Modeling of low-frequency pulmonary impedance in dogs.

The mechanical impedance of the lungs (ZL) was measured in open-chest dogs with small-amplitude pseudorandom volume oscillations between 0.125 and 5 Hz, at mean transpulmonary pressures (Ptp) of 0.2, 0.4, and 0.8 kPa. At the lowest frequencies, the pulmonary resistance showed a marked negative frequency dependence and mirrored the changes in the reactance with altered Ptp. The ZL data were evaluated on the basis of two models, each containing the same airway compartment with a resistance and an inertance. The tissue impedance (Zti) in model 1 was represented with two compliances and a resistance (L. E. Mount. J. Physiol. Lond. 127: 157-167, 1955), whereas in model 2 a two-parameter formulation implying rate-independent dissipated work and frequency-dependent elastance (J. Hildebrandt. J. Appl. Physiol. 28: 365-372, 1970) was employed. The estimation of model parameters showed that model 2 was superior to model 1 in both fitting performance and parameter insensitivity to weighting in the fitting criterion. The model 2 coefficients of damping and elastance, characterizing the real and imaginary parts of Zti, respectively, depended on the lung distension and were closely correlated. Although ZL exhibited a slight dependence on the peak-to-peak volume excursion, at a given oscillatory volume no inconsistency with linear tissue viscoelasticity was detected.