COPD (chronic obstructive pulmonary disease)
Also known as: chronic obstructive pulmonary disease
COPD is a progressive obstructive lung disease characterized by persistent airflow limitation that is not fully reversible. It encompasses chronic bronchitis and emphysema, and is most often caused by long-term cigarette smoking.
COPD is defined physiologically by airflow obstruction: a reduced FEV1/FVC ratio, typically below 0.70, that does not normalize with bronchodilators. Lung volumes move in the opposite direction from restrictive disease — total lung capacity, functional residual capacity, and residual volume all increase because air is trapped distal to collapsing airways. The DLCO is reduced in emphysema, where alveolar surface area is destroyed, but relatively preserved in chronic bronchitis.
The two classic phenotypes differ in mechanism. Chronic bronchitis is defined clinically as a productive cough for at least 3 months in each of 2 consecutive years; the pathology is mucus gland hyperplasia with an increased Reid index, and the physiology is hypoxemia with hypercapnia from V/Q mismatch. Emphysema is defined pathologically as permanent enlargement of airspaces distal to the terminal bronchiole with alveolar wall destruction; loss of elastic recoil causes expiratory collapse and air trapping. Centriacinar emphysema affecting upper lobes is the smoking-related pattern; panacinar emphysema affecting lower lobes points to alpha-1 antitrypsin deficiency, particularly in a younger patient or one with concurrent liver disease.
Cigarette smoke drives disease through protease–antiprotease imbalance and oxidative injury: recruited neutrophils and macrophages release elastase, which degrades alveolar walls when antiprotease defense is overwhelmed or genetically deficient. Complications include pulmonary hypertension and cor pulmonale, secondary polycythemia from chronic hypoxemia, spontaneous pneumothorax from ruptured bullae, and acute exacerbations often precipitated by infection.
Historical exam shorthand contrasts the "blue bloater" of chronic bronchitis — cyanotic, obese, hypercapnic — with the "pink puffer" of emphysema — thin, barrel-chested, pursed-lip breathing, relatively preserved oxygenation. Most real patients have features of both.
USMLE Step 1 tests COPD in respiratory pathology, and questions usually hinge on interpreting pulmonary function tests, distinguishing centriacinar from panacinar distribution, recognizing alpha-1 antitrypsin deficiency, or separating obstructive from restrictive patterns.
Key takeaways
- COPD is defined by a reduced FEV1/FVC ratio that does not fully reverse with bronchodilators, with increased TLC, FRC, and residual volume.
- Chronic bronchitis is a clinical diagnosis (productive cough 3 months a year for 2 years) with an elevated Reid index; emphysema is a pathologic one.
- Centriacinar upper-lobe emphysema is smoking-related; panacinar lower-lobe emphysema suggests alpha-1 antitrypsin deficiency.
- DLCO falls in emphysema because alveolar surface area is destroyed, but is relatively preserved in chronic bronchitis.
- Complications include cor pulmonale, secondary polycythemia, pneumothorax from bullae, and infection-triggered exacerbations.