Hypothermia

Quick Reference

Critical Alerts

• "Not dead until warm and dead": Hypothermic patients may survive prolonged resuscitation with excellent neurological outcomes even after hours of arrest[1,2]
• Handle gently: Cold myocardium is extremely irritable and prone to ventricular fibrillation with minimal stimulation[3]
• Rewarming is definitive treatment: Goal is core temperature > 35°C; method depends on severity and hemodynamic stability[4]
• Potassium predicts viability: K+ > 12 mEq/L suggests nonsurvivable cellular death; K+ less than 8 mEq/L associated with better prognosis in cardiac arrest[5,6]
• ECMO for severe hypothermic arrest: Best survival rates (47-100%) when K+ less than 12 mEq/L and no obvious fatal trauma[5,7]
• Modified defibrillation protocol: Single shock for VF if less than 30°C; defibrillation less effective until rewarmed > 30°C[8,9]
• Medication dosing altered: Withhold adrenaline/vasopressors if less than 30°C; double intervals between doses at 30-35°C[8,9]

Temperature-Based Classification

Key Diagnostics

Emergency Treatments by Severity

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Definition

Overview

Hypothermia is defined as core body temperature less than 35°C (95°F).[1,4] It represents a state of inadequate heat production relative to heat loss, resulting in a progressive decline in core temperature that affects all organ systems. Accidental hypothermia occurs unintentionally from environmental cold exposure, while therapeutic hypothermia is induced for neuroprotection. This topic focuses on accidental hypothermia in adults.

Hypothermia exists on a spectrum from mild discomfort to life-threatening cardiovascular collapse and cardiac arrest. The key principle of management is that hypothermia itself is reversible, and with appropriate rewarming, patients can achieve complete recovery even after prolonged cardiac arrest—hence the maxim "not dead until warm and dead."[1,2]

Classification Systems

Temperature-Based Classification (Primary System): The most widely used system categorizes hypothermia by measured core temperature:[4,10]

Swiss Staging System (Clinical Assessment): Useful when core temperature cannot be measured immediately in prehospital settings:[11]

Epidemiology

Incidence and Mortality:

• Approximately 1,500 deaths annually in the United States are attributed to hypothermia[12]
• True incidence likely underestimated as hypothermia often not recognized or documented
• Case fatality rate varies widely (3-85%) depending on severity, comorbidities, and access to advanced rewarming techniques[4,7]
• In-hospital mortality for severe hypothermia with cardiac arrest: 50-80% without ECMO; improves to 20-53% with ECMO rewarming[5,7]

Demographics and Risk Groups:

• Age: Bimodal distribution—elderly (> 65 years) and young adults (outdoor recreators)
• Gender: Male predominance (approximately 2:1 ratio)
• Urban vs wilderness: Urban hypothermia often associated with homelessness, substance abuse; wilderness hypothermia with recreation/occupational exposure
• Seasonal: Peak in winter months, but can occur year-round
• Geographic: Higher incidence in northern latitudes and mountainous regions

High-Risk Populations:

Etiology

Primary Categories:

1. Environmental (Accidental) Hypothermia (Most Common):

• Cold ambient temperature exposure
• Inadequate protective clothing
• Wet clothing (accelerates heat loss 25-fold in water vs air)
• Wind chill effect
• Immersion in cold water (causes rapid cooling)

2. Immersion Hypothermia:

• Cold water submersion/near-drowning
• Extremely rapid heat loss (water conducts heat 25 times faster than air)
• May be protective in drowning by reducing cerebral metabolic demands[14]

3. Medical Conditions Impairing Thermoregulation:

4. Drug and Toxin-Induced:

• Alcohol: Vasodilation, impaired judgment, hypoglycemia, inhibits shivering (most common toxin)[13]
• Sedative-hypnotics: Benzodiazepines, barbiturates impairing awareness
• Antipsychotics: Impair hypothalamic thermoregulation
• Anesthetics: General anesthesia, regional blocks
• Opioids: Reduced consciousness, impaired judgment
• Cannabis: Impaired judgment
• Carbon monoxide: Impaired oxygen delivery

5. Iatrogenic:

• Perioperative hypothermia (cold OR, anesthesia, IV fluids)
• Massive transfusion with cold blood products
• Cold intravenous fluids in ED/ICU
• Heat-loss during resuscitation/trauma

6. Trauma-Associated ("Lethal Triad"):

• Hypothermia + acidosis + coagulopathy
• Blood loss and hypovolemia
• Environmental exposure at scene
• Prolonged extrication

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Pathophysiology

Normal Thermoregulation

The human body maintains core temperature at 36.5-37.5°C through:

• Hypothalamic thermoregulation: Preoptic area of anterior hypothalamus acts as thermostat
• Heat production: Basal metabolism, shivering thermogenesis, brown adipose tissue (infants)
• Heat conservation: Vasoconstriction, behavioral changes (seeking warmth, adding clothing)
• Heat loss mechanisms: Radiation (60%), convection (15%), conduction (3%), evaporation (22%)

Thermoregulatory Failure in Hypothermia

Compensatory Phase (Mild Hypothermia):

1. Peripheral vasoconstriction: Reduces heat loss by decreasing cutaneous blood flow
2. Shivering: Increases metabolic heat production up to 5-fold; maximum at 35°C
3. Behavioral: Seeking warmth, adding clothing, increasing activity
4. Hormonal: Increased thyroxine, catecholamines, cortisol

Decompensation Phase (Moderate-Severe):

• Below 32°C: Shivering decreases and ceases less than 30-32°C
• Metabolic rate progressively decreases (~6% per 1°C drop in temperature)[15]
• Compensatory mechanisms fail
• Core temperature drops progressively

System-Specific Pathophysiology

Cardiovascular System (Most Critical):

Progressive Temperature-Dependent Changes:[3,4]

Mechanisms of Cardiac Dysfunction:

• Reduced conduction velocity: Prolonged PR, QRS, QT intervals on ECG
• Myocardial irritability: Cold myocardium extremely sensitive to mechanical stimulation (rough handling, central line insertion, intubation can trigger VF)[3]
• Decreased contractility: Direct temperature-dependent myocardial depression
• Arrhythmogenesis: Re-entry circuits, early/late afterdepolarizations
• Reduced responsiveness: Decreased sensitivity to chronotropic/inotropic agents

ECG Changes (Osborn/J Waves):[16]

• Positive deflection at the junction (J point) of QRS complex and ST segment
• Appears as "hump" or "camel hump" on ECG
• Height inversely proportional to core temperature
• Best seen in leads II and V3-V6
• Not pathognomonic: also seen in subarachnoid hemorrhage, hypercalcemia, brain injury, Brugada syndrome
• Mechanism: Transmural voltage gradient from early repolarization

Respiratory System:

• Progressive hypoventilation: Respiratory rate and tidal volume decrease
• Decreased respiratory drive: Central depression
• Bronchorrhea: Increased secretions
• Decreased cough reflex: Risk of aspiration
• Decreased ciliary function: Impaired clearance
• May appear apneic: At severe hypothermia; careful assessment required before declaring respiratory arrest
• Decreased oxygen consumption: Metabolic demands decrease proportionally with temperature

Neurological System: Temperature-Dependent Consciousness Changes:[15]

• 35-34°C: Confusion, amnesia, dysarthria, apathy
• 34-32°C: Altered judgment, "paradoxical undressing" (25% of cases—patient feels warm, removes clothing)
• 32-30°C: Stupor, decreasing level of consciousness
• 30-28°C: Progressive coma
• less than 28°C: Coma, absent reflexes
• less than 20°C: Flat EEG (potentially reversible with rewarming)[17]
• Pupillary findings: Fixed dilated pupils possible at profound hypothermia; do NOT indicate brain death

Cerebral Protection:

• Decreased cerebral metabolic rate for oxygen (CMRO₂) by ~6-7% per 1°C decrease[15]
• At 28°C: CMRO₂ reduced approximately 50%
• At 18°C: CMRO₂ reduced up to 75%
• Protective in cardiac arrest: Prolonged tolerance to circulatory arrest (up to hours in some cases)[1,2]

Renal System:

• "Cold diuresis": Peripheral vasoconstriction shifts blood centrally → increased central venous pressure → suppression of ADH and atrial natriuretic peptide release → increased urine output[18]
• Intravascular volume depletion: Can be significant (up to 25% plasma volume loss)
• Decreased GFR: At severe hypothermia
• Electrolyte shifts: Variable potassium (initially intracellular shift, then release with acidosis/cellular damage)

Metabolic/Endocrine:

• Decreased basal metabolic rate: ~6% per 1°C temperature drop[15]
• Hyperglycemia initially: Catecholamine-induced glycogenolysis, decreased insulin secretion/sensitivity
• Hypoglycemia later: Glycogen depletion, decreased gluconeogenesis
• Decreased drug metabolism: Accumulation of medications with potential toxicity during rewarming
• Thyroid function: Often normal in accidental hypothermia unless underlying hypothyroidism

Hematological/Coagulation:

• Hemoconcentration: From cold diuresis and plasma volume loss
• Coagulopathy: Temperature-dependent enzyme dysfunction (coagulation cascade enzymes less active at low temperature)[19]
  • PT/PTT measured at 37°C in lab may appear normal but functionally prolonged at patient's actual temperature
  • Platelet dysfunction
  • Fibrinolysis activation
• Left shift of oxygen-hemoglobin dissociation curve: Hemoglobin holds oxygen more tightly (reduced tissue oxygen delivery)

Acid-Base Balance:

• Initial: Respiratory alkalosis (from hyperventilation/shivering)
• Progressive: Metabolic acidosis (lactic acid from shivering, tissue hypoperfusion, decreased hepatic metabolism)
• Severe: Combined metabolic and respiratory acidosis
• pH less than 6.5: Associated with extremely poor prognosis[4]

Temperature Correction of Blood Gases (Controversial):

• Blood gas analyzers warm samples to 37°C
• "Alpha-stat" strategy: Use uncorrected values (most common approach)
• "pH-stat" strategy: Correct values to patient's actual temperature
• Clinical significance: Most centers use uncorrected values for decision-making[4]

Afterdrop Phenomenon

Definition: Continued core temperature decline during rewarming

Mechanisms:[20]

1. Return of cold peripheral blood: Vasodilation during rewarming brings cold blood from extremities to core
2. Conductive heat transfer: Cold skin/extremities conduct heat away from core
3. Cessation of shivering: Premature termination of endogenous heat production

Clinical Significance:

• Can cause deterioration during early rewarming
• May trigger arrhythmias
• Associated with increased mortality if severe

Prevention Strategies:

• Rewarm trunk/core first (prevent extremity vasodilation)
• Avoid active external rewarming of extremities in severe hypothermia
• Minimize patient movement
• Use active internal rewarming methods for severe cases

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Clinical Presentation

History Taking

Environmental Exposure Assessment:

• Duration of exposure: Minutes to hours to days
• Environment: Air vs water immersion (critical distinction—water causes 25× faster heat loss)
• Ambient temperature: Actual temperature and wind chill
• Wet vs dry: Wet clothing dramatically accelerates heat loss
• Protective measures: Type of clothing, shelter, sleeping bag
• Activity level: Exertion vs immobility
• Access to heat source: Ability to escape cold

Predisposing Factors:

Substance Use:

• Alcohol: Present in 35-40% of urban hypothermia cases[12,13]
• Recent alcohol consumption and quantity
• Other substances: opioids, benzodiazepines, cannabis

Symptoms by Severity

Mild Hypothermia (32-35°C):

• Neurological: Confusion, poor judgment, slurred speech, amnesia, apathy, fatigue
• Motor: Ataxia, poor coordination, stumbling gait
• Shivering: Vigorous and persistent
• Sensation: Cold, numbness
• Behavioral: Irritability, combativeness

Moderate Hypothermia (28-32°C):

• Neurological: Progressive obtundation, stupor, decreasing level of consciousness
• Shivering: Decreased or absent (critical sign of progression)
• Motor: Severe incoordination, muscle rigidity
• Paradoxical undressing: May remove clothing (approximately 25% of cases)
• "Terminal burrowing": Attempt to hide in small enclosed spaces (primitive survival behavior)

Severe Hypothermia (less than 28°C):

• Neurological: Coma, unresponsive, absent reflexes
• Pupils: May be fixed and dilated (does NOT indicate death or brain injury—can be reversible)[17]
• Shivering: Absent
• Appearance: May appear dead—pallor, cold, rigid, imperceptible vital signs
• Cardiovascular: Severe bradycardia, hypotension, arrhythmias, cardiac arrest

Physical Examination

Initial Assessment (Primary Survey):

Airway:

• Often patent in mild-moderate hypothermia
• Risk of aspiration (decreased gag reflex, bronchorrhea)
• Handle gently during airway maneuvers (can precipitate VF)

Breathing:

• Assess for 30-60 seconds (respiratory rate may be very slow—4-6 breaths/min)
• May appear apneic but still breathing
• Look for subtle chest rise, feel for air movement, listen for breath sounds

Circulation:

• Pulse check: Palpate central pulse (carotid/femoral) for 30-60 seconds before declaring cardiac arrest[8]
• Peripheral pulses may not be palpable even with cardiac output (severe vasoconstriction)
• Bradycardia expected and may not respond to atropine
• Blood pressure: Hypotension in moderate-severe cases; may be difficult to measure

Disability (Neurological):

• Glasgow Coma Scale (expect low score in moderate-severe)
• Pupil examination (may be fixed/dilated—not prognostic)
• Blood glucose (point-of-care testing)

Exposure/Environment:

• Remove wet clothing (cut off to minimize movement)
• Dry patient thoroughly
• Prevent further heat loss

Core Temperature Measurement (MANDATORY):

Methods:[4,10]

Critical: Standard thermometers often only measure down to 34°C; must use low-reading thermometer capable of measuring less than 25°C

Secondary Survey:

Skin:

• Color: Pale, cyanotic, waxy
• Temperature: Cold to touch centrally
• Frostbite: Check extremities, nose, ears for cold injury
• Edema: "Puffy" appearance from cold diuresis and capillary leak

Cardiovascular:

• Heart rate: Progressive bradycardia with decreasing temperature
• Rhythm: Monitor continuous ECG
• Arrhythmias: Atrial fibrillation (most common at 30-32°C), ventricular arrhythmias (less than 28°C)
• Heart sounds: May be distant or difficult to auscultate

Respiratory:

• Respiratory rate: Decreased (may be less than 10 breaths/min)
• Breath sounds: Bronchorrhea may cause crackles/rhonchi
• Work of breathing: Decreased effort

Abdominal:

• Often benign
• Ileus common
• Pancreatitis can occur (check lipase)
• Hypothermia-induced gastric ulceration (rare)

Neurological:

• Reflexes: Decreased or absent (deep tendon reflexes, gag, corneal)
• Tone: May be rigid (do not confuse with rigor mortis)
• Pupils: May be dilated and unreactive (reversible)

Musculoskeletal:

• Muscle rigidity (not rigor mortis)
• Decreased voluntary movement
• Assess for trauma/fractures

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Red Flags

Life-Threatening Conditions Requiring Immediate Action

Handling Precautions

"Handle Gently" Principle:[3]

• Cold myocardium extremely irritable—VF can be triggered by:
  • Rough patient movement
  • Aggressive chest compressions (if pulse present)
  • Central line insertion
  • Endotracheal intubation
  • Chest tube insertion
  • Vigorous fluid boluses

Safe Patient Handling:

• Move patient smoothly and deliberately
• Maintain horizontal position (avoid sudden positional changes)
• Minimize unnecessary procedures if hemodynamically stable
• If intubation required: preoxygenate well, gentle technique, have defibrillator ready
• If central access required: use ultrasound guidance, gentle technique

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Differential Diagnosis

Primary vs Secondary Hypothermia

Primary (Accidental) Environmental Hypothermia:

• Cold exposure is primary event
• Normal thermoregulation overwhelmed by environment
• History of exposure typically clear

Secondary Hypothermia (Underlying condition causing hypothermia): Must consider especially in:

• Urban settings without obvious environmental exposure
• Patients found indoors
• Temperature not proportional to environmental conditions

Conditions Causing or Mimicking Hypothermia

Mimics of Death in Severe Hypothermia

Profound hypothermia can mimic death. Do not pronounce dead based on:[17]

• Fixed dilated pupils: Reversible with rewarming
• Apparent apnea: Respirations may be less than 2/min—observe 60 seconds
• Absent peripheral pulses: Severe vasoconstriction; check central pulses for 60 seconds
• Muscle rigidity: Not rigor mortis; cold-induced
• Flat EEG: Can be reversible at temperatures less than 20°C

Consider termination only if:[5,6]

• K+ > 12 mEq/L (suggests cellular membrane disruption/death)
• Obvious fatal injuries (decapitation, massive trauma incompatible with life)
• Ice formation in airway (frozen solid)
• Asphyxiation confirmed (avalanche burial > 60 min with airway obstruction)
• Unsafe scene for rescuers

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Diagnostic Approach

Initial ED Evaluation

Immediate Actions (First 5 Minutes):

1. Remove all wet clothing (cut off to minimize movement)
2. Measure core temperature with low-reading thermometer
3. Continuous cardiac monitoring (watch for arrhythmias)
4. Handle patient gently (minimize stimulation)
5. Assess for signs of life (pulse 60 seconds, respirations 60 seconds)
6. Establish IV access (preferably peripheral initially)
7. Begin rewarming based on severity

Laboratory Studies

Essential Initial Labs:

Additional Labs Based on Clinical Suspicion:

• TSH, Free T4: If suspect myxedema coma
• Cortisol: If suspect adrenal insufficiency (give hydrocortisone empirically if unstable)
• Troponin: Often elevated (does not necessarily indicate MI; can be from cold injury)
• Toxicology screen: If overdose suspected
• Blood cultures: If sepsis suspected
• Fibrinogen, D-dimer: If DIC suspected

Serial Monitoring:

• Core temperature every 15-30 minutes during active rewarming
• Continuous cardiac monitoring
• Glucose every 1-2 hours
• Electrolytes, particularly potassium, every 2-4 hours
• ABG/VBG every 1-2 hours in severe cases

Electrocardiogram

Progressive ECG Changes with Decreasing Temperature:[16]

Osborn (J) Waves:[16]

• Appearance: Positive deflection immediately after QRS complex at the J point (QRS-ST junction)
• Morphology: Appears as "hump," "hook," or "camel hump" deformity
• Amplitude: Height inversely proportional to temperature (higher waves = lower temperature)
• Best leads: Most prominent in leads II, V3, V4, V5, V6
• Onset: Typically appear at temperatures less than 32-33°C
• Mechanism: Transmural voltage gradient from differential action potential duration between epicardium and endocardium
• Specificity: NOT pathognomonic for hypothermia—also seen in:
  • Subarachnoid hemorrhage
  • Hypercalcemia
  • Brain injury
  • Brugada syndrome
  • Vasospastic angina
  • Normal variant (early repolarization)

Clinical Significance of ECG:

• Confirms hypothermia diagnosis when J waves present
• Helps estimate severity (wave amplitude correlates with temperature)
• Identifies arrhythmias requiring treatment
• DO NOT delay rewarming to obtain 12-lead ECG

Imaging

Chest X-Ray:

• Indications: All moderate-severe cases
• Findings:
  • Pulmonary edema (can occur from cold-induced cardiac dysfunction)
  • Aspiration pneumonia (decreased gag reflex)
  • Underlying pneumonia/sepsis
  • "Halo sign" around heart (rare—pericardial fluid)

CT Brain (Non-Contrast):

• Indications: Altered mental status disproportionate to temperature, focal neurological signs, trauma
• Findings: Intracranial hemorrhage, stroke, mass lesion, evidence of trauma

CT Chest/Abdomen/Pelvis:

• Indications: Trauma, sepsis workup, abdominal pain
• Findings: Traumatic injuries, infection source, pancreatitis

Bedside Ultrasound (POCUS):

• Cardiac: Assess for cardiac activity in apparent arrest (may detect contractility when pulse not palpable), pericardial effusion
• Lung: Assess for pneumothorax, pulmonary edema, consolidation
• IVC: Assess volume status (often collapsed from cold diuresis)
• FAST: Trauma evaluation if applicable

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Treatment

General Principles

Core Management Tenets:[4,8,10]

1. Prevent further heat loss: Remove wet clothing, dry patient, insulation
2. Rewarm: Method depends on severity and hemodynamic stability
3. Handle gently: Avoid precipitating VF in severe cases
4. Cardiovascular support: Warm IV fluids; vasopressors cautiously
5. Correct reversible causes: Hypoglycemia, volume depletion, underlying conditions
6. Monitor continuously: Temperature, cardiac rhythm, labs
7. Avoid overly rapid rewarming: Risk of afterdrop, arrhythmias
8. Treat underlying etiology: Sepsis, endocrine emergencies, overdose

Rewarming Rate Considerations:

• Goal: Raise core temperature > 35°C (ideally 35-36°C)
• Optimal rate: 1-2°C per hour for moderate-severe hypothermia[20]
• Too rapid: Risk of arrhythmias, afterdrop, hemodynamic instability
• Too slow: Prolonged exposure to complications of hypothermia
• Cardiac arrest: Fastest possible rewarming (ECMO 10-12°C/hr)[7]

Rewarming Methods

Passive External Rewarming

Indications:

• Mild hypothermia (32-35°C) in otherwise healthy patients
• Intact shivering response
• Hemodynamically stable

Technique:

• Remove wet clothing
• Dry patient thoroughly
• Insulating blankets (multiple layers)
• Warm environment (room temperature 24-26°C)
• Patient generates own heat through shivering

Expected Rate: 0.5-2°C per hour

Advantages: Simple, non-invasive, low risk Disadvantages: Slow, requires intact thermoregulation

Active External Rewarming

Indications:

• Mild to moderate hypothermia (28-35°C)
• Inadequate response to passive rewarming
• Hemodynamically stable
• Can be combined with active internal methods

Methods:

Critical Principles:

• Focus on trunk rewarming: Apply heat to chest, abdomen, back
• AVOID isolated extremity rewarming: Causes vasodilation → afterdrop, shock
• Monitor skin: Frequent assessment for burns

Active Internal (Core) Rewarming

Indications:

• Severe hypothermia (less than 28°C)
• Hemodynamic instability at any temperature
• Cardiac arrest
• Inadequate response to external rewarming

Methods (Invasiveness Increases):

1. Warmed IV Fluids (Low Invasiveness):

• Technique: Normal saline or Lactated Ringer's warmed to 40-42°C via fluid warmer
• Rate: Contributes ~1°C per hour when combined with other methods
• Volume: Replace cold diuresis losses (often 500-1000 mL initial bolus, then ongoing)
• Access: Peripheral or central IV
• Advantages: Easy, universally available
• Limitations: Minimal rewarming as sole method

2. Warmed Humidified Oxygen (Low Invasiveness):

• Technique: Inspired gases warmed to 40-45°C via heated humidifier
• Rate: 1-2°C per hour contribution
• Delivery: Facemask, NIPPV, or via endotracheal tube
• Advantages: Non-invasive, reduces heat loss via respiration
• Limitations: Minimal rewarming alone

3. Gastric/Bladder Lavage (Moderate Invasiveness):

• Technique: Warm saline (40-42°C) instilled via NGT or Foley catheter
• Rate: Minimal contribution
• Advantages: Simple
• Limitations: Ineffective as primary method; rarely used

4. Peritoneal Lavage (Moderate-High Invasiveness):

• Technique: Peritoneal dialysis catheter placed; warmed crystalloid (40-42°C) instilled into peritoneum; 10-20 min dwell time, then drain; repeat cycles
• Rate: 1-3°C per hour
• Advantages: Relatively rapid; available in most hospitals
• Disadvantages: Invasive; risk of perforation; less effective than ECMO
• Use: When ECMO unavailable and severe hypothermia with instability

5. Thoracic (Pleural) Lavage (High Invasiveness):

• Technique: Bilateral chest tubes placed; warmed saline instilled; allows direct cardiac warming
• Rate: 3-5°C per hour
• Indications: Cardiac arrest when ECMO unavailable; rarely used
• Advantages: Faster than peritoneal lavage
• Disadvantages: Highly invasive; risk of pneumothorax, bleeding; requires procedural expertise
• Technique details: Left anterior chest tube (4th ICS, anterior axillary line), right posterior tube; lavage with 300-500 mL boluses

6. Extracorporeal Membrane Oxygenation (ECMO) / Cardiopulmonary Bypass (Highest Invasiveness):

GOLD STANDARD for Severe Hypothermic Cardiac Arrest[5,7]

Indications:

• Hypothermic cardiac arrest (VF/VT, asystole, PEA)
• Severe hypothermia (less than 28°C) with hemodynamic instability unresponsive to other measures
• Potassium less than 12 mEq/L[5,6]

Contraindications:

• K+ > 12 mEq/L (suggests cellular death, poor prognosis)[5,6]
• Obvious fatal trauma incompatible with life
• Asphyxia confirmed (avalanche burial > 60 min with airway obstruction)
• Advanced directives against resuscitation

Technique:

• Veno-arterial ECMO (VA-ECMO): Femoral vein cannulation (drainage), femoral artery cannulation (return)
• Cardiopulmonary bypass: Full sternotomy with central cannulation (used in some centers)

Rewarming Rate: 10-12°C per hour (fastest method)

Survival Rates:[5,7]

• With ECMO: 47-100% survival in appropriate patients (K+ less than 12 mEq/L, no fatal trauma)
• Without ECMO: 0-20% survival in hypothermic cardiac arrest

Advantages:

• Fastest rewarming
• Provides full cardiopulmonary support during rewarming
• Allows controlled correction of acidosis, electrolytes
• Excellent neurological outcomes in survivors

Disadvantages:

• Requires specialized center and expertise
• Not universally available
• Highly resource-intensive
• Risks: Bleeding, vascular injury, infection

Transfer Considerations:

• If ECMO not available at presenting hospital and patient meets criteria → transfer to ECMO-capable center
• Continue CPR during transfer (mechanical CPR device ideal)
• Goal transfer time less than 6 hours if possible
• Coordinate with receiving center before transport

Cardiac Arrest Management

CPR Modifications for Hypothermia[8,9]

Assessment:

• Pulse check: Palpate central pulse (carotid or femoral) for 30-60 seconds (not standard 10 seconds)
• Reason: Severe bradycardia and low cardiac output may make pulse difficult to detect
• Use ultrasound: If available, use cardiac POCUS to confirm absence of cardiac activity

CPR Technique:

• Standard chest compressions: 100-120/min, depth 5-6 cm
• Continuous CPR: Do not stop for rewarming procedures
• Mechanical CPR device: Consider (allows hands-free during rewarming, transport)
• Ventilation: Standard 10 breaths/min if intubated; 30:2 if not intubated

Intubation:

• Indications: Cardiac arrest, inability to protect airway, severe respiratory depression
• Technique: Gentle (risk of VF); preoxygenate; have defibrillator ready
• Drug-assisted: Consider (use reduced doses); avoid if possible in borderline cases

Defibrillation Protocol[8,9]

Standard ACLS (Temperature ≥30°C):

• Defibrillate VF/pulseless VT immediately
• Repeat shocks as per ACLS algorithm
• Adrenaline/amiodarone as per standard protocol

Modified Protocol for Severe Hypothermia (Temperature less than 30°C):

• Initial shock: Attempt single defibrillation for VF/VT
• If unsuccessful: Defer further shocks until core temperature > 30°C
• Rationale: Defibrillation often ineffective at temperatures less than 30°C; myocardium refractory to electrical therapy
• Focus on rewarming: Prioritize rapid rewarming to > 30°C; VF often converts spontaneously with rewarming

Medication Protocol[8,9]

Temperature less than 30°C:

• WITHHOLD adrenaline/epinephrine
• WITHHOLD vasopressors (noradrenaline, vasopressin)
• WITHHOLD antiarrhythmics (amiodarone, lidocaine)
• Rationale:
  • Drugs ineffective at low temperature (receptor dysfunction, altered pharmacokinetics)
  • Drugs accumulate and may cause toxicity during rewarming
  • Cold heart not responsive to medications

Temperature 30-35°C:

• DOUBLE the interval between drug doses
• Example: Adrenaline every 6-10 minutes (instead of 3-5 minutes)
• Rationale: Reduced metabolism and clearance

Temperature ≥35°C:

• Resume standard ACLS protocols
• Normal medication dosing and intervals

Exception—Medications Always Indicated:

• Glucose: If hypoglycemic
• Thiamine: If suspected Wernicke's or alcoholism
• Naloxone: If suspected opioid overdose (though may be less effective until warmed)

Duration of Resuscitation

Principle: "Not dead until warm and dead"[1,2]

Continue resuscitation until:

• Core temperature > 32-35°C achieved AND
• Still no return of spontaneous circulation (ROSC)

OR termination criteria met:

• K+ > 12 mEq/L[5,6]
• Obvious fatal injuries
• Confirmed asphyxiation
• Unsafe scene
• Valid advance directive

Rationale:

• Hypothermia protective to brain and vital organs
• Patients have survived with good neurological outcome after prolonged CPR (> 6 hours documented)[1,2]
• Rewarming is definitive therapy for hypothermic arrest

Documented Survivals:

• Successful resuscitation after 8.5 hours of CPR (core temp 13.7°C)[1]
• Multiple reports of intact neurological recovery after hours of arrest

Non-Arrest Arrhythmia Management

Atrial Fibrillation:[4]

• Most common arrhythmia at temperatures 30-32°C
• Usually spontaneously converts with rewarming
• Do NOT cardiovert unless hemodynamically unstable
• Avoid antiarrhythmics until temperature > 35°C
• Anticoagulation: Generally not indicated acutely

Bradycardia:

• Expected and physiological in hypothermia
• Do NOT treat with atropine (usually ineffective and unnecessary)
• Do NOT pace unless severe bradycardia with hemodynamic compromise after rewarming initiated
• Rewarming is treatment

Ventricular Arrhythmias (Non-Arrest):

• VT with pulse: Treat as per cardiac arrest protocol (single shock if less than 30°C, then rewarm)
• PVCs: Do not treat; often resolve with rewarming

Supportive Care

Fluid Resuscitation:

• Volume depletion common: Cold diuresis causes significant intravascular volume loss (up to 25% plasma volume)
• Initial: 500-1000 mL warmed crystalloid bolus (normal saline or LR, 40-42°C)
• Ongoing: Titrate to urine output, blood pressure, clinical perfusion
• Caution: Avoid excessive fluids (risk of pulmonary edema as heart rewarms and contractility improves)
• Use warmed fluids: All IV fluids should be warmed to 40-42°C

Vasopressors (Use Cautiously):

• Generally avoid until temperature > 30°C
• If required (refractory shock): Use lowest effective dose
• Expect poor response at low temperatures
• Risk of accumulation: May cause hypertensive crisis during rewarming
• Preferred agent: Noradrenaline (if absolutely necessary)

Glucose Management:

• Monitor frequently: Every 1-2 hours
• Hypoglycemia: Treat immediately with IV dextrose (D50W 50 mL or D10W 250 mL)
• Hyperglycemia: Usually transient; avoid insulin unless severe (> 300 mg/dL) as hypoglycemia may develop with rewarming

Electrolyte Management:

• Potassium: Monitor closely; DO NOT aggressively correct elevated K+ (may shift back intracellularly with rewarming); treat life-threatening hyperkalemia (> 6.5-7 mEq/L with ECG changes)
• Sodium: Correct hyponatremia slowly
• Calcium, Magnesium: Replete if low

Acid-Base Management:

• Do NOT aggressively correct acidosis with bicarbonate
• Rewarming corrects acidosis: As metabolism normalizes
• Bicarbonate indications: pH less than 7.0 with severe hyperkalemia (to shift K+ intracellularly)

Coagulopathy Management:[19]

• Avoid unnecessary procedures: Risk of bleeding
• Transfuse if active bleeding: Platelets, FFP, cryoprecipitate as indicated
• Temperature correction: Coagulopathy improves with rewarming
• Trauma patients: Aggressive correction of coagulopathy; consider tranexamic acid

Airway/Ventilation:

• Intubation indications: Cardiac arrest, GCS ≤8, inability to protect airway, severe respiratory depression
• Preoxygenate well
• Gentle technique: Risk of precipitating VF
• Ventilator settings: Moderate tidal volumes (6-8 mL/kg IBW); adjust for normal pH (uncorrected)

Renal Protection:

• Maintain urine output: > 0.5 mL/kg/hr
• Rhabdomyolysis: If CK > 5000, consider aggressive fluids, urinary alkalinization
• Avoid nephrotoxins

Specific Etiologies

Hypothyroidism/Myxedema Coma:

• Empiric treatment: If suspected, do NOT wait for labs
• Levothyroxine: 200-400 mcg IV loading dose, then 50-100 mcg daily
• Hydrocortisone: 100 mg IV q8h (give BEFORE levothyroxine to avoid precipitating adrenal crisis)
• Supportive: Fluid resuscitation, rewarming, glucose

Adrenal Crisis:

• Hydrocortisone: 100 mg IV bolus, then 50-100 mg IV q6-8h
• Fluid resuscitation: Aggressive with normal saline
• Dextrose: For hypoglycemia
• Electrolytes: Correct hyponatremia, hyperkalemia

Sepsis/Septic Shock:

• Antibiotics: Early broad-spectrum (within 1 hour)
• Source control: Identify and treat source
• Fluids: 30 mL/kg crystalloid bolus
• Vasopressors: If refractory hypotension after fluids
• Rewarming: As per hypothermia protocol

Alcohol Intoxication:

• Thiamine: 500 mg IV (before dextrose)
• Dextrose: If hypoglycemic
• Folic acid: 1 mg IV
• Supportive care
• Withdrawal prophylaxis: Consider benzodiazepines if chronic use

Trauma ("Lethal Triad"):[19]

• Damage control surgery: Abbreviated operation, control hemorrhage/contamination, temporary closure
• Aggressive rewarming: Active internal methods
• Massive transfusion protocol: 1:1:1 ratio (pRBCs:FFP:platelets)
• Correct coagulopathy: Tranexamic acid, factor concentrates if indicated
• Avoid excessive crystalloid: Worsens acidosis, dilutes coagulation factors

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Disposition

Admission Criteria

ICU Admission (High Dependency Required):

• Core temperature less than 32°C (moderate-severe hypothermia)
• Cardiac arrest or hemodynamic instability
• Arrhythmias requiring monitoring/treatment
• Active internal rewarming (especially ECMO)
• Significant comorbidities (sepsis, myxedema, adrenal crisis)
• Altered mental status not improving with rewarming
• Electrolyte disturbances (K+ > 6 mEq/L or less than 2.5 mEq/L)
• Rhabdomyolysis with acute kidney injury
• Need for mechanical ventilation

Telemetry/Step-Down:

• Mild-moderate hypothermia (28-34°C) with clinical improvement
• Hemodynamically stable
• No active arrhythmias
• Able to rewarm with external methods
• Requires cardiac monitoring during rewarming

General Medical Floor:

• Mild hypothermia (> 32°C) with clinical improvement
• Hemodynamically stable, normal mental status
• Underlying medical condition requiring treatment (pneumonia, UTI)
• Social factors preventing safe discharge (homelessness, inadequate heating)

Discharge Criteria

Safe for Discharge (All Must Be Met):

• Core temperature > 35°C and stable
• Hemodynamically stable (normal HR, BP)
• No arrhythmias on monitoring
• Normal mental status (return to baseline)
• No active medical issues requiring admission
• Able to ambulate safely
• Safe environment assured: Access to warm shelter, heating, appropriate clothing
• Follow-up arranged
• Patient understands warning signs and when to return

Relative Contraindications to Discharge:

• Homelessness without shelter placement
• Inadequate home heating
• Lives alone without support (elderly)
• Ongoing substance abuse without detox plan
• Psychiatric illness without safety plan
• Concern for self-harm or neglect

Follow-Up

Transfer Considerations

Transfer to Higher Level of Care (ECMO-Capable Center):

• Severe hypothermia (less than 28°C) with cardiac arrest or severe instability
• Presenting hospital lacks ECMO capability
• Patient meets criteria for ECMO (K+ less than 12 mEq/L, no obvious fatal injuries)
• Coordinate early with receiving center
• Continue CPR during transfer (mechanical CPR device ideal)
• Active rewarming during transport: Forced-air warming, warm IV fluids
• Goal transport time: less than 6 hours if possible

Pre-Transfer Stabilization:

• Intubate if not already done (airway protection during transport)
• Establish reliable IV access (preferably 2 large-bore peripheral or central line)
• Begin active rewarming (forced-air warming, warm fluids)
• Continuous cardiac monitoring
• Communicate patient details, labs, temperature trends to receiving team

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Patient Education

Prevention Strategies

General Public Education:

Clothing and Gear:

• Layer clothing: Multiple thin layers trap heat better than single thick layer
• Stay dry: Wet clothing loses insulating properties; carry rain gear
• Cover extremities: Hat, gloves, warm socks, insulated boots (50% of heat loss from head/neck)
• Avoid cotton: Retains moisture; prefer wool or synthetic materials

Behavioral:

• Limit outdoor time: During extreme cold (less than 0°C with wind chill)
• Recognize early signs: Shivering, numbness, confusion—seek warmth immediately
• Buddy system: Never hike/work outdoors alone in cold weather
• Avoid alcohol: Impairs judgment and causes vasodilation (increases heat loss)
• Stay active: Movement generates heat, but avoid sweating (wet clothing)
• Eat regularly: Maintain energy stores for thermogenesis

Elderly-Specific:

• Home heating: Keep thermostat at least 68-70°F (20-21°C)
• Check heating system: Before winter; ensure functional
• Dress warmly indoors: Layers even inside home
• Neighborhood checks: Family/friends check on elderly during cold weather
• Medication review: Discuss with doctor medications that impair thermoregulation (antipsychotics, sedatives)

Outdoor Workers and Recreators:

• Check weather forecast: Before outdoor activities; avoid outings during extreme cold warnings
• Tell someone: Itinerary, expected return time
• Carry emergency supplies: Extra clothing, fire-starting materials, emergency blanket, high-energy food
• Communication device: Cell phone (insulated to preserve battery), satellite communicator for remote areas
• Know your limits: Turn back if conditions worsen or fatigue sets in
• Recognize terrain: Avoid water crossings that may result in wet clothing

Homeless and Vulnerable Populations:

• Warming centers: Know locations; utilize during extreme cold
• Social services: Connect with case management for housing assistance
• Emergency shelters: Accept shelter during cold weather even if usually avoid
• Soup kitchens: Hot meals help maintain body temperature
• Carry blankets: Insulated emergency blankets, sleeping bags

Warning Signs to Seek Help

Early Warning Signs (Seek Warm Environment):

• Shivering
• Feeling very cold, unable to get warm
• Numbness in extremities
• Clumsiness, poor coordination
• Fatigue, drowsiness

Emergency Signs (Call 911):

• Confusion, slurred speech, irrational behavior
• Shivering stops (sign of progression to severe hypothermia)
• Extreme drowsiness, difficulty staying awake
• Slow, shallow breathing
• Weak pulse
• Loss of consciousness

First Aid for Hypothermia (Bystander)

If Someone Appears Hypothermic:

1. Call for help: 911 or emergency services
2. Move to warm environment: If safe to do so
3. Remove wet clothing: Replace with dry clothing or blankets
4. Insulate from ground: Place blankets/sleeping pad under person
5. Cover person: Multiple layers of blankets, focus on trunk and head
6. Warm beverages: If person conscious and able to swallow (NOT alcohol)
7. Do NOT:
   • Rub or massage extremities (can cause afterdrop)
   • Apply direct heat (heating pads, hot water bottles) to extremities
   • Give alcohol
   • Assume person is dead (may appear dead but be viable)

If Unconscious/No Pulse:

• Call 911 immediately
• Begin CPR if no pulse detected after 30-60 seconds
• Continue CPR until help arrives: Prolonged resuscitation warranted
• Do NOT stop based on appearance (fixed pupils, rigidity not indicative of death)

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Special Populations

Elderly

Increased Vulnerability:

• Impaired thermoregulation: Decreased ability to vasoconstrict, reduced shivering
• Blunted awareness: May not perceive cold
• Comorbidities: Diabetes, cardiovascular disease, dementia
• Medications: Sedatives, antipsychotics, beta-blockers impair thermoregulation
• Social isolation: Living alone, unable to call for help
• Inadequate heating: Financial constraints, thermostat set too low

Indoor Hypothermia:

• Can occur at home with inadequate heating
• Gradual onset over days
• Often presents with altered mental status, falls

Management Considerations:

• Higher morbidity/mortality: More aggressive monitoring
• Assess for underlying illness: Often precipitating factor (sepsis, stroke, MI)
• Medication review: Adjust drugs that impair thermoregulation
• Disposition: Lower threshold for admission
• Social work: Home safety evaluation, heating assistance programs

Neonates and Infants

(Note: This topic focuses on adults; neonatal hypothermia is a separate entity)

If Encountered:

• Large surface area to body mass ratio (rapid heat loss)
• Limited metabolic reserves
• Dependent on caregivers
• Immediate warming critical; skin-to-skin contact effective
• Transfer to pediatric facility

Alcohol and Substance Users

Epidemiology:

• Alcohol involved in 35-40% of urban hypothermia cases[12,13]
• Increased incidence in homeless with substance abuse

Mechanisms:

• Vasodilation: Ethanol causes peripheral vasodilation → increased heat loss
• Impaired judgment: Failure to seek shelter, remove wet clothing
• Decreased shivering: Ethanol may inhibit shivering response
• Hypoglycemia: Alcohol-induced hypoglycemia worsens hypothermia
• Immobility: Passing out outdoors

Management:

• Thiamine: Always give before dextrose (prevent Wernicke's encephalopathy)
• Dextrose: Treat hypoglycemia
• Alcohol withdrawal: May manifest during rewarming; benzodiazepines for prophylaxis/treatment
• Social work: Addiction services, detox program, shelter
• Psychiatric evaluation: Assess for depression, suicidality

Cold Water Immersion and Drowning

Unique Physiology:

• Rapid cooling: Water conducts heat 25× faster than air
• "Dive reflex": Bradycardia, peripheral vasoconstriction, blood shift to vital organs
• Neuroprotection: Hypothermia may protect brain during submersion[14]
• Best outcomes: Children in very cold water (less than 5°C) with rapid submersion (reduced struggle, rapid cooling)

Management:

• Aggressive resuscitation: Excellent neurological outcomes possible even after prolonged submersion (> 60 min in cold water)
• ECMO ideal: Best rewarming method for cold water drowning with cardiac arrest[14]
• "Not dead until warm and dead": Applies especially to cold water drowning
• Pulmonary complications: Aspiration, ARDS common; often require ventilatory support

Survival Factors:

• Water temperature (colder better for neuroprotection)
• Submersion duration
• Age (children better outcomes)
• Witnessed vs unwitnessed
• Immediate bystander CPR

Avalanche Burial

Pathophysiology:

• Dual threats: Asphyxia (airway obstruction with snow) AND hypothermia
• Primary cause of death: Asphyxia in first 35 minutes; hypothermia if prolonged burial with air pocket[21]

Prognostic Factors:[21]

Management:

• Rapid extrication: Time-critical
• Assess airway: Clear snow/debris from airway
• Check for signs of life: Pulse 60 seconds
• Resuscitation: If signs of life or burial less than 60 min with air pocket
• ECMO: If hypothermic arrest and K+ less than 12 mEq/L
• Termination: Consider if asystole, burial > 60 min with airway completely obstructed, K+ > 12 mEq/L

Trauma Patients

"Lethal Triad" of Trauma:[19]

• Hypothermia + Acidosis + Coagulopathy
• Synergistic: Each worsens the others
• Associated with > 50% mortality if all three present

Mechanisms of Hypothermia in Trauma:

• Environmental exposure at scene (especially prolonged extrication)
• Blood loss and hypovolemia
• Cold IV fluids and blood products
• Open body cavities during surgery
• Impaired thermoregulation from CNS injury

Prevention:

• Prehospital: Remove wet clothing, insulate, forced-air warming during transport
• ED: Warm environment, warmed fluids (40-42°C), minimize exposure during exam
• OR: Increased room temperature (24-26°C), forced-air warming, warmed irrigation fluids, warm IV fluids/blood

Management:

• Damage control surgery: Abbreviated operation; control hemorrhage, contamination; pack; temporary closure; rewarm in ICU; return to OR when physiologically recovered
• Aggressive rewarming: Active internal methods; consider ECMO for severe hypothermia with arrest
• Massive transfusion protocol: 1:1:1 ratio (pRBCs:FFP:platelets); use rapid infuser with warmer
• Correct coagulopathy: Tranexamic acid (1g IV within 3 hours of injury); factor concentrates if refractory
• Avoid excessive crystalloid: Worsens hypothermia, acidosis, coagulopathy

Hypothermia Threshold:

• less than 35°C significantly increases mortality
• less than 32°C: Damage control surgery mandatory; rewarm before definitive repair

---

Pitfalls and Controversies

Common Pitfalls

Diagnostic:

• ❌ Using standard thermometer: Does not read below 34°C; hypothermia severity underestimated
• ❌ Relying on peripheral temperature: Oral, axillary, tympanic unreliable; must measure core
• ❌ Declaring death prematurely: Based on fixed pupils, apparent apnea, rigidity—all reversible
• ❌ Missing underlying etiology: Assuming environmental cause; failing to consider sepsis, hypothyroidism, adrenal crisis

Management:

• ❌ Rough handling: Precipitating VF in severe hypothermia
• ❌ Aggressive medication dosing: Drugs ineffective and accumulate at less than 30°C; toxicity during rewarming
• ❌ Multiple defibrillation attempts at less than 30°C: Ineffective; delay rewarming
• ❌ Extremity-only rewarming: Causes afterdrop, cardiovascular collapse
• ❌ Terminating resuscitation too early: Before adequately rewarmed to > 32°C
• ❌ Over-correcting electrolytes: Potassium shifts back intracellularly with rewarming
• ❌ Excessive fluid administration: Pulmonary edema when cardiac function recovers

Disposition:

• ❌ Discharging homeless patient without shelter plan: High risk recurrence
• ❌ Inadequate follow-up: Failing to address underlying risk factors

Controversies and Evolving Evidence

Temperature Correction of Blood Gases:

• Controversy: Should ABG values measured at 37°C be corrected to patient's actual temperature?
• Current practice: Most centers use uncorrected ("alpha-stat") values[4]
• Rationale: Simpler; outcome data not clearly different

Optimal Rewarming Rate:

• Traditional: Concern about overly rapid rewarming causing arrhythmias, afterdrop
• Current evidence: Faster rewarming (with ECMO) associated with better outcomes in cardiac arrest[7]
• Consensus: As fast as safely achievable in cardiac arrest; controlled 1-2°C/hr in non-arrest

ECMO vs Other Rewarming Methods:

• Clear evidence: ECMO superior to other methods in hypothermic cardiac arrest[5,7]
• Challenge: Resource availability; not all centers have ECMO capability
• Recommendation: Transfer to ECMO center if meets criteria and feasible

Potassium Threshold for Termination:

• Cutoff: K+ > 12 mEq/L widely cited[5,6]
• Controversy: Some survivors reported with K+ > 10 mEq/L
• Nuance: Use in conjunction with other factors (trauma, asphyxia duration); not absolute

Role of Extracorporeal CPR (ECPR) in Hypothermia:

• Growing evidence: ECPR (ECMO during CPR) for hypothermic arrest has excellent outcomes
• Question: Should all hypothermic arrests be transported to ECMO centers?
• Challenge: Logistics, distance, resources

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Quality Metrics and Performance Indicators

Key Performance Indicators

Documentation Requirements

Essential Elements:

• Core temperature: Exact value, method (rectal/esophageal/bladder), time measured
• Estimated exposure duration: Based on history
• Rewarming method: Specific technique used
• Serial temperatures: Every 15-30 minutes during active rewarming
• Cardiac rhythm: Continuous monitoring strip; note arrhythmias
• Laboratory trends: Potassium, glucose, ABG q1-2h in severe cases
• Response to treatment: Hemodynamic changes, mental status improvement
• ECMO consideration: Documented if indicated; reason if not pursued (unavailable, K+ > 12, etc.)
• Disposition and rationale: Why admitted to ICU vs floor vs discharge
• Follow-up plan: Specific appointments, social work involvement

Handoff Communication (ICU/Transfer):

• Severity classification (mild/moderate/severe)
• Lowest temperature recorded
• Rewarming method and rate achieved
• Current temperature and trend
• Potassium level
• Arrhythmias encountered
• Underlying etiology if identified
• Social factors (housing, substance use)

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Key Clinical Pearls

Diagnostic Pearls

• Always measure core temperature with low-reading thermometer; oral/axillary/tympanic unreliable and underestimate severity
• J waves are classic but not specific: Also seen in SAH, hypercalcemia, Brugada, early repolarization
• Fixed dilated pupils do NOT indicate death: Reversible finding in profound hypothermia[17]
• Potassium is the most important prognostic marker: K+ > 12 mEq/L suggests non-survivability; K+ less than 8 mEq/L better prognosis in arrest[5,6]
• Look for underlying cause: Especially in urban settings—sepsis, hypothyroidism, adrenal crisis, overdose, not just environmental
• Alcohol is the most common toxin: Present in ~40% of urban hypothermia cases[12,13]
• Assess for 60 seconds: Pulse and respirations may be very slow—do not declare arrest prematurely

Treatment Pearls

• Handle gently: Cold myocardium extremely prone to VF with rough handling, procedures, movement[3]
• Rewarm trunk first: Prevent afterdrop from peripheral vasodilation
• ECMO is gold standard for hypothermic arrest: 47-100% survival if K+ less than 12 mEq/L; transfer if not available locally[5,7]
• Single shock for VF if less than 30°C, then rewarm: Defibrillation ineffective until temperature > 30°C[8]
• Withhold drugs if less than 30°C: Adrenaline, vasopressors, antiarrhythmics ineffective and accumulate; double intervals at 30-35°C[8,9]
• Atrial fibrillation usually self-resolves: With rewarming; do not cardiovert acutely
• Do not aggressively correct potassium: Will shift intracellularly with rewarming
• Avoid extremity-only rewarming: Causes afterdrop and cardiovascular collapse

Disposition Pearls

• "Not dead until warm and dead": Prolonged resuscitation warranted; patients survived > 6 hours CPR with intact neuro[1,2]
• Exceptions to prolonged resuscitation: K+ > 12 mEq/L, obvious fatal trauma, confirmed asphyxiation (avalanche > 60 min with obstructed airway), unsafe scene[5,6,21]
• Transfer for ECMO if indicated: Best option for severe hypothermic arrest; coordinate early
• Address social factors: Housing, heating, shelter resources critical to prevent recurrence
• Homeless patients: Mandatory social work involvement before discharge; need shelter placement

System Pearls

• Hypothermia is reversible: With appropriate rewarming, complete recovery possible even after prolonged arrest
• ECMO centers should be identified: Know your regional ECMO-capable centers; have transfer protocols
• Warming centers during cold weather: Public health measure; publicize locations during extreme cold
• Multidisciplinary approach: ED, ICU, social work, cardiology, ECMO team coordination essential

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References

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2. Walpoth BH, Walpoth-Aslan BN, Mattle HP, et al. Outcome of survivors of accidental deep hypothermia and circulatory arrest treated with extracorporeal blood warming. N Engl J Med. 1997;337(21):1500-1505. doi:10.1056/NEJM199711203372103

3. Paal P, Brugger H, Strapazzon G. Accidental hypothermia. Handb Clin Neurol. 2018;157:547-563. doi:10.1016/B978-0-444-64074-1.00033-1

4. Brown DJA, Brugger H, Boyd J, Paal P. Accidental hypothermia. N Engl J Med. 2012;367(20):1930-1938. doi:10.1056/NEJMra1114208

5. Pasquier M, Hugli O, Paal P, et al. Hypothermia outcome prediction after extracorporeal life support for hypothermic cardiac arrest patients: The HOPE score. Resuscitation. 2018;126:58-64. doi:10.1016/j.resuscitation.2018.02.026

6. Mair P, Kornberger E, Furtwaengler W, Balogh D, Antretter H. Prognostic markers in patients with severe accidental hypothermia and cardiocirculatory arrest. Resuscitation. 1994;27(1):47-54. doi:10.1016/0300-9572(94)90021-3

7. Ruttmann E, Weissenbacher A, Ulmer H, et al. Prolonged extracorporeal membrane oxygenation-assisted support provides improved survival in hypothermic patients with cardiocirculatory arrest. J Thorac Cardiovasc Surg. 2007;134(3):594-600. doi:10.1016/j.jtcvs.2007.03.049

8. Truhlář A, Deakin CD, Soar J, et al. European Resuscitation Council Guidelines for Resuscitation 2015: Section 4. Cardiac arrest in special circumstances. Resuscitation. 2015;95:148-201. doi:10.1016/j.resuscitation.2015.07.017

9. Vanden Hoek TL, Morrison LJ, Shuster M, et al. Part 12: Cardiac arrest in special situations: 2010 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation. 2010;122(18 Suppl 3):S829-S861. doi:10.1161/CIRCULATIONAHA.110.971069

10. Dow J, Giesbrecht GG, Danzl DF, et al. Wilderness Medical Society Clinical Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2019 Update. Wilderness Environ Med. 2019;30(4S):S47-S69. doi:10.1016/j.wem.2019.10.002

11. Durrer B, Brugger H, Syme D. The medical on-site treatment of hypothermia: ICAR-MEDCOM recommendation. High Alt Med Biol. 2003;4(1):99-103. doi:10.1089/152702903321489031

12. Centers for Disease Control and Prevention (CDC). Hypothermia-related deaths—United States, 2003-2004. MMWR Morb Mortal Wkly Rep. 2005;54(7):173-175.

13. Vassal T, Benoit-Gonin B, Carrat F, Guidet B, Maury E, Offenstadt G. Severe accidental hypothermia treated in an ICU: prognosis and outcome. Chest. 2001;120(6):1998-2003. doi:10.1378/chest.120.6.1998

14. Quan L, Mack CD, Schiff MA. Association of water temperature and submersion duration and drowning outcome. Resuscitation. 2014;85(6):790-794. doi:10.1016/j.resuscitation.2014.02.024

15. Polderman KH. Mechanisms of action, physiological effects, and complications of hypothermia. Crit Care Med. 2009;37(7 Suppl):S186-S202. doi:10.1097/CCM.0b013e3181aa5241

16. Emslie-Smith D, Sladden GE, Stirling GR. The significance of changes in the electrocardiogram in hypothermia. Br Heart J. 1959;21(3):343-351. doi:10.1136/hrt.21.3.343

17. Steen PA, Milde JH, Michenfelder JD. The detrimental effects of prolonged hypothermia and rewarming in the dog. Anesthesiology. 1980;52(3):224-230. doi:10.1097/00000542-198003000-00006

18. Pretorius T, Bristow GK, Steinman AM, Giesbrecht GG. Thermal effects of whole head submersion in cold water on nonshivering humans. J Appl Physiol. 2006;101(2):669-675. doi:10.1152/japplphysiol.01241.2005

19. van Veelen MJ, Brodmann Maeder M. Hypothermia in Trauma. Int J Environ Res Public Health. 2021;18(16):8719. doi:10.3390/ijerph18168719

20. Kornberger E, Schwarz B, Lindner KH, Mair P. Forced air surface rewarming in patients with severe accidental hypothermia. Resuscitation. 1999;41(2):105-111. doi:10.1016/s0300-9572(99)00067-x

21. Brugger H, Durrer B, Elsensohn F, et al. Resuscitation of avalanche victims: Evidence-based guidelines of the International Commission for Mountain Emergency Medicine (ICAR MEDCOM): intended for physicians and other advanced life support personnel. Resuscitation. 2013;84(5):539-546. doi:10.1016/j.resuscitation.2012.10.020

22. Hara C, Taira T, Inoue A, et al. Association Between Rewarming Rate and Survival and Neurologic Outcome of Accidental Hypothermia. Crit Care Med. 2025;53(7):e1416-e1425. doi:10.1097/CCM.0000000000006712