Which parameter is most sensitive to small airways obstruction and may be abnormal when FEV1 is normal?

Which parameter is most sensitive to small airways obstruction and may be abnormal when FEV1 is normal?

• A. PEFR
• B. DLCO
• C. FEF25-75%
• D. MIP

Answer: (C)

Small airways disease first reduces the flow during the middle portion of a forced exhale. The flow in this middle phase depends more on the small airways, so when obstruction starts there, the mid‑expiratory flow (FEF25-75%) drops while the overall FEV1 can still be normal. That makes FEF25-75% a sensitive early indicator of small airways obstruction and explains why it may be abnormal even when FEV1 is within normal limits. Peak expiratory flow rate is highly effort‑dependent and reflects flow in the larger airways, so it may stay normal despite small airways involvement. DLCO measures gas transfer across the alveolar–capillary membrane, not airway caliber, so it isn’t a good marker of small airway obstruction. Max inspiratory pressure assesses inspiratory muscle strength, not airway patency.

Small airways disease first reduces the flow during the middle portion of a forced exhale. The flow in this middle phase depends more on the small airways, so when obstruction starts there, the mid‑expiratory flow (FEF25-75%) drops while the overall FEV1 can still be normal. That makes FEF25-75% a sensitive early indicator of small airways obstruction and explains why it may be abnormal even when FEV1 is within normal limits.

Peak expiratory flow rate is highly effort‑dependent and reflects flow in the larger airways, so it may stay normal despite small airways involvement. DLCO measures gas transfer across the alveolar–capillary membrane, not airway caliber, so it isn’t a good marker of small airway obstruction. Max inspiratory pressure assesses inspiratory muscle strength, not airway patency.