Overview
Hypothermia
Quick Reference
Critical Alerts
- "Not dead until warm and dead": Hypothermic patients may survive prolonged resuscitation
- Handle gently: Cold myocardium is irritable and prone to VF
- Rewarming is treatment: Goal is core temperature >35°C
- Potassium predicts survival: K+ >12 mEq/L suggests nonsurvivable; K+ <8 has better prognosis
- ECMO/Bypass for severe: Best survival in cardiac arrest with accidental hypothermia
- One shock for VF if <30°C: Defibrillation less effective until rewarmed
Key Diagnostics
| Test | Finding | Significance |
|---|---|---|
| Core temperature | <35°C | Defines hypothermia |
| ECG | Osborn (J) waves, bradycardia, arrhythmias | Classic findings |
| Glucose | Hypo- or hyperglycemia | Common |
| Potassium | Elevated or low | Prognostic importance |
| ABG | Acidosis | May be respiratory or metabolic |
| Coagulation | Prolonged (temperature-dependent) | Coagulopathy common |
Emergency Treatments
| Severity | Rewarming Method |
|---|---|
| Mild (32-35°C) | Passive external rewarming |
| Moderate (28-32°C) | Active external rewarming |
| Severe (<28°C) | Active internal rewarming |
| Cardiac arrest | ECMO/Cardiopulmonary bypass |
Definition
Overview
Hypothermia is defined as core body temperature <35°C (95°F). It occurs when heat loss exceeds heat production and can range from mild discomfort to life-threatening cardiac arrest. Management focuses on rewarming while supporting cardiovascular function and preventing further heat loss.
Classification
By Core Temperature:
| Category | Temperature | Clinical Features |
|---|---|---|
| Mild | 32-35°C (89.6-95°F) | Shivering, confusion, tachycardia |
| Moderate | 28-32°C (82.4-89.6°F) | Decreased shivering, obtundation, bradycardia |
| Severe | <28°C (82.4°F) | Absent shivering, coma, VF risk |
| Profound | <20°C (68°F) | Usually asystole, appears dead |
Swiss Staging System (Clinical Assessment When Cant Measure Temp):
| Stage | Clinical Features | Estimated Temperature |
|---|---|---|
| HT I | Conscious, shivering | 35-32°C |
| HT II | Impaired consciousness, not shivering | 32-28°C |
| HT III | Unconscious, vital signs present | 28-24°C |
| HT IV | Apparent death, no vital signs | <24°C |
| HT V | Death due to irreversible hypothermia | K+ >2 |
Epidemiology
- Deaths: ~1,500 deaths/year in US from hypothermia
- Risk groups: Elderly, homeless, outdoor workers, psychiatric patients, substance users
- Associated factors: Alcohol (35-40%), trauma, drowning
- Survival: Excellent outcomes possible with appropriate rewarming
Etiology
Causes of Hypothermia:
| Category | Examples |
|---|---|
| Environmental (accidental) | Cold exposure, immersion, mountaineering |
| Immersion | Near-drowning, cold water immersion |
| Medical conditions | Hypothyroidism, hypoadrenalism, sepsis, CNS disorders |
| Drug-induced | Alcohol, sedatives, anesthetics |
| Iatrogenic | Surgical, blood transfusion, IV fluids |
| Trauma | Hemorrhage, burns, immobility |
Contributing Factors:
- Age extremes (elderly, neonates)
- Psychiatric illness
- Substance abuse (especially alcohol)
- Malnutrition
- Wet clothing
- Inadequate shelter
- Immobility
Pathophysiology
Thermoregulation Failure
- Heat loss mechanisms: Radiation (most), conduction, convection, evaporation
- Compensatory responses: Shivering, vasoconstriction, behavioral changes
- Failure of compensation: Core temperature drops
Physiological Effects by System
Cardiovascular:
| Temperature | Effect |
|---|---|
| 35°C | Tachycardia, increased BP |
| 32°C | Bradycardia, decreased BP |
| 30°C | Atrial fibrillation |
| <28°C | VF threshold lowered, refractory arrhythmias |
| <24°C | Asystole |
Neurological:
- 34°C: Confusion, amnesia
- 30°C: Stupor
- 28°C: Loss of consciousness
- <20°C: Flat EEG (reversible)
Respiratory:
- Decreased respiratory rate and tidal volume
- May appear apneic at severe hypothermia
- Bronchorrhea
Metabolic:
- Decreased basal metabolic rate (~6% per 1°C drop)
- Hyperglycemia initially (catecholamines)
- Hypoglycemia later (glycogen depletion)
- Acidosis (lactic from shivering, then respiratory)
Hematological:
- Coagulopathy (enzyme dysfunction at low temps)
- "Cold diuresis" leading to hypovolemia
- Platelet dysfunction
- Hemoconcentration
J Wave (Osborn Wave)
- Positive deflection at J point (junction of QRS and ST)
- Height correlates inversely with temperature
- Classic but not pathognomonic
- Also seen in subarachnoid hemorrhage, hypercalcemia
Clinical Presentation
Symptoms
Mild (32-35°C):
Moderate (28-32°C):
Severe (<28°C):
History
Key Questions:
Physical Examination
Vital Signs:
General:
Cardiovascular:
Neurological:
Shivering (vigorous)
Common presentation.
Cold sensation
Common presentation.
Poor coordination
Common presentation.
Slurred speech
Common presentation.
Confusion
Common presentation.
Fatigue
Common presentation.
Red Flags
Life-Threatening Conditions
| Finding | Concern | Action |
|---|---|---|
| Core temp <28°C | Severe hypothermia, VF risk | Handle gently, active internal rewarming |
| VF/VT | Cardiac arrest | CPR, single defibrillation, rewarm |
| Asystole | Cardiac arrest | CPR, rewarm before pronouncing |
| K+ >2 mEq/L | Poor prognosis | Consider termination after discussion |
| Trauma + hypothermia | "Lethal triad" (hypothermia, acidosis, coagulopathy) | Damage control surgery, rewarm |
| pH <6.5 | Severe acidosis | Very poor prognosis |
Handle With Care
- Cold myocardium is very irritable
- Rough handling can precipitate VF
- Move patient gently
- Maintain horizontal position
Differential Diagnosis
Considerations Beyond Cold Exposure
| Diagnosis | Clinical Clues | Evaluation |
|---|---|---|
| Sepsis | Infection source, warm or cold extremes | Cultures, lactate |
| Hypothyroidism (myxedema coma) | Thyroid history, edema, constipation | TSH, free T4 |
| Hypoadrenalism | Hypotension, hypoglycemia, pigmentation | Cortisol |
| Hypoglycemia | Known DM, medications | Fingerstick glucose |
| Drug overdose | History, toxidrome | Tox screen |
| Severe malnutrition | Cachexia | History, albumin |
| CVA | Focal signs | CT head |
| DKA/HHS | Hyperglycemia | Glucose, ABG |
Diagnostic Approach
Temperature Measurement
Accurate Core Temperature is Essential:
| Method | Notes |
|---|---|
| Rectal | Reliable; may lag behind true core |
| Esophageal | Most accurate; requires intubation |
| Bladder | Via catheter; reliable |
| Oral, axillary, tympanic | Inaccurate in hypothermia; avoid |
Minimum Reading Thermometers:
- Standard thermometers don't read below 34°C
- Use low-reading or electronic thermometers
Laboratory Studies
| Test | Purpose | Findings |
|---|---|---|
| Glucose | Hypo/hyperglycemia | Must treat hypoglycemia |
| Electrolytes | K+ is prognostic | K+ may be falsely low or high |
| ABG | Acid-base status | May not correct for temperature |
| CBC | Hemoconcentration | Falsely elevated Hct |
| Coagulation | Coagulopathy | Often prolonged at low temp |
| Lactate | Tissue hypoperfusion | May be elevated |
| TSH | Hypothyroidism | If history suggests |
| Cortisol | Adrenal insufficiency | Consider |
| Drug levels | Overdose | If suspected |
| Lipase | Pancreatitis | Can be induced by hypothermia |
Temperature Correction of ABG:
- Labs run samples at 37°C
- Correcting to patient temperature is controversial
- Many centers use uncorrected ("alpha-stat") values
ECG Findings
- Sinus bradycardia
- Prolonged PR, QRS, QT intervals
- Osborn (J) waves
- Atrial fibrillation
- Ventricular fibrillation (<28°C)
- Asystole (<24°C)
Treatment
Principles of Management
- Prevent further heat loss: Remove wet clothing, insulation
- Rewarm: Method depends on severity
- Cardiovascular support: Fluids, vasopressors cautiously
- Handle gently: Avoid precipitating arrhythmias
- Avoid overcorrection: Rebound hyperthermia
- Treat underlying cause: Infection, overdose, etc.
Rewarming Methods
Passive External Rewarming (Mild Hypothermia):
- Remove wet clothing
- Insulating blankets
- Warm environment
- Patient generates own heat via shivering
- Rate: 0.5-2°C per hour
Active External Rewarming (Mild-Moderate):
| Method | Description |
|---|---|
| Warm blankets | Forced-air warming (Bair Hugger) preferred |
| Heating pads/blankets | Risk of burns; monitor closely |
| Warm water immersion | Impractical in ED |
- Focus heat on trunk (avoid extremity rewarming alone → afterdrop)
Active Internal Rewarming (Severe):
| Method | Invasiveness | Rewarming Rate |
|---|---|---|
| Warm IV fluids (40-42°C) | Low | ~1°C/hr |
| Warm humidified oxygen | Low | Minimal alone |
| Gastric/bladder lavage | Moderate | Variable |
| Peritoneal lavage | Moderate | 1-3°C/hr |
| Thoracic lavage | High | 3-5°C/hr |
| ECMO/Cardiopulmonary bypass | High | 10-12°C/hr |
ECMO/Cardiopulmonary Bypass
Indications:
- Hypothermic cardiac arrest
- Severe hypothermia with hemodynamic instability
- Potassium <8-12 mEq/L (viable)
Gold Standard for Hypothermic Cardiac Arrest:
- Survival rates 50-100% with ECMO in appropriate patients
- Superior to other rewarming methods
- Provides circulatory support during rewarming
- Transfer to ECMO-capable center if available
Cardiac Arrest Management
CPR:
- Full CPR if no pulse palpable
- May be difficult to detect pulse in severe hypothermia
- Continue CPR during transport and rewarming
Medications:
- Drugs may be ineffective until rewarmed
- Drugs accumulate at low temperatures → toxicity during rewarming
- Withhold or space medications if <30°C
- Resume standard dosing once >30°C
Defibrillation:
| Temperature | Approach |
|---|---|
| <30°C | Single shock for VF; then rewarm before repeat |
| 30-35°C | Standard ACLS |
Duration of Resuscitation:
- "Not dead until warm and dead"
- Continue until core temp >32°C
- Exception: K+ >12 mEq/L suggests cellular death
Cardiovascular Support
Fluids:
- Warm IV fluids (40-42°C)
- Replace intravascular volume (cold diuresis causes hypovolemia)
- Normal saline or LR
Vasopressors:
- Use cautiously
- May accumulate and cause toxicity during rewarming
- Heart may not respond normally
Arrhythmias:
- Atrial fibrillation usually converts spontaneously with rewarming
- Avoid aggressive treatment until rewarmed
Avoiding Afterdrop
- Refers to continued core temperature drop during rewarming
- Caused by: Cold blood from periphery returning to core
- Prevention: Rewarm trunk first; passive rewarming of extremities
Disposition
ICU Admission Criteria
- Moderate-severe hypothermia (<32°C)
- Cardiac arrest or arrhythmias
- Need for active internal rewarming
- Hemodynamic instability
- Significant comorbidities
Floor Admission
- Mild hypothermia with comorbidities
- Social factors preventing safe discharge
- Need for observation
Discharge Criteria
- Mild hypothermia (>32°C) that resolves with passive rewarming
- No arrhythmias
- Normal mental status
- Able to ensure safe warm environment
- No underlying medical cause requiring admission
Follow-Up
| Situation | Follow-Up |
|---|---|
| Simple environmental | PCP if underlying factors need addressing |
| Underlying medical cause | Appropriate specialty follow-up |
| Social factors (homelessness) | Social work, shelter resources |
| Frostbite | Wound care, possible surgery follow-up |
Patient Education
Prevention
General Public:
- Dress in layers; keep dry
- Limit time outdoors in extreme cold
- Recognize early signs (shivering, confusion)
- Buddy system for outdoor activities
- Avoid alcohol before/during cold exposure
- Eat regularly to maintain energy stores
High-Risk Individuals:
- Elderly: Keep home heated adequately (at least 68°F)
- Check on elderly neighbors during cold spells
- Recognize medications that impair thermoregulation
Outdoor Workers/Recreators:
- Know the weather conditions
- Carry emergency supplies
- Tell someone your plans and expected return
- Carry communication device
Warning Signs
- Shivering
- Feeling very cold
- Confusion, slurred speech
- Drowsiness
- Loss of coordination
- Slow breathing
Special Populations
Elderly
- Impaired thermoregulation
- Less able to recognize cold
- More susceptible due to medications
- Higher mortality
- Indoor hypothermia possible
Neonates
- Large surface area to volume ratio
- Limited metabolic reserve
- Dependent on caregivers for warmth
Alcohol and Drug Users
- Impaired vasoconstriction
- Impaired judgment
- May fall asleep outdoors
- Higher incidence of hypothermia
Drowning/Submersion
- Rapid heat loss in water
- Cold water may be protective (slows metabolic rate)
- Best outcomes in children with cold water submersion
- Aggressive resuscitation indicated
Avalanche Burial
- Asphyxia often the cause of death, not hypothermia
- Air pocket presence improves survival
- Duration of burial and final temperature predict outcome
Trauma Patients
- "Lethal triad": Hypothermia, acidosis, coagulopathy
- Damage control surgery
- Active rewarming essential
- Avoid excessive crystalloid
Quality Metrics
Performance Indicators
| Metric | Target | Rationale |
|---|---|---|
| Core temperature measured | 100% | Required for diagnosis |
| Low-reading thermometer used | 100% | Standard thermometers insufficient |
| Potassium checked | 100% | Prognostic importance |
| Rewarming initiated in ED | 100% | Definitive treatment |
| ECMO consideration if <28°C cardiac arrest | 100% | Best outcomes |
| Wet clothing removed | 100% | Prevent continued heat loss |
Documentation Requirements
- Core temperature (method specified)
- Estimated duration of exposure
- Rewarming method and rate
- Serial temperatures
- Laboratory trends
- Rhythm and response to treatment
- Disposition and rationale
Key Clinical Pearls
Diagnostic Pearls
- Measure core temperature: Oral and tympanic are unreliable
- J waves are classic but not specific: Also in SAH, hypercalcemia
- Pupils may be fixed and dilated: Does NOT indicate death
- Potassium is prognostic: K+ >12 suggests non-survivability
- Look for underlying cause: Hypothyroidism, drugs, sepsis
- Alcohol is major risk factor: Vasodilation and impaired judgment
Treatment Pearls
- Handle gently: Cold heart is prone to VF
- Rewarm trunk first: Prevent afterdrop
- ECMO for cardiac arrest if available: Best survival
- Single shock for VF <30°C, then rewarm: Defibrillation less effective until warm
- Drugs accumulate: Space out ACLS medications if <30°C
- Arrhythmias may resolve with rewarming: Don't over-treat
Disposition Pearls
- "Not dead until warm and dead": Prolonged resuscitation warranted
- Exception: K+ >12, avalanche burial >60 min with asystole, obvious fatal injuries
- Transfer for ECMO if needed: Best option for severe hypothermia with arrest
- Address social factors: Housing, winter shelters
References
- Brown DJA, et al. Accidental hypothermia. N Engl J Med. 2012;367(20):1930-1938.
- Paal P, et al. Accidental hypothermia – an update. Scand J Trauma Resusc Emerg Med. 2016;24:111.
- Truhlář A, et al. European Resuscitation Council Guidelines 2021: Cardiac arrest in special circumstances. Resuscitation. 2021;161:152-219.
- Kornberger E, et al. Forced air surface rewarming in patients with severe accidental hypothermia. Resuscitation. 1999;41(2):105-111.
- Walpoth BH, 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.
- Brugger H, et al. Resuscitation of avalanche victims: Evidence-based guidelines. Resuscitation. 2013;84(5):539-546.
- Danzl DF, Pozos RS. Accidental hypothermia. N Engl J Med. 1994;331(26):1756-1760.
- UpToDate. Accidental hypothermia in adults. 2024.