(See also Overview of Cold Injury.)
Primary hypothermia causes about 600 deaths each year in the US. Hypothermia also has a significant and underrecognized effect on mortality risk in cardiovascular and neurologic disorders.
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.
Dow J, Giesbrecht GG, Danzl DF, et al: Wilderness Medical Society Clinical Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2019 Update. Wilderness Environ Med 30(4S):S47-S69, 2019. doi: 10.1016/j.wem.2019.10.002. Epub 2019 Nov 15. PMID: 31740369.
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 31° 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.
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.
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.
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
For a given degree and duration of hypothermia, children are more likely to recover than adults.
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 hypothermia than in severe hyperthermia. For stable patients, elevation of core temperature by 1° C/hour is acceptable.
Fluid resuscitation is essential for hypovolemia. Patients are given 1 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.
Active rewarming is required if patients have temperature < 32.2° C, cardiovascular instability, hormone insufficiency (such as hypoadrenalism or hypothyroidism), or hypothermia secondary to trauma, toxins, or predisposing disorders.
In moderate hypothermia, body temperature is at the warmer end of the range (28 to 32.2° C), and external rewarming with forced hot air enclosures may be used. External heat is best applied to the thorax because warming the extremities may increase metabolic demands on a depressed cardiovascular system.
In severe hypothermia, patients with lower temperatures (< 28° C), particularly those with low blood pressure or cardiac arrest, require core rewarming.
Core rewarming options include
Inhalation of heated (40 to 45° C), humidified oxygen via mask or endotracheal tube eliminates respiratory heat loss and can add 1 to 2° C/hour to the rewarming rate.
IV crystalloids or blood should be heated to 40 to 42° C, especially with massive volume resuscitations.
Closed thoracic lavage through 2 thoracostomy tubes (see How To Do Surgical Tube Thoracostomy) is very efficient in severe cases. Peritoneal lavage with dialysate heated to 40 to 45° C requires 2 catheters with outflow suction and is especially useful for severely hypothermic patients who have rhabdomyolysis, toxin ingestions, or electrolyte abnormalities. Heated lavage of the bladder or gastrointestinal tract transfers minimal heat.
There are 5 types of ECR: hemodialysis, venovenous, continuous arteriovenous, cardiopulmonary bypass, and extracorporeal membrane oxygenation. ECR measures require a prearranged protocol with appropriate specialists. Although they are intuitively attractive and heroic, these measures are not routinely available, and they are not commonly used in most hospitals.
Hypotension and bradycardia are expected when core temperature is low and, if due solely to hypothermia, need not be aggressively treated.
When needed, endotracheal intubation after oxygenation must be done gently to avoid precipitating a nonperfusing rhythm.
CPR should be withheld if patients have a perfusing rhythm unless true cardiac arrest is confirmed by absence of cardiac motion on bedside cardiac ultrasonography. Treat with fluids and active rewarming. Chest compressions are not done, because
Patients with a nonperfusing rhythm (ventricular fibrillation or asystole) require CPR. Chest compressions and endotracheal intubation are done. Defibrillation is difficult if body temperature is low; one attempt with a 2 watt sec/kg charge may be made, but if ineffective, further attempts are generally deferred until temperature reaches > 30° C.
Advanced life support should be continued until temperature reaches 32° C unless obviously lethal injuries or disorders are present. However, advanced cardiac life-support drugs (eg, antiarrhythmics, vasopressors, inotropes) are usually not given. Low-dose dopamine (1 to 5 mcg/kg/min) or other catecholamine infusions are typically reserved for patients who have disproportionately severe hypotension and who do not respond to fluid resuscitation and rewarming. Severe hyperkalemia (> 10 mEq/L [10 mmol/L]) during resuscitation typically indicates a fatal outcome and can guide resuscitation efforts.
Measure core temperature in the rectum or esophagus using an electronic thermometer or probe.
Above about 32° C, heated or forced-air blankets and warm drinks are adequate treatment.
Below about 32° C, active rewarming should be done, typically using forced-air hot air enclosures; heated, humidified oxygen; warm IV fluid; and sometimes heated lavage or extracorporeal methods (eg, cardiopulmonary bypass, hemodialysis).
At lower temperatures, patients are hypovolemic and require fluid resuscitation.
CPR is not done if there is a perfusing rhythm.
When CPR is done in patients with a nonperfusing rhythm, defibrillation is deferred (after one initial attempt) until temperature reaches about 30° C.
Advanced cardiac life-support drugs are usually not given.