Background: Small airways, the silent zone of the lung, are less investigated in interstitial lung disease (ILD). Impulse oscillometry (IOS) is a novel and easy technique to assess small airway dysfunction (SAD) with minimal patient effort.

Aim: We aimed to investigate whether IOS provides complementary data to pulmonary function tests (PFT) in patients with pulmonary fibrosis (PF-ILD), focusing on small airways.

Methods: IOS and PFT data of consecutive patients diagnosed with PF-ILD in our tertiary ILD clinic were collected and analyzed. Correlations between IOS and PFTs were investigated.

Results: Our data of 34 patients exhibited mild ILD disease (FVC 89.8 ±20.7%, DLCOSB 66.5±23.6%). SAD values were FEF25-75% 85.6±30.1% and RV/TLC 38.0±5.5%). Oscillometry parameters of resistance (R5), reactance (X5), resonant frequency (Fres) and the area of reactance (AX) correlated strongly with disease severity evaluated by FVC (r -0.588, p<0.001; r 0.671, p<0.001; r-0.562, p=0.001; r-0.515, p=0.002, respectively). SAD IOS parameter R5-19 was abnormal in 27% of patients, correlated with FEF25-75% (r-0.370, p=0.021) with AUC 0.738 (95%Cl 0.519-0.956). R5-19 did not correlate with smoking history but correlated with FVC (r -0.481, p=0.004). R5-19 was the only SAD parameter that correlated with the Composite Physiologic Index (CPI, r 0.338, p=0.04), while FEF 25-75% and RV/TLC% did not.

Conclusion: IOS is an easy technique revealing early mechanical alterations caused by PF-ILD and captures SAD, a marker of peripheral lung pathology, that may have prognostic implications for ILD patients.