Impulse oscillometry in assessing small airway function in asthma

Background Small airway dysfunction (SAD) can occur in asthma patients with normal spirometry, impacting symptoms and disease control. Small airway dysfunction is linked to more severe symptoms and worse spirometry, poor asthma control, nocturnal asthma, and exercise-induced asthma.Impulse oscillometry (IOS) assesses small airway function through a simple, noninvasive method that employs the forced oscillation technique. It requires minimal patient cooperation and is suitable for use in both children and adults. This method can assess obstructions in the large and small peripheral airways and has been used to measure bronchodilator response and bronchoprovocation testing. However, its utility in patients with SAD requires further investigation. Methods This study included 109 asthma patients with normal spirometry, who were divided into SAD (R5-R20 > 0.1 kPa·L⁻¹·s⁻¹) and normal groups (R5-R20 < 0.1 kPa·L⁻¹·s⁻¹) based on IOS results. Clinical data, IOS, and spirometry findings were compared between the two groups. Forty-three SAD patients were re-assessed after 3 months of treatment with standard inhaled corticosteroids and long-acting β₂ agonists. Findings SAD was identified in 56.9% of asthma patients with normal spirometry. The SAD group exhibited higher blood eosinophil counts (EOS), Z5%pred, R5%pred, R5-R20, and Fres (P<0.05), while FEV1%pred, MMEF%pred, MEF75%pred, MEF50%pred, MEF25%pred, and X5 were lower (P<0.05). Correlation analysis revealed that high R5-R20 correlated with higher Fres (ρ=0.812), R5%pred (ρ=0.637), and lower X5 (ρ=-0.643), MMEF%pred (ρ=-0.441), and FEV1%pred (ρ=-0.359). The AUCs for Fres, X5, MMEF%pred, and FEV1%pred were 0.940, 0.790, 0.702, and 0.684, respectively, with Fres demonstrating the highest diagnostic value for SAD (cutoff 14.385, 95% CI: 0.897-0.983, sensitivity 95.2%, specificity 83.0%). In the SAD group, significant reductions were observed in Z5%pred, R5%pred, R5-R20, Fres, and X5 after treatment (P<0.05). Interpretation Nearly 50% of asthma patients with normal spirometry exhibit small airway dysfunction, which can be detected using IOS. IOS provides higher sensitivity and specificity than spirometry for diagnosing SAD, with Fres being the most valuable parameter. Furthermore, IOS can effectively monitor asthma control.