Dehydration and Heat Injury

Last summer (July 1994) a well conditioned athlete entered a 1/2-Ironman distance race with hopes of putting in her best performance. She was well trained and had raced in the heat before. It turned out to be a very hot west Texas day (110 degrees F, 45% humidity). Things were going well until the run. After one mile, she experienced diarrhea and painful quadriceps muscle cramps. By mile 6 1/2 she had headaches and had stopped sweating. She managed a slow, painful finish, tried to hydrate, but became delirious. The medical staff was called. Intravenous fluids were started, but she deteriorated and began vomiting. After transfer to the hospital she had seizures. She experienced widespread muscle breakdown, severe electrolyte disturbance, kidney damage, and her lungs filled with fluid. She was placed on artificial life support and was given a 50:50 chance of survival. Fortunately, she lived, but is still recovering.

This case indicates that dehydration and heat injury are potentially very serious consequences of competing in triathlons. No one knows the exact number of triathletes treated each year for dehydration, but it is the most common ailment treated in the medical tent. Almost all triathletes are dehydrated to some degree when they finish a race. The longer the race the bigger the problem. The purpose of this article is to familiarize you with conditions that lead to dehydration and heat illness, and to show you ways to prevent it.

Thermal regulation As can be seen in figure 1, body temperature is raised by environmental conditions and exercising muscle. Cooling is accomplished primarily by the evaporation of sweat. The most important barrier against effective cooling is humidity.

Humidity is not your friend The rate of sweating is higher in humid conditions but the cooling is less. The reason is that because the air is already very saturated with water, sweat can't evaporate. Sweat that beads up and rolls off doesn't function in the cooling process. However, this "futile sweat" does deplete the body of vital water and salt. As dehydration progresses cooling becomes more difficult. Performance drops and heat injury becomes a real threat. Deaths have occurred when the air temperature was less than 75 degrees F (24 degrees C) but the relative humidity was above 95%.

Recognizing symptoms of heat injury There are three stages to heat illness; heat cramps, heat exhaustion, and heat stroke -- listed in order of increasing severity. Often the border between them is blurred into a continuous spectrum. Heat cramps are due to muscle spasms and often occur in the arms, legs, or abdomen. They are thought to be caused by dehydration and loss of salt and other electrolytes. Heat exhaustion is due to more profound loss of water and electrolytes. It is characterized by generalized weakness, headache, dizziness, low blood pressure, elevated pulse, and temperature elevation as high as 104 degrees F (40 degrees C). Both can usually be treated by moving out of the sun, drinking fluids, and eating salty food.

Heatstroke is a life threatening condition and represents severe dehydration, high body temperature, and a shut-down of the cooling mechanisms. The athlete may be delirious or comatose, and half of the victims have stopped sweating. The pulse is rapid and weak, the blood pressure is low and body temperature is greater than 105°ree;F (40.6°ree;C) and may reach as high as 110°ree;F (43°ree;C). The oral temperature is notoriously inaccurate in these circumstances. Damage to the brain, heart, lungs, kidneys and other organs may occur. Sometimes despite the best medical care, death is the end result.



Choice of fluid

A full discussion of all the available beverages and the science behind their content is beyond the scope of this article. For races lasting less than 1 hour, water alone is adequate. For longer races, there are many commercially available sports drinks. The most important features are taste, carbohydrate and sodium content.

Taste is important, because if you don't like it you won't drink it. Don't try a new sports drink on race day. Carbohydrate content in the range of 4 - 8% is best for endurance races. Levels above 10% are poorly absorbed and can cause diarrhea. Most sports drinks have a sodium content in the range of 10 - 20 mmol/liter (Gatorade is 23 mmol/liter = 1.3 grams of salt per liter). Higher levels are better for salt replacement, but tend to be less palatable. There doesn't appear to be any advantage gained from adding other electrolytes (e.g.. potassium, magnesium) since the diet is usually adequate to replace these.

Why is Salt important

Sweat contains between 2.25 to 3.4 grams of sodium chloride per liter. A sweat rate of 1 liter per hour would thus cause a salt loss of 27- 40 grams for a 12 hour race. Failing to replace salt during the race can result in hyponatremia (low salt concentration in the blood). From experience with the Hawaiian Ironman, Hiller (1) has recommended that athletes ingest an average of 1 gram of sodium per hour for hot races lasting longer than 4 hours. (2.5 grams of sodium chloride has 1 gram of sodium and 1.5 grams of chloride. One teaspoon of salt weighs approx. 6.6 grams) It is also advisable to increase salt intake for several days before a long race.

Exercise intensity and duration

The higher the intensity of exercise the greater heat production by the muscles. Overheating causes more sweat production. The net fluid lost per hour is greater for an Olympic distance triathlon than an ultradistance event. However, the much longer exercise time in an ultradistance race causes a greater problem with dehydration.

Never forget that how hard you push yourself during a race is under your direct control. You can hammer, dehydrate, not finish and hurt yourself, or you can slow down, finish the race and survive. When faced with unusual circumstances be conservative and cautious.

Know your body

There is large variability between individuals with respect to net water loss while exercising in the heat. This depends upon sweat rate, rate of fluid ingestion, rate of gastric emptying, type of fluid ingested, percentage body fat, and many other variables. Because of this there is no simple answer for which fluid to drink, how much, and how often. So how do you know what is right for you? The best way to determine this is to test yourself (see fluid balance test) > Often we spend a lot of money and time on equipment, but neglect determining our body's needs. You should become familiar with what you need to do to stay hydrated under a variety of conditions. Keep a training log about your experiences. Change only one variable at a time to develop a plan that works.