The intra-breath changes of reactance at low frequency in impulse oscillometry: differentiating obstructive airway disease and Interstitial Lung Disease.

Background: Impulse Oscillometry (IOS) helps understanding the mechanical properties of the respiratory system. Evaluating its role in distinguishing Obstructive Airway Disease (OAD) from Restrictive Lung Disease (RLD) remains underexplored.

Methods: We included the consecutive patients of both OAD (asthma and COPD), ILD (representing RLD) observing guideline recommendations and a cohort of 'healthy' subjects (asymptomatic, no disease clinico-radiologically and on spirometry). They underwent spirometry and impulse oscillometry concurrently on the same sitting. The comparison between the three groups included IOS derived R5, R10, R15, R20, R5-10, R5-15, R5-20, R10-20, X5, Rin5 (R5 inspiration), Rex5 (R5 expiration), Xin5 (X5 inspiration) and Xex5 (X5 expiration), Fres and AX. We proposed Δintrabreath-X5 (Xin5-Xex5) and a 'reactance variance index' (RVI) [(Xin5-Xex5)/Xex5 × 100] in the effort. Those showing highly significant difference (p ≤ 0.0001) were analysed with the ROC curves to determine the best differentiating values.

Results: We included 104 OAD (COPD: 53, Asthma: 51), 75 RLDs and 43 healthy controls. Multiple resistance (R5, R5-10, R5-15, R5-20, and Rex5 Xex5) and reactance related (X5, Fres and AX) parameters displayed significant difference (p < 0.0001). The Xex5 and R5 turned out to be the best discriminators, with areas under the curve of 0.9244 and 0.9292, showing sensitivities of 88.57% and 85.85%, and specificities of 95.35% and 100%, respectively, using cut-off values of -2.49 and 5.04 in differentiating OAD from healthy subjects. Of the resistance factors (R5, R10, R15, R20, R5-15, R5-20, R-in-5) significantly differed between ILD and healthy cohorts; the R5 had the highest discriminating power (sensitivity of 64% and specificity of 100% to differentiate them. The Δintrabreath-X5 could best distinguish ILD from OAD with 88.06% sensitivity and 80.95% specificity, closely followed by Xin5/Xex5 and the RVI.

Conclusion: IOS can acceptably distinguish OAD from ILD using Δintrabreath-X5. The observation demands further validation.