In a patient with severe emphysema, spirometry shows FEV1/FVC = 0.45, FVC = 80% predicted, TLC = 145% predicted, DLCO = 35% predicted. The markedly reduced DLCO (diffusing capacity) in emphysema is due to:
- A Increased airway resistance preventing CO from reaching alveoli
- B Loss of alveolar surface area from septal destruction, reducing the area available for gas diffusion ✓
- C Increased alveolar membrane thickness from fibrosis
- D Elevated pulmonary artery pressure reducing capillary transit time
Explanation
DLCO (diffusing capacity for CO) measures the transfer of CO from alveolar gas to haemoglobin, reflecting alveolar surface area, membrane thickness, and pulmonary capillary blood volume. In emphysema, alveolar septal destruction reduces the alveolar surface area available for gas exchange, markedly reducing DLCO — characteristically to <40% predicted in advanced disease. This is in contrast to asthma (normal/elevated DLCO) and pulmonary fibrosis (low DLCO due to membrane thickening). The combination of obstruction (low FEV1/FVC) with low DLCO strongly suggests emphysema.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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