Heat Illness for Runners: Causes and Prevention
by Dr. Cathy Fieseler
As the temperature rises, the risk of heat illness increases. During exercise, a significant amount of heat is generated. In cool weather, this generated heat is transferred to the air. In hot weather, this transfer of heat is inhibited, causing a greater increase in the body temperature. Sweating occurs to help regulate body temperature; the evaporation of sweat from the skin results in dissipation of heat. As the humidity rises, there is a decreased rate of evaporation of sweat, diminishing the cooling effect.
Exercising in the heat places a great stress on the body. Performance is decreased; heart rate is increased compared to the same level of activity at a cooler temperature. The risks of dehydration and heat illness increase as the temperature and humidity increase.
The body will adapt to repeated episodes of exercise in the heat to reduce the impact of heat on performance and the risk of heat illness (the risk of dehydration is not reduced). With acclimatization, the amount of fluid circulating in the body is increased to meet the demands of exercising muscles. Sweating will occur earlier in the course of exercise and at a lower core temperature. The rate of sweat production is increased and the sodium content of the sweat is decreased. These adaptations will occur over a 7 to 15 day period and will persist for up to 3 weeks after leaving a hot environment.
No one is immune to the risk of heat illness, but the risk is greater in those who are out of shape, overweight, dehydrated, ill, taking certain medications or supplements, or are not acclimatized to the heat. Wearing excessive clothing (i.e. tights) or a helmet and padding decreases the body’s ability to dissipate heat resulting in an increase in core temperature.
Heat illness from exertion is a spectrum of disorders ranging from heat cramps to heat stroke. Dehydration is a major factor in each of these disorders; dehydration negatively impacts the body’s ability to dissipate heat and athletic performance. Symptoms of dehydration may include thirst, irritability, dizziness, nausea, vomiting, cramps, chills, weakness and fatigue. Checking weight prior to and following exercise provides guidelines for assessing the degree of dehydration; each pound that was lost represents a 16 ounce fluid deficit that needs to be replaced.
The exact cause of muscle cramps is not truly known, but cramping tends to occur later in activity, in association with fatigue and dehydration. Athletes who lose a lot of salt in their sweat (white crust on clothing and skin) often experience muscle cramps.
Gentle stretching and massage may help reduce pain from cramping. Lost fluids and salt must be replaced in the diet.
As a result of strenuous activity in the heat, the body may not be able to deal with the increasing demands on the cardiac and circulatory systems, causing heat exhaustion. Symptoms may include severe fatigue, dizziness, fainting, chills or goose bumps, dehydration, headache, nausea, vomiting, diarrhea and stomach or muscle cramps. The core (rectal) temperature is elevated, but typically less than 104o F (40oC).
The athlete should be removed from the heat and cooled off. Lay him/her with the legs elevated. Ice packs on the groin, armpits and neck may be used for cooling. Have the athlete drink cool water or sports drink. If disorientation develops, vomiting prevents rehydration or any symptoms worsen, the athlete should be transported to the emergency room.
The athlete should be evaluated by a physician prior to return to exercise following an episode of heat exhaustion. A gradual return to full activity and intensity is recommended.
Heat stroke due to exertion is the result of the breakdown of the thermoregulatory system; this causes severe stress on the circulatory, metabolic and nervous systems. The body temperature rises to extreme levels, causing cell and tissue damage and could result in death.
This is a medical emergency. The core (rectal) temperature is usually greater than 104oF (40oC); temperatures taken in the ear, armpit or orally do not adequately reflect the body temperature. The athlete has mental status changes; he/she may be unconscious, irritable, convulsing, confused or disoriented. Typically the athlete is sweating in the case of exertional heat stroke. Any of the symptoms listed for heat exhaustion may also be present.
The most important part of treatment is to start cooling the athlete immediately as an ambulance is called. Place ice on the neck, groin and armpits, spray with cold water or immerse in a tub of cold water. Monitor the breathing, pulse and temperature closely. Stop the cooling when the core temperature reaches 101o F. The athlete should be transported to the emergency room.
The risk of heat illness can be decreased by slowly increasing activities in the hot weather. Checking your weight prior to and following exercise allows rehydration prior to the next workout. Add salt to your food while exercising in the heat. Limit exercise if you are sick. Stay away from supplements containing stimulants. The risk of heat illness is increased by medications such as certain antidepressants, anticholinergics, antihistamines, diuretics, and beta blockers; use of alcohol and many abused drugs; sleep deprivation; certain chronic medical conditions and obesity. Children and the elderly are at increased risk of heat illness. Be smart and stay safe. Don’t tough it out if you start to feel ill; early treatment of heat illness will prevent a potentially fatal outcome.
Dr. Fieseler is the Director of Sports Medicine for the Trinity Mother Frances Health System in Tyler, TX. She is a regular contributor to Running Times and has served as the chairperson for the sports medicine committee of the Road Runners Club of America. For more information on Dr. Fieseler’s medical services, click on Dr. Cathy Fieseler.
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