Updated: 1/24/2020

Pressure-Volume Curve

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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 
  • ↓ 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

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