dive medicine 

Heat Exhaustion and Heatstroke

By Paul S. Auerbach, M.D.

I remember vividly a dive trip to the Philippines, where we often had to shuttle from our large transoceanic vessel in small outboard wave runners to distant dive sites a half-hour away. We would don our dark-black wet suits, and suit up with tanks and fins by midway through the ride. It was very hot and humid, and no one carried any water to drink for the “quick trip” to the dive site. With the torrid sun beating down, the spray thrown up by the bouncing bow was a welcome cold jolt to counter the stifling heat. On at least one occasion, the heat overwhelmed us. We stopped the boat, stripped down and jumped into the ocean to cool off. One elderly diver had taken anti-diarrhea medication. The drug had the side effect of inhibiting his ability to sweat, and one particularly hot boat ride rendered him nauseated, weak and confused. He had become a victim of heat exhaustion.


Heat exhaustion and heatstroke are part of the same continuum, but of differing severity. Heat exhaustion is illness caused by an elevation of body temperature. It does not result in permanent damage. Heatstroke is life-threatening and can permanently disable the victim. Divers are predisposed to heat-related illnesses: They are often in the sun, may be dehydrated, wear restrictive garments (particularly heavy wet suits) that absorb heat but don’t allow its dissipation, use drugs that might inhibit sweating and undertake vigorous physical activity in hot and humid climates without a proper acclimatization (physiological adaptation) period. In this article, I will discuss heat exposure on land as well as at sea, because dive trips are often land-based in the tropics, and many diving adventures are combined with explorations and athletic activities on land.
The signs and symptoms of heat exhaustion are minor confusion, irrational behavior, a rapid, weak pulse, dizziness, nausea, diarrhea, headache and mild temperature elevation (up to 105˚F/41˚C). (For information about how to measure a person’s temperature, see the “Dive Medicine” column in the May 1999 issue of Dive Training.) It is important to note that sweating may be present or absent and that the skin of the victim may feel cool to the touch.
The signs and symptoms of heatstroke are extreme confusion, unconsciousness, low blood pressure, seizures, increased bleeding (bruising, vomiting blood, bloody urine), diarrhea, vomiting, shortness of breath, darkened urine and major core body temperature elevation. (Up to 115˚F/46.1˚C has been observed.) Again, it is important to note that sweating may be present or absent. The skin will usually be warm or hot to the touch when a victim suffers heat exhaustion or heatstroke, but again, this is not absolutely constant. It is rare for someone to feel cool externally when his temperature exceeds 105˚F, but it is not impossible.

Treating Heat-Related Illness

The most important aspect of therapy is to lower the temperature as quickly as possible. The body may lose its ability to control its own temperature when it approaches 106˚F/41.1˚C, so from that point upward, temperature can skyrocket. Help the victim to get fresh air. If oxygen is available and there is someone trained in oxygen therapy, administer low-flow oxygen to the victim. Do not give liquids by mouth unless the victim is awake and capable of purposeful swallowing. Cooled liquids do not assist the cooling process enough to risk choking the uncooperative or confused victim.

Muscle Cramps

Muscle cramps in a warm environment accompany overuse or water and salt losses in the individual who exerts strenuously. A well-trained athlete can lose 2 to 3 liters of sweat per hour (a potential 20-gram sodium loss each day). In most cases, cramps are caused by replacement of water without adequate salt intake. Divers may get cramps by repetitive use of certain muscle groups, usually the legs when kicking vigorously underwater.
Treatment for cramps consists of gentle motion, massage and stretching of the affected muscles, accompanied by fluid and salt replacement. This may be done by drinking water and balanced salt solutions or sports beverages prior to and during heavy exertion. With proper fluid and electrolyte replacement, salt tablets (which irritate the lining of the stomach and can cause nausea) are usually unnecessary. Do not drink seawater in an effort to replace salt.


Fainting due to heat exposure occurs when a person (particularly an elder) adapts by dilating blood vessels in the skin and superficial muscles in order to deliver warm blood to the surface of the body, where the excess heat energy can be delivered back to the environment. The expansion of the superficial blood vessels allows a greater-than-normal proportion of the circulating blood volume to be away from the central circulation (which supplies, among other organs, the brain).
This lack of sufficient central pressure is worsened when a person is on his feet for a prolonged period of time, as gravity allows a significant blood volume to pool in the lower limbs. Combined with fatigue and mild dehydration, the diversion of blood leads to a fainting episode, as not enough blood (with oxygen and glucose) is pumped to the brain. Dehydration can also stimulate the vagus nerve, which causes the heart rate to slow (“vasovagal” episode).
A victim who has suffered a fainting episode in the heat should be examined for any head or neck injuries, as well as other possible breaks or cuts. Other causes of fainting (low blood sugar, abnormal heart rhythm, etc.) must be considered. If fainting is due to the heat, the victim will reawaken shortly, as assuming a horizontal position returns blood to the brain and solves the major problem. In general, body temperature is not elevated.
The victim of a fainting spell due to heat should be rested in a horizontal position for 15 to 30 minutes and should not immediately assume a standing posture without first sitting for five minutes. He should be encouraged to consume a pint or two of cool, sweetened liquid (such as GatoradeTM). To avoid further episodes, efforts should be made to avoid dehydration, missing meals or standing in one position for a prolonged period. A person who feels lightheaded or dizzy should immediately lie down, or at least sit and lower his head to a position between his knees.

Avoiding Heat Illness

1. Avoid dehydration. Drink a pint of liquid 10 to 15 minutes before beginning vigorous exercise. Drink at least a pint to a quart of liquid with adequate electrolyte supplementation (see below) each hour during heavy exercise with sweating in a hot climate. Adequate water ingested during exercise is not harmful, does not cause cramps and will prevent a large percentage of cases of heat illness. Encourage rest and fluid breaks.
Divers are at significant risk for dehydration, because their thirst often does not keep up with the amount of fluid lost due to breathing dry air from a tank through a regulator, sweating in a hot climate, and excessive urinary losses because of the cold and pressure associated with being underwater. The water loss due to diving is not associated with the same degree of salt loss as is seen with extreme sweating.
Urine color is an excellent general measure of your state of hydration. If the urine becomes darkened or scant, fluid requirements are not being met. The urine color should be observed in the evening (or any other time that is at least three to four hours after a dive) in order to allow it to reflect the effect, if any, of dehydration. The urine color immediately after a dive, when you feel a great urge to urinate, will often be clear because it represents a lot of water without salt and waste products of metabolism. You can be misled if you don’t allow a few hours after the dive to check out the color.
For the most part, people outdoors (which includes people diving) should consume at least 3 quarts/2.8 l of fluid each day to replenish fluid lost through urination, exhaled moisture, skin evaporation and defecation. With moderate activity, this should be increased to at least 4 to 5 quarts/3.8 to 4.7 l. Again, do not rely on thirst as an absolute guide to fluid requirements.
With a normal diet, there is no need to take salt tablets. Electrolyte requirements can be met with food salted to taste. Electrolyte- and sugar-enriched drinks, such as Gatorade, should be used when normal meals cannot be eaten or when sweating is excessive. Coffee, tea and alcohol-containing beverages cause increased fluid loss through excessive urination (diuretic effect) and should be avoided. That beer after you dive may taste good, but it is not an effective way to rehydrate.
2.Be watchful of the very young and very old. Their bodies do not regulate body temperature well and can rapidly become too hot or too cold. Do not bundle up infants in warm weather.
3.Stay in shape. Obesity, lack of conditioning, insufficient rest, ingestion of alcohol and certain drugs all contribute to an increased risk for heat illness.
4.Condition yourself for the environment. Gradual increased exposure to work in a hot environment will allow an individual to acclimatize.
5.Wear clothing appropriate for the environment. Dress in layers, so that clothing can be added or shed as necessary. Clothing should be lightweight and absorbent. Wear a loosely fitted, broad-brimmed hat.
6.Towel off the face and scalp frequently. Fifty percent of sweating occurs from these areas.
7.Keep out of the sun on a hot day. Resting on hot ground increases heat stress. The sun can heat the ground by more than 40˚F/4˚C above the air temperature.
8.Avoid taking drugs that inhibit the sweating process (e.g., atropine, antispasmodics, anti-motion sickness), diminish cardiac output (“beta blockers”), disrupt certain features of physiologic activity (antidepressants, antihistamines), increase muscle activity (hallucinogens, cocaine), or promote dehydration (diuretics).
One of the fringe benefits to summertime or tropical diving is the sunshine and warm breezes. It’s fine to soak up the warmth, but do it with a full understanding that you must manage your environment properly in order to have a safe dive trip.
Divers are predisposed to heat-related illnesses: They are often in the sun, may be dehydrated, wear restrictive garments (particularly heavy wet suits) that absorb heat but don’t  allow its dissipation,   use drugs that might inhibit sweating and undertake vigorous physical activity in hot and humid climates without a proper acclimatization (physiological adaptation) period.


Cooling the Victim

1.Remove the victim from obvious sources of heat. Shield the victim from direct sunlight and remove his or her clothing.
2.Wet down the victim and begin to fan him vigorously. Evaporation is a very efficient method of heat removal. Use cool or tepid water. Do not be concerned with shivering, so long as you continue to aggressively cool the victim. Do not sponge the victim with alcohol, because this can be absorbed through the skin and become toxic.
3.Place ice packs in the armpits, behind the neck and in the groin.
4.Recheck the temperature every five to 10 minutes to avoid cooling much below 98.6˚F/37˚C. When you have cooled the victim to 99.5 to 100˚F/37.5 to 37.8˚C, taper the cooling effort. After the victim is cooled, recheck the temperature every 30 minutes for three to four hours, as there may be a “rebound” temperature rise.
5.If the victim is alert, begin to correct dehydration. Be certain that the concentration of carbohydrate or sugar in the beverage does not exceed 6 percent so as not to inhibit intestinal absorption. Try to get 1 to 2 liters of fluid into the victim over the first few hours. For every pound of weight loss attributed to sweating, have the victim ingest a pint of fluid. This may take up to 36 hours.