Long-term variability of impulse oscillometry defined small airway dysfunction in participants with and without chronic obstructive pulmonary disease

Background Small airway dysfunction (SAD), as assessed by impulse oscillometry (IOS) (IOS-SAD), exhibits temporal variability and its long-term fluctuations may be linked to distinct clinical phenotypes. We investigated the associations of long-term variability in IOS-SAD with adverse clinical outcomes among participants with and without chronic obstructive pulmonary disease (COPD). Methods The baseline and 2-year follow-up data from the early COPD cohort were retrospectively analysed. SAD was defined based on the following criteria: difference from R5 to R20 (R5-R20) >upper limit of normal (ULN) (R5-R20-SAD), or reactance at 5 Hz (X5) ULN. Individuals were classified into three groups based on SAD variability over three visits at 2-year follow-up: (1) consistent SAD (SAD at every visit), (2) inconsistent SAD (SAD at some, but not all, visits) and (3) never SAD (no SAD at any visit). Differences in the rate of spirometric lung function decline and exacerbation rate were compared across long-term IOS-SAD variability groups in participants with and without COPD. Results In individuals with COPD, the consistent and inconsistent SAD groups assessed were associated with a faster spirometric lung function decline and a higher risk of exacerbations. In individuals without COPD, the consistent R5-R20-SAD group was associated with a faster decline in spirometric lung function and progression towards COPD. Conclusions SAD, even diagnosed once, characterises an airway behaviour with a higher proportion of higher risk of moderate or severe exacerbations. Consistent SAD was associated with accelerated spirometric lung function decline, increased exacerbation risk and progression to spirometry-defined COPD. Trial registration number ChiCTR1900024643.