Hypothermia

(Accidental Hypothermia)

Hypothermia is a core body temperature < 35° C. Symptoms progress from shivering to lethargy to confusion, coma, and death. Diagnosis is by measurement of core body temperature. Treatment requires a warm environment and insulating blankets. Severe hypothermia occasionally requires external heat sources directed at the body core (eg, body cavity lavage, cardiopulmonary bypass).

Geriatric Essentials

• Risk factors for hypothermia in the elderly include reduced physical activity, immobility, dementia, undernutrition, many other common disorders, and many commonly used drugs.
• Elderly patients may not recognize that they are cold and may not shiver. Symptoms may be nonspecific, and the diagnosis is easily missed.
• Elderly patients with hypothermia require more extensive routine laboratory testing than younger patients.
• Because hypothyroidism is common among the elderly, it should always be excluded as a cause or contributor.
• In stable elderly patients with hypothermia, warming should proceed slowly (<= 0.6° C/h). Treatment for unstable elderly patients is the same as that for younger patients.
• Elderly patients undergoing surgery require special precautions to prevent hypothermia.

Hypothermia is known to cause about 600 deaths each year in the US, but this figure is probably an underestimate. For elderly patients with diagnosed hypothermia, the mortality rate is 50%. Mortality rate increases with aging, particularly after age 75. Mortality correlates more closely with comorbidities than with degree of hypothermia. Hypothermia significantly increases the mortality rate for cardiovascular and neurologic disorders, but this effect is underrecognized.

Etiology

For hypothermia to occur, body heat loss must exceed body heat production. Environmental factors that increase loss of body heat include being exposed to cold ambient temperatures or windchill, lying on a cold surface, being wet, and wearing thin or porous clothing.

Most episodes of hypothermia are initiated outdoors by ambient temperatures < 15.5° C, but frail elderly people may become hypothermic while indoors at temperatures as warm as 22 to 24° C. Iatrogenic hypothermia can occur when unclothed elderly patients undergo surgery in a cool operating room.

Many age-related physiologic changes, including a diminished perception of cold, predispose elderly people to hypothermia. Decreased responsiveness to endogenous catecholamines reduces the vasoconstrictor and shivering responses to cold. A decrease in lean body mass reduces the efficiency of shivering for producing heat. Reduced physical activity and caloric intake lower the metabolic rate, decreasing endogenous heat production. Other common risk factors in the elderly include certain drugs (eg, alcohol, antidepressants, barbiturates, benzodiazepines, opioids, phenothiazines, reserpine) and disorders that decrease heat production, increase heat loss, impair thermoregulation, or reduce physical activity (see Table 67-2). Diabetes increases risk 6-fold in the elderly.

Pathophysiology

Cold may initially stimulate the cardiovascular system, increasing cardiac output and renal blood flow. Diuresis results from the increased renal blood flow, as well as from decreased secretion and renal responsiveness to ADH. After initial cardiovascular stimulation, hypothermia slows all physiologic functions, including cardiac conduction and contractility, nerve conduction, mental acuity, neuromuscular reaction time, GI motility, hepatic drug metabolism, and overall metabolic rate. Respiration and the cough reflex are depressed, leading to atelectasis and often pneumonia. Endogenous heat production becomes disordered when body temperature is below about 30° C; at that point, the body usually depends on an external heat source for warming. Pancreatitis can occur. Fluid leaks into interstitial tissues. Heart failure occurs and, as fluid leaks across the pulmonary capillary membrane, leads to pulmonary edema. Initial diuresis combined with later fluid leakage into the interstitial tissues causes hypovolemia. Vasoconstriction can occur and, combined with hypovolemia, results in systemic lactic acidosis and may decrease renal blood flow enough to cause acute tubular necrosis.

Symptoms and Signs

Symptoms and signs of hypothermia are insidious and may be transient. Elderly people with a body temperature between 35° and 36.1° C often report feeling cold, but patients with a lower body temperature usually do not, although they feel cool to the touch. Many people have cold hands or feet in winter, but patients with hypothermia also have cold abdomens and backs. Their skin has a cadaveric pallor and chill, and pressure points have erythematous, bullous, or purpuric patches. Subcutaneous tissues are firm, probably because of edema, which also causes puffiness, especially of the face. Shivering may not occur. Instead, marked rigidity develops accompanied by a generalized increase in muscle tone; occasionally, a fine tremor is present.

Neurologic findings include thick, slow speech and ataxic gait; deep tendon reflexes are depressed. Sleepiness and confusion may progress to coma. Pathologic reflexes and extensor plantar responses may be present, and pupils may be dilated and sluggishly reactive. Focal signs, seizures, paralysis, and sensory loss may also occur.

Cardiovascular findings are tachycardia and elevated BP initially. Later, hypotension and progressive sinus bradycardia occur. Severe hypothermia can lower BP and heart rate to barely detectable levels; rarely, these findings lead to an erroneous pronouncement of death. Various cardiac arrhythmias (eg, atrial fibrillation and flutter, premature ventricular beats, idioventricular rhythm) may occur. Cardiac arrest due to ventricular fibrillation or asystole is increasingly likely as body temperature falls to < 30° C. Vasoconstriction, if present, may mask hypovolemia, resulting in sudden shock or cardiac arrest when peripheral vasculature dilates during warming (rewarming collapse).

GI findings can include abdominal distention, diminished or absent bowel sounds, and, less often, vomiting. Pancreatitis is usually not apparent until after warming.

Diagnosis

Hypothermia is often and easily missed. Clinical findings are usually nonspecific and can suggest other disorders (eg, MI, infection, hypoglycemia). Hypothermia should be suspected in patients who have altered mental status, bradycardia, or other suggestive symptoms or signs or who are at high risk.

Diagnosis is by core body temperature measurement. Electronic thermometers are preferred because standard mercury thermometers have a lower limit of 34° C; low-temperature mercury thermometers are also available. Rectal or esophageal probes are most accurate.

Laboratory testing should include CBC; platelet count; PT and PTT/INR; measurement of BUN, creatinine, electrolyte, CPK, plasma glucose, and serum lipase and amylase levels; thyroid and liver function tests; ABGs; ECG; and chest and abdominal x-rays. Because hypothyroidism is common among the elderly, it should always be excluded as a cause or contributor. Cardiac rhythm is monitored continuously, and core body temperature is measured continuously with an indwelling probe until temperature is > 35° C.

Laboratory findings are usually nonspecific. Hemoconcentration, leukocytosis, acidosis, thrombocytopenia, and hyperglycemia are common. If hypoglycemia is detected, drug-induced hypoglycemia or glycogen depletion is likely to be the cause of hypothermia. ABG results are interpreted without correction for body temperature.

On ECG, a J (junctional, or Osborn) wave, although present in only about 1/3 of patients, is somewhat specific for hypothermia (see Figure 67-1). In many patients who are not shivering, ECG shows fine regular oscillation of the baseline produced by increased muscle tone with an imperceptible tremor. Abdominal x-rays may show gastric distention or ileus.

Treatment

Hypothermia, even if mild, should be treated in the hospital, usually in an ICU. Wet clothing is removed, and the skin is dried. Patients must be moved gently because movement or excessive stimulation of patients may trigger arrhythmias.

Treatment includes body warming and supportive measures for the complications of hypothermia. Contributing disorders are treated. Most drugs are avoided as long as hypothermia is present. They are ineffective and poorly metabolized; as a result, enhanced and undesirable rebound effects may occur as patients are warmed. Because most patients are hypovolemic, IV fluids are given, usually 1 L of 0.9% normal saline solution warmed to 37 to 45° C.

In elderly patients, rapid warming with an external heat source (active external warming; usually used for younger patients) is avoided if possible because it can lead to a syndrome of severe hypotension, new cardiac arrhythmias, and worsening metabolic abnormalities, culminating in death. Therefore, stable elderly patients are warmed by conserving endogenous heat production and minimizing external heat loss (passive warming)--a slow method. For passive warming, body temperature increases about 0.6° C/h. Room temperature is kept > 21.1° C, and blankets or more effective insulating materials are used to retain body heat.

Passive warming often does not raise a body temperature that is <= 30° C; in such cases, active external and active core warming is necessary. Forced-air warming systems can provide external heat. Techniques for active core warming include use of heated, moist inspired air and lavage of the peritoneal or sometimes thoracic cavity with heated fluid. Heated arteriovenous or venovenous circuits (as in hemodialysis) are more rapidly effective but more invasive; cardiopulmonary bypass is even more effective and invasive. If a nonperfusing rhythm (eg, ventricular fibrillation, asystole) occurs at a temperature < 30° C, warming must occur as quickly as possible because, at such a temperature, the heart is unresponsive to electrical defibrillation and because drug therapy for most arrhythmias is ineffective.

CPR is not done if patients have perfusing cardiac rhythms, regardless of whether pulses are palpable. Patients with ventricular fibrillation or asystole require CPR and endotracheal intubation. Defibrillation may be attempted starting at 2 joules/kg. If a single stacked sequence (200/300/360 joules) is ineffective, further attempts are deferred until temperature increases. Resuscitation should be continued, and patients are not pronounced dead until temperature reaches 35° C, unless obviously lethal injuries or disorders are present. Drug therapy for arrhythmias is withheld.

Although hyperglycemia is common among patients with hypothermia, insulin is rarely given unless plasma glucose levels are > 400 mg/dL (> 22.2 mmol/L); insulin is ineffective at low temperatures. Production of endogenous insulin during warming may lead to hypoglycemia, even when exogenous insulin is not given. Plasma glucose should be monitored regularly.

Prevention

When outdoors in cold weather, elderly people should wear several layers of warm clothing and protect themselves against moisture and wind. Clothing that insulates even when wet (eg, made of wool or polypropylene) should be worn. Gloves and socks should be kept as dry as possible; insulated boots that do not impede circulation should be worn in very cold weather. A warm head covering is particularly important because 30% of body heat is lost from the head.

Indoors, elderly people with risk factors for hypothermia should keep room temperatures >= 18.3° C. Room temperature should be checked daily with a reliable thermometer separate from the thermostat, especially during very cold weather. Extra clothing, particularly for hands, feet, and head, should be worn indoors. Adequate caloric intake and, if possible, exercise help sustain endogenous heat production. Drugs that increase risk should be stopped whenever possible.

For elderly patients undergoing surgery, warming the operating room, inspired gases, IV solutions, and antibacterial solutions used to prepare the surgical site and using forced-air warming blankets can minimize the risk of hypothermia. Shivering, which sometimes accompanies hypothermia, should be prevented if possible. It increases O₂ consumption. If O₂ demand exceeds supply, hypoxemia, acidosis, and circulatory changes occur, and elderly patients may be unable to compensate. Then, surgical morbidity and mortality increase.

This topic was last updated February 2006.