Surgical Critical Care: Mechanical Ventilation – Modes

Ventilator Modes

Non-Controlled Ventilation

  • Continuous Positive Airway Pressure (CPAP)
  • Pressure Support Ventilation (PSV)
  • PSV Variations:
    • CPAP with PSV (Similar to BPAP)
    • Automatic Tube Compensation
    • Volume Support Ventilation (VSV)

Volume-Controlled Ventilation

  • Controlled Mechanical Ventilation (CMV)
  • Assist-Control Ventilation (AC)
  • Synchronized Intermittent Mechanical Ventilation (SIMV)

Pressure-Controlled Ventilation

  • Same Modes as Volume-Controlled Ventilation (CMV, AC & SIMV)

Advanced Modes

  • Airway Pressure Release Ventilation (APRV)
  • High-Frequency Oscillatory Ventilation (HFOV)
  • Adaptive Support Ventilation (ASV)
  • Neurally Adjusted Ventilatory Assist (NAVA) Ventilation

Non-Controlled Ventilation

Modes

  • Continuous Positive Airway Pressure (CPAP)
    • Provides a Continuous Pressure Level (Similar to PEEP)
    • No Additional Support Provided to Patient-Triggered Breaths
    • All Breaths are Patient-Triggered – Patient Determines Rate & Volume
  • Pressure Support Ventilation (PSV)
    • Additional Pressure Support Provided to Patient-Triggered Breaths
    • All Breaths are Patient-Triggered – Patient Determines Rate & Volume
  • PSV Variations:
    • CPAP with PSV (Similar to BPAP)
      • Pressure Support is in Addition to PEEP
      • PSV 10/5 Indicates 5 cm H2O of PEEP & 15 cm H2O During Support
    • Automatic Tube Compensation
      • Continuously Adjusts Pressure Support to the Level Needed to Overcome the Endotracheal Tube
    • Volume Support Ventilation (VSV)
      • Continuously Adjusts Pressure Support to the Level Needed to Achieve a Target Tidal Volume
      • Newer Technique & Some Evidence Suggests Decreased Weaning Time & Total Ventilation Time

Use

  • Used Primarily for Ventilator Weaning – Pressure Used to Overcome the Resistance of the Endotracheal Tube
  • Disadvantages:
    • Higher Work of Breathing & Can Cause Respiratory Fatigue
    • Can Result in CO2 Retention & Acidosis
    • Poor Choice for Full Ventilatory Support

Volume-Controlled Ventilation

Controlled Mechanical Ventilation (CMV)

  • Mechanism:
    • Sets a Controlled Minute Ventilation (Rate & Volume)
    • Does Not Allow Any Patient-Triggered Breaths
  • Advantages:
    • Lowest Work of Breathing
    • Set Minute Ventilation is Easily Adjusted
  • Disadvantages:
    • Less Comfortable
    • May See Ventilator Dyssynchrony with Wasted Effort
    • May Require Deeper Sedation or Paralytics

Assist-Control Ventilation (AC)

  • Mechanism:
    • Sets a Minimum Minute Ventilation (Rate & Volume)
    • Allows Patient-Triggered Breaths in Addition to Set Minimum Breaths
    • Patient-Triggered Breaths are at the Set Volume
  • Advantages:
    • Increased Comfort
    • Allows Sedation Weaning
    • Improved Ventilator Synchrony
    • Lower Work of Breathing than SIMV
  • Disadvantages:
    • Higher Work of Breathing than CMV
    • Can Hyperventilate with “Breath-Stacking”

Synchronized Intermittent Mechanical Ventilation (SIMV)

  • Mechanism:
    • Sets a Minimum Minute Ventilation (Rate & Volume)
    • Allows Patient-Triggered Breaths in Addition to Set Minimum Breaths
    • Patient-Triggered Breaths are at a Patient’s Own Volume
  • Advantages:
    • May Allow Exercise of Respiratory Muscles (No Proven Evidence of Any Advantage to SIMV)
  • Disadvantages:
    • Higher Work of Breathing & Can Cause Respiratory Fatigue
    • Less Comfortable
    • Can Hyperventilate with “Breath-Stacking”

Volume-Controlled Ventilation

Pressure-Controlled Ventilation

Pressure-Controlled Ventilation

  • Used Much Less Often than Volume-Controlled Ventilation
  • Mechanism:
    • Provides a Set Airway Pressure for Given Inspiratory Time
    • Uses Inspiratory Pressure Level Instead of Tidal Volume
  • Delivered Using Same Modes as Volume-Controlled Ventilation (CMV, AC & SIMV)
  • Advantages:
    • Peak Inspiratory Pressure (PIP) is Constant (Inspiratory Pressure + PEEP)
      • Decreased Risk for Barotrauma by Lower PIP
    • Increased Mean Airway Pressure & Duration of Alveolar Recruitment
    • Increased Comfort & Decreased Work of Breathing
    • May Allows Better Synchrony with the Ventilator
  • Disadvantages:
    • Tidal Volume & Minute Ventilation is Variable

Volume- vs Pressure-Controlled Ventilation

Advanced Modes of Ventilation

Airway Pressure Release Ventilation (APRV)

  • Mechanism:
    • Maintains High Pressure (P High) for an Extended Time (T High) to Optimize Oxygenation
    • Pressure Released (P Low) for Short Time Period of Time (T Low) to Allow Ventilation
    • Longer Inspiratory/Expiratory Ratio: 80-95%
    • Spontaneous Breathing is Permitted but Will Have Low Tidal Volumes at the Higher Pressures
  • Most Commonly Used for Severe ARDS When Having Difficulty Oxygenating on Other Modes
  • Advantages:
    • Maximize Alveolar Recruitment & Oxygenation
    • Lung-Protective (Lower Peak Pressures & Less Barotrauma)
    • More Comfortable than CMV – Allows Decreased Sedation
  • Disadvantages:
    • Not Ideal if Requiring Heavy Sedation – Spontaneous Breathing is Important for Ventilation
    • Generally Avoided in Severe Obstructive Airway Disease – Risks Hyperinflation with Increased Pressure & Barotrauma
    • Generally Avoided for High Ventilatory Requirements

High-Frequency Oscillatory Ventilation (HFOV)

  • Mechanism:
    • Very High Respiratory Rate (Up to 900 Breaths per Minute) by a High-Frequency Oscillatory Pump
      • Rate Set to 3-15 Hertz
    • Rate is So Fast That the Airway Pressure Merely Oscillates Around a Constant Mean Airway Pressure
      • Tidal Volumes are Very Small
  • Most Commonly Used for Severe ARDS Only When Having Difficulty Oxygenating on Other Modes – Should Not Be Used Routinely (May Actually Increase Mortality)
  • Advantages:
    • Maintains Alveolar Recruitment & Oxygenation
    • Lung-Protective (Lower Peak Pressures & Less Barotrauma)
  • Disadvantages:
    • Significant Discomfort & Requires Heavy Sedation or Paralysis
    • Decreased Expiratory Time Creates Risk for Hyperinflation with Increased Pressure & Barotrauma

Adaptive Support Ventilation (ASV)

  • Mechanism:
    • Continual Adjustments are Automatically Made to Respiratory Rate and Inspiratory Pressure to Achieve a Goal Minimum Minute Ventilation (MMV)
      • Optimal Settings Determined by the “Otis Equation” to Minimize Work of Breathing
      • Accounts for Respiratory Mechanics (Resistance, Compliance, Dead Space – Calculated)
    • Patient-Triggered Breaths are Given Pressure Support
    • Pressure-Controlled Breaths are Given as Needed to Achieve a Calculated Respiratory Rate
  • Settings:
    • Ventilate by Setting a Percentage of Minimum Volume (MinVol) of Desired Minute Ventilation
      • 100% Normal, 120% in ARDS, 90% in Asthma, 110% in Others
      • Add 20% for Fevers > 101.3 F
    • Oxygenate by Setting PEEP and FiO2
  • Advantages:
    • Decreased Work of Breathing
    • Attempts Lung Protective Strategies to Prevent Volutrauma, Barotrauma & Auto PEEP
    • May Decrease Ventilator Weaning Time in COPD

Neurally Adjusted Ventilatory Assist (NAVA) Ventilation

  • Mechanism:
    • Catheter Implanted in a Gastric Tube Detects Electrical Discharge in the Diaphragm
    • Diaphragmatic Excitation Triggers a Mechanical Breath
    • The Degree of Assist Varies by the Amplitude of the Electrical Discharge
      • Tidal Volume Continuously Varies
  • Advantages:
    • Neural-Ventilator Coupling (Time Between Spontaneous Breath & Delivered Mechanical Breath) is Faster than the Conventional Modes
    • May Increase Ventilator Synchrony
  • Disadvantages:
    • Requires Spontaneous Breathing – Unable to Use with Heavy Sedation or Blunted Respiratory Drive