DYNAMIC VISCOELASTIC NONLINEARITY OF LUNG PARENCHYMAL TISSUE

To investigate the contribution of nonlinear tissue viscoelasticity to the dynamic behavior of lung, time and frequency responses of isolate d parenchymal strips of degassed dog lungs were investigated. The stri ps were subjected to loading and unloading stretch steps for 60 s and to sinusoidal oscillations (0.03-3 Hz) of different stretch amplitudes (Delta lambda = 0.05, 0.1, and 0.2) and at different operating stress es (T-o = 0.5, 1, and 2 kPa). Elastance (E) increased linearly with th e logarithm of frequency (approximate to 10% per frequency decade), an d resistance (R) decreased hyperbolically with frequency. Both E and R varied Little with Delta lambda but they increased proportionally wit h T-o. Hysteresivity (eta = R x 2 pi x frequency/E) ranged from 0.07 t o 0.10. In agreement with the frequency response, the magnitude of the unit step response increased with T-o and was higher when loading tha n when unloading, and the stress relaxation ratio (similar to 0.10) di d not vary greatly with T-o or with Delta lambda. The time and frequen cy behavior of the strips were interpreted in terms of the quasilinear viscoelastic model of Navajas et al. (J. Appl. Physiol. 73: 2681-2692 , 1992). The model explains most of the dependencies of step and oscil latory responses on the measurement conditions, in particular the prop ortional dependence of E and R on T-o. According to the model, about t wo-thirds of energy dissipated during oscillation arises from tissue v iscoelasticity. The remaining dissipated energy could be accounted for by plasticity. Thus the effect of nonlinear elasticity on the dynamic behavior of lung tissue can be empirically described by a simple quas ilinear model characterized by only two parameters.