Separation of airway and tissue properties by transfer respiratory impedance and thoracic gas volume in reversible airway obstruction.

The aim of this study was to establish the ability to estimate separate airway and tissue properties from transfer respiratory impedance (Zrs,tr) data in the presence of airway obstruction. Zrs,tr, thoracic gas volume (TGV) and airway resistance (Raw,pleth) were measured in the presence of obstruction and after use of a bronchodilator (BD) in 13 normal or asymptomatic asthmatic adults and 28 children with symptomatic asthma. An analytical approach was used to solve the equations of a simplified variant of DuBois' model, including airway resistance (Raw*) and inertance (Iaw), tissue compliance (Ct) and resistance (Rt) and pulmonary gas compliance (Cg). The equations of the model could not be reliably solved in four children before BD. Mean Raw,pleth was not different from mean Raw* in adults before (mean +/- SEM) (3.4 +/- 0.5 vs 3.1 +/- 0.3 hPa.s.L-1) or after BD (1.4 +/- 0.2 vs 1.8 +/- 0.2 hPa.s.L-1), or in children after BD (2.9 +/- 0.3 vs 3.2 +/- 0.2 hPa.s.L-1, respectively). In children before BD, Raw* was significantly underestimated compared with Raw,pleth (3.8 +/- 0.4 vs 5.4 +/- 0.6 hPa.s.L-1). Overall, a significant positive correlation was found between the difference [Raw,pleth - Raw*] and Raw,pleth (r = 0.82). In adults, BD induced a decrease in Raw* and Rt, an increase in Ct, and no change in Iaw. In children after BD, there was no significant change in Raw* or Ct, whilst Rt decreased and Iaw increased. Taking Raw,pleth as the gold standard, it is concluded that coherent estimation of parameters of DuBois' model may be obtained from combined Zrs,tr and TGV measurements in normal subjects and moderately obstructed adults, but not in children with significant airway obstruction. This seems to be due to the systematic under-estimation of Raw*.