Background.  Measurement of resistance (Rrs) and reactance (Xrs) of the respiratory system by impulse oscillometry (IOS) provides information on respiratory system mechanics with minimal patient participation. It is unclear how IOS measurements vary over time in IPF, and how they correlate with spirometry.

Methods. IPF patients (n=52) were included in a multicenter prospective cohort study. Spirometry and IOS were performed at baseline, 3 months and 6 months (n=44). Respiratory system compliance (C) was calculated from IOS data using the eRIC model. Patients with FVC decline ?5% predicted at 6 months were classified as fast decliners. 

Results. At baseline, Rrs5 Z-score (median [IQR]) was higher than reference values (0.77 [0.29;1.71]) and Xrs5 Z-score was lower (-0.62 [-1.82;0.31]). Weak correlations (Spearman?s Rho <0.5) were observed between Rrs, Xrs, and C and either FVC, DLco, Gay?s score or dyspnea. Both Rrs5 and Rrs20 decreased from baseline (0.40 kPa.s.L-1 [0.30;0.45] and 0.28 kPa.s.L-1 [0.23;0.32]) to 3 months (0.35 kPa.s.L-1[0.36;0.44], p=0.043 and 0.26 kPa.L-1 [0.22;0.32], p=0.049) and 6 months (0.35 kPa.s.L-1[0.37;0.42], p=0.051 and 0.25 kPa.L-1 [0.22;0.33], p=0.043). There was no change in Xrs and compliance. The 6-month variation in FVC was +100 mL [-20;+200]. Fourteen patients were fast decliners. Neither baseline IOS measurements nor their 3-month variation discriminated fast decliners from slow decliners.

Conclusion. In 44 IPF patients with mostly stable disease, Rrs5 and Rrs20 decreased over 6 months, consistent with progressive increase of airway caliber in IPF. Oscillometry may be more sensitive than spirometry to prospectively detect changes in lung function in IPF.