Exercise in The Heat and Heat Disorders

This blog entry is dedicated to Dr. Fred Morrison a Floridian. His email inspired me to write about exercising in the heat and how heat affects the body. Thanks Fred!

Exercise in The Heat and Heat Disorders

The principal means by which the body loses heat during exercise or exposure to heat are 1. Circulatory adjustments of increased skin blood flow resulting from cutaneous vasodilation, and 2. Evaporative cooling resulting from increased secretion of sweat. Internal body heat carried by the blood (circulatory convection) to the surface, where conduction, convection, radiation and particularly evaporation take place. The cooled blood then returns to the warmer core and the cycle is repeated. The body temperature changes that occur during exercise in a comfortable environment (room temeperature).  Internal or rectal temperature increases to a new level during the first 30 minutes or so of work and remains at this new level until work is terminated. AT the same time, skin temperature decreases slightly primarily as a result of increased convective and evaporative cooling. The net result of theses changes is an increase in the thermal gradient between the skin and the core, which facilitates  heat loss in the manner previously described.

In a cold or cool environment, exercise that can be maintained for an hour or more is seldom limited by an  excessive increase in internal or rectal temperature. Under these environmental conditions, nearly all the metabolic heat produced can be easily dissipated by the circulatory and sudomotor (sweating) adjustments referred to earlier. Even in sever, short term work, when heat production may well exceed the heat-dissipating  capacity made possible by these adjustments, exhaustion usually results from the buildup of anaerobic metabolites (mainly lactic acid) before rectal temperature can reach a limiting or dangerous level. The elevation of rectal temperature during exercise, although proportional to the intensity of work (and therefore to metabolic rate), is independent of environmental temperatures ranging from cold to moderately warm.

Exercise In The Heat

As stated earlier, environmental heat reduces the thermal gradient between the environmental and the skin surface, and between the surface and the body core, thus imposing an added resistance to body heat loss. We have seen that body heat can actually be gained when the temperature of the environment is greater than that of our skin. By the same token, increased humidity imposes a heat-loss barrier to the evaporative mechanism by decreasing the vapor pressure gradient between moisture in the air and the sweat on our skin. Such a heat-loss barrier cause an excessive increase in rectal temperature and severely limit’s the capacity for work. 

Circulatory System and Sweating Mechanism

The reduced thermal and vapor pressure gradient of hot, humid environments greatly increase the demands placed on the circulatory system and sweating mechanism.  This is evidenced by greater increases in heart rate and sweating during exercise in hot as compared to cool environments. More blood must be circulated and more sweat secreted by the sweat glands to lose any given quantity of heat. Note should  also be made of the effects of hot, dry environments on the magnitude of these receptors. Even though the temperature is high, the low relative humidity considerably reduces the heat stress because evaporation of sweat is more efficient. The major circulatory demands while working in the hear are 1. A large blood flow through the working muscles to provide for the increased respiratory exchange of o oxygen and carbon dioxide, and to carry away the increased heat produced there, and 2. As previously indicated, a large skin blood flow to cool the blood and supply the sweat glands with water. 

Water and Salt Requirement

The high sweat rates required for adequate evaporative cooling during exposure to heat (.5 to 2.0 liters per hour) can lead to excessive losses of water (dehydration) and of salt and other electrolytes. When this occurs, work performance and tolerance to heat are greatly reduced; hypothermia (excessive internal body temperature) with predisposition to serious heat disorders is imminent.

The most serious consequence of profuse sweating is loss of body water. This leads to a decrease in blood volume and, if sever enough, to a decrease in sweating rate and evaporative cooling. The decrease in blood volume an evaporative cooling, in turn, cause added circulatory strain with eventual circulatory collapse and an excessive rise in rectal temperature. The best replacement fluid is one that contains as much salt and water as is lost through sweating, that is, about 1 to 2 grams of salt per liter of water. Several such types of replacement fluids– which have been flavored for palatability — are available commercially, Gatorade, smart water, function water, vitamin water are some examples. When these liquids are used, salt tablets should never be taken. Salt tablets should not be taken by athletes unless a clinical test shows an electrolyte imbalance. Fluids should be administered during as well as after prolonged work bouts in the heat. Adequate hydration by voluntary intake (thirst mechanism) alone takes several days. Therefore, during day to day heat exposures it might be necessary to insist on the drinking of some liquid even though there is no apparent thirst.

Heat Disorders In Athletics

The seriousness of overexposure to heat while exercising is exemplified not only by a decrease in work performance, but also by a predisposition to heat illness. These disorders are categorized in ascending severity as 1. Heat cramps, 2. Heat syncope, 3. Heat exhaustion– either salt-depletion or water depletion, and 4. Heat stroke. Special note is made of the possibility of exercise induced hypothermia.. Exercise induced hypothermia is relatively rare, but does occur. 

1. Heat cramps: Heat cramps are characterized by muscle spasms or twitching in the arms, legs and possibly, abdomen and usually occur in the unacclimatized individual.

2. Heat syncope: Heat syncope is characterized by a general weakness and fatigue, hypotension (low blood pressure), occasionally blurred vision, pallor (paleness), syncope (brief loss of consciousness), and elevated skin and core temperature. Heat syncope usually occurs in the unacclimatized.

3. Heat Exhaustion (Water Depletion). Water-depletion heat exhaustion is characterized by reduced sweating, although there is a large weight loss, dry tongue and mouth (“cotton mouth”), thirst, elevated skin and core temperature, weakness, loss of coordination, and dullness. Another sign is that the urine is very concentrated and almost an orange color. Water depletion heat exhaustion can occur in the acclimatized individual.

4. Salt-Depletion Heat Exhaustion. Salt depletion heat exhaustion is characterized by headache, dizziness, fatigue, nausea, possible vomiting and diarrhea, syncope, and muscle cramps. Salt-depletion heat exhaustion is slow acting  in that it usually takes 3 to 5 days  to develop. It can occur in an acclimatized individual.

5. Heat Stroke: I cannot emphasize the following too much. Heat stroke is a life-threatening medical emergency. The sweating mechanism has become fatigued although some sweating may still be occurring. Additionally, there are elevated skin and core temperatures (core temperature may well exceed 105 F or 40.5 C), muscle flaccidity involuntary limb movement, seizures and coma, vomiting and diarrhea, and tachycardia (rapid shallow heart beat). The individual may be irrational and hallucinating if not in a coma. Heat stroke may occur to any individual under the proper conditions. The appearance of any one of these procedures begun immediately. The athlete should never be left alone to “rest”. 

The most common denominater for all these conditions are 1. Heat exposure, 2. Loss of water and electrolytes, and 3. Heat storage, usually reflected by a high core temperature (hypothermia). However, the single most important factor, from a clinical stand point is loss of body water. Inattention to heat cramps, heat syncope, and heat exhaustion can lead to heat stroke and finally to death because of irreversible damage to the central nervous system. Even in those who do recover from heat stroke there often is some permanent damage to the thermoregulatory center in the hypothalamus. As a result of this damage, the hypothalamus loses some of its integrity or ability to regulate body temperature. This leads to decreased heat conductance form the body core to the periphery and explains why many who have survived heat stroke are more prone to future heat disorders. 

Normally, a person will voluntarily stop working and seek shelter from the heat when cramps, heat exhaustion, or syncope sets in. However, highly competitive athletes are more vulnerable to heat disorders in general and heat stroke in particular for several reasons 1. They are highly competitive (motivated) and therefore more likely to overextend themselves, 2. They have a sense of immortality 3, they sometimes are required to wear heavy protective equipment, which adds resistance to heat dissipation by reducing available evaporative surface, and 4. Incomprehensible as it may seem, the coach may deny the athlete water during prolonged contests or practice sessions, which lowers their resistance to heat tolerance. These factors, either singularly or combined, are as pertinent to environmental conditions that are usually considered “comfortable” as they are to hot environments. For example, rectal temperature equal to or greater than 40 degrees C or 104 degrees F are not uncommon, even in athletes who compete at environmental temperatures as low as 5 degrees to 16 degrees C, 41-61 degrees F.

Prevention of Heat Disorders

The occurrence of heat disorders can be greatly reduced by: 1. Adequate electrolyte (salt) and water replacement, 2. Acclimatization to heat, and 3. Awareness of the limitations imposed by the combination of exercise, clothing and environmental heat.

Salt and Water Replacement

Water and salt replacement during and following work in the heat is absolutely essential. It is not unusual for an athlete to lose 5 to 15 pounds (mostly water loss I.e. sweat) during each practice or during a game. Awareness needs to be taken by the coaches and the athlete as to how much weight they are losing.  It is not a bad idea to weigh athletes before practice, mid practice and at the end of practice during really hot days to determine the water loss.

Water Replacement

The availability of water should be unrestricted at all times during scheduled practices and games. The super-hydrated athlete suffers no impairment of efficiency.  However, large amounts of water should not be consumed all at once because the athlete may feel bloated under these circumstances. The best procedure is to schedule frequent water breaks as well as to encourage the drinking of water.

For athletic teams, water consumption can be facilitated by maintaining several water stations strategically located around the practice field. This allows the player convenient access to water. Frequent trips to the water tanks and drinking small amounts  are ideal. This procedure is physiologically more sensible than having a break every hour or so, during which the athletic might gulp  large amounts. Also, it allows for some efficient use of practice time. Ice water buckets, pressurized garden-spray containers, and thermos jugs are containers that can be properly located and adequately maintained. 

What the Athlete Should Drink

Many liquids may consumed to replace lost water and satisfy thirst,  but  what is needed most is drink that will provide for hydration without “lying in the stomach” for too long. A cold drink that is hypo tonic and has a concentration of sugar below that which retards gastic emptying is ideal.  The best drink should contain less than 2.5% sugar. Having made these observations, we must emphasize that, under most circumstances, water  is the best and most available drink. If modest dehydration occurs during a practice, fluids can be consumed over the next 24 hours to re-hydrate the athlete. 

Guidelines for fluid intake for athlete

Content of drink

The drink should be:

hypo tonic (fe solid particles per unit of water) low in sugar content (less than 2.5 grams per 100 ml of water)

Low in sugar content (less than 2.5 grams per 100 ml of water.

Cold (roughly 45-55 degree F, or 8-13 degree C

Palatable (it will be consumed in volumes ranging from 100 to 400 ml, or 3 to 10 ounces

Amount to  be ingested before competition 

Drink 400-600 ml ( 13.5 -20 ounces) of the above drink about 1 hour before the start of competition.

Amount to be ingested during competition

Drink 100-200 ml (3-6.5 ounces every 10-15 minutes)

Postcompetition diet

Following competition, modest salting of foods and the ingestion of drinks with essential minerals can adequately replace the electrolytes (sodium and potassium) lost in sweat.

Detection of dehydration

The athlete should keep a record of his or her early morning body weight (taken immediately after rising, after urinating and before breakfast) to detect symptoms of a condition of chronic dehydration.

Value of drinks

Drinks are of significant value in races or sporting events lasting 50-60 minutes.

Drinks are of significant value during long practice sessions under warm conditions in both individual and team sport settings. 

Daryl Conant, M.Ed