Hyperthermia

Hyperthermia is an abnormally high body temperature due to inadequate or inappropriate responses of heat-regulating mechanisms.

Physiologic changes resulting from normal aging and from disorders that become more common with aging combine to impair heat regulation in the elderly. Certain drugs may impair heat regulation. About 80% of people who die of heatstroke are > 50. Also, the number of deaths resulting from most other disorders increases during hot, humid weather, particularly among the elderly. For example, deaths attributed to diabetes, pulmonary disorders, and hypertension increase by > 50% during heat waves.

Heat Exhaustion

Heat exhaustion is a syndrome of weakness, malaise, nausea, and other nonspecific symptoms caused by heat exposure; it is not life threatening. Thermoregulation is not impaired. Diagnosis is by history and physical examination. Treatment is replacement of fluids and electrolytes.

Heat exhaustion results from depletion of fluids and electrolytes, which may occur during exertion or not. Symptoms are nonspecific and may not be recognized at first; they may include anorexia, thirst, nausea, vomiting, light-headedness, weakness, and muscle cramping. If unrecognized, symptoms may worsen. Orthostatic hypotension and syncope may occur. Body temperature may be normal or increased up to 40° C. Usually, heat exhaustion does not affect mental status. Morbidity is rare.

Diagnosis

Diagnosis is by history and physical examination. Serum electrolytes are measured if patients have certain conditions that require IV fluid replacement (eg, cardiac, renal, or metabolic disorders; diuretic use). Other testing is necessary if indicated to investigate an alternate diagnosis (eg, hypoglycemia, MI) suggested by symptoms.

Treatment

Because elderly patients tend to present late and with severe symptoms (eg, severe muscle cramping, syncope), treatment is usually IV administration of 0.9% normal saline solution. Elderly patients, particularly those who may be unable to tolerate large volumes of IV fluids (eg, patients with heart failure or renal insufficiency), require a lower fluid volume and lower initial administration rate than the volume and rate (500 mL/h) used for young, healthy patients. However, volume and rate should be adjusted based on the patient's clinical response, and fluid composition should be adjusted based on serum electrolyte levels. If symptoms are mild (eg, anorexia, thirst, nausea, light-headedness, mild weakness or muscle cramping) and patients are physiologically vigorous, oral replacement therapy may be tried. Oral replacement is with electrolyte-rich beverages, such as a sports drink or a solution made of 2 tsp of salt per liter of water. Usually, a volume of at least 500 to 600 mL, which accelerates gastric emptying, is given.

Heatstroke

Heatstroke is hyperthermia accompanied by impaired thermoregulation and a systemic inflammatory response that causes multiple organ dysfunction and often death. Symptoms include temperature > 40° C and altered mental status; in elderly patients, sweating is often absent. Diagnosis is by history and physical examination. Treatment is immediate external cooling, IV fluid replacement, and supportive care.

Geriatric Essentials

• Heatstroke usually develops over days, usually during heat waves, and most often affects elderly people who do not have air conditioning.
• In elderly people with heatstroke, heart rate may be inappropriately slow despite hypovolemia.

Etiology

Heatstroke can be exertional or classical. Exertional heatstroke occurs after hours of intense exertion in a hot, humid environment and tends to occur in young people, particularly athletes, military recruits, and factory workers. Classical heatstroke occurs after days of heat exposure, typically during heat waves, and tends to affect sedentary, elderly people who do not have air conditioning and who have limited access to fluids. Risk factors for classical heatstroke in the elderly include low socioeconomic status, impaired self-care ability, alcoholism, other comorbid mental or physical disorders (eg, cardiovascular or cerebrovascular disease, diabetes, COPD), and use of certain drugs. Overall, heatstroke occurs much more often in people > 65 than in younger people.

Many drugs, particularly psychoactive ones, can predispose to heatstroke (see Table 67-1). Anticholinergics, cyclic antidepressants, antihistamines, and phenothiazines impair hypothalamic function centrally and sweat output peripherally. In addition, these drugs, as well as opioids, sedative-hypnotics, and alcohol, reduce a person's awareness of heat and ability to respond to heat stress. Amphetamines can raise body temperature by acting directly on the hypothalamus. Diuretics (by causing additional fluid loss) and -blockers (by impairing cardiovascular responsiveness) can also increase risk of heatstroke.

Pathophysiology

Organ dysfunction may involve the CNS, kidneys, lungs (acute respiratory distress syndrome), heart, coagulation system, and occasionally liver. In the brain, edema, patchy congestion, and diffuse petechial hemorrhages are common. Rhabdomyolysis can occur, resulting in acute renal failure. Aldosterone secretion may be increased. Lactate levels increase, resulting in metabolic acidosis and a compensatory respiratory alkalosis. The coagulation cascade may become activated; rarely, full-blown disseminated intravascular coagulation results.

Symptoms and Signs

Heatstroke is characterized by a body temperature usually > 40° C and global brain dysfunction (eg, delirium, seizures, lethargy, stupor, coma). Light-headedness, dizziness, headache, weakness, dyspnea, nausea, or loss of consciousness may precede development of full-blown heatstroke. In elderly people with classical heatstroke, the skin may be hot and dry. Because onset is slower and recognition may be delayed, classical heatstroke may result in greater fluid loss than exertional heatstroke. Also, although hypovolemia is present, cardiac output, peripheral resistance, or both may be inappropriately low, leading to the so-called hypodynamic cardiovascular response. In this response, heart rate is slow, pulse is usually slow and thready, and BP may be low or imperceptible.

Risk of death is highest in elderly patients with the highest fever, the most severe hypotension, the greatest neurologic impairment, and the most severe comorbidities. Up to 20% of survivors have permanent brain damage. Renal insufficiency may become permanent, and body temperature may be labile for weeks.

Diagnosis

Diagnosis is usually clear from the history and physical examination. Core body temperature is measured rectally or esophageally using an electronic thermometer or a mercury thermometer with an extended upper range. However, infection (eg, sepsis, meningitis) should be considered, and antibiotics must often be given until infection can be ruled out. A thorough drug history can help elucidate factors that caused or contributed to development of heatstroke.

CBC is done, and Ca levels are measured. Other laboratory testing and common findings include the following:

• PT and PTT/INR: Elevated, often with a decreased fibrinogen level
• Electrolytes: Hypokalemia and high anion gap metabolic acidosis
• BUN and creatinine: Elevated
• CPK: Elevated, possibly reflecting rhabdomyolysis or cardiac muscle necrosis
• Liver function tests: Elevated transaminases or bilirubin
• ECG (usually): Sometimes changes in ST segments and T waves, premature ventricular contractions, supraventricular tachycardias, and conduction abnormalities
• Urinalysis by urethral catheter: Proteinuria

The urethral catheter is also used to monitor urine output. Cultures of blood, urine, or CSF may be necessary to exclude infection.

Treatment

Heatstroke is a medical emergency requiring immediate cooling measures. Fanning the patient continuously while applying ice to the neck, axillae, and groin or while spraying the skin with cool water is usually effective. Immersion in cool or ice water is effective but may interfere with monitoring the patient. Rectal temperature should be continuously monitored during cooling. Cooling is stopped when body temperature reaches about 38.8° C to avoid overcooling and hypothermia. Because heart failure or dehydration may occur and may be clinically unapparent, some experts recommend central venous pressure monitoring to guide fluid replacement. Supportive therapy for secondary organ dysfunction is important. Patients are hospitalized in an ICU.

Prevention

Prevention of classical heatstroke requires both individual efforts and public health measures. Caregivers should be alert to symptoms of heatstroke in elderly people. In very hot weather, caregivers should try to move elderly people, especially those at high risk, to an air-conditioned environment, even if only for brief periods. If such a move is impossible, providing fans may help. However, during heat waves, elderly people may need to move to temporary shelters. Also, elderly people should wear light, porous clothing and keep their windows open at night and shaded during the day; adequate fluid intake (even when they are not thirsty) and avoidance of exercise are also important during heat waves. Drugs that predispose to heatstroke should be avoided or given in reduced doses whenever possible.

This topic was last updated February 2006.