Respiratory system reactance reflects communicating lung volume in chronic obstructive pulmonary disease.

Respiratory system reactance (Xrs) measured by the forced oscillation technique (FOT) is theoretically and experimentally related to lung volume. In chronic obstructive pulmonary disease (COPD), the absolute volume measured by body plethysmography includes a proportion which is inaccessible to pressure oscillations applied via the mouth - that is, a 'non-communicating' lung volume. We hypothesized that, in COPD, the presence of non-communicating lung would disrupt the expected Xrs-volume relationship when compared to plethysmographic functional residual capacity (FRCpleth ). Instead, Xrs would relate to estimates of non-communicating volume, namely expiratory reserve volume (ERV) and single-breath alveolar volume (VASB ). We examined FOT and lung function data from people with COPD (n=51) and from healthy volunteers (n=40). In healthy volunteers, we observed an expected inverse relationship between reactance at 5 Hz (X5) and FRCpleth . In contrast, there was no such relationship between Xrs and FRCpleth in COPD subjects. However, there was an inverse relationship between Xrs and both ERV and VASB . Hence the theoretical Xrs-volume relationship is present in COPD, but only when considering the communicating volume rather than the absolute lung volume. These findings confirm the role of reduced communicating lung volume as an important determinant of Xrs, and therefore advance our understanding and interpretation of FOT measurements in COPD.