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 (Rrs) during breathing (within-breath changes in Rrs, ΔRrs) 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 ΔRrs 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. Rrs was recorded during tidal breathing and then analyzed as whole-breath Rrs (Rrs at 5 Hz, R5; Rrs at 20 Hz, R20; and their difference, R5−R20) or as ΔRrs, the difference between the expiratory and inspiratory Rrs (ΔR5, ΔR20 and ΔR5–R20). The percentage of the low attenuation area (LAA%) and airway wall area (WA%) was quantified by computed tomography analysis, and their contributions to ΔRrs were examined. Results Seventy-five COPD patients were recruited. LAA% was negatively correlated with ΔR5 and ΔR5−R20 (P = 0.0002 and P = 0.0033, respectively); meanwhile, WA% in B10 was positively correlated with ΔR5 and ΔR5−R20 (P = 0.0057 and P < 0.0001, respectively). Multivariate analysis revealed that the contribution of both LAA% and WA% in B10 to ΔRrs was independent of the severity of airflow limitations. Conclusion This study shows that emphysema suppresses ΔRrs in COPD patients, while airway disease increases ΔRrs in these patients.