Emphysema and airway disease affect within‐breath changes in respiratory resistance in COPD patients

Background and objective Chronic obstructive pulmonary disease ( COPD ) is characterized by a mixture of emphysema and airway disease. The forced oscillation technique ( FOT ) has been applied to COPD patients to clarify changes in respiratory mechanics; dynamic changes in respiratory resistance ( R rs) during breathing (within‐breath changes in R rs, Δ R rs) are characteristic of COPD . However, the pathophysiological significance of these changes is unknown. The aim of this study was to assess how emphysema and airway disease influence Δ R rs in COPD patients. Methods In this cross‐sectional study, stable COPD patients were recruited and underwent respiratory impedance measurements with a commercially available FOT device. R rs was recorded during tidal breathing and then analyzed as whole‐breath R rs ( R rs at 5 Hz, R 5; R rs at 20 Hz, R 20; and their difference, R 5− R 20) or as Δ R rs, the difference between the expiratory and inspiratory R rs (Δ R 5, Δ R 20 and Δ R 5– R 20). The percentage of the low attenuation area ( LAA %) and airway wall area ( WA %) was quantified by computed tomography analysis, and their contributions to Δ R rs were examined. Results Seventy‐five COPD patients were recruited. LAA % was negatively correlated with Δ R 5 and Δ R 5− R 20 ( P  = 0.0002 and P  = 0.0033, respectively); meanwhile, WA % in B 10 was positively correlated with Δ R 5 and Δ R 5− R 20 ( P  = 0.0057 and P  < 0.0001, respectively). Multivariate analysis revealed that the contribution of both LAA % and WA % in B 10 to Δ R rs was independent of the severity of airflow limitations. Conclusion This study shows that emphysema suppresses Δ R rs in COPD patients, while airway disease increases Δ R rs in these patients.