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Heat Stroke and Hyponatremia

The most likely cause of death during hot weather sports is heat stroke, when the body temperature rises so high that it cooks the brain (Medicine and Science in Sports and Exercise, July 2008). Nobody should ever die of heat stroke because your body sends you warning signals as your temperature rises. Those most likely to suffer heat stroke are those who have arteriosclerosis, are overweight or are in poor shape. The treatment for a person who collapses from heat stroke is immediate immersion in cold water.

In 1965, I almost died from heat stroke in an unimportant local race in Arlington, Virginia. I am still embarrassed by the stupidity that I showed when I ignored all of the warning signs as my temperature continued to climb.

Signs of impending heat stroke
First your muscles are affected, then your circulation and then your brain. As your temperature starts to rise, your muscles feel like a hot poker is pressing against them.

It is normal for intense exercise to make your muscles burn, but hard exercise does not cause painful burning that feels like fire. Furthermore, the burning of hard exercise is relieved by slowing down. The muscle burning of impending heat stroke does not go away when you slow down.

As your temperature rises further, the air that you breathe feels like it's coming from a furnace and no matter how rapidly and deeply you try to breathe, you can't take in enough air. When you exercise intensely, you can become very short of breath, but the air you breathe will not burn your lungs. Burning in your lungs, not relieved by slowing down, signals impending heat stroke.

When you feel that the air is so hot that it burns your lungs, stop exercising. This sign means that your heart cannot pump enough blood from your exercising muscles to your skin, so heat is accumulating rapidly and your temperature is rising rapidly. Your temperature is now over 104 and continuing to exercise will raise your body temperature even further and it will start to cook your brain.

Your head will start to hurt, you'll hear a ringing in your ears, you may feel dizzy, you may have difficulty seeing and then you will end up unconscious. Your temperature is now over 106 and your brain is being cooked just as the colorless portion of an egg turns white when it hits the griddle.

When does heat stroke occur?
Almost all cases of heat stroke occur when you suddenly increase the intensity of your exercise, such as the finishing sprint of a long distance running or cycling race, or an intense run down the field in soccer.

How body temperature can rise uncontrollably
An excessive rise in body temperature is caused either by producing too much heat or by inability to dissipate the extra heat. When you exercise, almost 80 percent of the energy that is used to drive your muscles is lost as heat. That means that the harder you exercise, the more heat you produce.

During exercise, more than 70 percent of the energy used to drive your muscles is lost as heat. Your heart has to pump extra blood from your hot muscles to your skin where you sweat. Sweat evaporates and cools your skin to dissipate the heat. The harder you exercise, the more heat your muscles produce. Everyone who exercises, particularly in hot weather, has to sweat to keep the body temperature from rising too high.

Drugs can cause heat stroke
Heat stroke is more likely to be caused by inability to get rid of heat than by producing too much heat. Stimulants such as amphetamines or cocaine can kill athletes by preventing them from getting rid of heat by blocking sweating and blood flow to the skin. A single nasal dose of cocaine can block blood flow to the skin and sweating, to prevent a person from cooling his own body (Annals of Internal Medicine, June 4, 2002).

When a person passes out from heatstroke, get medical help immediately. Any delay in cooling can kill him. Carry the victim rapidly into the shade and place him on his back with his head down and feet up so blood can circulate to his brain. Cool him by pouring on him any liquids you can find or spray him with a hose. It doesn't make any difference what you pour on him: milk, Coca Cola, beer, or anything else. Evaporation of any liquid cools. As you cool him, he will then wake up and talk to you and act like nothing has happened. While he's sitting or lying there, his temperature can rise again and he can go into convulsions or pass out again, so he must be watched for at least an hour.

An athlete or exerciser who passes out from overheating should be immersed in cold water immediately to prevent brain and multiple organ damage. However, a heart attack can also cause a person to pass out and this should not be treated with cold water immersion. Therefore always get medical help immediately when you see a person pass out during exercise.

Heat stroke is caused by continuing to exercise intensely in spite of all the warning signals that the body presents. Dehydration also increases your risk for heat stroke.

When you compete in sports, you need to drink before you feel thirsty, because you slow down and lose power long before you have any signals to tell you that you are dehydrated. In warm weather, trail runners raced 12 km (7.2 miles) much faster when they took fluids (Journal of Athletic Training, March-April 2010). With fluids, they averaged 53.1 minutes compared to 55.7 minutes without fluids. Immediately after the race, the dehydrated runners had signs of greater body stress such as heart rates six beats per minute faster and intestinal temperatures .22 degrees C higher.


Thirst is a late sign of dehydration
You won't feel thirsty during exercise until you have lost between two and four pints, or two to four pounds. Thirst is a very late sign of dehydration. You sweat during exercise, and since sweat contains much less salt than your blood, you lose far more water than salt during exercise. As blood salt levels rise higher and higher, they trip off special osmoreceptors in your brain to tell you that you are thirsty. Since it takes a long time for blood salt levels to rise high enough to tell you that you are thirsty, you will be severely dehydrated long before you feel thirst.

You need more sugar in hot weather
During long sports competitions, you need to take sugar as well as fluid because running out of sugar stored in muscles slows you down. The only mineral that you need to replace during exercise is common table salt. Water or your favorite drink plus food containing sugar and salt are just as effective as any sports drink to maintain endurance and prevent heat exhaustion. The best exercise drink is the one that tastes best to you, because that's what you will drink the most (International Journal of Sport Nutrition and Exercise Metabolism, January 2002).


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Up to fifteen years ago, athletes were advised to drink as much as they could to insure that they did not lose any weight during endurance competitions. This caused a condition called HYPONATREMIA which has killed some novice cyclists, runners and athletes in other endurance sports. It occurs almost never in trained athletes because it is most likely to occur in people who slow down so much that they spend too much time drinking fluids and too little effort maintaining pace. During competitions, you work so hard to maintain pace that you have to conscientiously work just to drink enough.

How hyponatremia kills
Hyponatremia is caused by drinking too much fluid, not by excessive loss of salt in sweat or by the stress of exercising. The extra fluid expands blood volume and dilutes blood salt levels. This causes blood salt levels to drop too low, while brain salt levels remain normal. Fluid moves from an area of low salt concentration into areas with high salt levels, so fluid moves from the bloodstream into the brain, causing brain swelling. Since the brain is enclosed in the skull, which is a tight box, the brain expands and has nowhere to go, so it is squashed to cause headache, nausea, and blurred vision.

Blood tests only way to diagnose hyponatremia
Since the symptoms of hyponatremia are the same as those caused by pure dehydration with normal blood salt levels, the only way to diagnose the condition is with blood tests. As blood salt levels drop even lower, the person becomes confused, develops seizures and falls unconscious. You should suspect hyponatremia when the event takes more than four hours and the athlete has been drinking often during the event. Anyone who is confused, passes out or has seizures should be sent to a hospital immediately. Hyponatremia requires skilled management because the first impulse of an inexperienced physician is to give intravenous fluids, which dilute blood salt levels further, causing more brain swelling that can kill the patient.

How much fluid should you drink?
You will not become thirsty during exercise until you have lost between two and four pints of fluid, so you can't wait for thirst to encourage you to drink. Dehydration makes you tired and it is unlikely that you can replace the lost fluid during a race after you have become thirsty. The American College of Sports Medicine recommends a limit of 1200cc (5 cups, 2.5 pints, a little over 1 quart, or 2 average size water bottles) per hour, but for a person who is not exercising near his or her maximum, this could be too much (Clinical Journal of Sport Medicine, July-August 2005). A person exercising near his capacity and not slowed down by fatigue probably does not have to worry about limiting fluid intake. He is working so hard to maintain intensity that he doesn't have enough time to drink too much. On the other hand, people slowed down by fatigue or those who are out of shape should limit fluid intake, probably to less than two water bottles per hour.

Drink to avoid thirst
No studies show that forced drinking of fluids is any more effective than just drinking frequently to avoid thirst (Annals of Nutrition and Metabolism, November 2010). So current advice is to drink frequently, but just try to avoid feeling thirsty. You are in trouble with dehydration when you start to feel thirsty. Thirst is such a late sign of dehydration that once you feel thirsty, it is too late for you to catch up on your fluid needs during competition.


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May 22nd, 2011
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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