PFT: Obstructive Vs Restrictive

Obstructive lung diseases (asthma, COPD, bronchiectasis) limit airflow during exhalation due to airway narrowing from inflammation, mucus, smooth muscle constriction, or loss of elastic recoil (emphysema). Air trapping occurs because narrowed airways collapse during forced expiration, increasing residual volume (RV) and functional residual capacity (FRC). Restrictive lung diseases limit lung expansion during inhalation due to intrinsic causes (pulmonary fibrosis, sarcoidosis — reduced lung compliance) or extrinsic causes (chest wall deformity, obesity, neuromuscular weakness, pleural disease — mechanical limitation). The key spirometric distinction is FEV1/FVC ratio: obstructive diseases lower this ratio because FEV1 decreases disproportionately to FVC, while restrictive diseases maintain or increase the ratio because both FEV1 and FVC decrease proportionally (with FVC often decreasing more).