RESPIRATORY RESISTIVE IMPEDANCE IN OBSTRUCTIVE PATIENTS - LINEAR-REGRESSION ANALYSIS VS VISCOELASTIC MODELING

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 o bstructive patients, and to investigate the significance of parameters derived from this model. The study was performed in 38 patients, in t he basal state and after inhalation of 200 ng salbutamol. Impedance da ta measured over 332 Hz were fitted by a general four parameter viscoe lastic model describing gas redistribution, and completed by an inerti al component. This model yielded Newtonian resistance (Rmin) and maxim al resistance (Rmax = Rmin plus delayed resistance due to gas redistri bution), Resistive impedance data were also submitted to linear regres sion analysis over the 4-16 and 17-32 Hz frequency ranges, which, resp ectively, yielded resistive impedance extrapolated at 0 Hz (R0) and re sistive impedance estimated at 32 Hz (R32). R0 and R32 were compared t o Rmax and Rmin, respectively. The airway response to salbutamol inhal ation was assessed by the percentage changes in these parameters (R0%, R32%, Rmax%, and Rmin%, respectively). Significant linear correlation s (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. Th ese results demonstrate that resistive impedance extrapolated at zero frequency is equivalent to maximal resistive impedance, and can be pro posed as an index, not only of the level of airway obstruction, but al so of its reversibility.