Standard pulmonary function tests and respiratory oscillometry patterns in hypersensitivity pneumonitis and idiopathic pulmonary fibrosis

Background Hypersensitivity pneumonitis (HP) is an interstitial lung disease (ILD) caused by repeated exposure to inhaled antigens, leading to small airway and parenchymal inflammation. Diagnosis is based on a detailed clinical history, chest imaging and invasive tests such as bronchoalveolar lavage. Distinguishing HP from other ILDs is challenging. Respiratory oscillometry, a novel pulmonary function test (PFT), is highly sensitive to small airway abnormalities. Oscillometry measurement of reactance is strongly correlated with gender-age-physiology score, a prognostic tool used to predict mortality and disease severity in idiopathic pulmonary fibrosis (IPF). Objective To determine if oscillometry and standard PFT patterns are different in HP and IPF. Methods 39 HP (79.5% with fibrotic HP) were enrolled from October 2022 to December 2023 for oscillometry before clinically-indicated standard PFTs and compared with 39 age-matched and sex-matched patients with IPF who also had same day oscillometry and standard PFTs. The main oscillometry metrics of interest were R5-19 (the difference in resistance from 5 to 19 Hz, a metric of small airway function and ventilatory inhomogeneity that increases with worsening respiratory mechanics), X5 (reactance at 5 Hz) which primarily reflects respiratory elastance and AX (area of reactance), a summative measure of the respiratory system stiffness across a range of frequencies, that behaves similarly but in opposite direction to X5. Results Patients with HP exhibited higher residual volume/total lung capacity (RV/TLC), lower per cent predicted (%) forced expiratory volume in 1 s (FEV 1 ) and % predicted forced vital capacity (FVC) than IPF (p<0.05) while FEV 1 /FVC and %TLC were similar. Oscillometry showed higher R5-19 in HP. RV/TLC ratio correlated with AX (r 2 =0.72), X5 (r 2 =0.66) and R5-19 (r 2 =0.64). Conclusion Gas trapping (RV/TLC>0.40) is a feature of HP not observed in IPF. The strong correlations of RV/TLC with AX, X5 and R5-19 suggest that oscillometry can provide non-invasive markers of small airway obstruction in HP that can differentiate it from IPF.