Partitioning of respiratory mechanical impedance by absolute and differential body plethysmography

The authors have recently demonstrated the feasibility of partitioning total respiratory impedance (Z _rs) into its airway (Z _aw) and tissular (Z _ti) components by measuring alveolar gas compression (V _pt) plethysmographically during pressure oscillations at the airway opening (Peslin et al.). The aim of this study was to comparatively evaluate an alternative approach: the measurement of Z _rs and of the transfer function (FTF) between airway flow and body surface flow obtained by absolute body plethysmography. The two approaches are theoretically equivalent, provided thermal and other artifacts are properly eliminated. Z _rs and V _pt (method 1) and Z _rs and FTF (method 2) were measured in 11 healthy subjects from 4 to 29 Hz, using a pressure-type and a flow-type plethysmograph, respectively. Inspired gas was conditioned to body temperature and pressure, saturated with water vapor in both instances to minimize thermal factors. Z _aw and Z _ti spectra computed from both sets of data were quite similar in shape. Neither airway resistance nor tissue compliance differed significantly; tissue resistance, however, was about 14% lower with method 1, which may be due to imperfect gas conditioning. The reproducibility of the data was similar with the two approaches. It is concluded that absolute body plethysmography is as reliable as differential body plethysmography to partition Z _rs.