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.
See “ ” for details about .
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
|Mechanisms of heat loss|
|Mode of heat transfer||Definition||Comments|
|Conduction|| || |
|Convection|| || |
|Radiation|| || |
- 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
- Substance use
- Increasing age
- Psychiatric disease
- 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)
- 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
- Shivering thermogenesis
- Hypothermia affects all organ systems
Accidental hypothermia: an involuntary drop in core body temperature below 35°C (95°F)
Clinical features of hypothermia 
Mild hypothermia (32–35°C/ 89.6–95.0°F)
- Alert, impaired judgment
- Amnesia, dysarthria, ataxia
- Tachycardia, tachypnea
- Bleeding diathesis
Moderate hypothermia (28–32°C/ 82.4–89.6°F)
- Worsening CNS depression, e.g. lethargy, stupor
- Bradycardia, cardiac arrhythmias
- Dilated pupils
- Loss of shivering typically occurs 
- 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
- Pulmonary edema, apnea
- Oliguria 
- Rigidity (pseudo-rigor mortis)
- Pulselessness 
All patients: Measure core temperature and determine the . 
- 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
- Trauma patients: Obtain imaging to evaluate concomitant injuries.
- Indication: best initial test to monitor for arrhythmias
- ECG findings in hypothermia
Routine laboratory studies 
Obtain basic laboratory studies to rule out complications and guide resuscitation. Typical findings include:
- ABG: initial respiratory alkalosis followed by mixed acidosis
- Coagulation studies: may be normal despite clinically apparent coagulopathy 
- 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). 
Additional laboratory studies 
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 
- Cardiac biomarkers: to exclude myocardial infarction
- All patients: CXR
- Trauma patients
Management of hypothermia 
- Unresponsive patient: Check pulses for 60 seconds; start CPR if pulseless. (See “ ” for details).
- Intubate early if needed.
- Provide supplemental oxygen.
- Initiate IV fluid resuscitation preferably through 2 large-bore peripheral IVs.
- Remove wet clothing and ensure a warm environment.
- Ensure continuous core temperature monitoring during resuscitation.
- Start based on .
- Identify and treat reversible or time-sensitive underlying causes.
Rewarming techniques 
- 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.
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 .
Active internal rewarming
- Warmed IV fluids 
- Warm humidified air
- Body cavity irrigation with warmed fluid: e.g., peritoneal dialysis, thoracic lavage
- Extracorporeal blood rewarming 
If rewarming is unsuccessful, consider the possibility of an underlying infection, endocrine insufficiency, or insufficient resuscitation. 
Supportive care 
- 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.”
Hypothermic cardiac arrest 
- Vital signs assessment
- No signs of life: Start CPR and continue until fully rewarmed (see “ACLS” for details).
- Consider if:
Defibrillation is unlikely to be successful at core temperatures < 30°C (86°F). 
Other disease complications
- Arrhythmias 
- Coagulopathy: improves with rewarming; do not administer clotting products 
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. 
- During resuscitation
- After rewarming
- Related to ECLS: hemorrhage, vessel injury, distal limb ischemia
Acute management checklist for hypothermia
- 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.
Clinical features 
- Frostbite can occur with or without hypothermia.
Areas most frequently affected
- Face (nose, cheeks, chin)
- 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.
|Clinical features and stages of frostbite |
|Stage||Clinical features||Tissue loss|
Superficial or mild frostbite
|1st degree|| |
Deep or severe frostbite
|3rd degree|| || |
|4th degree|| |
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. 
- 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.
- Imaging has a limited role in an emergency setting.
- magnetic resonance angiography, or scintigraphy may be performed to predict tissue viability 4–24 hours after thawing.  ,
- Remove any wet clothing and jewelry.
- Ensure that the patient is in a warm environment.
- first, if present (e.g., cover with blankets)
- Rapid rewarming: Immerse the affected extremity in a warm (37–39°C) circulating water bath. 
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. 
Prioritize frostbite, as it is both common and life-threatening. occurring concurrently with
- NSAIDs: for all patients, typically ibuprofen 
- Intraarterial thrombolytic therapy: for severe frostbite 
- Do not debride hemorrhagic vesicles.
- Change dressings every 6 hours. 
- Drape sterile gauze loosely around thawed areas. 
- Elevate the affected limb to decrease edema.
- Give antibiotics to patients with severe trauma, cellulitis, or ; see “Antibiotics for acute open wounds.” 
- Ensure proper documentation of findings, e.g., with admission photographs.
- IV fluid resuscitation with warm (40–42°C/104.0–107.6°F) normal saline 
- Analgesia: Opiates may be needed, as rewarming can be excruciatingly painful.
- immunization status and degree of wound contamination : depending on the patient's
- Consult surgery for all patients with severe frostbite.
- Admit patients with deep frostbite for inpatient management.
- Ensure that patients who are discharged have a warm environment to return to.
- Loss of sensation
- Cold hypersensitivity
- Chronic pain
- Loss of limb or digits
Acute management checklist for frostbite
- Remove any wet clothing and jewelry and place the limb in a circulating water bath (40–42°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) 
Ischemic and neuropathic damage to the feet due to prolonged exposure to nonfreezing wet conditions
- Substance use
- Participation in military operations or recreational activities in cold, wet environments
- 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
- , if present.
- Allow the affected limb to air dry and gradually rewarm at room temperature.
- Provide analgesia as needed and elevate the affected limb. 
- Consider a podiatric or surgical consult if there is evidence of tissue necrosis.
- Prevention: keeping the feet warm and dry
- 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.
Perniosis (chillblains) 
- Higher incidence in women
- More common in individuals who smoke
Unknown, but associated conditions include:
- CTDs (especially systemic lupus erythematosus)
- Conditions associated with low BMI (e.g., anorexia nervosa, celiac disease)
- Not fully understood
- Likely mechanism: cold exposure → persistent vasoconstriction → inflammation of small blood vessels
- 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.
- Typically a clinical diagnosis
- Consider the following for atypical presentations:
- 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. 
- Symptomatic therapy: Topical corticosteroids and intense pulsed light therapy may help to relieve itching and erythema. 
- Mild necrosis of subcutaneous fat caused by prolonged exposure to cold temperatures
- May result in fat fibrosis, which can cause cosmetic concerns for patients