Surgical Critical Care: Mechanical Ventilation – Ventilator Management

Basic Management

General Indications for Mechanical Ventilation

  • Unable to Protect Airway (Trauma, Oropharyngeal Infection, etc.)
  • Hypoxemic Respiratory Failure with Inadequate Oxygenation
  • Hypercapnic Respiratory Failure with Inadequate Ventilation
  • Expectant Course – Anticipated Patient Decline

Primary Goals of Mechanical Ventilation

  • Oxygenation – Provide Oxygen to Lungs & Circulation
  • Ventilation – Exchange of Air Between the Lungs & Environment/Ventilator
    • Most Important Effect: Removal of CO2 from the Body
    • Does Not Increase Blood Oxygen Content

Settings & Modes

General Management

  • Increase Oxygenation (Increase Oxygen):
    • Increase FiO2 (Fastest)
    • Increase PEEP (Can Take Several Hours)
  • Increase Ventilation (Decrease CO2):
    • Increase Rate
    • Increase Tidal Volume
    • *COPD Exacerbation Can Cause Auto PEEP & Air Trapping
      • Increasing Rate May Actually Worsen Hypercarbia
      • Decreasing Inspiratory/Expiratory Time Can Improve Ventilation to Remove CO2

Ventilator Adjuncts

Prone Positioning

  • Can Improve Gas Exchange & Oxygenation
    • No Significant Change in Minute Ventilation or CO2 Clearance
  • Effect: Changes Gravitational Force
    • Redistributes Air Flow & Blood Flow More Evenly – Improves Gas Exchange
      • Reduces the Pleural Pressure Gradient from Nondependent to Dependent Regions
    • Decreases Lung Compression from Heart & Abdominal Organs – Reducing Areas of Collapse
    • Less Ventilatory Support Decreases Risk for Ventilator-Induced Lung Injury
    • Increased Blood Return to the Right Heart – Improved Heart Function
    • Improved Secretion Clearance from Mouth/Nose Facing Down
  • Prone Positioning Requires a Team of Trained Professionals with Risk for Dislodgement of Endotracheal Tubes or Other Medical Devices
    • Reintubation or CPR After Significantly More Difficult if Patient is in the Prone Position

Inhaled Pulmonary Vasodilators

  • Effects:
    • Inhalation Keeps it Selective to the Pulmonary Vasculature that is Better Ventilated
    • Mechanism:
      • Improved Ventilation-Perfusion (V/Q) Matching & Arterial Oxygenation
        • Blood Flow is Preferentially Directed to Better Ventilated Areas
      • Decreased Intrapulmonary (Right-to-Left) Shunting
      • Decreased Pulmonary Vascular Resistance
    • Typically Has Minimal Systemic Effects
    • Side Effects:
      • Increased Preload Can Worsen Cardiogenic Pulmonary Edema from Left Heart Failure
      • Small Anti-Platelet Effect (Clinical Impact is Unclear)
  • Indications:
    • ARDS with Worsening Oxygenation Despite Lung-Protective Ventilation Strategies
    • Pulmonary Hypertension
    • Acute Right Heart Dysfunction
    • Hypoxemia Due to Pulmonary Vasoconstriction
  • Drugs:
    • Nitric Oxide
      • Potent Vasodilator
    • Aerosolized Epoprostenol Sodium (Flolan/Veletri)
      • Synthetic Prostacyclin Causes Vasodilation
      • Generally Much Less Expensive than Nitric Oxide
    • Less Common:
      • Nitroglycerine – Metabolized to Nitric Oxide
      • Milrinone – Phosphodiesterase Inhibitor

Lung Recruitment Maneuvers

  • Definition: Temporary Sustained Increases in Airway Pressure to Open (Recruit) Collapsed Alveoli
  • Primarily Used in Severe ARDS if there is Concern for Alveolar De-Recruitment
  • Causes of De-Recruitment (Alveoli Collapse):
    • Low PEEP
    • Low Tidal Volumes
    • High FiO2 – Due to Absorption Atelectasis
  • Methods: Generally Done Using Pressure-Controlled Ventilation
    • PEEP 40 cm H2O for 30-40 Seconds with Respiratory Rate at Zero
    • Slow Increase in Inspiratory Pressure Over a Longer Period of Time Up to 40 cm H2O
    • Peak Pressure 50 cm H2O with PEEP Above Upper Inflection Point for 2 Minutes
  • Complications:
    • Lung Injury or Barotrauma (Pneumothorax)
    • Decreased Preload
    • Benefit May Be Short-Lived
    • Significant Discomfort (May Require Sedation)
  • Staircase Recruitment Maneuver
    • Identifies the De-Recruitment Point & Optimal PEEP
    • Method:
      • Set Inspiratory Pressure to 15 cm H2O Above Peep
      • Increase PEEP Every 2 Minutes (20 > 30 > 40 cm H2O)
      • Then Reduce PEEP Every 3 Minutes (25 > 22.5 > 20 > 17.5 cm H2O)
        • Decrease Until SaO2 ≥ 1% from the Maximum SaO2 Observed (De-Recruitment Point)
      • Increase PEEP to 40 cm H2O for One Minute
      • Then Return PEEP to 2.5 cm H2O Above the De-Recruitment Point (Optimal PEEP)
      • May Increase Mortality in Severe ARDS