Factors affecting Lung Compliance

• Factors causing Reduced Lung Compliance:

  • Low lung volumes (functional residual capacity is reduced)
  • Atelectasis / Post surgery
  • High lung volumes
  • Hyperinflation
  • Reduced pulmonary surfactant
  • Pulmonary oedema
  • Consolidation
  • Interstitial fibrosis
  • E.g. idiopathic pulmonary fibrosis
  • Pleural effusion
• Closing Volume

  • Functional residual capacity is amount of air left in lungs after tidal expiration
  • FRC = Expiratory reserve volume (can expire) + Reserve volume (cannot expire)
  • The closing volume is the point in functional residual capacity at which dynamic compression (collapse) of the airways begins
  • CV increases with age, smoking, lung disease and position (supine > upright)
  • Closing volume plus residual volume is called closing capacity
• Clinical Relevance of closing volume

  1. Low lung volumes and atelectasis associated with decrease in functional residual capacity ->
  2. As functional residual capacity falls closing volume is reached ->
  3. Collapse of dependent alveoli ->
  4. Inspiration moves below lower inflection point on pressure/volume curve ->
  5. So large amount of pressure must be generated to move a very small amount of air ->
  6. BIG increase in work of breathing and O2 consumption
• Pulmonary Surfactant:

  • Pulmonary surfactant is a surface-active lipoprotein complex
  • Secreted by type 2 alveolar cells
  • The main lipid component of surfactant (dipalmitoylphosphatidylcholine) reduces surface tension
  • Surfactant therefore causes an increase in pulmonary compliance
  • Prevents lung collapsing at end of expiration
• Clinical Relevance of Pulmonary Surfactant

  • Decrease in surfactant production ->
  • Increase in surface tension ->
  • Decrease in lung compliance ->
  • Atelectasis ->
  • Increase in work of breathing / O2 consumption
• Increased Lung Compliance:

  • Emphysema
  • Aging