A 52-year-old woman with systemic sclerosis presents with progressive dyspnea on exertion for 3 months. Chest X-ray is normal. Spirometry shows FEV₁ 88% predicted, FVC 90% predicted, FEV₁/FVC ratio 0.82. TLC is 95% predicted. However, DLCO is 55% predicted with DLCO/VA (KCO) also reduced. The spirometry and lung volumes pattern shown in the diagram corresponds to the finding marked **A** — isolated DLCO reduction with normal spirometry. Which of the following is the most likely mechanism underlying this pattern in this patient?

Explanation

## Why "Reduced pulmonary capillary blood volume due to pulmonary vascular disease (PAH)" is right The pattern marked **A** — isolated DLCO reduction with normal spirometry and normal lung volumes — is the hallmark of pulmonary vascular disease, particularly PAH. According to the Roughton-Forster equation (1/DLCO = 1/DM + 1/[θ × Vc]), DLCO is directly proportional to pulmonary capillary blood volume (Vc). In PAH and other pulmonary vascular diseases, Vc is reduced due to loss of perfusable capillaries, resulting in reduced DLCO while alveolar-capillary membrane surface area and hemoglobin remain intact. The reduced DLCO/VA (KCO) confirms a true gas-exchange defect rather than apparent reduction from small lungs. This patient's scleroderma is a well-recognized risk factor for PAH, and the clinical presentation (unexplained dyspnea, normal spirometry, isolated DLCO reduction) mandates echocardiography and right heart catheterization to confirm pulmonary hypertension (Murray and Nadel's 7e Ch 25, Ch 58; Harrison's 21e Ch 283). ## Why each distractor is wrong - **Loss of alveolar surface area due to emphysematous destruction**: Emphysema causes DLCO reduction with obstruction (reduced FEV₁/FVC), not normal spirometry. This patient's FEV₁/FVC is preserved at 0.82, ruling out obstructive disease. - **Thickened alveolar-capillary membrane due to parenchymal fibrosis**: Parenchymal ILD (including scleroderma-associated pulmonary fibrosis) typically reduces DLCO WITH restriction (reduced TLC and/or FVC). This patient's TLC is 95% predicted and FVC is 90% predicted — normal lung volumes exclude significant parenchymal disease as the primary mechanism. - **Reduced hemoglobin concentration causing apparent diffusion impairment**: Anemia causes apparent DLCO reduction but DLCO/VA (KCO) is preserved or elevated because the alveolar volume sampled is normal. This patient has reduced DLCO/VA, indicating a true gas-exchange defect, not anemia. **High-Yield:** Isolated DLCO reduction + normal spirometry + normal volumes = pulmonary vascular disease (PAH, CTEPH, PE, hepatopulmonary syndrome) until proven otherwise; next step is echocardiography. [cite:Murray and Nadel's Textbook of Respiratory Medicine 7e Ch 25, Ch 58; Harrison's Principles of Internal Medicine 21e Ch 283]