Estimating central and peripheral respiratory resistance: an alternative analysis.

Pimmel et al (J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 45: 375--380, 1978) recently presented an analysis of the frequency dependence of respiratory resistance (Rrs) based on a simple electrical analog of the respiratory system that allows estimation of the central (Rc) and peripheral (Rp) components of Rrs. The method by which they determine these parameters from the experimental data is based on a number of unproven assumptions. Using the same electrical analog, we present an analysis that allows calculation of these parameters, as well as the corner frequency of the network (f1), without need for similar assumptions. Our technique is based on fitting the resistances (RTh) measured over a range of frequencies (f) to the exact solution of the network given by RTh = Rc + Rpf1(2)/f2 + f1(2)). Using the transformation X = 1/(f2 + f1(2), the equation becomes a linear relationship between RTh and X allowing the resistances to be determined by linear regression. Reanalysis of Pimmel et al.'s data demonstrated that the assumptions of a constant f1, and the equivalence of RTh at 0 Hz to RTh at 1 Hz in invalid under certain conditions. Thus, if one is to use the electrical analog to partition Rrs into its central and peripheral components, one should use the analytic approach suggested here that does not rely on these assumptions.