Respiratory resistive impedance in obstructive patients: linear regression analysis vs viscoelastic modelling.

The aim of this study was to test the ability of a simple two segment model to describe the frequency dependence of resistive impedance in obstructive patients, and to investigate the significance of parameters derived from this model. The study was performed in 38 patients, in the basal state and after inhalation of 200 micrograms salbutamol. Impedance data measured over 4-32 Hz were fitted by a general four parameter viscoelastic model describing gas redistribution, and completed by an inertial component. This model yielded Newtonian resistance (Rmin) and maximal resistance (Rmax = Rmin plus delayed resistance due to gas redistribution). Resistive impedance data were also submitted to linear regression analysis over the 4-16 and 17-32 Hz frequency ranges, which respectively, yielded resistive impedance extrapolated at 0 Hz (R0) and resistive impedance estimated at 32 Hz (R32). R0 and R32 were compared to Rmax and Rmin, respectively. The airway response to salbutamol inhalation was assessed by the percentage changes in these parameters (R0%, R32%, Rmax%, and Rmin%, respectively). Significant linear correlations (p < 0.0001) were found between R0 and Rmax, R32 and Rmin, and R0% and Rmax%. Furthermore, the linear regression lines of R0 vs Rmax, and R0% vs Rmax%, were not significantly different from the identity line. These results demonstrate that resistive impedance extrapolated at zero frequency is equivalent to maximal resistive impedance, and can be proposed as an index, not only of the level of airway obstruction, but also of its reversibility.