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 isolated parenchymal strips of degassed dog lungs were investigated. The strips were subjected to loading and unloading stretch steps for 60 s and to sinusoidal oscillations (0.03-3 Hz) of different stretch amplitudes (Δλ = 0.05, 0.1, and 0.2) and at different operating stresses (T₀ = 0.5, 1, and 2 kPa). Elastance (E) increased linearly with the logarithm of frequency (≃10% per frequency decade), and resistance (R) decreased hyperbolically with frequency. Both E and R varied little with Δλ but they increased proportionally with T₀. Hysteresivity (η = R x 2 π x frequency/E) ranged from 0.07 to 0.10. In agreement with the frequency response, the magnitude of the unit step response increased with T₀ and was higher when loading than when unloading, and the stress relaxation ratio (~0.10) did not vary greatly with T₀ or with Δλ. The time and frequency 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 oscillatory responses on the measurement conditions, in particular the proportional dependence of E and R on T₀. According to the model, about two-thirds of energy dissipated during oscillation arises from tissue viscoelasticity. 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 quasilinear model characterized by only two parameters.