DESCRIPTION

• Mild hypothermia is characterized by core temperature of 32° to 35°C (89.6°-95°F), tachypnea, tachycardia, ataxia, and dysarthria; shivering generates heat.
• Moderate hypothermia occurs when core temperatures range from 28° to 32°C (82.4°-89.6°F); the patient exhibits loss of shivering, dysrhythmias, Osborn J waves on ECG, and depressed mentation.
• Severe hypothermia occurs below 28°C (82.4°F), and is characterized by loss of reflexes, coma, hypotension, acidemia, ventricular fibrillation, and asystole.
• Profound hypothermia is reached at 14° to 20°C (57.2°-68°F); virtually all patients are asystolic.

PATHOPHYSIOLOGY

• Temperature regulation in humans occurs via the hypothalamic-pituitary system, regulating vasoconstriction, muscle tone, and metabolism.
• Below a core temperature of about 30°C, humans lose the ability to produce and conserve heat.

EPIDEMIOLOGY

When related to poisoning, hypothermia is usually the result of low ambient temperature combined with the victim's inability to leave the environment (e.g., depressed mental status and poorly heated apartment).

Section Outline:

• DESCRIPTION
• PATHOPHYSIOLOGY
• EPIDEMIOLOGY

DIFFERENTIAL DIAGNOSIS

Further information on each poison is available in SECTION IV, CHEMICAL AND BIOLOGICAL AGENTS.

• Toxicologic causes are numerous.
  • Ethanol intoxication may be assessed using blood alcohol level.
  • Carbon monoxide is associated with coma, acidosis, and increased carboxyhemoglobin level.
  • Opioids cause small pupils and respiratory depression; evidence of chronic drug abuse may be apparent.
  • Sedative-hypnotic agents can usually be detected on urine drug screen.
  • Phenothiazines produce ECG abnormalities and perhaps anticholinergic effects.
  • Hallucinogens or over-the-counter antihistamines are often abused by minors and may facilitate development of environmental hypothermia.
  • Oral hypoglycemic agents impair response to hypothermia.
• Most cases of nontoxicologic hypothermia are environmental and have an apparent cause.
• Any condition that causes altered mental status may lead to hypothermia by preventing the patient from leaving the hypothermic environment.

SIGNS AND SYMPTOMS

Physical signs may help reveal the poison involved when they occur in the setting of hypothermia.

Vital Signs

Initial tachycardia is followed by bradycardia and hypotension.

HEENT

• Pinpoint pupils suggest an opioid, clonidine, or imidazoline drug.
• Dilated pupils indicate hypoxia, cold, or anticholinergic effect.

Dermatologic

Bullae may develop with coma associated with carbon monoxide or sedative-hypnotic agents.

Pulmonary

Loss of protective airway reflexes may allow aspiration.

Gastrointestinal

• Ileus usually develops.
• Opioids or anticholinergic drugs are other causes of decreased bowel sounds.

Hematologic

• Hemoconcentration is common.
• Disseminated intravascular coagulation may be present in severe cases.

Fluids and Electrolytes

• Acute hypothermia causes a "cold diuresis," resulting in volume depletion.
• Respiratory alkalosis may occur initially but will become respiratory acidosis if hypothermia is severe.
• Metabolic acidosis resulting from lactic acid accumulation is common.
• Hyper- or hypoglycemia may occur; hypoglycemia may indicate oral hypoglycemic ingestion.
• Hypokalemia occurs due to reversible redistribution of potassium into muscle; supplementation should be avoided.

Neurologic

• Mental status. Initial CNS stimulation is followed by depression, causing a progressive depressed mentation with ataxia and dysarthria; many patients are comatose below 30°C.
• Reflexes. Hyperreflexia is present at core temperatures from 32°C to 35°C, hyporeflexia from 26°C to 32°C; reflexes are absent below 26°C, the knee jerk being the last to go.

PROCEDURES AND LABORATORY TESTS

Essential Tests

• Rectal temperature should be taken to confirm hypothermia.
• Mild hypothermia requires no special laboratory testing in itself; appropriate laboratory tests are determined by the drugs involved.

Recommended Tests

• Serum electrolytes, glucose, BUN, creatinine. Hypokalemia and hypoglycemia are common. Hypoglycemia may indicate oral hypoglycemic ingestion.
• Complete blood count. Hemoconcentration is common.
• Arterial blood gases. Respiratory alkalosis followed by respiratory acidosis may occur.
• Amylase to check for hyperamylasemia in severe cases.
• ECG with continuous monitoring:
  • ECG effects may obscure conduction abnormalities caused by toxicant.
  • Severity of conduction abnormality is related to severity of hypothermia: bradycardia, QRS widening, prolonged QT, repolarization abnormalities with variable effects on the ST segment and T wave.
  • The Osborn wave (J wave) is a hump at the J point immediately after the QRS complex, the size increasing with temperature depression.
  • J waves are also associated with CNS lesions, focal cardiac ischemia, and sepsis; they may be present in young healthy persons.
• Serum acetaminophen and aspirin levels and urine drug screen in an overdose setting to detect occult overdose with analgesic medication and to screen for poisoning with sedative-hypnotic or drugs of abuse as cause.

Section Outline:

• DIFFERENTIAL DIAGNOSIS
• SIGNS AND SYMPTOMS
  • Vital Signs
  • HEENT
  • Dermatologic
  • Pulmonary
  • Gastrointestinal
  • Hematologic
  • Fluids and Electrolytes
  • Neurologic
• PROCEDURES AND LABORATORY TESTS
  • Essential Tests
  • Recommended Tests

• Treatment is directed toward preventing further heat loss, rewarming, and to preventing complications, especially cardiac dysrhythmias.
• Volume resuscitation should be performed with warmed dextrose 5% and normal saline, or with normal saline alone.
• Rewarming and specific treatments should be initiated while supportive care continues.
• Dose and time of exposure should be determined for all substances that could be involved.

DIRECTING PATIENT COURSE

Health-care provider should call poison control center when:

• cause of hypothermia is unclear.
• coingestant, drug interaction, or underlying disease presents unusual problems.

DECONTAMINATION

• Emesis should not be induced if patient is hypothermic.
• Gastric lavage can be performed in pediatric (tube size 24-32 French) or adult (tube size 36-42 French) patients for large ingestion presenting within 1 hour of ingestion or if serious effects are present.
• One dose of activated charcoal (1-2 g/kg) should be administered without a cathartic if a substantial ingestion has occurred within the previous few hours.

ADJUNCTIVE TREATMENT

Indications for Rewarming

• Mildly hypothermic patients (above 32°C) can be rewarmed with external noninvasive methods such as a blanket, heated humidified oxygen, and warm intravenous fluids.
• Moderate hypothermia (28°-32°C) should be treated with airway rewarming and warm intravenous fluids for all patients, possibly supplemented by gastric or peritoneal lavage if rewarming is proceeding at less than 1°C per hour.
• Severe hypothermia (below 28°C) mandates aggressive therapy and active core rewarming.
• If the core temperature is less than 25°C, femoral-femoral bypass should be considered if the facilities are available.
• For patients in cardiac arrest, femoral-femoral bypass is the rewarming method of choice.
• Open pleural lavage for direct cardiac rewarming should be considered if the core temperature remains below 28°C after 1 hour of bypass.

Methods for Rewarming

• Passive external rewarming consists of insulation of the patient.
• Active external rewarming uses convective blankets or other external heat sources for rewarming; radiant warmers work only if the patient is fully uncovered.
• Active core rewarming
  • Airway warming: nebulizer or ventilator is modified to give 100% humidified air warmed to 40° to 45°C.
  • Warm intravenous fluid (40°-42°C) is administered.
  • Body cavity lavage should be considered; peritoneal lavage and pleural lavage with tube thoracostomy can be useful in patients with severe effects.
  • Gastrointestinal or bladder irrigation also may rewarm central organs but are associated with risks of electrolyte imbalance.

Pharmacologic Treatment

• Target organs become progressively less responsive to medications as the core temperature decreases.
• Excessive pharmacologic manipulation of the vasoconstricted and depressed cardiovascular system should be avoided.
• Large doses of exogenous insulin or digoxin are ineffective at lower temperatures but can produce a toxic reaction as rewarming progresses.
• Infusion of low doses of catecholamine are indicated in patients who have lower blood pressure than would be expected for that degree of hypothermia and who are not responding to crystalloid infusion and rewarming.

Section Outline:

• DIRECTING PATIENT COURSE
• DECONTAMINATION
• ADJUNCTIVE TREATMENT
  • Indications for Rewarming
  • Methods for Rewarming
  • Pharmacologic Treatment

PATIENT MONITORING

• Continuous cardiac and respiratory monitoring should be performed.
• Arterial pressure monitoring (A line) may be needed in severe cases.

EXPECTED COURSE AND PROGNOSIS

• Failure to restore a circulating cardiac rhythm within about 30 minutes after rewarming to 32° to 35°C makes further efforts unlikely to be successful.
• Disseminated intravascular coagulopathy, cardiac dysrhythmia, rhabdomyolysis, and other complications may become apparent during rewarming of patients with moderate to severe hypothermia.

DISCHARGE CRITERIA/INSTRUCTIONS

Asymptomatic patients who suffered mild hypothermia may be discharged following a 6-hour observation period, decontamination, and psychiatric evaluation, if needed.

Section Outline:

• PATIENT MONITORING
• EXPECTED COURSE AND PROGNOSIS
• DISCHARGE CRITERIA/INSTRUCTIONS

• Cardiovascular depression can persist on rewarming.
• Volume redistribution (to the core blood vessels) and cold diuresis can lead to decrease in blood pressure with rewarming and peripheral vasodilation.
• Jostling or vigorously moving the patient should be avoided as this may trigger ventricular fibrillation.

General symptoms: hypothermia (not associated with environment).

See Also: SECTION II, Bradycardia and Hypotension chapters, and SECTION IV, Chapters on individual agents.

RECOMMENDED READING

Danzl DF, Pozos RS. Accidental hypothermia. N Engl J Med 1994;331:1756-1760.

Danzl DF, Pozos RS, Hamlet MP. Accidental hypothermia. In: Auerbach PS, ed. Wilderness medicine, 3rd ed. St. Louis: Mosby, 1995:51-103.

Jolly BT, KT Ghezzi. Accidental hypothermia. Emerg Med Clin North Am 1992;10:311-319.

Author: Gayle E. Long

Reviewer: Richard C. Dart