II. Definitions
- Hypothermia
- Unintentional drop in core Temperature <95 F (35 C)
III. Epidemiology
- U.S. Deaths: 1500 per year (50% are over age 65 years)
- Hypothermia accounts for 3 fold more deaths/year in U.S. than Heat Related Illness
IV. Risk Factors
- Extremes of age (esp. elderly)
- Elderly are also at highest risk of Hypothermia related mortality
-
Alcohol Intoxication or other Chemical Intoxication
- Associated with up to two-thirds of Accidental Hypothermia cases in some studies
- Comorbid illness
- Poor
- Homeless
- Rural home
- Cold AND wet environments
- Acute Trauma
- Cold environment outdoor activity
- Military
- Sailing
- Skiing and boating
- Mountain Climbing
- Swimming
V. Causes
VI. Pathophysiology
- Optimal Body Temperature is within 2-3 degrees F of 98.6 F (or 1-2 degrees C of 37 C)
VII. Mechanism
- Background
- Skin is responsible for 90% of total heat loss
- Vasodilation (e.g. Alcohol) further increases significant heat loss
- Radiative heat loss (50% of heat loss)
- Majority of radiative heat loss via head (60%)
- Conductive heat loss (2-3% of heat loss)
- Wet clothing: Heat loss increases x5
- Cold water immersion: Heat loss increases x25
- Frostbite may occur within seconds of contact with cold bare metal
- Convective heat loss (10%)
- Important in windy conditions
- Convective loss increases with shivering
- Evaporative heat loss (Up to 27%)
- Respiratory heat loss (Up to 9%)
VIII. Exam: Core Temperature
- Precautions
- Oral or infrared tympanic Thermometers should not be used
- Many standard Thermometers do not read Temperatures below 94 F (34.4 C)
- Use calibrated, low-reading thermistor
- Utility
- Most useful to define when core Temperature >32 degrees C (90 F)
- Methods
- Esophageal (preferred, esp. if intubated)
- Insert to level distal to carina
- Altered when using warm, humidified oxygen
- Bladder
- Second most accurate (behind esophageal)
- Altered with peritoneal or Bladder lavage
- Rectal
- Insert to at least 15 cm depth
- Reading may lag true core Temperature by as much as 1 hour during rewarming efforts
- Altered if inserted into cold stool, or by Peritoneal Lavage fluid Temperature
- Esophageal (preferred, esp. if intubated)
IX. Exam: Other Vital Signs
-
Pulses
- Palpate femoral artery or Carotid Artery for 60 seconds (Heart Rate may drop to 0-10 in severe Hypothermia)
-
Oxygen Saturation
- Forehead Pulse Oximetry
X. Signs: General
- Altered Mental Status
- Shivering
- Flushing
- Facial Edema
- Initial Tachycardia progresses to Bradycardia
- Hypotension
- Paradoxical undressing
- Respiratory depression
- Ataxia
- Decreased Corneal Reflex
XI. Signs: Mild Hypothermia (Hypothermia Stage I)
- Core Temperature: 95 to 90 F (35 to 32 C)
- Initial reaction to cold
- Involuntary shivering
- Increased Respiratory Rate, pulse and Blood Pressure (Catecholamine induced)
- Vasoconstriction
- Later with glycogen depletion and Fatigue
- Hypovolemia
- Cold diuresis (urine frequency)
- Cold water immersion is associated with >3 fold greater Enuresis
- Risk of Hypovolemia
- Amnesia
- Ataxia
- Apathy
- Fine motor skill difficulty
- Poor judgment
- Irritability
XII. Signs: Moderate Hypothermia (Hypothermia Stage II)
- Core Temperature: 89.6 to 82.4 F (32 to 28 C)
- Shivering response stops at Body Temperature of 86 F (30 C)
-
Cardiac Arrhythmia (esp. if Electrolyte abnormality or acidosis)
- Atrial Arrhythmia (esp. Atrial Fibrillation)
- EKG with J Wave or Osborn Wave
- QT Prolongation
- Bradycardia
- Neurologic changes
- Cerebral Blood Flow drops 6-7% with each 1 C below 30 C
- Dysarthria
- Agitation or confusion
- Altered Level of Consciousness to stupor
- Mydriasis (Pupil Dilation)
- Hyporeflexia or loss of reflexes
- Loss of voluntary control
- Paradoxical undressing
- Hypercoagulable
- Decreased Respiratory Rate, pulse and Blood Pressure
- Bradycardia: Heart Rate decreases by 50%
- Respiratory Acidosis may occur (due to Bradypnea or apnea)
XIII. Signs: Severe Hypothermia (Hypothermia Stage III)
- Core Temperature: 82.4 to 75.2 degrees F (28 to 24 C)
-
Cardiac Arrhythmia
- Ventricular Arrhythmias (e.g. Ventricular Tachycardia, Ventricular Fibrillation)
- Neurologic Changes
- Coma with no response to pain
- Pupils do not react and no Corneal Reflex
- Muscle rigidity (Rhabdomyolysis may occur)
- Flat or decreased Electroencephalogram (EEG) activity
- Apnea
- Hemodynamic and Electrolytes
- Major acid-base disturbance (Metabolic Acidosis)
- Oliguria
- Disseminated Intravascular Coagulation may occur
- Profoundly decreased Respiratory Rate, pulse and Blood Pressure
- Significant Hypotension and pulses may barely be palpable
XIV. Signs: Profound or Very Severe Hypothermia (Hypothermia Stage IV)
- Core Temperature: < 75.2 degrees F (24.0 C)
- Brainstem reflexes absent
- EEG flat tracing
- Vital Signs absent
- Asystole
XV. Staging: Swiss Hypothermia Staging System
- Stage 1: Conscious and Shivering
- Suspected core Temperature: 89.6 to 95 F (32 to 35 C)
- Stage 2: Altered Mental Status and not shivering
- Suspected core Temperature: 82.4 to 89.6 F (28 to 32 C)
- Stage 3: Unconscious and not shivering, but Vital Signs present
- Suspected core Temperature: 75.2 to 82.4 F (24 to 28 C)
- Stage 4: No Vital Signs
- Suspected core Temperature: <75.2 F (<24 C)
XVI. Labs
- Bedside Glucose
- Hypoglycemia and Hyperglycemia may occur
- Initial Hyperglycemia (impaired Insulin release and activity, increased sympathetic tone)
- Avoid correcting Hyperglycemia (unless severe) until patient rewarmed to >86 F (30 C)
- Hypoglycemia occurs with rewarming or with gradual onset Hypothermia (glycogen depletion)
- Basic metabolic panel (consider comprehensive metabolic panel to include LFTs)
- Acute Renal Failure
- Cold diuresis (distal tubules fail to reabsorb water, despite Vasopressin) and Dehydration results
- Rhabdomyolysis causes Acute Tubular Necrosis
- Cold-Induced decreased Cardiac Function results in decreased Glomerular Filtration Rate
- Serum Potassium
- Monitor closely for both Hyperkalemia and Hypokalemia (may change rapidly with rewarming)
- Hypokalemia is a response to cooling with intracellular Potassium shift and Na-K pump dysfunction
- Hyperkalemia occurs with acidosis and cell death (marker of worse prognosis)
- Serum Glucose (see above)
- Other Electrolytes (Na, Ca, Mg, Cl) are typically stable at core Temperatures above 77 F (25 C)
- Acute Renal Failure
-
Creatine Phosphokinase (CPK)
- Evaluate for Rhabdomyolysis
- Arterial Blood Gas
-
Complete Blood Count
- Hematocrit rises 2% for each 1 C drop
- White Blood Cell Count transiently increases with shivering, and then decreases as Hypothermia advances
- Thrombocytopenia
- Coagulation studies (INR, PTT, Fibrinogen)
- Often normal despite cold-induced Coagulopathy (but may also be markedly abnormal)
- Coagulation studies are typically run at room Temperature, and appear falsely normal
- Even minor drops in core Temperature, reduce Clotting Cascade factor activity significantly
- After rewarming, Coagulation Factor (and Platelet) activity may not return to normal for >1 hour
- Consider Fibrinogen in severe Hypothermia (to assess for DIC)
- Serum Lipase
- Acute Pancreatitis is common in severe Hypothermia
- Serum lactate
- May be used to help guide fluid Resuscitation
- Other labs to consider (contributing factors, or markers of systemic dysfunction)
- Serum Troponin
- Urine Tox Screen
- Thyroid Stimulating Hormone
- ACTH and Cortisol levels (for Adrenal Insufficiency)
- Consider Stress Dose Steroids (Hydrocortisone) in refractory Hypothermia
XVII. Differential Diagnosis
- See Hypothermia Causes
- Consider alternative causes when Body Temperature is refractory to rewarming measures
- Endocrine Disorder (e.g. Hypothyroidism, Adrenal Insufficiency)
- Sepsis
XVIII. Diagnostics: Electrocardiogram (EKG)
- Classic EKG Triad
- J Waves (Osborn Wave)
- Sinus Bradycardia
- Muscle Tremor artifact
-
General findings
- J Waves (Osborn Wave)
- PR Prolongation (AV Block)
- QRS prolongation (and QRS amplitude decreased)
- QT Prolongation
- Findings may mimic Acute Coronary Syndrome (ST changes, T Wave Inversion)
-
Dysrhythmias
- Asystole
- Sinus Bradycardia
- Heart Rate decreases with Temperature
- In primary Hypothermia, with core temp 82.4 F or 28 C, Heart Rate of 30-40 bpm is expected
- Hypothermia without Bradycardia suggests possible Secondary Hypothermia cause
- Atrial Fibrillation
- In Hypothermia, Heart Rate with Atrial Fibrillation is typically 60-80 bpm
- Ventricular Tachycardia or Ventricular Fibrillation (esp. below 80.6 F or 27 C)
- Increased risk with Electrolyte abnormalities (e.g. Hyperkalemia or Hypokalemia)
XIX. Management
- See Hypothermia Management
- See Rewarming Methods in Hypothermia
- See Hypothermia Management in the Wilderness
- See ABC Management
- See Trauma Evaluation
- Initial measures
- Immediately move to warm environment
- Cut off or remove all wet and cold clothing and apply warm blankets
- Start warmed IV fluids and warmed, humidified oxygen
- Field Triage
- Mild Hypothermia
- Initiate Passive Rewarming and Active External Rewarming
- Transport to hospital if any associated injury (e.g. Frostbite, Trauma, Drowning)
- Moderate to severe Hypothermia
- Initiate Passive Rewarming and Active External Rewarming
- Transport to hospital capable of invasive rewarming
- If hemodynamic instability or core Temperature <82 F (28 C)
- Transport to ECMO capable facility if available (otherwise to nearest hospital with ICU)
- Mild Hypothermia
-
General Resuscitation
- Feel for a pulse for 1 minute at femoral or carotid (weak, slow pulses are common)
- Start CPR if pulseless and perform as would in normothermia
- Amplify QRS Complex on highest setting (typically low amplitude spikes in Hypothermia)
- Monitoring with End-Tidal CO2, Bedside Ultrasound
- Defibrillation (when indicated) is often unsuccessful at core Temperature <86 F (30 C)
- Attempt Defibrillation at maximum joules at lower Temperature
- Reattempt Defibrillation once Temperature increases above 86 F (30 C)
- Other ACLS algorithms may be followed when core Temperature >86 F (30 C)
- However, when core Temperature 86-96.8 F (30-35 C), double interval between doses
- Anti-arrhythmics are ineffective when core Temperature <86 F (30 C)
- Cardiac pacing may be used in refractory Bradycardia with Hypotension despite rewarming
-
Advanced Airway Management
- Same Advanced Airway indications for normothermic patients (regardless of myocardial irritability)
- Trismus refractory to paralytics may require Nasotracheal Intubation or Cricothyrotomy
- Endotracheal Tube cuff should be underinflated to allow for expansion with re-heating
- Use lower doses and longer intervals of Anesthetic and neuromuscular agents
- Decrease ventilation rates to 4-5 breaths per min with Advanced Airway (8-10 without)
- Maintains cerebral Blood Flow, and oxygen demand, CO2 retention is lower in Hypothermia
- Access and Medications
- Femoral Central Line is preferred over IJ or Subclavian (less myocardial irritability risk)
- Medications should be delivered intravenously (not IM, SQ or PO/NG due to poor absorption)
- Consider secondary causes when patient fails to rewarm at expected rate (e.g. 1 C/hour in mild Hypothermia)
- Hypoglycemia
- Alcohol Intoxication
- Myxedema Coma
- May present with Altered Level of Consciousness and what is presumed to be Accidental Hypothermia
- May also present with Bradycardia, Hypotension and Hypoglycemia
- Cold Temperatures may also trigger Myxedema Coma, especially in the elderly
- Obtain TSH and Consider empiric Thyroxine
- Adrenal Insufficiency (Addison's Disease)
- Consider stress dose Corticosteroids
- Sepsis
- Consider infection source evaluation, Blood Cultures and empiric Antibiotics
XX. Complications
-
Arrhythmia (Ventricular Fibrillation, Ventricular Tachycardia, Atrial Fibrillation)
- Risk of Rescue Collapse (Cardiac Arrest during patient extrication and transport) due to myocardial irritability
-
Acute Coronary Syndrome
- Cold-related Vasoconstriction increases cardiac workload
- Increased Afterload is also a risk for Congestive Heart Failure
-
Coagulopathy
- Usually resolves with rewarming
- Coagulation Factor replacement is not typically recommended
- Coagulation labs may be normal (PTT, INR, Platelets) despite severe cold-induced Coagulopathy
- Microinfarctions are common in severe Hypothermia
- Related to increased cryofibrinogen resulting in increased blood viscosity
- Multisystem organ failure (esp. Trauma patients)
- Provoked by Hypothermia, Coagulopathy and acidosis
XXI. Precautions: Cardinal Rules
- ECMO is very effective in increasing survival rates from severe Hypothermia
- Not dead until warm and dead unless already dead
- Patients have survived after low of 55.6 F (13 C), and after 6 hours of CPR
- Do not cease Resuscitation until rewarmed
- Reevaluate after core temp >89.6 F (32 C)
- See Hypothermia Management for exceptions
XXII. Prognosis: Poor Prognostic Factors
- Elderly
- Asphyxia
- Out-of hospital Cardiac Arrest
- Low or absent Blood Pressure
- Blood Urea Nitrogen increased
- Disseminated Intravascular Coagulation
- Hyperkalemia (cell lysis related)
- Ammonia >250 mmol/L
- Endotracheal Intubation required
XXIII. Prevention
XXIV. References
- Bazzoli (2024) Crit Dec Emerg Med, Winter Edition, p 4-11
- Civitarese and Sciano (2018) Crit Dec Emerg Med 32(2): 3-16
- Herbert and Brown in Herbert (2014) EM:Rap 14(1):1-4
- Danzl in Marx (2002) Rosen's Emergency Med, p. 1979-96
- Danzl in Auerbach (2001) Wilderness Med, p. 135-77
- Zink (2020) Crit Dec Emerg Med 34(3): 19-27
- McCullough (2004) Am Fam Physician 70:2325-32 [PubMed]