Small airway dysfunction is an early physiological characteristic of idiopathic pulmonary fibrosis : a retrospective cohort study

Abstract

Background: Recently, the central position of small airways in the pathogenesis of idiopathic pulmonary fibrosis(IPF) has been gradually recognized and accepted with evidence of small airway anatomical and genetic abnormalities. However, the small airway physiology of IPF patients remained unclear. This study aimed to assess the small airway physiology of IPF patients using pulmonary function test (PFT). Methods: We retrospectively analyzed 138 IPF patients' and 186 control patients' small airway lung function data and medical records. A 1:1 propensity score match(PSM) for age, sex, smoking status and BMI was completed priors to the group comparison between IPF patients and controls. In addition, patients were divided into three groups according to FVC decline degree with cut-off values of 75% and 55%, and the small airway function was compared between the three groups. The impulse oscillometry system (IOS) diagnosis value was evaluated in 60 IPF patients. Results: The total cohort comprised 138 subjects, with 53(38.4%) diagnosed with SAD. After PSM, IPF patients were predisposed to SAD compared with controls (38.0% vs.15.2%, p=0.001). Compared small airway function between patients with different disease severities, we found even for patients in the mild disease group whose FVC is almost normal, 30.6% (n=22) were diagnosed with spirometry-SAD. And we found MEF50(98.2% vs. 87.4% vs. 52.4%, p=0.001), MEF25(74.2% vs.71.9% vs. 61.3%, p=0.062) and MMEF(77.2% vs. 74.6% vs. 44.6%, p=0.002) were deteriorated with the severity of disease. The incidence of spirometry-SAD in more severe patients was higher than those with less severe disease(p=0.021). Applying IOS to 60 IPF patients, 41(68.3%) were diagnosed with IOS-SAD. There was poor concordance between spirometry and IOS for the diagnosis of SAD (Kappa value=-0.068, p=0.542). IOS showed higher sensitivity than spirometry. For patients with abnormal lung function (FVC<80%),this advantage could be even more obvious. Conclusions: This study revealed that SAD was a common lesion of IPF patients. It should be emphasized that SAD could occur at the early stages of disease when the lung capacity has not decreased, and the incidence and degree of dysfunction were progressively aggravated with disease progression. Additionally, IOS was a powerful complementary tool for diagnosing SAD, especially for IPF patients with decreased lung function.