Dynamic viscoelastic nonlinearity of lung parenchymal tissue.
Navajas, Daniel, Geoffrey N. Maksym, and Jason H. T. Bates.
Meakins-Christie Laboratories. McGill University, Montreal, Quebec,
Canada H2X 2P2; and Lab. Biofisica i Bioenginyeria. Facultat
Medicina. 08028 Barcelona Spain
APStracts 2:0091A, 1995.
To investigate the contribution of nonlinear tissue viscoelasticity to
the dynamic behaviour 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 to 3 Hz) of different
stretch amplitudes ([delta]M = 0.05, 0.1 and 0.2) and at different
operating stresses (To = 0.5, 1 and 2 kPa). Elastance (E) increased
linearly with the logarithm of frequency ( nearly equal to 10% per
frequency decade) and resistance (R) decreased hyperbolically with
frequency. Both, E and R varied little with [delta]M but they
increased proportionally with To. Hysteresivity ([theta] =
Rx2[pi]xfrequency/E) ranged from 0.07 to 0.10. In agreement with the
frequency response, the magnitude of the unit step response increased
with To and was higher when loading than when unloading and the
stress relaxation ratio ( nearly equal to 0.10) did not vary greatly
with To or with [delta]M. The time and frequency behaviour of the
strips were interpreted in terms of the quasi-linear viscoelastic
model of Navajas et al. (J. Appl. Physiol. 73:2681-2692, 1992). The
model explains most of the dependences of step and oscillatory
responses on the measurement conditions, in particular the
proportional dependence of E and R on To. According to the model,
nearly equal to 2/3 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 quasi-linear model characterized by only two parameters.
Received 7 February 1994; accepted in final form 28 February
1995.
APS Manuscript Number A140-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 21 March 1995.