Severe hypothermia is defined as a core body temperature <28°C (82.4°F), though some classifications use <30°C (86°F). [1-2] It is a life-threatening emergency with high risk of cardiac arrest, but the reduced metabolic demand confers neuroprotection, making full neurological recovery possible even after prolonged resuscitation. [1][3-4]
The following rewarming algorithm provides a clinical decision framework:
1. History
- Duration and nature of cold exposure (outdoor, submersion, indoor)
- Witnessed vs. unwitnessed collapse; estimated down time
- Submersion time and presence of air pocket (avalanche)
- Alcohol or drug use prior to exposure
- Preceding symptoms: confusion, stumbling, paradoxical undressing
- Ability to shiver (absent in severe hypothermia — shivering ceases ~30°C) [6]
- Pre-existing medical conditions: hypothyroidism, adrenal insufficiency, diabetes, CNS disorders
- Medications: sedatives, opioids, antipsychotics, beta-blockers
- Trauma history — concurrent injuries
2. Alarm Features
- Unresponsiveness or coma (core temp <28°C) [6]
- Absent pulse or respirations — may mimic death; check for up to 60 seconds [1][4]
- Ventricular fibrillation or asystole on monitor [1][7]
- Fixed, dilated pupils — NOT a reliable sign of death in hypothermia [1]
- Apparent rigor mortis — NOT reliable in hypothermia [1]
- SBP <90 mmHg or ventricular dysrhythmias → imminent cardiac arrest risk [8]
- Serum potassium ≥12 mmol/L → consider futility of resuscitation [1][3]
- Frozen solid body → withhold CPR [1]
3. Medications
- Cardiovascular drugs may be ineffective in the hypothermic heart — unresponsive to vasopressors, antiarrhythmics, and pacemaker stimulation [9]
- The 2025 AHA Guidelines make no recommendation for or against antiarrhythmic medications in hypothermic cardiac arrest due to lack of human evidence [1]
- Drug metabolism is decreased and protein binding increased in hypothermia; drugs given during hypothermia may accumulate to toxic levels upon rewarming [6]
- QT-prolonging drugs should be avoided — hypothermia already prolongs QT and increases arrhythmia risk [10]
- Amiodarone is less effective in hypothermia and carries risk of torsades de pointes [6]
- WMS guideline: at core temp <30°C, consider withholding epinephrine and limiting to a maximum of 3 defibrillation attempts until rewarmed [11]
- Warmed IV normal saline (40–42°C) for volume resuscitation [12]
- Dextrose-containing fluids for concurrent hypoglycemia
4. Diet
- Not applicable in the acute severe hypothermia setting
- During recovery: warm, sugar-containing fluids once alert and able to swallow safely
- Address nutritional depletion — glycogen stores are exhausted during prolonged cold exposure
- Avoid alcohol (vasodilation worsens heat loss)
5. Review of Systems
- Neuro: level of consciousness (GCS/AVPU), pupil reactivity, shivering status, confusion, coma
- Cardiac: palpitations, chest pain, syncope
- Respiratory: dyspnea, apnea
- GI: ileus symptoms (decreased motility occurs <35°C) [13]
- Renal: urine output (cold diuresis leads to hypovolemia) [14]
- MSK: muscle rigidity, rhabdomyolysis symptoms
- Skin: frostbite, color changes, frozen extremities
- Endocrine: symptoms of hypothyroidism, adrenal insufficiency
6. Collateral History and Family History
- Witnesses to the exposure event — duration, circumstances, submersion
- Psychiatric history (suicidal intent, wandering behavior)
- History of hypothyroidism or adrenal insufficiency in patient or family
- Substance use history — alcohol, opioids, sedatives
- Living situation — homelessness, inadequate heating
- Prior episodes of hypothermia
7. Risk Factors
- Extremes of age — elderly (impaired thermoregulation) and infants (high BSA:mass ratio, no shivering) [1][15]
- Alcohol and drug intoxication — vasodilation, impaired judgment, sedation [2][6]
- Homelessness and outdoor exposure [2]
- Endocrine disorders: hypothyroidism, adrenal insufficiency, hypopituitarism [16-17]
- Sepsis — hypothermia is an independent predictor of mortality in septic patients [18-19]
- Trauma — especially with hemorrhage and immobilization
- CNS disorders — stroke, spinal cord injury, impaired thermoregulation
- Psychiatric illness — dementia, psychosis
- Low BMI, chronic cardiovascular disease, hypertension [19]
- Submersion/drowning, avalanche burial
8. Differential Diagnosis
- Primary environmental hypothermia (cold exposure)
- Myxedema coma — hypothermia + altered mental status + bradycardia in a patient with hypothyroidism; mortality 30–60% [16]
- Sepsis — hypothermia can be the presenting sign, especially in elderly [18][20]
- Adrenal crisis — hypotension, altered mental status, hypothermia
- Hypoglycemia — impaired thermogenesis
- Drug/toxin exposure — opioids, sedatives, ethanol, phenothiazines
- CNS pathology — stroke, traumatic brain injury, spinal cord injury
- Diabetic ketoacidosis — can present with hypothermia [16]
- Cardiac arrest from other causes mimicking hypothermic arrest
9. Past Medical History
- Prior hypothermia episodes
- Hypothyroidism, adrenal insufficiency, diabetes
- Cardiovascular disease, heart failure
- Psychiatric disorders, dementia
- Substance use disorders
- Prior frostbite or cold injuries
- Medications affecting thermoregulation
10. Physical Exam
- Core temperature: esophageal probe (most accurate in intubated patients); epitympanic thermistor; avoid peripheral/infrared thermometers [2]
- Vitals: bradycardia, hypotension, bradypnea — all expected; pulse may require 60 seconds to detect [1][4]
- Neuro: GCS, pupil reactivity (fixed dilated pupils are NOT reliable for death determination), muscle rigidity, absent shivering [1]
- Cardiac: irregular rhythm, muffled heart sounds
- Skin: cold, pale, cyanotic; check for frostbite; paradoxical undressing [3]
- Abdomen: decreased bowel sounds (ileus)
- Extremities: rigidity, edema, signs of rhabdomyolysis
- Assess for trauma — concurrent injuries are common
11. Lab Studies
- Serum potassium — critical for prognostication; K⁺ ≥12 mmol/L suggests futility. Initial hypothermia causes hypokalemia (intracellular shift); terminal cardiac arrest causes hyperkalemia (cell lysis) [1][3][21-22]
- ABG/VBG — mixed respiratory-metabolic acidosis expected [6][14]
- Glucose — hypoglycemia common (glycogen depletion); insulin resistance occurs [13]
- Lactate — elevated; prognostic marker [1]
- CBC — hemoconcentration, thrombocytopenia [23]
- Coagulation studies — PT/PTT prolonged 40–60% in moderate hypothermia; standard labs run at 37°C underestimate the true coagulopathy [14][24-25]
- BMP — electrolyte derangements, renal function (cold diuresis, rhabdomyolysis)
- CK — rhabdomyolysis screening
- TSH, free T4 — rule out myxedema coma, especially if hypothermia is unexplained or in a warm climate [16]
- Cortisol — if adrenal insufficiency suspected
- Toxicology screen — alcohol level, drug screen
- Lipase, LFTs — hepatic and pancreatic function may be impaired
12. Imaging
- Chest X-ray — evaluate for aspiration pneumonia, pulmonary edema, trauma
- CT head — if altered mental status is disproportionate to temperature or trauma suspected
- CT chest/abdomen/pelvis — if trauma is suspected
- POCUS/bedside echo — assess cardiac activity, wall motion, volume status; critical for distinguishing true asystole from very slow rhythm
- Imaging is generally secondary to resuscitation and rewarming priorities
13. Special Tests
- HOPE Score (Hypothermia Outcome Prediction after ECLS): includes age, sex, asphyxia mechanism, CPR duration, serum K⁺, and temperature. A score <0.1 predicts mortality in adults on ECLS [1]
- ICE Score (International Accidental Hypothermia ECLS): includes sex, asphyxia, and serum K⁺. Score >12 predicts survival [1]
- Revised Swiss Staging System — clinical staging (AVPU-based) when core temperature measurement is unavailable [2]
- Point-of-care ultrasound — cardiac activity assessment
- End-tidal CO₂ — confirms ventilation and can help assess perfusion during CPR [4]
- Viscoelastic testing (TEG/ROTEM) — can be run at patient temperature to quantify true coagulopathy [24-25]
14. ECG
- Osborn (J) waves — pathognomonic positive deflections at QRS-ST junction; amplitude correlates with degree of hypothermia; present in ~53% of hypothermic patients [7][16]
- Sinus bradycardia — progressive with cooling
- QT prolongation — present in ~49% of cases [7]
- PR prolongation — AV conduction delay [7][26]
- QRS widening [26]
- Atrial fibrillation — common below 32°C; present in ~21% of hypothermic patients [7][27]
- Ventricular fibrillation — high risk below 28°C; can be triggered by movement, acidosis, hypoxia [6]
- Asystole — may represent viable but profoundly hypothermic heart [1]
- Pitfall: Osborn waves can mimic ST-elevation MI → avoid unnecessary cath lab activation or reperfusion therapy [16]
- Pitfall: Brugada-pattern morphology can appear with hypothermia [27]
15. Assessment
Severe hypothermia (<28°C) represents a life-threatening emergency with absent shivering, progressive loss of consciousness to coma, and high risk of VF or asystole. [1][6] Key clinical pearls:
- Metabolism drops ~7% per 1°C decrease in core temperature, providing neuroprotection [14]
- "No one is dead until warm and dead" — do not terminate resuscitation until rewarmed to ≥32°C unless obvious lethal injury or K⁺ ≥12 mmol/L [1][3]
- Survival with full neurological recovery has been reported after CPR durations exceeding 6 hours [3]
- The coldest reported survival from accidental hypothermia is 11.8°C [4]
- Always search for secondary causes (sepsis, myxedema, toxins, trauma) — severe hypothermia is frequently preceded by other disorders [9][16]
- Afterdrop — core temperature may continue to fall after rescue due to cold peripheral blood returning to the core; avoid unnecessary movement and peripheral rewarming [12]
16. Treatment Plan
Initial stabilization
- Remove from cold environment; remove wet clothing
- Handle gently — rough handling can trigger VF [11-12]
- Keep patient horizontal to prevent afterdrop [12]
- Insulate with vapor barrier + dry insulating layer (Hibler method) [15]
- Secure airway if GCS warrants; use warmed humidified oxygen
Rewarming strategy (tiered approach)
- Active external rewarming: forced-air warming blankets (Bair Hugger), applied to trunk (not extremities initially) [1][12]
- Minimally invasive core rewarming: warmed IV NS at 40–42°C, warmed humidified O₂ [12]
- Invasive core rewarming: thoracic/peritoneal warm water lavage (42°C) [1][9]
- ECLS/ECMO — gold standard for severe hypothermia with cardiac arrest; ECMO preferred over cardiopulmonary bypass (41% greater survival probability) [1-2]
Cardiac arrest management
- Begin CPR immediately unless obviously dead (frozen solid, lethal injury) [1]
- Attempt defibrillation once for VF/VT; if unsuccessful, defer additional attempts until core temp ≥30°C (WMS) or allow up to 3 attempts (ERC) [1][11]
- Mechanical CPR devices preferred for prolonged transport [3][8]
- Vasopressors/antiarrhythmics: limited evidence; AHA makes no specific recommendation [1][9]
- Transfer to ECLS-capable center as soon as possible [1][3][8]
- Target ECLS rewarming rate <5°C/hour — faster rates associated with worse neurological outcomes [1]
Concurrent management
- Correct hypoglycemia with dextrose
- Volume resuscitate with warmed NS (cold diuresis causes hypovolemia)
- Monitor and correct electrolytes carefully — K⁺ may rise rapidly during rewarming [13][21-22]
- Address coagulopathy — rewarming is the primary treatment; consider blood products if actively bleeding [14][24]
- Treat underlying cause if secondary hypothermia (levothyroxine + hydrocortisone for myxedema coma, antibiotics for sepsis) [16]
17. Disposition
- All patients with severe hypothermia require ICU admission [3][8]
- Cardiac arrest or hemodynamic instability → transfer to ECLS/ECMO-capable center [1][3][8][11]
- If ECLS not available locally, initiate CPR and conventional rewarming while arranging transfer; check serum K⁺ en route if possible [3]
- Patients with cardiac instability (SBP <90, ventricular arrhythmias) or core temp <28°C should bypass non-ECLS hospitals [3][8]
- Contact receiving hospital in advance to ensure ECMO availability [3]
- Consult cardiac surgery or ECMO team early
- Consult endocrinology if myxedema coma or adrenal crisis suspected
- Consult toxicology if ingestion suspected
- Consult trauma surgery if concurrent injuries
18. Follow Up / Return Precautions
- Survivors require monitoring for:
- Rewarming-related complications: hyperkalemia, DIC, rhabdomyolysis, acute kidney injury, cerebral edema
- Cardiac arrhythmias during and after rewarming
- Aspiration pneumonia
- Compartment syndrome and frostbite sequelae
- Post-discharge: address underlying risk factors (housing, substance use, endocrine disorders, medication review)
- Educate on cold exposure prevention: layered clothing, avoiding alcohol in cold, buddy systems
- Patients with secondary hypothermia need follow-up for the underlying condition (thyroid function, adrenal testing, etc.)
- Return immediately for recurrent confusion, chest pain, dyspnea, decreased urine output, or new extremity pain/color changes
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