Significances of spirometry and impulse oscillometry for detecting small airway disorders assessed with endobronchial optical coherence tomography in COPD.

Background: Spirometry confers limited value for identifying small-airway disorders (SADs) in early-stage COPD, which can be detected with impulse oscillometry (IOS) and endobronchial optical coherence tomography (EB-OCT). Whether IOS is useful for reflecting small-airway morphological abnormalities in COPD remains unclear.

Objectives: To compare the diagnostic value of spirometry and IOS for identifying SADs in heavy-smokers and COPD based on the objective assessment with EB-OCT.

Methods: We recruited 59 COPD patients (stage I, n=17; stage II, n=18; stage III-IV, n=24), 26 heavy-smokers and 21 never-smokers. Assessments of clinical characteristics, spirometry, IOS and EB-OCT were performed. Receiver operation characteristic curve was employed to demonstrate the diagnostic value of IOS and spirometric parameters.

Results: More advanced staging of COPD was associated with greater abnormality of IOS and spirometric parameters. Resonant frequency (Fres) and peripheral airway resistance (R5 -R20 ) conferred greater diagnostic values than forced expiratory volume in one second (FEV1 %) and maximal (mid-)expiratory flow (MMEF%) predicted in discriminating SADs in never-smokers from heavy-smokers (area under curve [AUC]: 0.771 and 0.753 vs 0.570 and 0.558, respectively), and heavy-smokers from patients with stage I COPD (AUC: 0.726 and 0.633 vs 0.548 and 0.567, respectively). The combination of IOS (Fres and R5 -R20 ) and spirometric parameters (FEV1 % and MMEF% predicted) contributed to a further increase in the diagnostic value for identifying SADs in early-stage COPD. Small airway wall area percentage (Aw% 7-9), an EB-OCT parameter, correlated significantly with Fres and R5 -R20 in COPD and heavy-smokers, whereas EB-OCT parameters correlated with FEV1 % and MMEF% in advanced, rather than early-stage, COPD.

Conclusions: IOS parameters correlated with the degree of morphologic abnormalities of small airways assessed with EB-OCT in COPD and heavy-smokers. Fres and R5 -R20 might be sensitive parameters that reliably reflect SADs in heavy-smokers and early-stage COPD.