eMedicine Specialties > Emergency Medicine > Environmental

Hypothermia

Author: Jamie Alison Edelstein, MD, Staff Physician, Department of Emergency Medicine, State University of New York, Kings County Hospital Center
Coauthor(s): James Li, MD, Former Assistant Professor, Division of Emergency Medicine, Harvard Medical School; Board of Directors, Remote Medicine; Mark A Silverberg, MD, FACEP, MMB, Assistant Professor, Assistant Residency Director, Department of Emergency Medicine, State University of New York Downstate College of Medicine; Consulting Staff, Department of Emergency Medicine, Staten Island University Hospital, Kings County Hospital, University Hospital, State University of New York Downstate at Brooklyn; Wyatt Decker, MD, Chair and Program Director, Department of Emergency Medicine, Mayo Clinic
Contributor Information and Disclosures

Updated: Oct 29, 2009

Introduction

Background

Hypothermia describes a state in which the body's mechanism for temperature regulation is overwhelmed in the face of a cold stressor. Hypothermia is classified as accidental or intentional, primary or secondary, and by the degree of hypothermia. 

Accidental hypothermia generally results from unanticipated exposure in an inadequately prepared person; examples include inadequate shelter for a homeless person, someone caught in a winter storm or motor vehicle accident, or an outdoor sport enthusiast caught off guard by the elements. Intentional hypothermia is an induced state generally directed at neuroprotection after an at-risk situation (usually after cardiac arrest, see Therapeutic Hypothermia).¹ Primary hypothermia is due to environmental exposure, with no underlying medical condition causing disruption of temperature regulation.² Secondary hypothermia is low body temperature resulting from a medical illness lowering the temperature set-point.

Many patients have recovered from severe hypothermia, so early recognition and prompt initiation of optimal treatment is paramount.

Systemic hypothermia may also be accompanied by localized cold injury (see Frostbite).

Pathophysiology

The body's core temperature is tightly regulated in the "thermoneutral zone" between 36.5°C and 37.5°C, outside of which thermoregulatory responses are usually activated. The body maintains a stable core temperature through balancing heat production and heat loss. At rest, humans produce 40-60 kilocalories (kcal) of heat per square meter of body surface area through generation by cellular metabolism, most prominently in the liver and the heart. Heat production increases with striated muscle contraction; shivering increases the rate of heat production 2-5 times.

Heat loss occurs via several mechanisms, the most significant of which, under dry conditions, is radiation (55-65% of heat loss). Conduction and convection account for about 15% of additional heat loss, and respiration and evaporation account for the remainder. Conductive and convective heat loss, or direct transfer of heat to another object or circulating air, respectively, are the most common causes of accidental hypothermia. Conduction is a particularly significant mechanism of heat loss in drowning/immersion accidents as thermal conductivity of water is up to 30 times that of air.

The hypothalamus controls thermoregulation via increased heat conservation (peripheral vasoconstriction and behavior responses) and heat production (shivering and increasing levels of thyroxine and epinephrine). Alterations of the CNS may impair these mechanisms. The threshold for shivering is 1 degree lower than that of vasoconstriction and is considered a last resort mechanism by the body to maintain temperature.³ The mechanisms for heat preservation may be overwhelmed in the face of cold stress and core temperature can drop secondary to fatigue or glycogen depletion.

Hypothermia affects virtually all organ systems. Perhaps the most significant effects are seen in the cardiovascular system and the CNS. Hypothermia results in decreased depolarization of cardiac pacemaker cells, causing bradycardia. Since this bradycardia is not vagally mediated, it can be refractory to standard therapies such as atropine. Mean arterial pressure and cardiac output decrease, and an electrocardiogram (ECG) may show characteristic J or Osborne waves (see Media file 1). While generally associated with hypothermia, the J wave may be a normal variant and is seen occasionally in sepsis and myocardial ischemia.

Atrial and ventricular arrhythmias can result from hypothermia; asystole and ventricular fibrillation have been noted to begin spontaneously at core temperatures below 25-28°C.

Hypothermia progressively depresses the CNS, decreasing CNS metabolism in a linear fashion as the core temperature drops. At core temperatures less than 33°C, brain electrical activity becomes abnormal; between 19°C and 20°C, an electroencephalogram (EEG) may appear consistent with brain death. Tissues have decreased oxygen consumption at lower temperatures; it is not clear whether this is due to decreases in metabolic rate at lower temperatures or a greater hemoglobin affinity for oxygen coupled with impaired oxygen extraction of hypothermic tissues.

The term "core temperature after drop" refers to a further decrease in core temperature and associated clinical deterioration of a patient after rewarming has been initiated. The current theory of this documented phenomenon is that as peripheral tissues are warmed, vasodilation allows cooler blood in the extremities to circulate back into the body core. Other mechanisms may be in operation as well. Some believe that after drop is most likely to occur in patients with frostbite or long-standing hypothermia.

Frequency

United States

Accurately estimating the incidence of hypothermia is impossible, as hospital encounters only represent the "tip of the iceberg" in that they reflect the more severe cases. Even so, the number of emergency department encounters for hypothermia is growing, as ever-growing numbers of people take to the outdoors in search of adventure. Hypothermia is also a disease of urban settings. Societal problems with alcoholism, mental illness, and homelessness create a steady stream of these cases to inner-city hospitals. Although most cases occur in regions of the country with severe winter weather, other areas with milder climates also experience cases on a regular basis. This is especially true in milder climates that experience rapid climate changes either due to seasonal changes or day-to-night changes secondary to altitude. According to current data, states with the highest overall death rates for hypothermia are Alaska, New Mexico, North Dakota, and Montana.

The greatest number of cases of hypothermia occur in an urban setting and are related to environmental exposure attributed to alcoholism, illicit drug use, or mental illness, often exacerbated by concurrent homelessness. This is simply due to the fact that more people are found in the urban regions rather than rural areas.

A second affected group includes people in an outdoor setting for work or pleasure, including hunters, skiers, climbers, boaters/rafters, and swimmers.

Mortality/Morbidity

According to one study, overall in-patient mortality in hypothermic patients was 12%. Most people tolerate mild hypothermia (32-35°C body temperature) fairly well, which is not associated with significant morbidity or mortality. In contrast, a multicenter survey found a 21% mortality rate for patients with moderate hypothermia (28-32°C body temperature). Mortality is even higher in severe hypothermia (core temperature below 28°C). Despite hospital-based treatment, mortality from moderate or severe hypothermia approaches 40%. Patients experiencing concurrent infection account for most deaths due to hypothermia. Other comorbidities associated with higher mortality rates include homelessness, alcoholism, psychiatric disease, and advanced age.

"Indoor hypothermia" is more likely to occur in patients with significant medical comorbidities (alcoholism, sepsis, hypothyroidism/hypopituitarism) and tends to carry worse outcomes than exposure hypothermia.

According to current records, approximately 700 people die in the United States from accidental primary hypothermia each year.

Sex

The overall mortality rate from hypothermia is similar between men and women. Because of a higher incidence of exposure among males, men account for 65% of hypothermia-related deaths.

Age

Very young and elderly persons are at increased risk and may present to the emergency department with symptoms that are not clinically obvious or specific for hypothermia, such as altered mental status.

Older patients appear to be more likely to present with chronic or secondary hypothermia. Half of the recorded deaths from accidental hypothermia occurred in individuals older than 65 years.

Clinical

History

Hypothermia is usually readily apparent in the setting of severe environmental exposure. In elderly patients or "indoor" patients, or for a patient—particularly a wet patient, with exposure to less extreme cold, the history may be subtle and less obvious. These patients may have a higher mortality rate secondary to a longer time to diagnosis and increased age and fragility. Mild or moderate hypothermia can present with misleading symptoms, such as confusion, dizziness, chills, or dyspnea.

A patient's companions often note initial symptoms in the field. Symptoms can include mood change, irritability, poor judgment, and lassitude. Companions may note the patient to demonstrate paradoxical undressing (a severely hypothermic person removes clothing in response to prolonged cold stress) or rhythmic or repeated motions such as rocking. Slurred speech and ataxia may mimic a stroke, alcohol intoxication, or high-altitude cerebral edema. Similarly, profound hypothermia may present as coma or cardiac arrest.

In an urban environment, the use of alcohol or illicit drugs, overdose, psychiatric emergency, and major trauma all are associated with an increased risk of hypothermia.

Physical

The key to establishing a diagnosis of hypothermia is rapid determination of true core temperature. In the emergency department, core temperature is best measured using a low-reading temperature probe in the bladder or rectum or an esophageal probe. In the field, core temperature may be more difficult to establish reliably. A special low-reading thermometer can be used orally or rectally, but it may not reflect a true core temperature. Care should be taken not to rely on a temperature from a rectal thermometer lodged in stool because an inaccurately low core temperature can be recorded; the probe's reading will also lag behind the core temperature during rewarming. Additionally, a thermometer may become dislodged; be suspicious if a core temperature reading is identical to the room temperature.

• Obtaining a core temperature may help prevent erroneous diagnosis for patients with an altered mental status due to stroke, drug overdose, alcohol intoxication, or mental illness. Standard temperature measuring devices commonly used for triage may lack the capability to report unusually low temperature; obtain a core temperature reading for any patient suspected of being significantly hypothermic.
• At a given temperature, specific physical examination findings vary among patients. However, an examination does provide a frame of reference for dividing presenting symptoms into mild, moderate, and severe hypothermic signs.⁴
  • Mild hypothermia (32-35°C)
    • Between 34°C and 35°C, most people shiver vigorously, usually in all extremities.
    • As the temperature drops below 34°C, a patient may develop altered judgment, amnesia, and dysarthria. Respiratory rate may increase.
    • At approximately 33°C, ataxia and apathy may be seen. Patients generally are stable hemodynamically and able to compensate for the symptoms.
    • In this temperature range, the following may also be observed: hyperventilation, tachypnea, tachycardia, and cold diuresis as renal concentrating ability is compromised.
  • Moderate hypothermia (28-32°C)
    • Oxygen consumption decreases, and the CNS depresses further; hypoventilation, hyporeflexia, decreased renal flow, and paradoxical undressing may be noted.
    • Most patients with temperatures of 32°C or lower present in stupor.
    • As the core reaches temperatures of 31°C or below, the body loses its ability to generate heat by shivering.
    • At 30°C, patients develop a higher risk for arrhythmias. Atrial fibrillation and other atrial and ventricular rhythms become more likely. The pulse continues to slow progressively, and cardiac output is reduced. J wave may be seen on ECG in moderate hypothermia.
    • Between 28°C and 30°C, pupils may become markedly dilated and minimally responsive to light, a condition that can mimic brain death.
  • Severe hypothermia (<28°C)
    • At 28°C, the body becomes markedly susceptible to ventricular fibrillation and further depression of myocardial contractility.
    • Below 27 º C, 83% of patients are comatose.
    • Pulmonary edema, oliguria, coma, hypotension, rigidity, apnea, pulselessness, areflexia, unresponsiveness, fixed pupils, and decreased or absent activity on EEG are all seen.

Causes

• Decreased heat production
  • Several etiologies related to endocrine derangements may cause decreased heat production. These include hypopituitarism, hypoadrenalism, and hypothyroidism. Consider all these conditions in patients presenting with unexplained hypothermia who fail to rewarm with standard therapy.
  • Other causes include severe malnutrition or hypoglycemia and neuromuscular inefficiencies seen in the extremes of age.
• Increased heat loss
  • This category includes accidental hypothermia due to both immersion etiologies and nonimmersion etiologies and is the most common form of hypothermia encountered in the emergency department.
  • Patients may present with induced vasodilatation from pharmacologic or toxicologic agents.
  • Erythrodermas, such as burns or psoriasis, that decrease the body's ability to preserve heat, or iatrogenic etiologies, such as cold infusions, overenthusiastic treatment of heatstroke, or emergency deliveries, may cause hypothermia due to increased heat loss.
• Impaired thermoregulation
  • A variety of causes may be associated with impaired thermoregulation, but, generally, it is associated with failure of the hypothalamus to regulate core body temperature.
  • This may occur with CNS trauma, strokes, toxicologic and metabolic derangements, intracranial bleeding, Parkinson disease, CNS tumors, Wernicke disease, and multiple sclerosis.
• Miscellaneous causes include sepsis, multiple trauma, pancreatitis, prolonged cardiac arrest, and uremia.
• Hypothermia may be related to drug administration; such medications include beta-blockers, clonidine, meperidine, neuroleptics, and general anesthetic agents. Ethanol, phenothiazines, and sedative-hypnotics also reduce the body’s ability to respond to low ambient temperatures.

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