Hypothermia

Hypothermia results when body heat loss exceeds body heat production. Hypothermia is most common during cold weather or immersion in cold water, but it may occur in warm climates when people lie immobile on a cool surface (eg, when they are intoxicated) or after very prolonged immersion in swimming-temperature water (eg, 20 to 24° C). Wet clothing and wind increase risk of hypothermia.

Conditions that cause loss of consciousness, immobility, or both (eg, trauma, hypoglycemia, seizure disorders, stroke, drug or alcohol intoxication) are common predisposing factors. Older people and the very young also are at high risk:

• Older people often have diminished temperature sensation and impaired mobility and communication, resulting in a tendency to remain in an overly cool environment. These impairments, combined with diminished subcutaneous fat, contribute to hypothermia in older people—sometimes even indoors in cool rooms.

• The very young have similarly diminished mobility and communication and have an increased surface area/mass ratio, which enhances heat loss.

Hypothermia slows all physiologic functions, including cardiovascular and respiratory systems, nerve conduction, mental acuity, neuromuscular reaction time, and metabolic rate. Thermoregulation ceases below about 30° C; the body must then depend on an external heat source for rewarming.

Renal cell dysfunction and decreased levels of vasopressin (ADH) lead to production of a large volume of dilute urine (cold diuresis). Diuresis plus fluid leakage into the interstitial tissues causes hypovolemia. Vasoconstriction, which occurs with hypothermia, may mask hypovolemia, which then manifests as sudden shock or cardiac arrest during rewarming (rewarming collapse) when peripheral vasculature dilates.

Immersion in cold water can trigger the diving reflex, which involves reflex vasoconstriction in visceral muscles; blood is shunted to essential organs (eg, heart, brain). The reflex is most pronounced in small children and may help protect them. Also, hypothermia due to total immersion in near-freezing water may protect the brain from hypoxia by decreasing metabolic demands. The decreased demand probably accounts for the occasional survival after prolonged cardiac arrest due to extreme hypothermia.

Intense shivering occurs initially, but it ceases below about 30° C, allowing body temperature to drop more precipitously. Central nervous system dysfunction progresses as body temperature decreases; people do not sense the cold. Lethargy and clumsiness are followed by confusion, irritability, sometimes hallucinations, and eventually coma. Pupils may become unreactive. Respirations and heartbeat slow and ultimately cease. Initially, sinus bradycardia is followed by slow atrial fibrillation; the terminal rhythm is ventricular fibrillation or asystole.

• Core temperature measurement

• Consideration of intoxication, myxedema, sepsis, hypoglycemia, and trauma

Diagnosis is by core temperature, not oral temperature. Electronic thermometers are preferred; many standard mercury thermometers have a lower limit of 34° C. Rectal and esophageal probes are most accurate.

There are a variety of core temperature thresholds for classifying severity of hypothermia. The Wilderness Medical Society Clinical Practice Guidelines uses the following (1):

• Mild: 32 to 35°C

• Moderate: 28 to 32°C

• Severe: < 28°C

Laboratory tests include complete blood count, glucose (including bedside measurement), electrolytes, blood urea nitrogen, creatinine, and arterial blood gases (ABGs). ABGs are not corrected for low temperature. An electroencephalogram (EEG) may show J (Osborn) waves (see figure Abnormal ECG showing J [Osborn] waves [V4]) and interval prolongation (PR, QRS, QT). If the cause of hypothermia is unclear, tests to detect contributing factors are done, including measuring the alcohol level and screening for drugs and thyroid function. Sepsis and occult head or skeletal trauma must be considered. Hypoadrenalism and hypothyroidism (including myxedema) can contribute and are often occult, with no history of cold intolerance, dry skin, arthralgias, or lassitude. Failure to rewarm is a clue. Myxedema characteristically prolongs the relaxation phase of the ankle reflex more than the contraction phase.

Abnormal ECG showing J (Osborn) waves (V4)

The J (Osborn) wave is visible as a hump at the junction of the QRS complex and ST segment. Computer programs cannot reliably differentiate J waveform abnormalities from myocardial injury current.

Patients who have been immersed in icy water for 1 hour or (rarely) longer have sometimes been successfully rewarmed without permanent brain damage (see Drowning: Prognosis), even when core temperatures were very low or when pupils were unreactive. Outcome is difficult to predict and cannot be based on the Glasgow Coma Scale. Grave prognostic markers include

• Evidence of cell lysis (serum potassium > 12 mEq/L [12 mmol/L])

• Intravascular thrombosis (fibrinogen < 50 mg/dL [1.47 micromol/L])

• A nonperfusing cardiac rhythm (ventricular fibrillation or asystole)

For a given degree and duration of hypothermia, children are more likely to recover than adults.

• Drying and insulation

• Fluid resuscitation

• Active rewarming unless hypothermia is mild, accidental, and uncomplicated

The first priority is to prevent further heat loss by removing wet clothing and insulating the patient. Subsequent measures depend on how severe hypothermia is and whether cardiovascular instability or cardiac arrest is present. Returning patients to a normal temperature is less urgent in mild hypothermia than in severe hyperthermia. For stable patients, elevation of core temperature by 1° C/hour is acceptable.

Fluid resuscitation is essential because patients are typically hypovolemic. Patients are given 500 cc to 2 L of 0.9% saline solution (20 mL/kg for children) IV; if possible, the solution is heated to 40 to 42° C. More fluid is given as needed to maintain perfusion.