Prevalence and features of IOS-defined small airway disease across asthma severities

Background: Impulse oscillometry (IOS) is a noninvasive method based on the forced oscillation technique able to detect small airway dysfunction (SAD) in asthma. We aimed to analyze the prevalence and the functional features of IOS-defined SAD across the different Global Initiative for Asthma (GINA) steps. Methods: A cross-sectional, single-center study in which 400 consecutive adult patients with physician-diagnosed, community-managed asthma underwent standard spirometry and IOS, and were stratified by stepwise GINA classification. SAD was defined by IOS as a fall in resistance from 5 to 20 Hz [R5–R20]>0.07kPa × s × L−1. Results: The prevalence of IOS-defined SAD ranged between 58.3% (GINA step 2) and 78.6% (GINA step 5), without statistically significant difference within GINA steps (p > 0.05 in all comparisons). Isolated SAD (i.e. without proximal airways involvement) was similarly represented across GINA steps 2–4. Peripheral airways resistance (R5-R20) tended to a progressive increase with the worsening of GINA steps, and was significantly higher in steps 4–5 compared to the other steps (p < 0.05). The proportion of patients with FEF25–75%-defined SAD (<60%) was lower than the IOS-defined one in GINA steps 2–4 (p < 0.05). Only non-significant or weak inverse correlations between R5-R20 and FEF25–75% were observed within each GINA step, with the exception of GINA step 5, which showed a strong, inverse correlation (r = −0.80, p = 0.0005). Conclusions: This study shows that first, IOS-defined SAD is overwhelmingly present across asthma severities; second, airways resistance increases with the worsening of GINA steps; and third, SAD may be overlooked by standard spirometry, especially in milder asthma.