Hypothermia and frostbite

Hypothermia is defined as a drop in core body temperature below 35°C (95°F). Impaired thermoregulation, decreased heat production, and increased heat loss can contribute to accidental hypothermia. Hypothermia is classified as mild, moderate, or severe based on core body temperature and clinical features, which range from shivering to progressive bradycardia, coma, and circulatory collapse. During the diagnostic assessment, the patient's core body temperature should be determined first, followed by an ECG. Further testing assesses for comorbidities and complications. Treatment involves rewarming and supportive care. Cardiac arrhythmias are the most common cause of death in hypothermia. Frostbite is a tissue injury that can occur after exposure to freezing temperatures and typically affects the face, ears, fingers, and/or toes; frostbite can occur with or without hypothermia. Mild frostbite is reversible, while severe cases may require amputation. Nonfreezing cold injuries are typically less severe and include immersion injuries, panniculitis, cold urticaria, and pernio. Preventive measures include avoiding cold temperatures, wearing layers, keeping skin dry, and rewarming quickly if numbness or pain develops in the extremities.

See “Targeted temperature management” for details about therapeutic hypothermia.

Primary (environmental exposure to cold) or secondary (underlying condition with inadequate temperature regulation) etiologies which result in:

• Increased heat loss: drugs (induced vasodilation), erythroderma (burns, psoriasis), surgery, sepsis, multiple trauma

• Decreased heat production: endocrine disorders (hypopituitarism, hypoadrenalism, and hypothyroidism), severe malnutrition, hypoglycemia, damage to posterior hypothalamic nucleus, and neuromuscular inefficiencies
• Impaired thermoregulation: damage to the preoptic nucleus of the hypothalamus due to CNS trauma, strokes, toxicologic and metabolic derangements, intracranial bleeding, Parkinson disease, CNS tumors, Wernicke disease, or multiple sclerosis
• Intentional hypothermia: therapeutic neuroprotection in intubated patients post-cardiac arrest
• Risk factors
  • Substance use
  • Increasing age
  • Homelessness
  • Psychiatric disease
  • Certain medications (e.g. antihypertensives, diuretics, clonidine)

References:^[1][2]

• The body loses heat through radiation; (most significant means of heat loss), conduction, convection, and direct contact with cold surfaces.
• The hypothalamus attempts to maintain a temperature of approximately 36.5°C (97.7°F) to 37.5°C (99.5°F) by:
  • Conserving heat (peripheral vasoconstriction – direct and sympathetic)
  • Cold-induced thermogenesis (increasing heat production)
    • Shivering thermogenesis
      • Involuntary, rapid oscillations of skeletal muscles that use ATP and generate heat
      • Shivering can increase heat production by up to 400%
      • Primary means of maintaining core body temperature in cold environments
      • Does not occur in infants (due to skeletal muscle immaturity)
      • Cannot be sustained indefinitely due to discomfort and fatigue
    • Non-shivering thermogenesis
      • Increased heat production by brown adipose tissue
      • Increases heat production by 10–30% following acute cold exposure
      • Important for cold acclimation following sustained cold exposure
      • Primary means of maintaining core body temperature among infants
• Hypothermia affects all organ systems
  • General tissue oxygen demand decreases by ∼ 6% per degree Celsius below 35°C.
  • Weakened cellular immune response
  • Cardiovascular effects: ↓ depolarization of cardiac cells → ↓ cardiac output and ↓ mean arterial pressure
  • CNS effects: ↓ CNS metabolism

References:^[2][3]

Definition ^[4]

Accidental hypothermia: an involuntary drop in core body temperature below 35°C (95°F)

Clinical features of hypothermia ^[5][6]

Clinical findings, including level of consciousness (LOC), correlate with core body temperature, and together can be used to define stages of hypothermia. ^[7][8][9]

Mild hypothermia (32–35°C/ 89.6–95.0°F)

• Alert, impaired judgment
• Amnesia, dysarthria, ataxia
• Tachycardia, tachypnea
• Shivering
• Bleeding diathesis

Moderate hypothermia (28–32°C/ 82.4–89.6°F)

• Worsening CNS depression, e.g. lethargy, stupor
• Hypoventilation
• Bradycardia, cardiac arrhythmias
• Hyporeflexia
• Dilated pupils
• Loss of shivering typically occurs ^[6]
• Cold diuresis: Peripheral vasoconstriction in hypothermia increases central and renal blood flow, which causes antidiuretic hormone suppression, resulting in diluted urine.
• Paradoxical undressing: the abnormal removal of clothing by patients despite low ambient temperature
• Ileus, pancreatitis

The presence or absence of shivering does not accurately reflect the stage of hypothermia.

Severe hypothermia (< 28°C/ < 82.4°F)

• Coma, areflexia
• Fixed and dilated pupils
• Ventricular fibrillation
• Hypotension
• Pulmonary edema, apnea
• Oliguria ^[10]
• Rigidity (pseudo-rigor mortis)
• Pulselessness ^[11]

Diagnostics ^[5][11]

Approach

• All patients: Measure core temperature and determine the stage of hypothermia. ^[6]
  • First-line: esophageal temperature probe
  • Second-line: rectal or bladder thermometer
  • Avoid tympanic or oral thermometers, as they do not reflect core temperature.
• Moderate or severe hypothermia
  • Obtain routine diagnostics, including ECG, basic laboratory studies, ABG, and CXR.
  • Consider additional diagnostics (e.g., serum toxicology, blood cultures) if LOC is inconsistent with core temperature.
• Trauma patients: Obtain imaging to evaluate concomitant injuries.

Consider alternate diagnoses (e.g., sepsis or stroke) if the clinical features are inconsistent with the body temperature (e.g., coma at a core temperature of 32°C).

ECG ^[12]

• Indication: best initial test to monitor for arrhythmias
• ECG findings in hypothermia
  • Heart blocks and dysrhythmias: variable; depends on core temperature
  • J wave (Osborn wave): elevated J point ^[5][11]
  • Prolongation of all ECG intervals

Routine laboratory studies ^[5][11][13]

Obtain basic laboratory studies to rule out complications and guide resuscitation. Typical findings include:

• CBC
  • ↑ Hematocrit due to hemoconcentration
  • ↓ WBC and platelets secondary to sequestration
• CMP
  • Variable electrolyte disturbances, e.g., ↑ K⁺
  • Hyperglycemia that progresses to hypoglycemia
• ABG: initial respiratory alkalosis followed by mixed acidosis
• Coagulation studies: may be normal despite clinically apparent coagulopathy ^[14][15]
• Lipase: may be increased as a result of ischemic pancreatitis
• Serum creatine kinase: may be elevated secondary to rhabdomyolysis

Use clinical findings to guide management, as cold and/or rewarmed blood samples may yield inaccurate results (e.g., due to hemolysis). ^[11]

The lethal triad of hypothermia, acidosis, and coagulopathy is associated with poor outcomes in patients with cold exposure and severe trauma. ^[14]

Additional laboratory studies ^[5]

Consider the following to identify underlying etiologies or differential diagnoses:

• Serum ethanol and urine toxicology: to assess for intoxication
• Blood cultures: for patients with suspected sepsis
• TFT: to exclude hypothyroidism and myxedema coma
• Serum cortisol level: to assess for adrenal insufficiency ^[1]
• Cardiac biomarkers: to exclude myocardial infarction

Imaging ^[5]

• All patients: CXR
• Trauma patients
  • FAST scan and CT head
  • See also “Urgent diagnostics for trauma patients.”

Management of hypothermia ^[4][5][6]

Initial resuscitation: ABCDE approach

• Unresponsive patient: Check pulses for 60 seconds; start CPR if pulseless. (See “Hypothermic cardiac arrest” for details).
• Intubate early if needed.
• Provide supplemental oxygen.
• Initiate IV fluid resuscitation preferably through 2 large-bore peripheral IVs.
  • Alternative routes: IO access or femoral line
  • Use warmed (40–42°C) normal saline.
  • Vasopressors may have limited benefit because of vasoconstriction due to hypothermia.
• Remove wet clothing and ensure a warm environment.
• Ensure continuous core temperature monitoring during resuscitation.
• Start rewarming techniques based on hypothermia stage.
• Identify and treat reversible or time-sensitive underlying causes.

Rewarming techniques ^[4][5][16]

Passive rewarming

• Patients with mild hypothermia may only require passive rewarming.
• Insulation (e.g., with blankets) allows patients to retain body heat.
• Active movement can increase heat generation.

Active rewarming

• Indications
  • Moderate to severe hypothermia
  • Cardiovascular instability
  • Unsuccessful passive rewarming
  • Impaired thermoregulation (e.g., due to stroke, endocrine dysfunction)
• Rate of rewarming
  • A rate of 1–2°C/hour is thought to be safe in hemodynamically stable patients.
  • Patients with cardiovascular instability should be rewarmed more quickly.
• Active external rewarming
  • Methods include warming blankets, radiant heat, and forced warm air.
  • Rewarm the torso (e.g., axillae, chest, back) before the extremities to minimize the risk of afterdrop.
• Active internal rewarming
  • Warmed IV fluids ^[4]
  • Warm humidified air
  • Body cavity irrigation with warmed fluid: e.g., peritoneal dialysis, thoracic lavage
  • Extracorporeal blood rewarming ^[4][11]
    • Indications include:
      • Hypothermic cardiac arrest
      • Unsuccessful rewarming with other techniques
      • Respiratory failure
      • Refractory acidosis
    • Options include venovenous rewarming, hemodialysis, and extracorporeal life support (ECLS).

If rewarming is unsuccessful, consider the possibility of an underlying infection, endocrine insufficiency, or insufficient resuscitation. ^[5]

Supportive care ^[5][16]

• Handle patients with severe hypothermia gently to avoid precipitating ventricular arrythmias.
• Identify, treat and prevent disease and treatment complications (see “Complications”).
• Avoid administering medications in patients with severe hypothermia, if feasible.
• Avoid oral and intramuscular routes of administration.
• Identify and treat related trauma and cold injuries; see “Frostbite” and “Management of trauma patients.”

Complications ^[11]

Hypothermic cardiac arrest ^[6][9]

Refers to cardiac arrest occurring after the development of hypothermia. Both ventricular fibrillation and asystole can occur spontaneously in severe hypothermia.

• Vital signs assessment
  • Perform in patients without signs of irreversible death.
  • Check pulse and breathing for up to 60 seconds.
  • Use echocardiography, EtCO₂ monitoring, or handheld Doppler to check for central pulses.
• No signs of life: Start CPR and continue until fully rewarmed (see “ACLS” for details).
  • Consider using a mechanical CPR device to compensate for chest wall rigidity.
  • Consider delaying shocks until core temperature is > 30°C.
  • Prevent further heat loss and initiate active internal rewarming, preferably using ECLS.
  • Address other reversible causes of cardiac arrest.
• Consider termination of CPR if:
  • Obvious signs of irreversible death are present
  • Cardiac arrest was witnessed and happened prior to cooling.
  • Serum potassium is > 12 mmol/L.
  • No signs of return of spontaneous circulation despite normothermia.

Initiate rewarming and do not withhold life-saving treatment from hypothermic patients who appear clinically dead (e.g., dilated pupils, areflexia, rigidity) without signs of irreversible death. ^[6][9]

Defibrillation is unlikely to be successful at core temperatures < 30°C (86°F). ^[6]

Other disease complications

• Arrhythmias ^[5][6]
  • Atrial arrhythmias are expected and typically resolve spontaneously.
  • The efficacy of antiarrhythmic medications is limited.
  • Ventricular dysrhythmias: Treat as hypothermic cardiac arrest.
• Coagulopathy: improves with rewarming; do not administer clotting products ^[14][15]

Patients with moderate to severe hypothermia may have arrhythmias that are unresponsive to defibrillation. Continue CPR and consider delaying further defibrillation until the patient's core body temperature is > 30°C. ^[6][15]

Treatment complications

• During resuscitation
  • Afterdrop: influx of cold, acidemic blood to the heart following increased peripheral perfusion ^[6]
  • Prolonged hypotension
  • Dysrhythmias
• After rewarming
  • Pulmonary edema, pneumonia
  • Seizure disorders, neuropathy, impaired cognition
  • Multiple organ failure
• Related to ECLS: hemorrhage, vessel injury, distal limb ischemia

To prevent afterdrop, avoid jostling patients during transport and rewarming the extremities before the core; afterdrop may exacerbate hypothermia and cause arrhythmias. ^[5]

Disposition

• Hemodynamic instability or cardiac arrest: transfer to a hospital with an ECLS program
• Moderate to severe hypothermia: consider ICU admission
• Mild hypothermia: may be discharged after rewarming if otherwise healthy

• Check central pulses using bedside echocardiography, EtCO₂, or Doppler.
• If pulseless, start CPR and continue until the patient is fully rewarmed; see “ACLS.”
• Remove wet clothing and ensure a warm environment.
• Assess airway patency and intubate early if needed.
• Obtain ECG and initiate cardiac monitoring.
• Initiate continuous monitoring of core temperature, preferably using an esophageal probe.
• Place two large-bore IVs and obtain laboratory studies.
• Initiate IV fluid resuscitation with warm (40–42°C) normal saline.
• Passive rewarming for all patients: using warm blankets, active movement
• Active external rewarming for moderate or severe hypothermia: e.g., using heating pads or forced warm air
• Active internal rewarming for unstable patients or if passive and active external rewarming are unsuccessful: e.g., using ECLS, thoracic or peritoneal lavage
• Assess for concomitant trauma and cold injuries; see “Frostbite” and “Management of trauma patients.”
• Admit patients with moderate or severe hypothermia to the ICU.

Definition

Severe localized tissue injury; due to freezing of interstitial and cellular spaces after prolonged exposure to very cold temperatures

Clinical features ^[1][5][16]

General features

• Frostbite can occur with or without hypothermia.
• Areas most frequently affected
  • Face (nose, cheeks, chin)
  • Ears
  • Fingers and toes
• Initial paresthesia is followed by numbness of the affected region.

Features by stage

• Frostbite is classically staged by degree of injury, similar to burns, but this can be difficult to assess prior to thawing.
• The simplified staging is easier to apply clinically.

The extent of tissue injury from frostbite is challenging to define initially. Accurate prognosis requires careful observation and may only be possible weeks to months after thawing. ^[5]

Diagnostics ^[5][16]

General principles

• Frostbite is a clinical diagnosis.
• Measure core temperature in all patients to rule out hypothermia.
• Consider imaging to evaluate for injury and determine tissue viability.
• Consider additional diagnostics for patients with concurrent hypothermia or other injuries.

Perform a pulse examination in all patients to assess for an underlying vascular cause (e.g., acute limb ischemia, vascular injury).

Imaging

• Imaging has a limited role in an emergency setting.
  • In all patients, consider obtaining an x-ray of the affected area.
  • Angiography or scintigraphy is indicated in severe frostbite if thrombolytic therapy is planned.
  • For trauma patients, see also “Urgent diagnostics for trauma patients.”
• Single-photon emission CT, magnetic resonance angiography, or scintigraphy may be performed to predict tissue viability 4–24 hours after thawing. ^[16]

Management ^[16][17]

Initial management

• Remove any wet clothing and jewelry.
• Ensure that the patient is in a warm environment.
• Treat hypothermia first, if present (e.g., cover with blankets)
• Rapid rewarming: Immerse the affected extremity in a warm (37–39°C) circulating water bath. ^[5]

Refreezing thawed tissue damages it further. If there is a risk of disruption to the thawing process, delay rewarming and keep frostbitten areas dry and insulated. ^[5]

Prioritize management of hypothermia occurring concurrently with frostbite, as it is both common and life-threatening.

Pharmacotherapy

• NSAIDs: for all patients, typically ibuprofen ^[16]
• Intraarterial thrombolytic therapy: for severe frostbite ^[18][19]

Wound care

• Do not debride hemorrhagic vesicles.
• Change dressings every 6 hours. ^[5]
• Drape sterile gauze loosely around thawed areas. ^[16][17]
• Elevate the affected limb to decrease edema.
• Give antibiotics to patients with severe trauma, cellulitis, or signs of sepsis; see “Antibiotics for acute open wounds.” ^[16]
• Ensure proper documentation of findings, e.g., with admission photographs.

Supportive therapy

• IV fluid resuscitation with warm (40–42°C/104.0–107.6°F) normal saline ^[5]
• Analgesia: Opiates may be needed, as rewarming can be excruciatingly painful.
• Tetanus prophylaxis: depending on the patient's immunization status and degree of wound contamination

Disposition ^[1][5][16]

• Consult surgery for all patients with severe frostbite.
  • Urgent indications include signs of sepsis and acute compartment syndrome.
  • In patients without sepsis, amputation is reserved until necrotic tissue is well demarcated.
• Admit patients with deep frostbite for inpatient management.
• Ensure that patients who are discharged have a warm environment to return to.

Tissue demarcation (e.g., to guide the need for amputation) may only occur 1–3 months after the frostbite injury. ^[16]

Complications ^[11]

• Loss of sensation
• Cold hypersensitivity
• Chronic pain
• Loss of limb or digits

• Remove any wet clothing and jewelry and place the limb in a circulating water bath (37–39°C).
• Provide analgesia with opioids as needed.
• Initiate treatment with NSAIDs, e.g., ibuprofen.
• Consider tetanus prophylaxis.
• Administer antibiotics if there are signs of sepsis or cellulitis.
• Wound care: Consider topical aloe vera, wrap sterile dressing loosely, and elevate the limb.
• Consult surgery early if there is concern for infection or compartment syndrome.
• For severe frostbite, obtain angiography or scintigraphy prior to thrombolysis.
• Ensure proper documentation of findings, e.g., with admission photographs.

Immersion injury (trench foot) ^[5][20][21]

Ischemic and neuropathic damage to the feet due to prolonged exposure to nonfreezing wet conditions

Risk factors

• Homelessness
• Substance use
• Participation in military operations or recreational activities in cold, wet environments

Clinical features

• Stage 1 (cold exposure): numbness, initial erythema followed by pallor
• Stage 2 (postexposure): numbness, mottled skin, weak pulses
• Stage 3 (hyperemic phase): hyperalgesia, erythema, edema
• Stage 4 (posthyperemic phase): cold sensitivity, chronic pain, hyperhidrosis
• Tissue necrosis: rare; commonly associated with pressure necrosis

Management

• Manage hypothermia, if present.
• Allow the affected limb to air dry and gradually rewarm at room temperature.
• Provide analgesia as needed and elevate the affected limb. ^[1][20]
• Consider a podiatric or surgical consult if there is evidence of tissue necrosis.

Disposition

• Admission is commonly necessary for analgesia administration.
• Discharge considerations
  • Ensure that patients can return to a warm and dry environment.
  • Consider early involvement of social care.

Unlike in frostbite injury, rapid rewarming should not be used for an immersion injury, as this can exacerbate the injury. ^[1][20]

Perniosis (chillblains) ^[22][23][24]

A seasonal condition characterized by the inflammation of small blood vessels; triggered by an abnormal reaction to cold and humid conditions

Epidemiology

• Higher incidence in women
• More common in individuals who smoke

Etiology

Unknown, but associated conditions include:

• CTDs (especially systemic lupus erythematosus)
• Dysproteinemias
• Conditions associated with low BMI (e.g., anorexia nervosa, celiac disease)

Pathophysiology

• Not fully understood
• Likely mechanism: cold exposure → persistent vasoconstriction → inflammation of small blood vessels

Clinical features

• Erythrocyanotic papules or nodules, predominantly on the hands, fingers, toes, legs, and face
• Lesions appear 12–24 hours after exposure to cold and last for 1–3 weeks.
• Lesions are accompanied by pain, itching, burning, and/or swelling.
• In severe cases: blisters, ulcerations.

Diagnostics

• Typically a clinical diagnosis
• Consider the following for atypical presentations:
  • CBC, ANA, antiphospholipid antibodies, cryoglobulins, and serum protein electrophoresis
  • Biopsy of lesions

Differential diagnoses

• Raynaud phenomenon
• Cold urticaria
• Cellulitis
• Contact dermatitis
• Acrocyanosis
• Acute thromboembolism

Treatment

• First-line
  • Protection from cold conditions (e.g., multiple layers of warm clothing, gloves)
  • Slow rewarming of the affected areas
• Second-line: Consider calcium channel blockers (e.g., nifedipine: ) for refractory perniosis. ^[24][25]
• Symptomatic therapy: Topical corticosteroids and intense pulsed light therapy may help to relieve itching and erythema. ^[24]

Panniculitis ^[11]

• Mild necrosis of subcutaneous fat caused by prolonged exposure to cold temperatures
• May result in fat fibrosis, which can cause cosmetic concerns for patients

Cold urticaria ^[26]

• Development of urticaria and/or angioedema after brief exposure to cold temperatures
• The diagnosis is mainly clinical and involves a cold stimulation test.
• Management includes antihistamines (e.g., loratadine ) and cold avoidance.
• May be accompanied by cold-induced anaphylaxis

General measures ^[27][28]

• Identify individuals at elevated risk, e.g., those with: ^[27]
  • Disease risk factors for hypothermia and frostbite
  • Participation in cold-weather sports
  • Prolonged unavoidable exposure to cold temperatures
• Advise measures to reduce risk.
  • Avoid alcohol and recreational drugs that impair sensation and/or affect decision-making.
  • Be aware of environmental factors that can lead to hypothermia and frostbite developing at warmer temperatures, e.g.: ^[27]
    • Wind chill factor
    • High altitude
    • Immersion in water
  • Do not touch or grip cold surfaces with bare skin.
  • Avoid emollients. ^[17]
  • Follow measures to minimize cold exposure.
  • In cold environments, follow measures to maintain or increase peripheral perfusion.

Hypothermia can occur in nonfreezing conditions if combined stressors like wind chill, wet skin, and exhaustion are present. ^[27][28]

Measures to minimize cold exposure ^[28]

• Avoid prolonged exposure to cold temperatures and unnecessary exposure to temperatures < 0°C.
• Protect skin from moisture, wind, and cold.
  • Shelter when necessary. ^[6]
  • Dry skin immediately if it becomes wet. ^[27]
  • Wear appropriate clothing, composed of clean and loose layers. ^[1][6][27]
    • Base layer: moisture-wicking material (e.g., polypropylene, polyester)
    • Mid-layer: wool or fleece
    • Outer layer: wind and water repellent with ventilation features
  • To protect the extremities: ^[1][27]
    • Keep hands and feet dry.
    • Change gloves and socks frequently.
    • Use portable heating devices. ^[1][27]
    • Layer gloves. ^[27]
    • Wear shoes large enough to allow movement of the toes if extra or thick socks are worn.

Measures to maintain or increase peripheral perfusion ^[28]

• Avoid nicotine and alcohol.
• Optimize management of medical conditions that limit peripheral perfusion.
• If feasible, avoid medications that impair peripheral perfusion. ^[1]
• Ensure adequate hydration and nutrition. ^[1][28]
• Exercise
• Use supplemental oxygen in hypoxic settings (e.g., at altitude).

While exercise can be preventive, sweat, damp clothes, and exhaustion increase the risk of hypothermia. ^[27]

In cold environments, individuals should regularly assess themselves and others for numbness or pain in the extremities, and rewarm as soon as possible (hands can be placed on the abdomen or in the axillae). ^[28]