Introduction and background: The forced oscillation technique (FOT) is a non-invasive method where oscillations are surimposed to spontaneous tidal breathing in order to assess respiratory resistance (Rrs) and reactance (Xrs), representing respiratory mechanics. 

Aim and objectives: To compare respiratory mechanics between patients with progressive pulmonary fibrosis (PPF), idiopathic pulmonary fibrosis (IPF) and stable ILDs.

Methods: This was a retrospective monocentric study (CHU Liège). Pulmonary function tests (PFTs) and FOT were performed in 160 patients with ILDs. Data were compared between patients with PPF, IPF and stable ILDs (respectively 45, 21 and 94 patients). Longitudinal follow-up of 40 patients was performed to determine the correlation between changes in FOT and PFTs values over time.

Results: In comparison to stable patients, PPFs showed significantly lower median Xrs at 5Hz during inspiratory phase (z-score at 0,31 cmH2O/(L/sec) versus -0,39 cmH2O/(L/sec), p=0,043), lower FEV1 (86% versus 80% expressed as predicted values, p=0,014), FVC (89% versus 75%, p=0,018), and DLCO (70% versus 51%, p<0,0001). DLCO values were the only significant difference between IPF and stable patients (47% versus 51%, p=0,003).

40 patients (25%) underwent longitudinal follow-up (with a median time of 272,5 days). Expiratory Xrs and whole-breath Xrs at 5Hz changes over time showed correlation with FEV1, CVF and DLCO changes. Inspiratory Rrs and Xrs changes at 5Hz also showed correlation with FEV1 and CVF but not with DLCO.

Conclusions: Oscillometry is a useful technique, in conjunction with other clinical and laboratory data, to support diagnosis and follow-up of patients with PPF.