Clinical meaning
Obstructive and restrictive lung diseases represent two fundamentally different mechanisms of pulmonary dysfunction distinguished by pulmonary function testing (PFT) patterns. Obstructive lung diseases — including COPD (chronic bronchitis and emphysema), asthma, and bronchiectasis — are characterized by increased airway resistance that limits expiratory airflow. In chronic bronchitis, chronic inflammation of the bronchial mucosa causes goblet cell hyperplasia, mucus hypersecretion, and submucosal gland hypertrophy, narrowing the airway lumen. In emphysema, proteolytic destruction of alveolar walls (primarily by neutrophil elastase and matrix metalloproteinases unopposed by alpha-1 antitrypsin) destroys the elastic recoil that normally tethers small airways open during expiration, leading to dynamic airway collapse, air trapping, and hyperinflation. The loss of alveolar surface area also reduces gas exchange capacity, reflected in decreased diffusing capacity (DLCO). In asthma, reversible bronchospasm, airway inflammation with eosinophilic infiltration, and mucus plugging cause episodic airflow obstruction; airway remodeling with subepithelial fibrosis can develop with chronic uncontrolled disease. The hallmark PFT finding in obstructive disease is a reduced FEV1/FVC ratio below 0.70, with increased residual volume (RV) and total lung capacity (TLC) from air trapping. Restrictive lung diseases limit lung expansion and are characterized by reduced total lung capacity (TLC below 80% predicted) with a preserved or increased FEV1/FVC ratio. Intrinsic (parenchymal) restriction includes idiopathic pulmonary fibrosis (IPF), where repetitive alveolar epithelial injury triggers aberrant fibroblast activation, excessive collagen deposition, and honeycombing that progressively stiffens the lung parenchyma, reducing compliance and impairing gas diffusion (decreased DLCO). Other parenchymal causes include sarcoidosis, hypersensitivity pneumonitis, pneumoconioses (asbestosis, silicosis), and drug-induced fibrosis (bleomycin, amiodarone, nitrofurantoin). Extrinsic (extrapulmonary) restriction results from chest wall deformities (kyphoscoliosis, obesity), neuromuscular weakness (ALS, myasthenia gravis, diaphragmatic paralysis), or pleural disease (effusion, fibrosis) that mechanically limit thoracic expansion. In extrinsic restriction, DLCO is typically normal because the lung parenchyma is intact. Mixed obstructive-restrictive patterns occur when both mechanisms coexist, as in a COPD patient with concurrent obesity or pulmonary fibrosis; full lung volume measurements (body plethysmography) are required to diagnose the restrictive component when obstruction is present. The flow-volume loop provides additional diagnostic information: obstructive disease shows a scooped or concave expiratory limb, while restrictive disease shows a narrow but normally shaped loop with reduced volumes. Understanding these distinct pathophysiological mechanisms is essential for accurate diagnosis, appropriate pharmacotherapy selection, and prognostication.