What are heat injuries?

Heat injuries, heat disorders, heat-related illness: all refer to the same entity namely a medical condition arising from failure of the body to properly regulate its heat level, resulting in over-heating. Heat injuries are common in physical activities of a strenuous nature (sports, outdoor activities, endurance events). These activities do not need to take place in an outdoor setting for someone to be at risk of developing a heat injury. It is important to realize that heat injuries can have very serious consequences including damage and failure of major organs in the body leading to death.

 

What causes heat injuries in active people?

In order to function properly, the body has to operate in a very narrow range of internal or core temperature. This optimal body temperature range is 36.1 – 37.8 degrees Celsius. In strenuous physical exercise particularly of an endurance nature, the body core temperature may rise above this optimal range. Heat injuries occur when the body is unable to manage this raised core temperature. Thus,

(1) Heat injuries result from an imbalance between the heat gained by the body during physical exercise, and attempts by the body to shed this heat.

(2) When a person exercises, heat is produced or gained by body as a result of the metabolic activities (eg. muscle contractions) and heat gained from the environment. The more intensely muscles contract and the longer they do so, the greater the heat generation within the body. The higher the ambient air temperature, the greater the heat gained by the body.

(3) Heat is lost from the body by several processes of which evaporation is the main means of heat loss (80%) when exercising. Evaporation takes the form of loss of water vapor in sweat. Anything that causes a person’s sweating response to be reduced will result in less heat loss, and therefore a build-up of heat in the body.

 

What factors place people at higher risk of developing heat injuries?

Certain factors can increase the risk of heat injuries.

(1) Weather conditions. Hot, humid and sunny environments are associated with higher incidence of heat injuries. The temperature of a person’s skin is lower than that of his body core, usually 30-32 deg C. Thus when the air temperature is higher than this, heat is gained by the body through radiation. If the air humidity is high, this reduces sweat evaporation and leads to decrease in heat loss through sweating. It should be appreciated that heat injuries can occur at night if the conditions of heat and humidity are high. In sunny weather, there is heat gain from the solar radiation. There is less heat gain and improved heat loss when the weather is cloudy, cool and breezy.

(2) Prolonged, high intensity physical exercise or activity. Such activity increases the body’s heat gain as a result of repeated muscle contractions, while the duration of the activity may prolong a person’s exposure to unfavorable environmental conditions.

(3) Insufficient hydration status. Hydration is important for maintaining the body’s blood volume, providing water for sweating, and overall for lowering of the body core temperature. In situations where a person is poorly hydrated or dehydrates as a result of physical exercise, a rise in body core temperature occurs which predisposes to heat injury.

(4) Illness. If a person has recently experienced or is experiencing an illness with fever, the body’s temperature regulation processes may be altered in their efficient and effective response to heat gain from exercise. This may result in higher than normal rises in body core temperature. A person who has had an illness with diarrhea or vomiting may have lost both water and electrolytes. This lowers the body’s hydration and electrolyte status, which may make the person more prone to heat injury.

(5) Insufficient cardiovascular & sweating response. Certain situations lower the efficiency of the body’s cardiovascular and sweating responses, thus increasing the risk of heat injuries. This includes poor physical fitness, insufficient physical conditioning to meet the demands of the exercise, insufficient acclimatization, and in the young & elderly. In addition, exercise in hot humid conditions increases the body’s oxygen uptake and the use of glycogen by the exercising muscles. This results in increased lactate production and earlier depletion of the glycogen stores, leading to premature physical fatigue and exhaustion.

 

What are the signs of heat injury?

Heat injuries represent a continuum of medical condition rather than the usual perception of distinct injuries: heat cramps, heat exhaustion, and heat stroke.

The current opinion on muscle cramps is that these arise from muscle fatigue and not from over-loss of electrolytes.

(1) Heat exhaustion is conventionally considered as due to the inability of the cardiovascular system to meet the needs of both the contracting muscles and the supply of blood to the skin for sweating processes. The common signs of this include thirst, fatigue, weakness, and headaches.

(2) Heat stroke is generally considered when the body core temperature exceeds 40 deg C, and represents a failure of the body’s temperature regulation mechanisms. The person will have experienced the signs of heat exhaustion and may then show signs of decreased sweating, feeling faint or dizzy, mental irritation (confusion, altered consciousness, irrational behavior), and finally collapse.

In addition, there may be feelings of thirst, by which time the body has already started to dehydrate. Salt staining of clothing indicates a loss of minerals associated with excessive sweating.

 

How is a heat injury treated?

Once a person shows signs of heat injury, treatment should begin immediately. This includes:

• Reducing the intensity of physical exertions, stopping if necessary
• Movement to a cooler location (eg. more shade, breezy, under a fan, in an airconditioned room)
• Consumption of water or electrolyte-containing fluids, preferably cooled to lower body core temperature and to improve absorption
• Removal of clothing to assist with cooling, wherever practical
• Application of cooling methods (eg. towels soaked in cold water, ice packs, immersion in cold water, etc)
• Medical treatment if required, including resuscitation.

What measures can prevent heat injuries?

The prevention of heat injuries involves all individuals who participate in strenuous physical exercise, as well as the planners and managers of physical exercise, sports and activities.

(1) Awareness of heat injuries. This requires a systematic education process including the causes, signs, treatment and prevention of heat injuries. Continuing awareness of heat injury risk throughout any physical exercise or activity will improve the vigilance and repeated reminders of prevention measures.

(2) Event timing. Plan to conduct activities in the cooler parts of the day (early morning, night). This planning should take into account the ambient weather conditions, and the nature of the event. For example, some triathlon events have been successfully conducted in hot humid environments while starting the event at midday. The thinking was that the relative coolness of the sea swim phase and adequate hydration while cooled by convection on the bike led to a run which took place as the sun was going down, creating less solar load than a run which ended at midday.

(3) Weather awareness. Monitoring of air temperature, relative humidity and solar radiation (combined as the Wet Bulb Globe Thermometer reading) will facilitate event timing, modification of physical exercise (eg. running distance, time of exposure to sun, etc), provision of preventive measures (eg. more drink stations), and timely reminders to participants.

(4) Individual participant measures (including event organizers and staff). These include:

• Wearing loose woven, light-coloured clothing. Synthetic materials with tight weaving should be avoided for participants and event staff.
• Regular and adequate hydration (see hydration guidelines below)
• Self-reporting of signs of not feeling well, especially signs of heat injury
• Matching of physical exertion level to fitness/conditioning status, weather conditions, and in light of recent illness
• Reducing exertion level or stopping the activity when feeling unwell.

In addition, people who are participating in physical activities in hotter conditions than they are usually accustomed to should spend time adapting to the new conditions. Such acclimatization allows the body to adjust such processes as blood flow and sweating to meet the higher heat loss demands. Guidelines for such acclimatization include the conduct of repeated, progressive and increasingly prolonged exercise bouts in the heat over a period of 5-10 days and at moderate to high exercise intensity.

(5) Event measures. Heat injuries can affect event staff as well as participants. Preventive measures may include:

• Provision of areas of shade (eg. tents) for participants to prepare and rest in, or shelters for event staff
• Drink stations at regular intervals during events (eg. running routes) as well as provision of fluids to event staff. Fluids may include water as well as sports drinks
• Cooling measures such as sponges or towels which have been immersed in cold water
• Electric fans in recovery or rest areas
• Regular breaks during prolonged activities during which rest is in cooler shaded areas and hydration fluids are provided
• Constant vigilance for signs of heat injury, as well as verbal checks with weaker or tiring participants

What is the importance of adequate hydration in preventing heat injuries?

 Adequate hydration is essential to the successful prevention of heat injuries. This involves both adequate levels of water as well as minerals (electrolytes):

(1) Water. The adequacy of hydration may be measured as the % of body weight lost during exercise. It is common for long distance runners to lose as much as 6-8% body weight. Water loss results in lowered circulating blood volume which reduces blood pressure as well as blood flow to the working muscles and sweat-producing skin. This results in increased heart rate as well as body core temperature. When water loss results in as little as 2% body weight decrease, there are early signs of dehydration as well as reduced aerobic endurance. Thus, hydration has an important performance goal as well as heat injury prevention.

(2) Electrolytes. Important minerals such as sodium (Na), potassium (K), and magnesium (Mg) are found in sweat. In some situations – intense physical exertion, prolonged exercise, hot weather – the body’s sweating rate increases and more electrolytes are lost in the sweat. It is important to replace these losses for both prevention and performance. Most sports drinks contain electrolytes for this purpose.

Some useful guidelines for successful hydration are:

• Hydrate before the physical exercise and as part of your recovery from the previous exercise or activity
• Drink right up to the commencement of the physical exercise
• Drink during the exercise, especially if this is prolonged
• Apart from water, it is recommended that your hydration fluid contains carbohydrate to replace energy used during the physical exertion, as well as electrolytes
• Cooler fluids assist in cooling the body, and are absorbed faster by the body
• In endurance sports events, practice hydration during training sessions.