A 38-year-old woman with progressive exertional dyspnea (NYHA class III), near-syncope on exertion, and a family history of pulmonary arterial hypertension is evaluated with pulmonary function testing. Spirometry shows FEV1 94% predicted, FVC 96% predicted, and FEV1/FVC 0.82 (normal). Lung volumes are normal (TLC 98%, RV 98% predicted). However, DLCO is markedly reduced at 42% predicted with reduced KCO (56% predicted). Right heart catheterization confirms mean PA pressure 52 mmHg, PAWP 9 mmHg, and PVR 9.2 Wood units, consistent with Group 1 pulmonary arterial hypertension. The pulmonary function pattern marked **A** in the diagram—normal FEV1/FVC with isolated DLCO reduction—is characteristic of which underlying pathophysiology in this patient?
A. Restrictive lung parenchymal fibrosis with proportional reduction in all lung volumes
B. Intrinsic vascular or alveolar-capillary membrane disease with preserved airway mechanics and lung volumes
C. Obstructive airway disease with emphysematous destruction of the capillary bed
D. Mixed obstructive-restrictive pattern with concurrent airway obstruction and parenchymal loss
Explanation
Why "Intrinsic vascular or alveolar-capillary membrane disease with preserved airway mechanics and lung volumes" is right
The pattern marked A—normal FEV1/FVC with isolated DLCO reduction—is the hallmark of pulmonary vascular disease, particularly Group 1 pulmonary arterial hypertension. The preserved spirometry (normal FEV1/FVC ratio) and normal lung volumes indicate intact airway mechanics and alveolar volume. The markedly reduced DLCO (42% predicted) with reduced KCO (56% predicted) indicates an intrinsic problem with the alveolar-capillary membrane or pulmonary vasculature rather than loss of alveolar surface area. In this patient with heritable PAH (BMPR2 mutation), the pathophysiology is pulmonary vascular remodeling and endothelial dysfunction, not airway obstruction or parenchymal fibrosis. The 2015 ESC/ERS Guidelines recognize this pattern as characteristic of Group 1 PAH, where DLCO reduction reflects impaired gas exchange across a structurally abnormal pulmonary vascular bed despite normal lung mechanics.
Why each distractor is wrong
Obstructive airway disease with emphysematous destruction of the capillary bed: Obstructive disease would show reduced FEV1/FVC ratio (below LLN), which this patient does not have. The flow-volume loop is normal with preserved peak flows and no scooping or coving. Emphysema would also show hyperinflation (elevated TLC and RV), which are normal here.
Restrictive lung parenchymal fibrosis with proportional reduction in all lung volumes: Restrictive disease would show reduced TLC and proportional reduction in FVC and DLCO. This patient has normal lung volumes (TLC 98%, RV 98% predicted), excluding true restrictive parenchymal disease. The normal FVC argues against fibrosis.
Mixed obstructive-restrictive pattern with concurrent airway obstruction and parenchymal loss: Mixed disease would show reduced FEV1/FVC ratio (obstructive component) and reduced TLC (restrictive component). This patient has normal FEV1/FVC and normal TLC, making this pattern incompatible with the findings.
High-YieldNEET PG
In pulmonary arterial hypertension, isolated DLCO reduction with normal spirometry and lung volumes is pathognomonic for vascular/capillary membrane disease, not airway or parenchymal disease—this pattern should trigger right heart catheterization and exclusion of thromboembolic disease.
Galie N, Humbert M, Vachiery JL, et al. 2015 ESC/ERS Guidelines for the diagnosis and treatment of pulmonary hypertension. Eur Respir J. 2015;46(4):903-975.
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