Surgical Critical Care: Accidental Hypothermia

Accidental Hypothermia

Definitions

  • Hypothermia Definition: Core Temperature < 35°C/95°F
  • Types:
    • Primary Hypothermia – Environmental Exposure
    • Secondary Hypothermia – Due to Illness or Substance Abuse

Severity

  • Normal Temperature: 98.6°F (37°C)
  • Cold Stressed: 95-98.6°F (35-37°C)
    • Not Hypothermic
  • Mild Hypothermia: 90-95°F (32-35°C)
  • Moderate Hypothermia: 82-90°F (28-32°C)
  • Severe Hypothermia: 68/75-82°F (20/24-28°C)
  • Profound Hypothermia: < 68-75°F (< 20-24°C)

Swiss Staging System

  • Pre-Hospital System to Estimate Core Temperature
  • HT I: Conscious & Shivering (90-95°F)
  • HT II: Impaired Consciousness, Not Shivering (82-90°F)
  • HT III: Unconscious, Not Shivering (75-82°F)
  • HT IV: No Vital Signs (56.7-75°F)
  • HT V: Death (< 56.7°F)

Presentation

  Mental Status Muscle Response Cardiovascular Changes
Cold Stressed Normal Shivering Normal
Mild Altered Shivering Tachycardia
Increased Cardiac Output
Moderate Agitated & Combative Muscle Spasticity Atrial Fibrillation
Hypotension (Decreased SVR)
Severe Comatose Flaccid Prolonged QRS
J Wave (Osborn Wave)
Ventricular Fibrillation
Profound Comatose Flaccid Cardiac Arrest

Physiologic Changes

  • Cardiovascular Effects:
    • The Heart is the Most Sensitive Organ to Hypothermia
    • Arrhythmias
      • Tachycardia (Mild) or Bradycardia (Moderate-Severe)
      • Ventricular Fibrillation
      • Cardiac Arrest
    • EKG Changes:
      • Prolonged QRS
      • Prolonged PR & QT Intervals
      • J Wave (Osborn Wave) – Positive Deflection After the QRS Complex
        • Seen Only in Severe Hypothermia
    • Vasoconstriction – Reduced Blood Flow to Peripheral Tissues to Preserve Core Heat
  • Pulmonary Effects:
    • Decreased CO2 Production
    • Metabolic Acidosis
    • Left-Shifted Oxygen-Hemoglobin Dissociation Curve – Increased Affinity
    • Hypoventilation (Moderate-Severe)
    • Pulmonary Edema (Severe)
  • Hematologic Effects:
    • Coagulopathy – Impaired Clotting Enzymes & Platelet Function
    • Increased Blood Viscosity
  • Heat Generation:
    • Shivering
    • Increased Thyroid Activity
    • Increased Catecholamine Activity
  • Electrolyte Changes:
    • Hypokalemia
    • Hypomagnesemia
    • Hypophosphatemia
  • Decreased Tissue Metabolism
  • Inhibited Neural Activity
  • Cold-Induced Diuresis
  • Decreased GFR

Temperature Measurement

  • Pulmonary Artery Catheter
    • Gold Standard Monitor
    • More Invasive & Rarely Used
  • Esophageal Probe
    • Inserted into the Lower Third of the Esophagus
    • Generally the Preferred Method to Monitor Rewarming Efforts in Severe Hypothermia
  • Rectal or Bladder Temperatures
    • Changes in Temperature are Delayed Behind Core Temperature Changes
    • Adequate for Mild-Moderate Hypothermia
    • Should Not be Used in Severe Hypothermia
  • Standard Thermometers
    • Minimum Reading Around 34°C
    • Insufficient in Monitoring Hypothermia & Rewarming Efforts

Treatment

  • Mild: Passive External Rewarming (Prevent Heat Loss)
    • Remove Wet Clothing
    • Blankets to Insulate
    • *Requires Physiologic Reserve to Generate Heat by Shivering or Increased Metabolism – Elderly Patients May Lack Ability & Require Active External Rewarming
  • Moderate: Active External Rewarming
    • Warmed Blankets
    • Heating Pads
    • Warmed Baths
    • Forced Warm Air
    • *Warm Trunk Before Extremities to Prevent “Afterdrop” – Rebound Drop in Core Temperature Due to Peripheral Vasodilation
  • Severe/Profound: Active Internal/Core Rewarming
    • Warmed IV Fluids
    • Warmed Peritoneal Lavage
    • Warmed Thoracic Lavage
    • Extracorporeal Blood Rewarming (Cardiopulmonary Bypass or ECMO)
      • Fastest/Most Effective Rewarming Tool
    • *Avoid Gastric or Colonic Irrigation – Can Cause Severe Electrolyte Fluctuations
  • If in Cardiac Arrest Do Not Stop CPR Until Normothermic

Rewarming Rates

  • Passive External Rewarming: 2°C/Hour
    • Highly Dependent on Metabolic Rate
  • Active External Rewarming: 2-3.4°C/Hour
  • Active Internal Rewarming:
    • Warmed IV Fluids: Prevents Heat Loss but Does Not Rewarm
    • Peritoneal Lavage: 1-3°C/Hour
    • Thoracic Lavage: 3°C/Hour
    • Extra-Corporeal Membrane Oxygenation (ECMO): 4-6°C/Hour
    • Cardiopulmonary Bypass (CPB): 9.5°C/Hour

Complications of Rewarming

  • Hypovolemia & Hypotension from Severe Dehydration & Fluid Shifts with Redistribution to Extremities
  • Severe Hyperkalemia from Intracellular Release
  • Arrhythmias
  • Rhabdomyolysis
  • Cerebral Edema
  • Seizures