Pressure-Volume Curve

Flow Volume Loops

• Describes change in lung volume and air flow during a respiratory cycle
• Curve begins just before expiration when 
  • there is no flow through the airways
  • lung volume is at a maximum
• Curve then proceeds through an organized set of steps including
  • expiration with increasing flow and decreasing lung volume
  • peak expiratory flow where expiration is the fastest
  • end expiration with decreasing flow and decreasing lung volume
  • cessation of expiration at residual volume with no flow
  • inspiration with increasing flow and increasing lung volume
  • peak inspiratory flow where inspiration is the fastest
  • end expiration with decreasing flow and increasing lung volume
  • return to the initial point of no flow and maximal volume
• Analyzing the structure of these curves can provide important insights into mechanisms of disease
  • upper airway obstruction will present with blunting of peak expiratory and inspiratory flow
    • curves will be flatter rather than proceeding to peaks
    • total tidal volumes may remain the same
  • obstructive lung disease will present with diminished expiratory flows and increased lung volumes
    • relatively smaller changes during inspiration
  • restrictive lung disease will present with smaller lung volumes during each respiratory cycle
• curve will not reach as large volumes as normal

Compliance

• Describes distensibility of respiratory system
• Describes change in lung volume for a given change in pressure (C = V/P) 
• ↑ compliance in emphysema and aging 
• ↓ compliance in pulmonary fibrosis, pulmonary edema, ARDS, and chest wall disease

Elastance

• Describes elastic properties (inverse of compliance, elastance = P/V)
• Lungs tend to collapse inward
• Chest wall tends to expand outward

Pressure-Volume Curve

•  V = FRC (functional residual capacity) 
  • FRC = volume in lungs at end of normal tidal expiration
  • airway pressure = atmospheric pressure = no airflow
  • collapsing force from lungs = expanding force from chest wall
  • resting volume when there is no airflow at the end of tidal expiration
  • combined lung and chest wall system is at equilibrium
• V < FRC 
  • e.g., end forced expiration
  • ↓ volume in lungs → ↓ collapsing force on lungs and ↑ expanding force on chest wall
  • combined lung and chest wall system “wants” to expand
• V > FRC
  • e.g., inspiration
  • ↑ volume in lungs → ↑ collapsing force on lungs and ↓ expanding force on chest wall
  • combined lung and chest wall system “wants” to collapse