Hypothermia

Accidental Hypothermia

David Ray Velez, MD

Table of Contents

Definition
Severity and Staging
Physiologic Changes
Presentation
Diagnosis and Temperature Measurement
Management

Definition

Hypothermia Definition: Core Temperature < 35°C/95°F

Types

  • Primary Hypothermia: Due to Environmental Exposure
  • Secondary Hypothermia: Due to Illness or Substance Abuse

Severity and Staging

Normal Temperature: 98.6°F (37°C)

Severity

  • Cold Stressed: 95-98.6°F (35-37°C)
    • Not “Hypothermic”
  • Mild Hypothermia (Stage I): 90-95°F (32-35°C)
  • Moderate Hypothermia (Stage II): 82-90°F (28-32°C)
  • Severe Hypothermia (Stage III): 68/75-82°F (20/24-28°C)
  • Profound Hypothermia (Stage IV): < 68-75°F (< 20-24°C)

Swiss Staging System

  • Pre-Hospital System to Estimate Core Temperature
  • HT I: Conscious and 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)
Mnemonic:
Difficult to Remember Exact Temperature Staging
°F °C Effects
Mild 90’s 30’s Shivering
Moderate 80’s 20’s (Upper) No Shivering
Severe 70’s 20’s (Middle) Hypotension
Profound 60’s 20’s (Lower) Arrest

Physiologic Changes

Cardiovascular Effects

  • The Heart is the Most Sensitive Organ to Hypothermia
  • Arrhythmias
    • Tachycardia (Mild) or Bradycardia (Moderate-Severe)
    • Atrial Fibrillation with Slow Ventricular Response
    • Ventricular Fibrillation
    • Cardiac Arrest
  • EKG Changes:
    • Prolonged QRS
    • Prolonged PR Interval
    • Prolonged QT Interval
    • J Wave (Osborn Wave) – Positive Deflection After the QRS Complex
      • Seen Only in Severe Hypothermia
      • Wave Size Correlates to the Degree of Hypothermia
      • No Specific Prognostic Value
  • Vasoconstriction – Reduced Blood Flow to Peripheral Tissues to Preserve Core Heat

Pulmonary Effects

  • Left-Shifted Oxygen-Hemoglobin Dissociation Curve – Increased Affinity
  • Hypoventilation
  • Pulmonary Edema

Hematologic Effects

  • Coagulopathy – Impaired Clotting and Platelet Function
  • Increased Blood Viscosity
  • Thromboembolism

Electrolyte Changes

  • Hypokalemia
  • Hypomagnesemia
  • Hypophosphatemia

Other Changes

  • Decreased Tissue Metabolism and CO2 Production
  • Metabolic Acidosis
  • Inhibited Neural Activity
  • Cold-Induced Diuresis
  • Decreased GFR

Physiologic Efforts to Generate Heat

  • Shivering
  • Increased Thyroid Activity
  • Increased Catecholamine Activity

J Wave (Osborn Wave) 1

Presentation

Stage°F°CMental StatusShivering ResponseCardiovascular Changes
Cold Stressed95-98.635-37NormalShiveringNormal
I: Mild90-9532-35NormalShiveringTachycardia, Tachypnea
II: Moderate82-9028-32AlteredNone (Spastic)Bradycardia, Bradypnea, Atrial Fibrillation
III: Severe68/75-8220/24-28UnconsciousNone (Flaccid)Hypotension, Prolonged QRS, J Waves, Ventricular Fibrillation
IV: Profound< 68-75< 20-24ComatoseNone (Flaccid)Cardiac Arrest

Cold Stressed is Not “Hypothermic”

Exact Physical Signs and Responses Vary Depending on the Publication

Diagnosis and Temperature Measurement

Pulmonary Artery Catheter

  • Gold Standard but More Invasive and 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 but Should Not be Used in Severe Hypothermia

Standard Thermometers

  • Minimum Reading Around 34°C
  • Insufficient in Monitoring Hypothermia and Rewarming Efforts

Management

Mild (< 95°F): 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 and Require Active External Rewarming

Moderate (< 90°F): 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 (< 82°F): Active Internal (Core) Rewarming

  • *See Techniques Below
  • Warmed IV Fluids
  • Warmed Bladder Lavage
  • 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 – “Not Dead Until Warm and Dead”

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 Generally Does Not Rewarm Well
    • 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 and Hypotension from Severe Dehydration and Fluid Shifts with Redistribution to Extremities
  • Severe Hyperkalemia from Intracellular Release
  • Arrhythmias
  • Rhabdomyolysis
  • Cerebral Edema
  • Seizures

Active Internal (Core) Rewarming Techniques

Warmed IV Fluids

  • Heated to 38-42°C
  • Avoid Giving Through a Central Line – Can Irritate the Heart
  • Highly Variable and Decreased Tubing Distance Can Improve Heating
  • Prevents Heat Loss but Generally Does Not Rewarm Well
  • High Volumes Can Risk Volume Overload

Warmed Bladder Lavage

  • Generally Less Effective than Peritoneal/Thoracic Lavage
  • Used Only if Large Volumes of Warmed Fluid are Not Available for Other Techniques
  • Instill 100-200 mL Warmed Fluid (Heated to 38-42°C) Through a Foley Catheter
  • Foley is Clamped and Temperature is Monitored Until the Temperature Equilibrates 2-3°C
  • Foley is Then Drained and Lavage is Repeated

Warmed Peritoneal Lavage

  • Catheter Placed Through the Abdominal Wall – Similar to a Diagnostic Peritoneal Lavage (DPL)
    • Nasogastric Tube and Foley are Placed to Decompress Prior to Catheter Placement
    • 2 cm Incision Made Above or Below the Umbilicus to Expose the Linea Alba
    • Divide the Fascia and Peritoneum
    • Catheter is Directed into the Abdomen/Pelvis and Fascia is Closed Around the Tube in Purse String Fashion
  • Single Catheter Method
    • Infuse 10-20 mL/kg of Warmed Fluid (Heated to 38-42°C) Through the Catheter
    • Let Fluid Instill for 10-30 Minutes
    • Drain Fluid and Then Repeat
  • Two Catheter Method
    • A Second Catheter is Placed So that One Can Continuously Instill Fluid While the Other is Left Open to Drain
    • Warmed Fluid (Heated to 38-42°C) is Instilled at 5-6 L/Hour

Warmed Thoracic Lavage

  • Chest Tubes are Generally Placed on the Right (Warmed Fluid Instillation on the Left Can Irritate the Heart)
  • Single Chest Tube Method
    • A Single Lateral Chest Tube is Placed
    • Approximately 1 L of Warmed Fluid (Heated to 38-42°C) is Instilled into the Chest
    • Let Fluid Instill for 10-30 Minutes
    • Drain Fluid and Then Repeat
  • Two Chest Tube Method
    • Two Chest Tubes are Placed (One Anterior and One Lateral) to Allow Continuous Flow
    • Warmed Fluid (Heated to 38-42°C) is Continuously Instilled Through the Anterior Chest Tube at a High Rate
    • The Lateral Chest Tube is Continuously Drained per a Pleur-Evac

Extracorporeal Blood Rewarming (Cardiopulmonary Bypass or ECMO)

  • The Fastest/Most Effective Rewarming Tool

References

  1. Munta K, Santosh P, Surath MR. Severe Hypothermia Causing Ventricular Arrhythmia in Organophosphorus Poisoning. Indian J Crit Care Med. 2017 Feb;21(2):99-101. (License: CC BY-NC-SA-3.0)