CHAPTER 303

Heat Disorde

Humans, who are warm-blooded animals, maintain their body temperature within 1 or 2 degrees of 98.6° F (37° C) as measured by mouth and 100.4° F (38° C) as measured rectally, despite large fluctuations in external temperatures. This internal temperature range must be maintained for the body to function normally. Body temperature that gets too high or too low can result in serious injury to organs or death.

Temperature Regulation: The body regulates its temperature by balancing heat production and heat loss.

One way the body produces heat is through chemical reactions (metabolism) resulting mostly from the conversion of food into energy. Heat is also produced through the work of muscles during physical activity.

The body cools itself by losing heat, mainly through the processes of radiation and sweating. Radiation, in which heat flows from warmer to cooler areas, is the main source of heat loss when the body is warmer than its environment. Sweating, in which the sweat glands produce moisture that cools the skin as it evaporates, is the main source of heat loss when the environmental temperature approaches body temperature and during exercise. However, humidity (moisture in the air) slows water evaporation, decreasing the effectiveness of sweating. Therefore, heat loss may be difficult in hot, humid weather.

Heat Disorders: There are several types of heat disorder:

Heat cramps

Heat exhaustion

Heatstroke (the most serious)

These types vary by their symptoms, whether (and by how much) body temperature is elevated, and by the severity of body fluid and salt depletion. Body fluid and salt depletion result from excessive sweating and can lead to low blood pressure and painful muscle contractions. Internal organs can be damaged if body temperature is very high for a long time.

Causes

Heat disorders are caused by excessive heat production, ineffective heat loss, or both.

Excessive heat production can be caused by the following:

Infections that cause fever

Increased thyroid activity, which speeds up the body’s metabolism

Strenuous muscle activity, which may occur during exercise or physical labor (particularly among obese people) or can result from disorders such as seizures, agitation, or alcohol or drug withdrawal

Certain stimulant drugs, such as cocaine, amphetamines, methylenedioxymethamphetamine (MDMA, or Ecstasy), monoamine oxidase inhibitors (a type of antidepressant), and phencyclidine (angel dust)

Overdoses of drugs that contain aspirin, because high doses of aspirin cause cells to produce excessive heat

Ineffective heat loss is most common in hot, humid conditions. The following also greatly interfere with heat loss:

Heavy, tight, clothing that does not breathe (that is, does not allow air and moisture to pass through easily). Wearing such clothing prevents sweat from evaporating from the skin surface and cooling the body.

Certain drugs, most often antipsychotic drugs and drugs with anticholinergic effects (see box on page 1897), may reduce sweating.

Certain disorders that affect the skin interfere with sweating. These include cystic fibrosis, systemic sclerosis (formerly called scleroderma), psoriasis, eczema, and severe sunburns.

Obesity interferes with heat loss because a thick layer of fat is a good insulator.

Mental states that interfere with sensible responses to heat. For example, elderly people with dementia and intoxicated people who are in a hot environment may not move to a cool environment, remove heavy clothing, or turn on an air conditioner.

The chance of developing heat disorders increases when exposure to heat occurs suddenly, such as when a child is left in a closed car on a hot summer day. In hot weather, the interior of a closed car can heat from 80 to 120° F (27 to 49° C) in 15 minutes. When people are gradually exposed to longer periods of heat and humidity, the body adjusts and is better able to maintain normal body temperature. This process is called acclimatization. Acclimatization occurs more rapidly in young or physically active people than in older or physically inactive people.

Factors that increase vulnerability to the effects of most heat disorders include the following:

Being very old or very young

Having certain medical conditions, such as those that involve malfunction of the heart, lungs, kidneys, or liver

Taking diuretics

Having imbalances in blood chemistries (electrolytes)

Prevention

Using common sense is the best way to prevent heat disorders. For example, children (and pets too) should never be left in enclosed, poorly ventilated spaces, such as a hot car, even for a few minutes. During excessively hot weather, the very old and the young should not remain in unventilated residences without air-conditioning.

During hot, humid weather, it is best to wear light, loose-fitting clothing made of cloth that breathes, such as cotton. Fluids and salts lost through sweating can usually be replaced by consuming water or lightly salted foods and beverages, such as sports beverages, salted tomato juice, or cool bouillon. Alcoholic beverages are not a good fluid replacement.

Exertion in the Heat: Strenuous exertion in a very hot environment should be avoided. When exertion in a hot environment cannot be avoided, drinking plenty of fluids and frequently cooling the skin by misting or wetting it with cool water can help keep body temperature near normal. To replace adequate amounts of fluids, drinking must continue even after thirst is quenched. Weight loss after exercise or work can be used to monitor dehydration. People who lose 2 to 3% of their body weight should be reminded to drink extra fluids and should be within about 2 pounds (1 kilogram) of starting weight before the next day’s exposure. People who lose at least 4% of their body weight should limit their activity for 1 day.

Ways to Help Prevent Heat Disorders

Ensure adequate ventilation or air-conditioning during heat waves, particularly for people who are very old or very young.

Avoid leaving children in automobiles in the hot sun, particularly with closed windows.

Avoid strenuous exertion in hot environments and poorly ventilated spaces.

Avoid inappropriately heavy, insulated clothing.

If exertion in heat is unavoidable, wear open-mesh clothing, use a fan, and drink every few hours regardless of thirst.

If 2% or more of body weight is lost during exercise or work, drink extra fluids.

If 4% or more of body weight is lost during exercise or work, limit activity for 1 day.

If large amounts of water are drunk, consume salts in fluids or food.

If prolonged exertion in heat is unavoidable, starting 10 to 14 days before maximum exertion is required, begin with moderate activity done for about 15 minutes a day, slowly increasing the intensity of the activity and the time spent doing it.

People engaged in outdoor activities who drink large quantities of water without salt may dilute the sodium in the blood (a condition called hyponatremia— see page 948), which may cause seizures and even death. Consuming salt, even in salty “junk” food, along with the water can alleviate this problem. Also, many commercially available drinks contain extra salt.

Slowly increasing the level and amount of work done in the heat eventually results in acclimatization, which enables people to work safely at temperatures that were previously dangerous. Progressing from 15 minutes per day of moderate activity (enough to stimulate sweating) during a hot time of day to 90 minutes of vigorous activity over 10 to 14 days is typically adequate.

Heat Cramps

Heat cramps are severe muscle spasms resulting from a combination of prolonged exercise, heavy sweating, and excessive water replacement in extreme heat.

During sweating, salts (electrolytes) and fluids are lost, but drinking large quantities of water dilutes the salts, causing cramps. Heavy sweating is most likely to occur on warm days, especially during strenuous exertion. Heat cramps are common among all of the following:

Manual laborers, such as engine-room personnel, steelworkers, roofers, and miners

Athletes, especially mountain climbers or skiers, whose many layers of clothing may keep them from noticing their heavy sweating, and tennis players and runners who do not take time to replace salts lost in sweat

Military trainees

Heat cramps are strong contractions in muscles of the hands, calves, feet, thighs, or arms. The contractions cause muscles to become hard, tense, and painful. Pain can be intense. Fever does not occur.

Mild heat cramps can be treated by drinking beverages that contain salt or by eating salty food. Drinking 1 to 2 quarts (about 1 to 2 liters) of a sports drink or water containing 2 teaspoons of salt is usually enough. Severe heat cramps are treated with fluids and salts given intravenously. Stretching the involved muscle often gives immediate relief of pain.

SPOTLIGHT ON AGING

There are several reasons why older people have particular difficulty when the temperature is high. As people are exposed to long periods of high heat and humidity, their body gradually adjusts (acclimates) and is better able to maintain usual body temperatures. Older people, however, are not as able as younger people to acclimate to higher temperatures and humidity. Older people tend to have difficulty increasing the flow of blood to all skin surfaces, and thus their body does not cool itself as readily.

Certain drugs, such as antipsychotics and antidepressants, and some disorders that affect the skin, such as systemic sclerosis and psoriasis, can also interfere with sweating. Other disorders such as heart failure can interfere with the body’s ability to cool itself. Aging also affects thirst; older people do not get thirsty as readily as younger people. Thus, older people tend to get dehydrated, which in turn means they are less able to sweat in warm surroundings.

Heat Exhaustion

Heat exhaustion is excessive loss of salts (electrolytes) and fluids due to heat, leading to decreased blood volume that causes many symptoms, sometimes including fainting or collapse.

Heat exhaustion is more severe than heat cramps. Fluids and salts are more depleted, and symptoms are more severe.

Symptoms and Diagnosis

Dizziness, light-headedness, weakness, fatigue, headache, blurred vision, muscle aches, or nausea and vomiting may develop. Muscle cramps may occur but often do not. People may feel faint or even lose consciousness when standing. Drenching sweats are common. Mild confusion may develop. The heart rate and breathing rate may become rapid. Blood pressure may become low. Body temperature is usually normal and if it is high, it is not higher than 104° F (40° C).

Heat exhaustion usually is diagnosed on the basis of the symptoms and occurrence after exposure to heat.

Treatment

Treatment involves replacing fluids and salts, usually intravenously, and removing people from the hot environment. Removing or loosening clothing and applying wet cloths or ice packs to the skin also aid cooling.

After receiving fluids, people usually recover rapidly and fully. If left untreated, heat exhaustion can lead to heatstroke.

Heatstroke

Heatstroke is a life-threatening condition that results in very high body temperature and malfunction of many organ systems.

Heatstroke can develop after hours of exertion in young athletes or after days of hot weather in rooms without air-conditioning in very old people.

Body temperature is higher than 104° F (40° C), and the brain malfunctions.

People should be cooled immediately.

Heatstroke is the most severe form of heat-induced illness. People with heatstroke are much sicker than people with other heat disorders. The following features in particular distinguish heatstroke from other heat disorders:

Body temperature is usually higher than 104° F.

Symptoms of brain malfunction develop.

Heatstroke may occur when people exert themselves in extreme heat or is in a closed, hot environment. For example, heatstroke can develop in young, healthy athletes, particularly those who are not acclimatized, after only hours of intense exertion in hot, humid weather. Heatstroke can also develop over days of hot weather when people, particularly older sedentary people, stay in rooms that are poorly ventilated and not air-conditioned. Older people, people who have certain medical conditions (such as those that involve malfunction of the heart, lungs, kidneys, or liver), and young children are most vulnerable to heatstroke.

Heatstroke occurs because the body cannot lose heat rapidly enough in conditions of extreme heat. Because the body cannot cool itself, body temperature continues to rise rapidly to dangerously high levels. Conditions that interfere with heat loss, including certain skin disorders, and drugs that decrease sweating increase the risk.

Heatstroke can temporarily or permanently damage vital organs, such as the heart, lungs, kidneys, liver, and brain. The higher the temperature, especially when higher than 106° F (41° C), the more rapidly problems develop. Death may occur.

Symptoms

Dizziness, light-headedness, weakness, fatigue, headache, blurred vision, muscle aches, nausea, and vomiting (which are also symptoms of heat exhaustion) are common warning symptoms. Affected people do not sense that body temperature is greatly elevated.

During heatstroke, the skin becomes hot, flushed, and dry. Sweating may not occur despite the heat. Because of brain malfunction, people may become confused and disoriented and may have seizures or go into a coma. The heart rate and breathing rate increase. The pulse rate is usually rapid. The blood pressure may be high or low. Body temperature usually exceeds 104° F and may be so high that it exceeds the markings on a typical thermometer.

Diagnosis

The diagnosis is usually evident. People have a high fever, symptoms of brain malfunction, and a history of being exposed to high heat and humidity. If the diagnosis is not evident, tests are done for other disorders that can cause similar symptoms, such as infections, stroke, and an overactive thyroid gland (hyperthyroidism).

Treatment

The body must be cooled immediately, and an ambulance should be called. While awaiting transportation to the hospital, people should be wrapped in cold, wet bedding or clothing; immersed in a lake, stream, or cool bathtub; or cooled by being immersed in ice-cold water. Misting the body with water and then blowing air across the body with a fan is also very effective. Using drugs (aspirin or acetaminophen) designed to treat a fever due to an infection is useless and should be avoided.

At the hospital, the body is usually cooled rapidly by removing the clothes and covering the exposed skin with water or occasionally ice. To speed evaporation and body cooling, a fan may be used to blow air on the body. Body temperature is measured frequently, often constantly. Cooled fluids may be given intravenously. To avoid overcooling, cooling is stopped when the body temperature is reduced to about 102° F (about 39° C).

Did You Know…

Drinking large amounts of plain water during exertion can dangerously dilute sodium in the bloodstream.

Seizures, coma, and malfunction of other organs may also need treatment. Heatstroke is best treated in an intensive care unit of a hospital.

Prognosis

The risk of death from heatstroke climbs depending on the following factors:

How old adults are

How young children are

How severe any conditions (such as heart, lungs, kidneys, or liver disorders) are

What the highest body temperature is

How long body temperature remains extremely high

In about 20% of people who survive, the brain may not fully recover, leaving a person with personality changes, clumsiness, or poor coordination. In some people, the kidneys do not fully recover. After recovery, body temperature may fluctuate abnormally for weeks.