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Systemic sclerosis-associated PAH — SCE Respiratory MCQ

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HardPulmonary Vascular DiseaseSystemic sclerosis-associated PAHSCE Respiratory

A symptomatic smoker has FEV1 2.10 L, forced vital capacity 3.00 L and relaxed vital capacity 3.60 L. The FEV1/FVC ratio is above its lower limit of normal, but FEV1/relaxed vital capacity is below it. What best explains this discordance?

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Correct answer: A — Forced expiratory compression lowered FVC, so relaxed vital capacity unmasks obstruction

In obstructive disease, the high intrathoracic pressure of forced expiration can cause dynamic airway narrowing, air trapping and a lower FVC than the relaxed vital capacity. Relating FEV1 to the largest valid vital capacity can therefore reveal obstruction that FEV1/FVC alone misses. Lung volumes are still needed if restriction is suspected; the discrepancy by itself does not diagnose neuromuscular disease.

Reference: ARTP statement on pulmonary function testing (Published 2020): https://bmjopenrespres.bmj.com/content/7/1/e000575; Study of slow vital capacity in detecting airway obstruction (Published 2014): https://pmc.ncbi.nlm.nih.gov/articles/PMC4075849/