Diagnosis and evaluation of small airway disease and COPD using impulse oscillometry.

Impulse oscillometry (IOS) is a sensitive tool for assessing small airway function in patients with chronic obstructive pulmonary disease (COPD). This study aimed to differentiate between COPD, small airway disease (SAD), and normal groups using IOS, and to evaluate the clinical applicability of IOS. This retrospective cohort study was conducted from January 2020 to February 2022. The eligible population comprised adult patients who simultaneously underwent IOS and pulmonary function tests. The diagnostic value of IOS in differentiating SAD and/or COPD from control was analyzed, and the correlations among IOS parameters, lung function, and radiological assessment results were determined. Among the enrolled 306 patients, 38 (12.4%) had SAD and 134 (43.8%) had COPD. The remainder comprised the control group without COPD and/or SAD. Abnormal airway resistance according to IOS parameters was detected in 17.2% of the patients in the control group, 47.2% of those in the SAD group, and 55.2% of those in the COPD group. Airway resistance estimated by IOS were significantly higher in the SAD and COPD groups than in the control group and correlated with lung function and radiological airway wall thickness. We developed a composite index called the IOS severity index (IOSsi) using IOS parameters that can predict SAD and COPD, and IOSsi showed significantly differentiation of SAD and/or COPD from control. Especially, IOSsi value ≥ 4 was associated with an increased risk of SAD and/or COPD and also with risk for moderate-to-severe exacerbation in patients with COPD. IOS may be a useful tool to differentiate disease status and evaluate disease severity and prognosis in patients with SAD and/or COPD, and a prognostic factor of COPD.