Toxicology – Hypothermia

Definition

Hypothermia is a core body temperature below 35°C (95°F).

A commonly used clinical classification is:

• Mild: 32–35°C
• Moderate: 28–32°C
• Severe: <28°C

Clinical findings do not always correspond perfectly to a specific temperature, so management should consider both core temperature and physiologic condition.

Pathophysiology

The hypothalamus normally maintains temperature through:

• Peripheral vasoconstriction
• Shivering
• Increased metabolic heat production
• Behavioral responses to cold

With progressive cooling:

Cold exposure → vasoconstriction + shivering → depletion of energy reserves → impaired thermoregulation → CNS/cardiovascular depression

At lower temperatures:

• Shivering eventually stops.
• Metabolic rate falls.
• Cardiac conduction slows.
• Myocardial irritability increases.
• Consciousness progressively decreases.

Toxicologic hypothermia commonly occurs when intoxication impairs consciousness or judgment, preventing the patient from recognizing or escaping a cold environment.

Major Toxicologic Causes

Important causes include:

• Ethanol
• Opioids
• Sedative-hypnotics
• Clonidine and imidazolines
• Antipsychotic medications
• Other CNS depressants

Many drugs cause hypothermia indirectly by producing:

• Immobility
• CNS depression
• Vasodilation
• Impaired behavioral responses
• Reduced shivering or metabolic heat production

Ethanol

Alcohol intoxication predisposes to hypothermia through:

• Impaired judgment
• CNS depression
• Reduced behavioral response to cold
• Peripheral vasodilation
• Prolonged environmental exposure

A blood ethanol concentration can document exposure but should not automatically be assumed to explain profound hypothermia or altered consciousness.

Opioids

Opioid poisoning may cause:

• CNS depression
• Respiratory depression
• Miosis
• Immobility

These effects increase the risk of environmental hypothermia.

When opioid-induced respiratory depression is suspected, naloxone should be used to restore adequate ventilation.

Sedative-Hypnotics

Benzodiazepines, barbiturates, and other sedatives can predispose to hypothermia through:

• CNS depression
• Immobility
• Reduced protective behavior
• Prolonged exposure to cold

Severe intoxication can additionally produce respiratory depression and hypotension.

Routine urine drug screens may miss clinically important sedatives and therefore cannot reliably exclude poisoning.

Carbon Monoxide

Carbon monoxide exposure should be considered when hypothermia occurs with:

• Headache
• Confusion
• Syncope
• Coma
• Metabolic/lactic acidosis
• Exposure to combustion products

Carboxyhemoglobin measurement by co-oximetry is used to confirm exposure.

Standard pulse oximetry may appear deceptively normal.

Hypoglycemic Agents

Hypoglycemia can contribute to:

• Altered mental status
• Reduced heat production
• Hypothermia

Therefore, bedside glucose should be checked early in every significantly hypothermic patient.

Nontoxicologic Causes

Important alternatives include:

• Environmental exposure
• Sepsis
• Hypoglycemia
• Hypothyroidism/myxedema coma
• Adrenal insufficiency
• CNS injury
• Trauma
• Stroke
• Malnutrition

Any disorder causing altered consciousness can indirectly produce hypothermia by preventing escape from a cold environment.

Mild Hypothermia

Typical findings include:

• Shivering
• Tachycardia
• Tachypnea
• Peripheral vasoconstriction
• Ataxia
• Dysarthria
• Impaired judgment

Patients are often still capable of generating substantial endogenous heat.

Moderate Hypothermia

As temperature falls:

• Shivering decreases or disappears
• Consciousness deteriorates
• Bradycardia develops
• Respiratory rate falls
• Cardiac conduction slows
• Dysrhythmias become increasingly possible

An Osborn (J) wave may appear on ECG.

Severe Hypothermia

Severe hypothermia can produce:

• Coma
• Marked bradycardia
• Hypotension
• Hypoventilation
• Loss of reflexes
• Ventricular dysrhythmias
• Ventricular fibrillation
• Asystole

Vital signs may become extremely difficult to detect.

Cardiovascular Effects

The usual progression is:

Early tachycardia → progressive bradycardia → conduction slowing → increasing dysrhythmia risk

ECG abnormalities can include:

• Sinus bradycardia
• PR prolongation
• QRS widening
• QT prolongation
• Atrial dysrhythmias
• Osborn waves
• Ventricular fibrillation

Osborn (J) Waves

The Osborn wave is a positive deflection near the J point, immediately following the QRS complex.

It is strongly associated with hypothermia but is not specific.

J waves can also occur in other clinical settings and their absence does not exclude significant hypothermia.

Neurologic Effects

Progressive cooling causes:

• Impaired judgment
• Ataxia
• Dysarthria
• Confusion
• Lethargy
• Loss of reflexes
• Coma

Profound CNS depression in severe hypothermia can mimic death.

Respiratory Effects

Early cooling may cause tachypnea.

With worsening hypothermia:

• Respiratory rate falls
• Tidal volume decreases
• Protective airway reflexes disappear
• Hypoventilation develops
• Aspiration risk increases

Cold Diuresis

Peripheral vasoconstriction shifts blood centrally, which promotes renal excretion of water and electrolytes.

This cold diuresis can produce significant intravascular volume depletion.

During rewarming, peripheral vasodilation may then reveal or worsen hypotension.

Acid-Base Abnormalities

Hypothermia can produce complex acid-base disturbances.

Possible abnormalities include:

• Early respiratory alkalosis
• Later respiratory acidosis from hypoventilation
• Lactic acidosis from impaired perfusion
• Mixed acid-base disorders

Blood-gas interpretation in severe hypothermia requires awareness that laboratory analyzers generally measure samples at standard temperature.

Potassium

Potassium concentrations can change substantially during cooling and rewarming.

Hypokalemia may occur from intracellular redistribution, but potassium management must be cautious because potassium can move back extracellularly during rewarming.

Marked hyperkalemia in profound hypothermia can also indicate severe cellular injury and may carry important prognostic information.

Glucose

Both hyperglycemia and hypoglycemia can occur.

Hypoglycemia is particularly important because it:

• Causes altered consciousness
• Impairs shivering
• Reduces heat production
• Is rapidly treatable

Laboratory Evaluation

Significant hypothermia may warrant:

• Bedside glucose
• Electrolytes
• Renal function
• CBC
• Creatine kinase
• Blood gas
• Lactate
• Coagulation studies
• ECG

Depending on circumstances:

• Ethanol concentration
• Carboxyhemoglobin
• Acetaminophen and salicylate concentrations
• Thyroid/adrenal testing
• Other targeted toxicology studies

Core Temperature

Accurate core-temperature measurement is essential.

In significant accidental hypothermia, a low-reading core thermometer should be used.

Depending on the clinical setting, appropriate measurement sites include:

• Rectal
• Esophageal
• Bladder

Peripheral skin, oral, or axillary measurements may substantially underestimate or inaccurately reflect core temperature.

Initial Management

Priorities are:

Prevent further cooling → stabilize ABCs → begin appropriate rewarming → treat the underlying cause

Initial measures include:

• Remove wet clothing.
• Insulate the patient.
• Move to a warm environment.
• Handle gently.
• Assess airway and ventilation.
• Check glucose.
• Establish vascular access when required.
• Begin continuous cardiac monitoring in significant hypothermia.

Gentle Handling

Patients with severe hypothermia should be handled carefully.

The severely cold myocardium is electrically unstable, and unnecessary vigorous movement has historically been associated with precipitation of dangerous dysrhythmias.

Necessary resuscitative procedures should still be performed.

Passive External Rewarming

Passive rewarming involves:

• Removing wet clothing
• Drying the patient
• Insulation with blankets
• Providing a warm environment

It is generally appropriate for mild hypothermia when the patient retains adequate endogenous heat production.

Active External Rewarming

Methods include:

• Forced-air warming blankets
• Other controlled external heating systems

Active external warming is particularly useful when passive measures alone are inadequate.

Heat is generally focused on the trunk/core rather than aggressively warming the extremities first.

Active Internal Rewarming

More severe hypothermia may require:

• Warmed IV fluids
• Warmed humidified respiratory gases as an adjunct
• Extracorporeal rewarming in severe cases

Older methods such as routine gastric, bladder, or peritoneal lavage have largely been displaced by more effective and safer contemporary approaches and are not routine rewarming methods.

Warmed IV Fluids

Warmed isotonic crystalloid is useful when fluid resuscitation is required.

Its primary roles are:

• Preventing additional cooling
• Treating hypovolemia

Warmed fluids alone generally provide relatively limited heat transfer and should not be considered sufficient treatment for profound hypothermia.

Extracorporeal Rewarming

Extracorporeal life support (ECLS), particularly VA-ECMO, provides the most effective rewarming for selected patients with severe hypothermia and cardiovascular instability or cardiac arrest.

It provides:

• Rapid core rewarming
• Oxygenation
• Circulatory support

Cardiopulmonary bypass is another extracorporeal option where ECMO is unavailable.

The source’s emphasis on femoral-femoral bypass and open pleural lavage reflects older practice; modern management generally favors ECMO/ECLS when available for hypothermic cardiac arrest.

Hypothermic Cardiac Arrest

Severe hypothermia can produce extraordinarily low metabolic requirements, meaning apparently lifeless patients may occasionally survive prolonged arrest when appropriately rewarmed.

Therefore, traditional prognostic rules used in normothermic cardiac arrest cannot simply be applied.

The older statement that resuscitation is futile if circulation does not return within about 30 minutes after rewarming is not considered a reliable modern stopping rule.

Decisions regarding termination of resuscitation should incorporate:

• Mechanism
• Core temperature
• Potassium and other prognostic markers
• Evidence of lethal injury
• Duration and circumstances of arrest
• Availability of extracorporeal rewarming

Defibrillation and Medications

The severely hypothermic myocardium may respond poorly to:

• Defibrillation
• Vasopressors
• Antiarrhythmic medications

Modern resuscitation follows hypothermia-specific cardiac-arrest protocols, with modifications according to core temperature.

Repeated accumulation of medications during profound hypothermia should be avoided because drug metabolism is markedly reduced and concentrations may rise during rewarming.

Bradycardia

Bradycardia is an expected physiologic response to significant hypothermia.

The primary treatment is generally:

Rewarming

Attempts to normalize the heart rate pharmacologically are often unnecessary unless another process is clearly contributing.

Decontamination

Do not induce vomiting.

Routine gastric lavage in a hypothermic poisoned patient is not standard modern management.

Activated charcoal may occasionally be appropriate after a selected ingestion, but airway safety is essential and rewarming/resuscitation takes priority.

Complications During Rewarming

Potential complications include:

• Hypotension
• Dysrhythmias
• Electrolyte shifts
• Rhabdomyolysis
• Acute kidney injury
• Coagulopathy
• Pulmonary edema
• Rewarming-related vasodilation

Patients with moderate or severe hypothermia therefore require continued monitoring even after the temperature begins to normalize.

Monitoring

Monitor:

• Core temperature
• ECG and cardiac rhythm
• Blood pressure/perfusion
• Respiratory function
• Glucose
• Electrolytes
• Renal function
• Acid-base status

Severe cases may require invasive hemodynamic monitoring and intensive care.

Key Points

• Hypothermia = core temperature <35°C.
• Toxicologic hypothermia commonly occurs because intoxication causes CNS depression, immobility, impaired judgment, or reduced ability to escape cold exposure.
• Important causes include ethanol, opioids, sedative-hypnotics, clonidine/imidazolines, and other CNS depressants.
• Progressive cooling produces CNS depression, bradycardia, conduction slowing, and increasing risk of ventricular dysrhythmias.
• Osborn (J) waves are classically associated with hypothermia but are not specific.
• Check glucose early because hypoglycemia can both cause and worsen hypothermia.
• Cold diuresis can cause substantial volume depletion.
• Mild cases may respond to passive/active external warming; severe unstable hypothermia may require extracorporeal rewarming.
• VA-ECMO/ECLS is preferred when available for selected patients with hypothermic cardiac arrest or profound cardiovascular instability.
• Bradycardia from hypothermia is primarily treated by rewarming, rather than attempts to force the heart rate back to normal.
• Profound hypothermia can mimic death; conventional normothermic cardiac-arrest prognostic rules should not be applied uncritically.
• Significant complications can emerge during rewarming, so moderate-to-severe cases require continued close monitoring.