A 58-year-old man with a 40 pack-year smoking history presents with progressive dyspnea on exertion over 6 months. He denies chest pain or orthopnea. On examination, he has diminished breath sounds bilaterally and prolonged expiration. Spirometry shows FEV₁ = 52% predicted, FVC = 78% predicted, FEV₁/FVC ratio = 0.58. DLCO is 65% predicted. What is the most likely diagnosis?

Explanation

## Spirometric Pattern Analysis **Key Point:** The FEV₁/FVC ratio <0.70 (here 0.58) is the hallmark of obstructive airway disease. FVC is relatively preserved at 78% predicted, confirming obstruction rather than restriction. ### Pattern Interpretation | Parameter | Value | Interpretation | | --- | --- | --- | | FEV₁ | 52% predicted | Moderately reduced | | FVC | 78% predicted | Relatively preserved | | FEV₁/FVC | 0.58 | **Obstructive pattern** | | DLCO | 65% predicted | Reduced (emphysematous component) | **High-Yield:** In obstructive disease, FEV₁ falls disproportionately because small airways collapse during forced expiration. FVC remains near-normal because total lung capacity is not reduced (may even be increased due to air trapping). **Clinical Pearl:** The reduced DLCO (65%) in this smoker with obstructive pattern strongly suggests emphysema as the underlying pathology — a key COPD phenotype. Chronic bronchitis alone would preserve DLCO. ### Why This Is COPD, Not Asthma - **Smoking history** (40 pack-years) — major COPD risk factor - **Insidious onset** over months — asthma typically presents acutely or with episodic symptoms - **Reduced DLCO** — asthma does not cause alveolar destruction; DLCO is normal - **Prolonged expiration on exam** — classic sign of small airway obstruction in COPD **Mnemonic: OBSTRUCTIVE SPIROMETRY** = FEV₁/FVC <0.70 with relatively preserved FVC [cite:Harrison 21e Ch 304]