Why does emphysema have increased lung compliance and fibrosis have decreased lung compliance?

Lung Compliance Differences in Emphysema and Pulmonary Fibrosis

Emphysema is characterized by increased lung compliance due to destruction of elastic tissue and loss of elastic recoil, while pulmonary fibrosis has decreased compliance due to stiffening of lung tissue from excessive collagen deposition and fibrotic changes.

Pathophysiological Basis for Compliance Changes

Emphysema: Increased Compliance

• Structural changes: Emphysema involves permanent destructive enlargement of air spaces, destruction of alveolar walls, and loss of pulmonary elastic recoil 1
• Elastic recoil loss: The reduced elastic recoil directly results in increased lung compliance, which allows for hyperinflation with increases in residual volume (RV) and total lung capacity (TLC) 2
• Mechanical consequences:
  • Hyperinflation leads to flattened diaphragm, creating mechanical disadvantage for respiratory muscles 1
  • Air trapping occurs due to collapse of small airways during expiration 2
  • Increased work of breathing despite the higher compliance due to mechanical inefficiency 2

Pulmonary Fibrosis: Decreased Compliance

• Structural changes: Fibrosis involves excessive collagen deposition, thickening of alveolar walls, and stiffening of lung tissue 3
• Mechanical properties: The stiffened lung tissue requires higher pressures to achieve the same volume change, defining decreased compliance 2
• Functional impact:
  • Reduced lung volumes, particularly TLC and vital capacity
  • Increased work of breathing due to the need for greater pressure generation to inflate stiff lungs
  • Restrictive ventilatory defect pattern on pulmonary function testing

Functional Consequences of Altered Compliance

Emphysema

• Breathing mechanics: Despite increased compliance, patients experience:
  • Dynamic hyperinflation during exercise due to air trapping 2
  • Increased work of breathing from inefficient diaphragm function 1
  • Reduced expiratory flow rates due to airway collapse 2
• Pulmonary function tests:
  • Increased TLC, RV, and FRC
  • Reduced FEV1 and FEV1/FVC ratio
  • Markedly decreased diffusing capacity (DLCO) 2

Pulmonary Fibrosis

• Breathing mechanics:
  • Higher inflation pressures needed for normal tidal volumes 2
  • Static inflation pressure for typical tidal volumes may exceed 25 cmH2O, implying lung compliance approaching 20 ml/cmH2O (less than one-fourth of normal) 2
  • Rapid, shallow breathing pattern to minimize work of breathing
• Pulmonary function tests:
  • Decreased TLC, vital capacity
  • Preserved or increased FEV1/FVC ratio
  • Reduced DLCO

Combined Pulmonary Fibrosis and Emphysema (CPFE)

Interestingly, when both conditions coexist (CPFE syndrome), the opposing effects on lung compliance can result in:

• Relatively preserved lung volumes and spirometric values despite severe disease 3
• Severely reduced diffusing capacity (DLCO) due to additive effects on gas exchange 3
• Lung elastance and compliance intermediate between those of patients with emphysema or fibrosis alone 3

Clinical Implications

• Emphysema: The increased compliance contributes to:

  • Dynamic hyperinflation during exercise
  • Air trapping
  • Increased work of breathing despite higher compliance
  • Flattened diaphragm with mechanical disadvantage 1

• Pulmonary fibrosis: The decreased compliance leads to:

  • Restrictive ventilatory pattern
  • Increased work of breathing
  • Rapid, shallow breathing pattern
  • Decreased exercise tolerance

Diagnostic Assessment

• Pulmonary function testing reveals the characteristic patterns:

  • Emphysema: Obstructive pattern with increased TLC and RV
  • Fibrosis: Restrictive pattern with decreased TLC and vital capacity
  • Both: Reduced DLCO 2

• Imaging findings reflect the compliance differences:

  • Emphysema: Hyperinflation with flattened diaphragm on chest radiography 1
  • Fibrosis: Reduced lung volumes with elevated diaphragm position

The understanding of these opposing compliance changes is fundamental to comprehending the pathophysiology, clinical presentation, and management approaches for these distinct pulmonary conditions.