Buddy Harris, 42, steps up to the plate during a neighborhood baseball game on a sweltering Saturday afternoon. He's already made several remarkable catches and hit a home run and two doubles. In between, he's drunk two beers in an attempt to stay cool. Now it's the bottom of the eighth inning. But before Mr. Harris can swing at the first pitch, he collapses.
You run out to the field and find him minimally responsive. He's breathing rapidly, and his skin is flushed and dry. Given these signs and Mr. Harris's exertions during the game, you suspect heatstroke. Do you know what to do next?
In this article, I'll discuss the signs, symptoms, and treatment of heatstroke and less-serious heat injuries (heat cramps and heat exhaustion). For a review of the physiology of heat injuries, see Hot, hot, hot.
The body releases 90% of the heat it produces through a combination of radiation and evaporation. Heat is transferred through the skin to water molecules in the surrounding air. When the ambient air temperature exceeds 95° F (35° C), almost all heat loss is accomplished by evaporation because radiation ceases. But when the relative humidity reaches 75%, evaporation becomes less effective and the risk of heat injuries rises.
The mechanism of heat injury is the same for all three types of heat injuries, with the difference being the severity of the heat injury. Patients who've had a heat injury once are more susceptible to heat injuries in the future.
Risk factors for heat injuries include inadequate acclimatization to a hot environment, infections, fever, recent illness or injury, obesity, dehydration, exertion, fatigue, heavy meals, and ingesting alcohol (a vasodilator) or recreational drugs. Commonly prescribed drugs such as anticholinergics, antidepressants, antihistamines, beta-blockers, diuretics, and phenothiazines also predispose patients to heat injuries by interfering with how the body deals with excess heat. Older adults and very young children are more prone to heat injuries because of their impaired ability to dissipate heat; they also may be unable to adequately communicate thirst to caregivers.
The best way to prevent heat injuries is to drink adequate amounts of water before, during, and after exercise or exposure to high temperatures. Other preventive measures include staying indoors between 10 a.m. and 2 p.m. on extremely hot days; wearing a broad-brimmed hat and lightweight, light-colored clothing; and avoiding alcohol.
Let's take a closer look at the three types of heat injuries.
Heat cramps, usually occurring during and after heavy exercise in a hot environment, are brief, painful muscle cramps caused by hyponatremia exacerbated by drinking water without taking in salt. Large muscle groups, particularly in the legs and abdomen, are most often affected. Severe cramping usually is accompanied by thirst.
Move the patient to a cool, shaded spot and provide water. Don't give salt tablets, which can cause nausea and vomiting. Instead, provide small amounts of simple carbohydrates, such as sugared carbonated drinks. Because sports drinks contain glucose, mix equal parts of the sports drink and water to prevent the patient from vomiting. Stretching the muscles also may help. Monitor the patient closely for worsening signs and symptoms. If the cramps don't respond to treatment or the patient doesn't feel better, take him to a hospital.
Excessive loss of salt, water, or both causes heat exhaustion. People who aren't acclimatized to a hot environment are at greater risk for heat exhaustion.
Someone suffering from heat exhaustion may be irritable, exhibit poor judgment, and experience headache, nausea, vomiting, diarrhea, intense thirst, vague malaise, and dizziness. His temperature probably will be elevated (100.4° F to 102.2° F [38° C to 39° C]), but it may be normal. Other signs of trouble include tachycardia, tachypnea, dry mucous membranes, and low urine output.
Cool, clammy skin indicates shock. Remember: The line between heat exhaustion and heatstroke is thin!
To treat heat exhaustion, move the patient to a cooler environment—a shaded area or an air-conditioned car or building—as quickly as possible. Remove as much of his clothing as practical and apply water to his skin. If water isn't available, use other liquids. If the patient is conscious and can swallow, provide small amounts of water or a half-strength sports drink at frequent intervals.
Call 911. When the paramedics arrive, they'll start an I.V. line and administer oxygen.
The classic signs and symptoms of heatstroke include early disorientation similar to that seen in heat exhaustion. Disorientation may range from irrational behavior to frank psychosis and unconsciousness. The patient's skin will be hot and flushed and may be dry because he may stop sweating. His temperature may be as high as 109.4° F (43° C).
Other signs include vomiting, diarrhea, tachycardia, and tachypnea, which may be profound; his respiratory rate may be as high as 60. The resulting respiratory alkalosis may cause tetany. Pulmonary edema also may occur.
Tachycardia, with rates that may be as high as 180 beats/minute, is universal. The ECG may show nonspecific ST-segment and T-wave changes and various atrial and ventricular dysrhythmias. Muscle breakdown caused by heatstroke is evidenced by elevated creatine kinase (CK) values, which can measure in the tens of thousands. (The normal range is 5 to 200 units/L.)
Caring for Mr. Harris
With this information in mind, let's consider Mr. Harris, the man who collapsed during a baseball game. He's minimally conscious, responding only to painful stimuli. Because no air-conditioned buildings are close by, Mr. Harris's teammates help you move him to the shade of the dugout and remove his shirt, pants, socks, and shoes. Another spectator has called 911. You sprinkle water from the team's cooler over Mr. Harris and begin to fan him to increase heat loss. Apply ice packs to his neck, axillae, and groin.
When the paramedics arrive soon after, they find that Mr. Harris's systolic BP is 80 mm Hg by palpation and his temperature is 105° F (40.6° C) by skin strip. They start two large-bore I.V. lines for fluid resuscitation with a crystalloid solution. Discontinue active cooling methods as his temperature approaches 102.2° F; you don't want to overcool him.
The ECG monitor shows sinus tachycardia at a rate of 145 beats/minute with frequent premature ventricular contractions. Mr. Harris is breathing adequately, so he's put on 100% oxygen via non-rebreather mask. During the ambulance trip to the hospital, a paramedic continues to fan him.
In the ED, a nurse inserts a nasogastric tube and begins iced gastric lavage. Mr. Harris's temperature is 106.4° F (41.3° C) rectally. An indwelling urinary catheter is placed to monitor fluid requirements and response. The nurse draws blood for lab studies, including arterial blood gas analysis, complete blood cell count, CK level, and comprehensive metabolic panel. He also obtains a urine sample to check for myoglobin.
The initial lab tests reveal a CK level of 78,200 units/L, consistent with severe heatstroke. Mr. Harris's serum sodium level is 128 mEq/L, indicating hyponatremia, and his blood glucose is 60 mg/dL, indicating hypoglycemia, both conditions associated with heatstroke.
If initial treatment doesn't lower Mr. Harris's temperature, the healthcare provider may consider immersing Mr. Harris in ice water. This lowers the patient's temperature but may cause shivering, which generates more heat. Shivering can be treated with I.V. chlorpromazine.
Mr. Harris's hyperthermia responds to the iced gastric lavage, sponging, and fanning. He'll be admitted to the ICU after he's stable because of the potential for severe complications: systemic inflammatory response syndrome, renal failure, respiratory failure, and liver failure. In the ICU, a pulmonary artery catheter may be inserted to monitor fluid balance.
After 2 days in the ICU, Mr. Harris is transferred to the step-down unit. He's discharged 2 days later, having suffered no permanent damage from his heatstroke.
Hot, hot, hot
Heat injuries occur when the body loses its ability to maintain a stable temperature (below 102.2° F [39° C]). This can happen when someone is exposed directly to an intense heat source (including hot summertime temperatures), when the body generates more heat than it can transfer to the environment, or (most commonly) both.
Coping with temperature rises
- Heat generated by the muscles during sustained exercise is transferred to the core and can raise body temperature 1.8° F (1° C) every 5 to 8 minutes.
- The anterior hypothalamus activates the autonomic nervous system, causing splanchnic and renal vasoconstriction and vasodilation of the peripheral and skin blood vessels. Increased blood flow to the skin lets heat dissipate into the environment through evaporation of sweat.
- Vasodilation causes the heart rate and stroke volume to rise in order to maintain cardiac output. Respiratory rate and depth also increase to meet the oxygen demand and help cool the body.
- Because sodium is lost in sweat, the kidneys retain sodium and water, decreasing urine output.
If the body can't lose enough heat to keep the temperature below 102.2° F, these changes occur:
- Vasodilation, coupled with fluid losses of up to 3 L/hour through sweating, causes massive hypovolemia.
- Compensatory responses can no longer maintain adequate cardiac output, and blood pressure drops, causing shock. Myocardial infarction or ventricular dysrhythmias are uncommon, but may occur because of the massively increased cardiac workload, especially if the patient is already compromised by heart disease.
- The increased respiratory rate may lead to respiratory alkalosis initially. As hypovolemia and shock develop, respiratory failure decreases heat loss through exhalation, and metabolic acidosis develops.
- As brain temperature increases, lethargy and poor judgment ensue—signs and symptoms that can be mistaken for effects of alcohol or drugs.
- With the loss of large amounts of water and the resultant hypovolemia and hypotension, the kidneys are prone to hypoperfusion and acute tubular necrosis, and a further drop in urine output occurs.
- Despite fluid volume losses, hyponatremia may occur from the loss of sodium in perspiration.
- The liver is very sensitive to temperature elevations. In addition, hypoperfusion causes prothrombin times to become prolonged, leading to bleeding problems. Hypoperfusion also can produce hypoglycemia in up to 20% of exertional heatstroke victims.
- Prolonged hypovolemia leads to ischemia of the gut, which can cause ulceration and bowel content leakage into the bloodstream.
- Rhabdomyolysis—the disintegration of muscle proteins—can lead to renal failure; hypoperfused kidneys can't deal with these large proteins.