Surgical Critical Care: Mechanical Ventilation –Airway Pressures

Airway Pressures

Pressure Gradients

  • Trans-Airway Pressure = Atmospheric Pressure – Alveolar Pressure
  • Trans-Thoracic Pressure = Alveolar Pressure – Body Surface Pressure
  • Trans-Pulmonary Pressure = Alveolar Pressure – Pleural Pressure
  • Trans-Respiratory Pressure = Atmospheric Pressure – Body Surface Pressure
    • Trans-Respiratory Pressure = Trans-Airway Pressure + Trans-Thoracic Pressure

Ventilator Pressure Evaluation

  • Peak Inspiratory Pressure (PIP) – Highest Pressure Seen During Inspiration
    • Maximum Acceptable PIP < 35-40 cm H2O
  • Plateau Pressure (Pplat) – Static Pressure at the End of Full Inspiration
    • Estimates Alveolar Pressure
    • Normal Plateau Pressure < 30 cm H2O
    • Measure with an “Inspiratory Hold” Maneuver on the Ventilator
      • Ventilation is Held for 2 Seconds After Inspiratory Flow is Complete to Evaluate Plateau Pressure

Relationships/Equations

  • PIP = PEEP + Elastic Pressure + Restrictive Pressure
    • Also: PIP = Pplat + Restrictive Pressure
  • Pplat = PEEP + Elastic Pressure
  • Restrictive Pressure = Flow x Resistance
  • Elastic Pressure = Volume x Elastance = Volume/Compliance

Compliance

  • Compliance = Change in Volume / Change in Pressure
    • Elastance = 1/Compliance
  • Dynamic Compliance = Tidal Volume / (Elastic Pressure + Restrictive Pressure)
    • Dynamic Compliance = Tidal Volume / (PIP – PEEP)
  • Static Compliance = Tidal Volume / Elastic Pressure
    • Static Compliance = Tidal Volume / (Pplat – PEEP)

Ventilator Pressures

Elevated Airway Pressures

Effects of High Pressure

  • High Airway Pressure Itself is Not Always Harmful – Unless it is Caused by High Alveolar Pressure
  • Effects of Elevated Alveolar Pressure:
    • Barotrauma (Causes Acute Lung Injury & Air Leaks)
    • Decreased Venous Return (Decreases Cardiac Output & Blood Pressure)
    • Decreased Ventilation

Causes of High Pressures

  • High Restrictive Pressure (High PIP with Normal Pplat):
    • Increased Flow
    • Increased Airway Resistance
      • Obstructed Endotracheal Tube
      • Displaced Endotracheal Tube
      • Tubing or Endotracheal Kinking
      • Pooling of Condensed Water in the Circuit
      • Bronchospasm
      • Aspiration, High Secretions or Mucous Plugging
  • High Elastic Pressure (High Pplat):
    • Increased Volume
      • Air Trapping
    • Decreased Compliance
      • Decreased Lung Compliance:
        • Atelectasis
        • Pulmonary Consolidation
        • Pulmonary Edema
        • Pleural Effusion
        • Pneumothorax
      • Decreased Chest Wall Compliance:
        • Abdominal Distention
        • Morbid Obesity
        • Inadequate Anesthesia
        • Kyphoscoliosis
        • Malignant Hyperthermia
      • Patient-Ventilator Dysynchrony
  • High PEEP

Evaluation of High Pressures of Unknown Cause

  • Disconnect from Ventilator & Manually Bag the Patient if Necessary
    • Can Evaluate Resistance While Bagging
  • Check the Ventilator for Correct Settings
  • Check the Circuit for Obstruction or Kinking
  • Pass a Suction Catheter Through the Endotracheal Tube to Assess Patency/Obstruction
  • Examine End-Tidal CO2
  • Chest X-Ray to Check Endotracheal Tube Position & Pulmonary Pathology
  • Watch for Ventilator Dyssynchrony
  • Physical Examination (Wheezing or Asymmetrical Chest Expansion)
  • Preform an “Inspiratory Hold” Maneuver to Differentiate Elastic from Restrictive Pressures