A 71-year-old former smoker (60 pack-years) presents with severe dyspnea at rest, pursed-lip breathing, barrel chest, and pedal edema. Examination reveals an elevated JVP, left parasternal heave, and a loud P2. Chest CT shows centrilobular emphysema with upper-lobe predominance and flattened hemidiaphragms. Spirometry reveals the pattern marked **A** in the diagram: FEV1/FVC ratio 0.34 (post-bronchodilator), FEV1 28% predicted, RV/TLC 0.62, and DLCO 38% predicted. ABG shows pH 7.36, PCO2 56, PO2 54 mm Hg. Which of the following best explains why the DLCO is reduced to 38% predicted in this patient's spirometric pattern marked **A**?
A. Interstitial fibrosis with thickened basement membrane reducing gas diffusion capacity
B. Reversible small-airway inflammation and mucus plugging responsive to bronchodilators
C. Pulmonary edema from acute left ventricular failure causing alveolar fluid accumulation
D. Emphysematous destruction of the alveolar-capillary membrane with obliteration of gas-exchange surface area
Explanation
Why "Emphysematous destruction of the alveolar-capillary membrane with obliteration of gas-exchange surface area" is right
The reduced DLCO (38% predicted) in this patient with the obstructive spirometric pattern A is the pathophysiologic hallmark of emphysema-dominant COPD. Emphysematous destruction permanently obliterates the alveolar-capillary membrane and reduces the surface area available for gas exchange. This is a fixed, irreversible process that distinguishes emphysema-predominant COPD (low DLCO) from pure chronic bronchitis (preserved DLCO), as emphasized in Harrison 21e Ch 286 and GOLD 2024 guidelines. The centrilobular emphysema with upper-lobe predominance on CT and the scooped-out expiratory limb on the flow-volume loop confirm the emphysematous component. The elevated RV/TLC (0.62) reflects gas trapping from small-airway collapse, but the low DLCO specifically reflects alveolar destruction.
Why each distractor is wrong
Reversible small-airway inflammation and mucus plugging responsive to bronchodilators: While small-airway disease contributes to airflow obstruction in COPD, it does not explain the reduced DLCO. The post-bronchodilator FEV1 improvement was <12% and <200 mL, confirming FIXED obstruction. Reversible inflammation would not cause permanent loss of gas-exchange surface.
Pulmonary edema from acute left ventricular failure causing alveolar fluid accumulation: Acute pulmonary edema would cause an acute restrictive pattern with elevated DLCO (due to blood in alveoli), not a chronic obstructive pattern with low DLCO. The ABG shows chronic compensated respiratory acidosis, not acute decompensation.
Interstitial fibrosis with thickened basement membrane reducing gas diffusion capacity: While interstitial fibrosis does reduce DLCO, it produces a restrictive spirometric pattern (elevated FEV1/FVC, reduced TLC), not the obstructive pattern A (FEV1/FVC 0.34, elevated RV/TLC). This patient's pattern is obstructive, not restrictive.
High-YieldNEET PG
In COPD, low DLCO (<60% predicted) indicates emphysema-predominant disease with alveolar destruction; preserved DLCO suggests chronic bronchitis or small-airway disease without emphysema.
Harrison 21e Ch 286; GOLD 2024 Report; ATS/ERS COPD Guidelines
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