Small Airway Dysfunction in Rheumatoid Arthritis-Associated Interstitial Lung Disease: A Single-Center Retrospective Observational Study

Background: Rheumatoid arthritis-associated interstitial lung disease (RA-ILD) is an important extra-articular manifestation of rheumatoid arthritis and is associated with impaired pulmonary function and poor clinical outcomes. Although interstitial abnormalities in RA-ILD have been widely studied, airway involvement, particularly small airway dysfunction, has received less attention. This study aimed to evaluate small airway function and impulse oscillometry parameters in patients with RA-ILD and to explore their potential associations with disease severity. Methods: This is a hospital-based cross-sectional comparative observational study using retrospectively collected clinical data. The patients with RA-ILD included in this study were treated at Beijing Jishuitan Hospital, Capital Medical University, between January 2021 and December 2025. Age- and sex-matched healthy individuals without pulmonary disease were included as controls. Clinical data, high-resolution computed tomography (HRCT) findings, pulmonary function parameters, and impulse oscillometry indices were collected. Small airway function was assessed using MEF25–75%, MEF50%, and MEF25%. Disease severity was evaluated using HRCT visual scores, the gender–age–physiology (GAP) score, and the composite physiologic index (CPI). Subgroup analyses were performed according to the presence of small airway dysfunction, HRCT subtype, and history of RA-related joint surgery. Correlations between airway function parameters and disease severity indices were analyzed. Multiple linear regression analyses were performed. Results: A total of 255 patients with RA-ILD were included, including 85 men (33.3%), with a mean age of 67.53 years. Compared with controls, patients with RA-ILD had significantly lower BMI, FVC%, TLC%, and DLCO% (all p < 0.05). Small airway function parameters, including MEF25–75%, MEF50%, and MEF25%, were significantly reduced in the RA-ILD group. Impulse oscillometry showed significantly higher Z5, R5–R20, R5/R20, and Fres, and significantly lower X5 in patients with RA-ILD compared with controls. Patients with RA-ILD and small airway dysfunction had lower FVC% than those without small airway dysfunction (87.7% vs. 95.46%, p = 0.015), and their GAP scores tended to be higher, although the difference did not reach statistical significance (2.59 vs. 1.79, p = 0.055). Among different ILD subtypes, only MEF50% differed significantly among the UIP, NSIP, and other-pattern groups. No significant differences in airway function or disease severity were observed between patients with and without a history of RA-related joint surgery. Correlation analysis showed that MEF25–75%, MEF50%, and MEF25% were negatively correlated with GAP score and positively correlated with FVC%. MEF25–75% and MEF50% were also positively correlated with DLCO%. Conclusions: In this study, patients with RA-ILD showed evidence of small airway dysfunction, with altered impulse oscillometry parameters suggesting increased airway resistance and reduced airway compliance. Small airway function parameters were associated with FVC% and GAP score, indicating that small airway dysfunction may be related to disease severity in RA-ILD.