(Heat Edema, Heat Syncope, Heat Cramps, Heat Exhaustion)
Heat illnesses are a spectrum of illnesses resulting from failure of the body’s normal thermoregulatory mechanisms after exposure to excessive heat. Most heat-related illness is mild; however, severe hyperthermia associated with heat stroke, neuroleptic malignant syndrome, or serotonin syndrome is a severe, life-threatening condition and should not be overlooked.
The milder forms of heat-related illness include heat edema, heat syncope (or presyncope), or heat cramps. These illnesses are usually found after prolonged exposure to excessive heat and humidity in patients who are unable to remove themselves from the situation.
Heat edema is dependent edema of the hands and feet that may last for a few weeks.
Heat syncope is postural syncope or presyncope related to excessive heat exposure.
Heat cramps are painful muscle cramps after vigorous exertion in hot environments (often several hours later) in the calves, thighs, and/or shoulders.
Heat exhaustion is a slightly more severe form of heat illness, although it too is usually self-limited if treated appropriately. Elderly patients (without air-conditioning on a hot, humid day), workmen, or athletes (exerting themselves in a hot climate while taking in an inadequate amount of fluid) may be more symptomatic, with fatigue, weakness, lightheadedness, headache, nausea, and vomiting in addition to orthostatic hypotension and painful muscle spasms. The patient may have a normal temperature, or the temperature may be elevated to 40° C (104° F), with tachycardia, clinical evidence of dehydration, and, often (especially with exertion), profuse sweating. Mental status is normal.
The severe forms of heat-related illness are characterized by alteration in mental status associated with hyperthermia (temperature greater than 40° C). Neuroleptic malignant syndrome and serotonin syndrome are not classified as heat-related illnesses but present with severe hyperthermia and altered mental status and can be easily confused with heat stroke.
What To Do:
Assess and monitor all patients with minor heat illness for the development of heat stroke. This is a much more serious form of heat illness, which is accompanied by a core temperature of more than 40° C and altered mental status manifested by delirium, seizures, or coma.
Remove patients with any form of heat illness from the hot environment. Most of the clothing should be removed to promote cooling, and a rectal temperature should be determined.
Obtain a careful history from the patient or witnesses, with special attention to the type and length of heat exposure, any underlying medical problems, and any medications being used that might predispose the patient to developing heat illness.
Perform a physical examination, looking for abnormal vital signs, associated medical illness, dehydration, and diaphoresis.
For heat edema, inform patients of the benign nature of this problem, and let them know that they can anticipate having this swelling for a few weeks. Advise them to keep their extremities elevated above the level of their heart as much as possible and, in severe cases, to use compressive stockings.
For heat syncope or presyncope, patients should rest and receive oral or intravenous rehydration. They should be thoroughly evaluated for any injury resulting from a fall, and all potentially serious causes of syncope should be considered (see Chapter 11).
For heat cramps alone, provide muscle stretching and massage, and administer an oral electrolyte solution (½ tsp table salt in 1 quart of water) or intravenous normal saline for rapid relief.
For true heat exhaustion, provide intravenous rehydration with normal saline or a glucose-in-hypotonic saline solution (1 L over 30 minutes). Obtain serum sodium, potassium, glucose, magnesium, calcium, and phosphorus levels, as well as hematocrit, blood urea nitrogen, and creatinine levels. Electrolyte abnormalities should be corrected appropriately. Rapid correction of hypernatremia can cause cerebral edema.
When there is hyperthermia, the patient should be sprayed or sponged with tepid or warm water (to prevent shivering) and then fanned to enhance evaporation and cooling. Ice water immersion is more effective in rapid cooling but poorly tolerated in most patients (especially elderly patients).
If not treated properly, heat exhaustion may evolve into heatstroke, which is a major medical emergency that may lead to cardiac arrhythmias, rhabdomyolysis, serum chemistry abnormalities, disseminated intravascular coagulation, irreversible shock, and death. Physical examination and laboratory analysis should provide the correct diagnosis.
When patients with minor forms of heat illness respond successfully to treatment, with vital signs returning to normal and symptoms relieved, they may be discharged with instructions on how to avoid future episodes and advised to continue adequate fluid intake over the next 24 to 48 hours. Elderly and mentally ill patients should be encouraged to maintain adequate fluid intake in the future, to prevent recurrence. People engaged in strenuous exercise in hot weather should be encouraged to drink water more frequently than thirst dictates. Runners should drink 100 to 300 mL of water or a hypotonic glucose-electrolyte solution (Gatorade and others) 10 to 15 minutes before beginning a race and should drink about 250 mL every 3 to 4 kilometers. Those who must work in a hot environment with high humidity should be encouraged to acclimate themselves over several weeks. Successive increments in the level of work performed in a hot environment result in adaptations that eventually allow a person to work safely at levels of heat that were previously intolerable or life threatening.
Elderly patients and their caretakers, as well as parents of small children, should be educated about high-risk situations and instructed about putting limits on activity during hot and humid days.
Admission should be considered for elderly patients who have chronic medical problems, significant electrolyte abnormalities, or risk for recurrence. All patients who are treated but do not have a complete resolution of their symptoms over several hours should also be admitted.
What Not To Do:
Do not do a comprehensive laboratory workup on young, healthy patients with minimal symptoms or minor heat-related illness.
Do not use pharmacologic agents that are designed to accelerate cooling. None have been found to be helpful. The role of antipyretic agents in heat illness has not been evaluated.
Do not continue therapeutic cooling techniques after the temperature reaches 38.5° C. Beyond this point, continued active cooling may result in hypothermia.
Do not recommend salt tablets to prevent heat illness. Fluid losses during exercise are much greater than electrolyte losses.
Do not overlook the possibility of neuroleptic malignant syndrome and serotonin syndrome with patients who have recently begun taking neuroleptic drugs or serotonergic agents.
Do not allow overhydration in athletes who are trying to prevent heat illness (especially women and slow runners). Severe cases of hyponatremia that resulted from excessive water consumption have been reported.
Control of thermoregulation resides within the hypothalamus, which stimulates cutaneous vasodilation and sweating through the autonomic nervous system in response to elevation of blood temperature. Blood flow to the skin may increase 20-fold. Cooling normally occurs by transfer of heat from the skin by radiation, convection, and evaporation. As the ambient temperature exceeds the body’s temperature, a rise in body temperature may occur in response to radiation and convection of heat from the environment. When the humidity rises, the body’s ability to cool through evaporation is diminished.
Dehydration and salt depletion impair thermoregulation by reducing the body’s ability to increase cardiac output needed to shunt heated blood from the core circulation to the dilated peripheral circulation. Cardiovascular disease and use of medications that impair cardiac function can also result in increased susceptibility to heat illness.
Although athletes are commonly thought to be most at risk for heat illness, children and the elderly, poor, and socially isolated are particularly vulnerable.
Compared with adults, children produce proportionately more metabolic heat, have a greater surface area-to-body mass ratio (which causes a greater heat gain from the environment on a hot day), and have a lower sweating capacity, which reduces their ability to dissipate heat through evaporation. These facts emphasize the danger of leaving a child unattended in a car during hot weather. A fatal event can occur within 20 minutes if normal heat loss mechanisms become overwhelmed.
Both children and young adults (most often athletes and laborers) are associated with exertional heat illness, where there has been intense strenuous activity in a hot, humid environment. Elderly, chronically ill, or sedentary adults, as well as small children, are associated with nonexertional heat illness. Environmental conditions, along with a predisposition for impaired thermoregulation, lead to heat illness in these patients. The elderly and infirm may have diminished cardiac output, a decreased ability to sweat, and decreased ability to vasoregulate. Medications may predispose them to heat illness because of negative effects on cardiac output (beta-blockers) or on sweating (anticholinergics) or because of volume depletion (diuretics). Nonexertional heat illness may be indolent in its onset and may be associated with significant volume depletion.
Heatstroke is the deadliest of heat illnesses. Treatment, especially aggressive cooling procedures and fluid replacement, must begin immediately to help ensure survival. Morbidity and mortality are directly associated with the duration of elevated core temperature. More intensive evaluation and treatment are required for these patients than is covered in this chapter. The most serious complications of heat stroke are those falling within the category of multiorgan dysfunction syndrome. They include encephalopathy, rhabdomyolysis, acute renal failure, acute respiratory distress syndrome, myocardial injury, hepatocellular injury, intestinal ischemia or infarction, pancreatic injury, and hemorrhagic complications, especially disseminated intravascular coagulation, with pronounced thrombocytopenia.