The hallmark of restrictive lung disease is a reduction in lung compliance, which refers to the lung’s ability to stretch and expand. This reduction is caused by various factors depending on the underlying cause of RLD:
Intrinsic RLD: Diseases like pulmonary fibrosis result in thickening of the alveolar walls due to fibrosis or inflammation. The accumulation of extracellular matrix proteins leads to lung tissue stiffening, which reduces compliance and limits gas exchange. This impairs oxygen transfer, causing hypoxemia; because carbon dioxide diffuses far more readily than oxygen, hypercapnia appears only in advanced disease.
Extrinsic RLD: In cases of chest wall deformities or obesity, the mechanical forces necessary for lung expansion are compromised. The increased load on the respiratory muscles leads to reduced tidal volumes and inefficient ventilation. Neuromuscular disorders further exacerbate this by weakening the muscles responsible for breathing, leading to hypoventilation and progressive respiratory failure.
| Feature | Intrinsic RLD | Extrinsic RLD |
|---|---|---|
| Site of the problem | Lung parenchyma itself | Structures outside the lungs that restrict expansion |
| Examples | Interstitial lung disease, pulmonary fibrosis, pneumonitis | Chest wall deformity (kyphoscoliosis), obesity, pleural effusion, neuromuscular disorders such as myasthenia gravis |
| Mechanism | Alveolar wall thickening from fibrosis or inflammation; extracellular matrix accumulation stiffens lung tissue | Mechanical forces for lung expansion are compromised; increased load on respiratory muscles |
| Functional consequence | Reduced compliance limits gas exchange, causing hypoxemia; carbon dioxide retention is a late finding | Reduced tidal volumes and inefficient ventilation; muscle weakness leads to hypoventilation and progressive respiratory failure |
