A patient with pulmonary fibrosis has FVC 60% predicted, FEV1 65% predicted, FEV1/FVC 92%, DLCO 45% predicted. Which mechanism best explains the reduced DLCO in this condition?

• A Ventilation-perfusion mismatch with relative alveolar hypoxia reducing diffusion gradient
• B Increased pulmonary capillary blood volume diluting inspired CO per unit alveolar volume
• C Thickened alveolar-capillary membrane with reduced alveolar surface area, impairing diffusion ✓
• D Obstructed airways reducing CO delivery to alveoli despite preserved membrane integrity

Explanation

In pulmonary fibrosis, interstitial scarring simultaneously thickens the alveolar-capillary membrane (increasing diffusion distance) and destroys alveolar architecture (reducing surface area available for gas exchange). Both changes reduce the diffusing capacity for CO (DLCO = Dm × Vc), which is characteristically and severely reduced in restrictive interstitial diseases. A low DLCO with a restrictive pattern (reduced FVC, preserved/high FEV1/FVC) is the hallmark spirometric-DLCO pattern of pulmonary fibrosis.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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