Airway Pressure

Airway Pressure

David Ray Velez, MD

Table of Contents

Definitions
Elevated Airway Pressure

Definitions

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 Pressures

  • 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: Describes the Ability of the Lung to Expand in Response to Pressure Changes
    • Compliance = Change in Volume / Change in Pressure = 1 / Elastance
    • Elastance = Change in Pressure / Change in Volume = 1 / Compliance
  • Dynamic Compliance: Describes Compliance During Respiration
    • Dynamic Compliance = Tidal Volume / (Elastic Pressure + Restrictive Pressure)
    • Dynamic Compliance = Tidal Volume / (PIP – PEEP)
  • Static Compliance: Describes Compliance When There is No Airflow
    • Static Compliance = Tidal Volume / Elastic Pressure
    • Static Compliance = Tidal Volume / (Pplat – PEEP)

Ventilator Pressures

Elevated Airway Pressure

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 and Air Leaks)
    • Decreased Venous Return (Decreases Cardiac Output and Blood Pressure)
    • Decreased Ventilation

Causes of High Restrictive Pressures (High PIP and 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

Causes of High Elastic Pressures (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 Can Cause an Elevated Pplat Measurement Even with Normal Elastic Pressures

Evaluation of High Pressures of Unknown Cause

  • Disconnect from the Ventilator and Manually Bag the Patient if Necessary
    • Can Physically 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