II. Definitions

  1. Hypothermia
    1. Unintentional drop in core Temperature <95 F (35 C)

III. Epidemiology

  1. U.S. Deaths: 1500 per year (50% are over age 65 years)
    1. Hypothermia accounts for 3 fold more deaths/year in U.S. than Heat Related Illness

IV. Risk Factors

  1. Extremes of age (esp. elderly)
    1. Elderly are also at highest risk of Hypothermia related mortality
  2. Alcohol Intoxication or other Chemical Intoxication
    1. Associated with up to two-thirds of Accidental Hypothermia cases in some studies
  3. Comorbid illness
  4. Poor
  5. Homeless
  6. Rural home
  7. Cold AND wet environments
  8. Acute Trauma
  9. Cold environment outdoor activity
    1. Military
    2. Sailing
    3. Skiing and boating
    4. Mountain Climbing
    5. Swimming

V. Causes

VI. Pathophysiology

  1. Optimal Body Temperature is within 2-3 degrees F of 98.6 F (or 1-2 degrees C of 37 C)

VII. Mechanism

  1. Background
    1. Skin is responsible for 90% of total heat loss
    2. Vasodilation (e.g. Alcohol) further increases significant heat loss
  2. Radiative heat loss (50% of heat loss)
    1. Majority of radiative heat loss via head (60%)
  3. Conductive heat loss (2-3% of heat loss)
    1. Wet clothing: Heat loss increases x5
    2. Cold water immersion: Heat loss increases x25
    3. Frostbite may occur within seconds of contact with cold bare metal
  4. Convective heat loss (10%)
    1. Important in windy conditions
    2. Convective loss increases with shivering
  5. Evaporative heat loss (Up to 27%)
  6. Respiratory heat loss (Up to 9%)

VIII. Exam: Core Temperature

  1. Precautions
    1. Oral or infrared tympanic Thermometers should not be used
    2. Many standard Thermometers do not read Temperatures below 94 F (34.4 C)
    3. Use calibrated, low-reading thermistor
  2. Utility
    1. Most useful to define when core Temperature >32 degrees C (90 F)
  3. Methods
    1. Esophageal (preferred, esp. if intubated)
      1. Insert to level distal to carina
      2. Altered when using warm, humidified oxygen
    2. Bladder
      1. Second most accurate (behind esophageal)
      2. Altered with peritoneal or Bladder lavage
    3. Rectal
      1. Insert to at least 15 cm depth
      2. Reading may lag true core Temperature by as much as 1 hour during rewarming efforts
      3. Altered if inserted into cold stool, or by Peritoneal Lavage fluid Temperature

IX. Exam: Other Vital Signs

  1. Pulses
    1. Palpate femoral artery or Carotid Artery for 60 seconds (Heart Rate may drop to 0-10 in severe Hypothermia)
  2. Oxygen Saturation
    1. Forehead Pulse Oximetry

X. Signs: General

  1. Altered Mental Status
  2. Shivering
  3. Flushing
  4. Facial Edema
  5. Initial Tachycardia progresses to Bradycardia
  6. Hypotension
  7. Paradoxical undressing
  8. Respiratory depression
  9. Ataxia
  10. Decreased Corneal Reflex

XI. Signs: Mild Hypothermia (Hypothermia Stage I)

  1. Core Temperature: 95 to 90 F (35 to 32 C)
  2. Initial reaction to cold
    1. Involuntary shivering
    2. Increased Respiratory Rate, pulse and Blood Pressure (Catecholamine induced)
    3. Vasoconstriction
  3. Later with glycogen depletion and Fatigue
    1. Hypovolemia
    2. Cold diuresis (urine frequency)
      1. Cold water immersion is associated with >3 fold greater Enuresis
      2. Risk of Hypovolemia
    3. Amnesia
    4. Ataxia
    5. Apathy
    6. Fine motor skill difficulty
    7. Poor judgment
    8. Irritability

XII. Signs: Moderate Hypothermia (Hypothermia Stage II)

  1. Core Temperature: 89.6 to 82.4 F (32 to 28 C)
  2. Shivering response stops at Body Temperature of 86 F (30 C)
  3. Cardiac Arrhythmia (esp. if Electrolyte abnormality or acidosis)
    1. Atrial Arrhythmia (esp. Atrial Fibrillation)
    2. EKG with J Wave or Osborn Wave
    3. QT Prolongation
    4. Bradycardia
  4. Neurologic changes
    1. Cerebral Blood Flow drops 6-7% with each 1 C below 30 C
    2. Dysarthria
    3. Agitation or confusion
    4. Altered Level of Consciousness to stupor
    5. Mydriasis (Pupil Dilation)
    6. Hyporeflexia or loss of reflexes
    7. Loss of voluntary control
    8. Paradoxical undressing
  5. Hypercoagulable
  6. Decreased Respiratory Rate, pulse and Blood Pressure
    1. Bradycardia: Heart Rate decreases by 50%
    2. Respiratory Acidosis may occur (due to Bradypnea or apnea)

XIII. Signs: Severe Hypothermia (Hypothermia Stage III)

  1. Core Temperature: 82.4 to 75.2 degrees F (28 to 24 C)
  2. Cardiac Arrhythmia
    1. Ventricular Arrhythmias (e.g. Ventricular Tachycardia, Ventricular Fibrillation)
  3. Neurologic Changes
    1. Coma with no response to pain
    2. Pupils do not react and no Corneal Reflex
    3. Muscle rigidity (Rhabdomyolysis may occur)
    4. Flat or decreased Electroencephalogram (EEG) activity
    5. Apnea
  4. Hemodynamic and Electrolytes
    1. Major acid-base disturbance (Metabolic Acidosis)
    2. Oliguria
    3. Disseminated Intravascular Coagulation may occur
  5. Profoundly decreased Respiratory Rate, pulse and Blood Pressure
    1. Significant Hypotension and pulses may barely be palpable

XIV. Signs: Profound or Very Severe Hypothermia (Hypothermia Stage IV)

  1. Core Temperature: < 75.2 degrees F (24.0 C)
  2. Brainstem reflexes absent
  3. EEG flat tracing
  4. Vital Signs absent
  5. Asystole

XV. Staging: Swiss Hypothermia Staging System

  1. Stage 1: Conscious and Shivering
    1. Suspected core Temperature: 89.6 to 95 F (32 to 35 C)
  2. Stage 2: Altered Mental Status and not shivering
    1. Suspected core Temperature: 82.4 to 89.6 F (28 to 32 C)
  3. Stage 3: Unconscious and not shivering, but Vital Signs present
    1. Suspected core Temperature: 75.2 to 82.4 F (24 to 28 C)
  4. Stage 4: No Vital Signs
    1. Suspected core Temperature: <75.2 F (<24 C)

XVI. Labs

  1. Bedside Glucose
    1. Hypoglycemia and Hyperglycemia may occur
    2. Initial Hyperglycemia (impaired Insulin release and activity, increased sympathetic tone)
      1. Avoid correcting Hyperglycemia (unless severe) until patient rewarmed to >86 F (30 C)
    3. Hypoglycemia occurs with rewarming or with gradual onset Hypothermia (glycogen depletion)
      1. See Hypoglycemia Management
  2. Basic metabolic panel (consider comprehensive metabolic panel to include LFTs)
    1. Acute Renal Failure
      1. Cold diuresis (distal tubules fail to reabsorb water, despite Vasopressin) and Dehydration results
      2. Rhabdomyolysis causes Acute Tubular Necrosis
      3. Cold-Induced decreased Cardiac Function results in decreased Glomerular Filtration Rate
    2. Serum Potassium
      1. Monitor closely for both Hyperkalemia and Hypokalemia (may change rapidly with rewarming)
      2. Hypokalemia is a response to cooling with intracellular Potassium shift and Na-K pump dysfunction
      3. Hyperkalemia occurs with acidosis and cell death (marker of worse prognosis)
    3. Serum Glucose (see above)
    4. Other Electrolytes (Na, Ca, Mg, Cl) are typically stable at core Temperatures above 77 F (25 C)
  3. Creatine Phosphokinase (CPK)
    1. Evaluate for Rhabdomyolysis
  4. Arterial Blood Gas
  5. Complete Blood Count
    1. Hematocrit rises 2% for each 1 C drop
    2. White Blood Cell Count transiently increases with shivering, and then decreases as Hypothermia advances
    3. Thrombocytopenia
  6. Coagulation studies (INR, PTT, Fibrinogen)
    1. Often normal despite cold-induced Coagulopathy (but may also be markedly abnormal)
    2. Coagulation studies are typically run at room Temperature, and appear falsely normal
    3. Even minor drops in core Temperature, reduce Clotting Cascade factor activity significantly
    4. After rewarming, Coagulation Factor (and Platelet) activity may not return to normal for >1 hour
    5. Consider Fibrinogen in severe Hypothermia (to assess for DIC)
  7. Serum Lipase
    1. Acute Pancreatitis is common in severe Hypothermia
  8. Serum lactate
    1. May be used to help guide fluid Resuscitation
  9. Other labs to consider (contributing factors, or markers of systemic dysfunction)
    1. Serum Troponin
    2. Urine Tox Screen
    3. Thyroid Stimulating Hormone
    4. ACTH and Cortisol levels (for Adrenal Insufficiency)
      1. Consider Stress Dose Steroids (Hydrocortisone) in refractory Hypothermia

XVII. Differential Diagnosis

  1. See Hypothermia Causes
  2. Consider alternative causes when Body Temperature is refractory to rewarming measures
    1. Endocrine Disorder (e.g. Hypothyroidism, Adrenal Insufficiency)
    2. Sepsis

XVIII. Diagnostics: Electrocardiogram (EKG)

  1. Classic EKG Triad
    1. J Waves (Osborn Wave)
    2. Sinus Bradycardia
    3. Muscle Tremor artifact
  2. General findings
    1. J Waves (Osborn Wave)
    2. PR Prolongation (AV Block)
    3. QRS prolongation (and QRS amplitude decreased)
    4. QT Prolongation
    5. Findings may mimic Acute Coronary Syndrome (ST changes, T Wave Inversion)
  3. Dysrhythmias
    1. Asystole
    2. Sinus Bradycardia
      1. Heart Rate decreases with Temperature
      2. In primary Hypothermia, with core temp 82.4 F or 28 C, Heart Rate of 30-40 bpm is expected
      3. Hypothermia without Bradycardia suggests possible Secondary Hypothermia cause
    3. Atrial Fibrillation
      1. In Hypothermia, Heart Rate with Atrial Fibrillation is typically 60-80 bpm
    4. Ventricular Tachycardia or Ventricular Fibrillation (esp. below 80.6 F or 27 C)
      1. Increased risk with Electrolyte abnormalities (e.g. Hyperkalemia or Hypokalemia)

XIX. Management

  1. See Hypothermia Management
  2. See Rewarming Methods in Hypothermia
  3. See Hypothermia Management in the Wilderness
  4. See ABC Management
  5. See Trauma Evaluation
  6. Initial measures
    1. Immediately move to warm environment
    2. Cut off or remove all wet and cold clothing and apply warm blankets
    3. Start warmed IV fluids and warmed, humidified oxygen
  7. Field Triage
    1. Mild Hypothermia
      1. Initiate Passive Rewarming and Active External Rewarming
      2. Transport to hospital if any associated injury (e.g. Frostbite, Trauma, Drowning)
    2. Moderate to severe Hypothermia
      1. Initiate Passive Rewarming and Active External Rewarming
      2. Transport to hospital capable of invasive rewarming
      3. If hemodynamic instability or core Temperature <82 F (28 C)
        1. Transport to ECMO capable facility if available (otherwise to nearest hospital with ICU)
  8. General Resuscitation
    1. Feel for a pulse for 1 minute at femoral or carotid (weak, slow pulses are common)
    2. Start CPR if pulseless and perform as would in normothermia
    3. Amplify QRS Complex on highest setting (typically low amplitude spikes in Hypothermia)
    4. Monitoring with End-Tidal CO2, Bedside Ultrasound
    5. Defibrillation (when indicated) is often unsuccessful at core Temperature <86 F (30 C)
      1. Attempt Defibrillation at maximum joules at lower Temperature
      2. Reattempt Defibrillation once Temperature increases above 86 F (30 C)
    6. Other ACLS algorithms may be followed when core Temperature >86 F (30 C)
      1. However, when core Temperature 86-96.8 F (30-35 C), double interval between doses
      2. Anti-arrhythmics are ineffective when core Temperature <86 F (30 C)
      3. Cardiac pacing may be used in refractory Bradycardia with Hypotension despite rewarming
  9. Advanced Airway Management
    1. Same Advanced Airway indications for normothermic patients (regardless of myocardial irritability)
    2. Trismus refractory to paralytics may require Nasotracheal Intubation or Cricothyrotomy
    3. Endotracheal Tube cuff should be underinflated to allow for expansion with re-heating
    4. Use lower doses and longer intervals of Anesthetic and neuromuscular agents
    5. Decrease ventilation rates to 4-5 breaths per min with Advanced Airway (8-10 without)
      1. Maintains cerebral Blood Flow, and oxygen demand, CO2 retention is lower in Hypothermia
  10. Access and Medications
    1. Femoral Central Line is preferred over IJ or Subclavian (less myocardial irritability risk)
    2. Medications should be delivered intravenously (not IM, SQ or PO/NG due to poor absorption)
  11. Consider secondary causes when patient fails to rewarm at expected rate (e.g. 1 C/hour in mild Hypothermia)
    1. Hypoglycemia
    2. Alcohol Intoxication
    3. Myxedema Coma
      1. May present with Altered Level of Consciousness and what is presumed to be Accidental Hypothermia
      2. May also present with Bradycardia, Hypotension and Hypoglycemia
      3. Cold Temperatures may also trigger Myxedema Coma, especially in the elderly
      4. Obtain TSH and Consider empiric Thyroxine
    4. Adrenal Insufficiency (Addison's Disease)
      1. Consider stress dose Corticosteroids
    5. Sepsis
      1. Consider infection source evaluation, Blood Cultures and empiric antibiotics

XX. Complications

  1. Arrhythmia (Ventricular Fibrillation, Ventricular Tachycardia, Atrial Fibrillation)
    1. Risk of Rescue Collapse (Cardiac Arrest during patient extrication and transport) due to myocardial irritability
  2. Acute Coronary Syndrome
    1. Cold-related Vasoconstriction increases cardiac workload
    2. Increased Afterload is also a risk for Congestive Heart Failure
  3. Coagulopathy
    1. Usually resolves with rewarming
    2. Coagulation Factor replacement is not typically recommended
    3. Coagulation labs may be normal (PTT, INR, Platelets) despite severe cold-induced Coagulopathy
    4. Microinfarctions are common in severe Hypothermia
      1. Related to increased cryofibrinogen resulting in increased blood viscosity
  4. Multisystem organ failure (esp. Trauma patients)
    1. Provoked by Hypothermia, Coagulopathy and acidosis

XXI. Precautions: Cardinal Rules

  1. ECMO is very effective in increasing survival rates from severe Hypothermia
    1. See Hypothermia Management
  2. Not dead until warm and dead unless already dead
    1. Patients have survived after low of 55.6 F (13 C), and after 6 hours of CPR
  3. Do not cease Resuscitation until rewarmed
    1. Reevaluate after core temp >89.6 F (32 C)
    2. See Hypothermia Management for exceptions

XXII. Prognosis: Poor Prognostic Factors

  1. Elderly
  2. Asphyxia
  3. Out-of hospital Cardiac Arrest
  4. Low or absent Blood Pressure
  5. Blood Urea Nitrogen increased
  6. Disseminated Intravascular Coagulation
  7. Hyperkalemia (cell lysis related)
  8. Ammonia >250 mmol/L
  9. Endotracheal Intubation required

XXIV. References

  1. Bazzoli (2024) Crit Dec Emerg Med, Winter Edition, p 4-11
  2. Civitarese and Sciano (2018) Crit Dec Emerg Med 32(2): 3-16
  3. Herbert and Brown in Herbert (2014) EM:Rap 14(1):1-4
  4. Danzl in Marx (2002) Rosen's Emergency Med, p. 1979-96
  5. Danzl in Auerbach (2001) Wilderness Med, p. 135-77
  6. Zink (2020) Crit Dec Emerg Med 34(3): 19-27
  7. McCullough (2004) Am Fam Physician 70:2325-32 [PubMed]

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