Ventilator Volume Loops

Ventilator Volume Loops

David Ray Velez, MD

Table of Contents

Pressure-Volume Loops
Flow-Volume Loops

Pressure-Volume Loops

Pressure-Volume Loop: A Graphical Display of the Relationship Between Pressure and Volume During the Respiratory Cycle

Display

  • The Graph Proceeds Counter-Clockwise When Mechanically Ventilated with Positive Pressure Ventilation
    • Spontaneous Breaths Proceed in a Reverse Clockwise Fashion
  • The Peak Represents the Tidal Volume (Vt) and Peak Inspiratory Pressure (PIP)

Pressure-Volume Loop

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
  • Changes in Compliance Can Be Reflected in the Pressure-Volume Loop
  • Increased Compliance – Results in a Left-Shift in the Curve
    • Less Pressure Required to Reach the Same Volume
  • Decreased Compliance – Results in a Right-Shift in the Curve
    • Greater Pressure Required to Reach the Same Volume

Compliance: A. Increased; B. Normal; C. Decreased

Pressure Evaluation

  • Lower Inflection Point: The Point Where a Change in Slope is Seen in the Lower Inspiratory Curve
    • Represents the Minimum Pressure Required for Alveolar Recruitment
    • Suggested as the Set PEEP in ARDS to Maintain FRC (Debated) – Prevents Injury and Atelectrauma from Constant Collapse and Reopening
  • Upper Inflection Point: The Point Where a Change in Slope is Seen in the Upper Inspiratory Curve
    • Represents the Pressure at Which Regional Overdistention is Seen
  • “Breaking” Point: Point Where Increased Pressure Does Not Correlate with Increased Volume
    • High Risk for Injury at Pressures Above this Point
    • Keep PIP Below this Point

Pressure-Volume Loop

Flow-Volume Loops

Flow-Volume Loop: A Graphical Display of the Relationship Between Flow and Volume During the Respiratory Cycle

Display

  • Conventionally, Inspiration is Below the X-Axis and Expiration is Above the X-Axis During Pulmonary Function Testing (PFT’s)
    • Note: Ventilators Often Show the Graph Reversed with the Inspiratory Limb Above the X-Axis
  • The Graph Proceeds in a Clockwise Fashion Regardless of Spontaneous or Positive Pressure Ventilation

Conventional Flow-Volume Loop

Abnormal Flow-Volume Loops

  • A. Normal
  • B. Variable Extra-Thoracic Obstruction
    • Flattened Inspiration
    • Causes: Unilateral Vocal Cord Dysfunction, Mobile Tumors (Lipoma)
  • C. Variable Intra-Thoracic Obstruction
    • Flattened Expiration
    • Causes: Tracheomalacia, Bronchogenic Cysts
  • D. Fixed Upper Airway Obstruction
    • Flattened Inspiration and Expiration
    • Causes: Tracheal Stenosis, Goiter, Rigid Mass
  • E. Lower Airway Obstruction
    • “Scooped Out” Expiration
    • Causes: COPD, Asthma
  • F. Restriction
    • Normal Shape with Reduced Volume
    • Causes: Fibrosis, Sarcoidosis, Restrictive Lung Disease

Abnormal Flow-Volume Loops

Dynamic Hyperinflation (Air Trapping)

Flow-Volume Loops on a Ventilator: A. Normal; B. Obstructive Pattern with Air Trapping (Fails to Return to Zero)