Disorders of Temperature Control: Hypothermia
Jiaying Zhang
Susanne Muehlschlegel
Mark M. Wilson
I. UNINTENTIONAL HYPOTHERMIA
A. General principles.
1. Hypothermia is defined as core temperature ≤ 35°C (95°F).
2. When body temperature decreases, the hypothalamus modulates autonomic tone causing cessation of sweat production, constriction of cutaneous vasculature, and increased heat production during the shivering phase (35°C to 30°C).
3. Temperature regulation declines with age as a result of deterioration in sensory afferent neurons, cortical function, and effector responses.
B. Etiology.
1. The most frequent causes of hypothermia are exposure to cold, use of certain drugs (alcohol, phenothiazines, barbiturates, neuroleptics, paralytics), and hypoglycemia.
2. Other common causes include hyperglycemia, hypothyroidism, adrenal insufficiency, central nervous system (CNS) disorders, extensive burns, sepsis, and trauma.
C. Pathogenesis.
1. The incidence of hypothermia doubles with every 5°C drop in ambient temperature. Wet clothing effectively loses up to 90% of its insulating value. Convective heat losses due to wind may increase losses to greater than five times baseline values.
2. Most sedative-hypnotic drugs cause hypothermia by inhibiting shivering and impairing capability for voluntary control of temperature.
3. Hypoglycemia lowers cerebral intracellular glucose concentrations and impairs hypothalamic function. In acute hypoglycemia, hypothermia occurs due to peripheral vasodilation and sweating.
4. Increasing degrees of hypothermia produce malignant dysrhythmias, depressed cardiac function, and hypotension. The electrocardiogram (ECG) in mild hypothermia shows bradycardia with prolongation of the PR interval, QRS complex, and QT interval. At temperatures <33°C, the ECG commonly shows a characteristic J-point elevation. At temperatures <30°C, first-degree block is common. Atrial fibrillation (AF) is extremely common at temperatures of 34°C to 25°C, and ventricular fibrillation (VF) is frequent at <28°C. Third-degree block and asystole are common when core temperatures drop to <20°C.
5. Pulmonary mechanics and gas exchange appear to change little with hypothermia. Both tidal volume and respiratory rate decline as core temperature lowers. At temperatures <24°C, respirations may cease.
6. As blood pressure decreases during the nonshivering phase (<30°C), renal blood flow may decrease by 75% to 85%, without a significant decrease in urine production. This process is termed cold diuresis and is due to a defect in tubular reabsorption. The net result is dehydration and a relatively hyperosmolar serum.
7. Hypothermia is neuroprotective; complete neurologic recovery has been described in hypothermic adults after 20 minutes of complete cardiac arrest and after up to 3.5 hours of cardiopulmonary resuscitation.
8. The white blood cell count in mild hypothermia remains normal to slightly elevated; it may drop severely at temperatures <28°C. The hematocrit usually rises in patients at a temperature of 30°C from dehydration and splenic contraction.
9. Hepatic dysfunction is common and involves synthetic and detoxification abnormalities. Ileus and pancreatitis can also occur.
10. Hypothermia directly suppresses the release of insulin and increases resistance to insulin’s action in the tissues. Any hyperglycemia, however, is usually mild.
11. The hypothermic host is prone to infections secondary to impairment of immune function and decrease in inflammatory responses, secretion of cytokines, and suppression of leukocyte migration and the function of phagocytic cells.
D. Diagnosis.
1. Hypothermia is diagnosed by history of exposure, a high-risk patient profile (elderly, alcoholic, diabetic, quadriplegic, or severely debilitated), clinical examination, and laboratory abnormalities.
2. Cool skin, muscle rigidity, shivering or muscle tremor, and acrocyanosis are present in most noncomatose patients.
3. Between 35°C and 32°C, the patient may be verbally responsive but incoherent and between 32°C and 27°C, stuporous or confused, and at temperatures <27°C, most patients are comatose but respond purposefully to noxious stimuli. Deep coma is uncommon but, when present, may be difficult to distinguish from death. The criteria for death cannot be applied until core temperature is back near 37°C.
4. Thermometers calibrated to record temperatures <35°C should be used; only sites that reflect core temperature should be measured (bladder, rectal, tympanic, esophageal, or great vessel sites).
E. Treatment.
1. Treatment should be aggressive. Wet clothes should be removed and replaced with dry ones. The victim should be insulated from cold and wind. Rough handling must be avoided; even minor manipulations can induce VF.
2. Fluid resuscitation, preferably through a central vein, should be attempted in all patients in hypothermic shock. Slightly hypotonic crystalloid fluids should be given after warming to at least room temperature. Pressor agents and procedures (intubation or catheter placement) should not be withheld because of a fear of dysrhythmia.
