D1.141 - Added value of impulse oscillometry in the early detection of small airway dysfunction in asthma

Small airway dysfunction (SAD) is a common feature of asthma across all severity levels and is associated with poorer clinical outcomes, yet remains difficult to assess in clinical practice. Conventional lung function tests, including spirometry and body plethysmography, assess small airway involvement only indirectly. This study aimed to compare impulse oscillometry (IOS) with spirometry and plethysmography in the assessment of SAD and its relationship with asthma severity.

73 participants were enrolled during outpatient visits: 20 healthy controls, 20 with mild–moderate asthma (GINA 1–4), and 33 with severe asthma (21 receiving biologic therapy and 12 biologic-naïve). Subjects with smoking-related lung disease or respiratory infections were excluded. Small airway dysfunction was defined as D5–20% ≥20% (IOS), FEF25–75 <65% (spirometry), and RV/TLC >40% (plethysmography). Spearman’s correlation and the Mann–Whitney U test were used for statistical analysis.

IOS (D5–20%) and spirometry (FEF25–75) correlated significantly with RV/TLC, with a stronger correlation for IOS. IOS detected the highest prevalence of small airway dysfunction: 20% in controls, 35% in mild–moderate asthma, and 66.7% in biologic-naïve severe asthma patients. Plethysmography showed a similar trend but lower prevalence rates. In patients receiving biologic therapy, IOS and plethysmography detected an equal proportion of dysfunction (42.9%). Spirometry did not demonstrate a consistent increase in dysfunction with asthma severity. Subjects with isolated IOS-defined dysfunction (D5–20% ≥20% with normal RV/TLC) exhibited a 15.8% lower mean FEV₁ and a 6.8% lower FEV₁/FVC ratio. Although not statistically significant (FEV₁ p=0.179; FEV₁/FVC p=0.106), these differences were clinically meaningful and consistent with early functional airway changes not detected by plethysmography.

IOS detects small airway dysfunction more consistently than spirometry and demonstrates a progressive increase in dysfunction with greater asthma severity. Unlike plethysmography, which mainly reflects later air trapping, IOS identifies early functional changes in the small airways, including isolated dysfunction not captured by conventional lung function tests. This early detection positions IOS to identify small airway involvement at a stage when targeted interventions may still prevent irreversible airway damage. Larger studies are needed to confirm these findings.