and muscle activity. It is possible to acclimatise the body to performing in hot conditions over time, due to adaptations within the respiratory, circulatory and musculoskeletal systems. Once acclimatised to training in hot weather, the body initiates the sweating response earlier, increases the amount we sweat and dilutes the sweat produced so that it contains less sodium.
Heat cramps have previously been known as miners cramps and fireman's cramps. They have historically been thought to be a result of dehydration and losses in sodium through sweating.
Cramps are strong involuntary muscular contractions which are usually short-lived and release on their own. They can be quite painful and occur most frequently in the Gastrocnemius (calf) and hamstrings (rear thigh). Gently stretching the muscle can help to relax a cramp.
Cramps can occur at rest or during activity in any environment. Therefore the old hypothesis that cramps are due to dehydration and a lack of sodium and other electrolytes lacks scientific back-up. Current research has shown no link between high temperature and the development of cramps. The current thought on the cause of cramps is alterations in spinal neural reflex activity which is caused by fatigue and is more common in some than others.
The term heat cramps is rapidly falling out of favour due to the lack of scientific evidence to support it.
Heat exhaustion is now more correctly known as exercise-associated collapse and is a form of heat injury, which is less severe than heat stroke. It usually occurs after the athlete has completed an event.
Exercise-associated collapse (heat exhaustion) following activity is thought to be due to something known as postural hypotension. This is low blood pressure caused by pooling of the blood in the limbs, instead of being returned back to the heart. During activity the repeated strong contractions of the working muscles act as a second pump, forcing blood back to the heart. When exercise suddenly stops, the efficiency of this venous return is decreased.
As collapse occurs after the cessation of exercise, it is clear that dehydration is not the cause. Individuals also do not display abnormally high temperatures, although this was previously thought to be the case because of the name 'heat exhaustion'. For this reason the term exercise-associated collapse is now more favorable.
Symptoms of heat exhaustion
- Nausea or vomiting
- Muscle cramps
- Excessive sweating
If left untreated and in high temperatures, exercise-associated collapse can progress and become heat stroke. This is a far more serious condition. A progression to heat stroke occurs when:
- Rectal temperature rises above 41 degrees centigrade
- Systolic blood pressure drops below 100 mmHg (i.e. 95/65, normal is 120/80)
- Heart rate is elevated beyond 100 bpm
Treatment of heat exhaustion
- Lay the individual on their back with the legs raised
- Give the individual a sports drink (provided there is no evidence of hyponatremia)
- Improvement should be noted within 10 minutes and the individual is usually able to stand and walk within 30 minutes
- Intravenous fluids are only administered if heart rate remains high and blood pressure does not rise.
Heatstroke is a life threatening condition which requires immediate medical attention. It is a form of hyperthermia (elevated body temperature), as are heat exhaustion and cramps, which are less severe forms.
Exercise in hot climates, especially those with high humidity, is a common cause of heat stroke. In these conditions the body struggles to cool itself due to the decreased effectiveness of sweat evaporation. Dehydration is also another common cause of heat stroke, where the athlete or other individual (commonly children, elderly and manual laborers working outside) has not taken in enough fluid to replace that lost during sweating.
What are the symptoms of heatstroke?
Although symptoms do vary slightly between individuals, these are the most common symptoms:
- High body temperature (rectal temperature above 41 degrees centigrade / 106 degrees Fahrenheit)
- Tacchycardia (high pulse rate)
- High breathing rate
- Confusion / irritation / aggression
- Hypotension (low blood pressure)
- Unconsciousness / coma
Some individuals may demonstrate the symptoms of heat cramps or exercise-associated collapse first, and if left untreated, this may progress on to become heatstroke.
How is heatstroke treated?
The aim of treatment of heatstroke is to reduce body temperature as soon as possible. It is the delay in initiating body cooling which causes heatstroke to become a potentially fatal condition. The faster this is done, the better the prognosis.
The first step is to notify the emergency services. Whilst waiting for their arrival, the following can be done to initiate cooling:
Place the individual in a bath of cold water for 5-10 minutes. During this time their body temperature should decrease considerably. This should not be continued for longer than 10 minutes, or if the athlete begins to shiver, as this may induce hypothermia (low body temperature). If a cool bath is not possible, other options include spraying the individual with cold water from a hose, and laying towels soaked in cold water over the body.
Give the athlete a cool drink (ideally a sports drink containing electrolytes) and fan them to help with evaporation of sweat.
Once emergency services arrive intravenous fluids may be given to combat any associated dehydration. Hospital admission occurs frequently following heatstroke, to ensure that cooling is maintained and there are no further complications.