Phenotypes of dynamic changes in impulse oscillometry during methacholine challenge

<bold>Background:</bold> Asthma demonstrates structural and functional heterogeneity. Whilst forced expiratory volume in 1s (FEV₁) is a measure of large airway calibre, impulse oscillometry (IOS) permits more detailed and sensitive passive measurements of large and small airway mechanics. <bold>Aim:</bold> To identify dynamic physiological phenotypes observed during bronchial challenge tests using IOS. <bold>Method:</bold> Symptomatic, untreated patients were recruited. Fractional exhaled nitric oxide (FeNO), skin prick test, spirometry, bronchodilator reversibility (BDR) and blood eosinophil counts were measured. IOS was performed at baseline and after final dose in a methacholine challenge. The dynamic IOS traces in those with a positive challenge (≥20% fall in FEV₁) were qualitatively classified into phenotypes by two assessors. <bold>Results:</bold> Thirty adults had a positive methacholine challenge. Five distinct phenotypes were identified (Figure). There was no difference amongst phenotypes in height, BMI, sex, FeNO, blood eosinophil counts, atopy, baseline spirometry or BDR. Type III and Type V were associated with similar fall in FEV₁ at challenge, despite marked differences observed on IOS (Figure). <bold>Conclusion:</bold> Dynamic physiological phenotypes can be identified by IOS in asthma; this may be reflective of structural and functional heterogeneity. Direct observation of ventilation defects on functional lung imaging may allow 3D characterisation of each phenotype. <fig><object-id>erj;64/suppl_68/PA3054/F1</object-id><object-id>F1</object-id><object-id>F1</object-id><graphic></graphic></fig>