Elevated AbsΔX5 Identifies a Distinct COPD Phenotype with Peripheral Airway Dysfunction and Longitudinal Improvement...

Impulse oscillometry (IOS) is increasingly used to assess small airway dysfunction (SAD) in chronic obstructive pulmonary disease (COPD). However, the physiological significance of the absolute inspiratory-expiratory reactance difference (AbsΔX5) remains unclear. We investigated whether elevated AbsΔX5 identifies a distinct COPD phenotype associated with baseline physiological abnormalities and longitudinal physiological changes.

We retrospectively analyzed patients with COPD who underwent serial IOS and spirometry between January 2023 and February 2025. COPD was defined as a post-bronchodilator FEV₁/FVC <0.70, and SAD as R5-R20 >0.07 kPa/L/s and/or AX >0.33 kPa/L. Patients were classified into COPD without SAD, SAD+/AbsΔX5 <0.10, and SAD+/AbsΔX5 ≥0.10. The 0.10 cutoff was determined by an exploratory threshold analysis. Baseline characteristics and longitudinal changes in IOS and post-bronchodilator spirometric parameters were compared.

Among 710 patients who underwent both IOS and spirometry, 112 met the diagnostic criteria for COPD. Patients were classified into COPD without SAD (n=36), SAD+/AbsΔX5 <0.10 (n=46), and SAD+/AbsΔX5 ≥0.10 (n=30). Baseline IOS abnormalities progressively worsened across the groups. Patients with SAD+/AbsΔX5 ≥0.10 had significantly higher R5-R20, AX, and Fres values and more negative X5 than the other groups. During follow-up, they showed greater improvements in R5, R5-R20, and AX, whereas spirometric changes were modest, except for a greater increase in FVC than in COPD without SAD.

Elevated AbsΔX5 was most clinically informative in patients with IOS-defined SAD, identifying a distinct COPD phenotype characterized by more severe baseline peripheral airway dysfunction and greater longitudinal improvement in IOS parameters.