AJP - Cell Physiology, Vol 261, Issue 2 C355-C363, Copyright © 1991 by American Physiological Society
Viscoelastic properties of the contracting detrusor. I. Theoretical basis
J. G. Venegas
Department of Biomedical Engineering, Massachusetts General Hospital, Boston.
This paper presents the theoretical basis for estimating the detrusor's
viscoelastic properties using the small-amplitude oscillatory perturbations
technique. Three possible configurations of the simplest second-order
lumped-parameter model of the bladder were analyzed to derive equations of
the parameters incremental resistance (R) and incremental elastance (K) in
terms of the experimentally measurable magnitude and phase of hydrodynamic
stiffness. In model I, single viscous, elastic, and inertial elements were
assumed to to be connected in series. In model III the elastic and viscous
elements were connected in series, but the inertial element was connected
in parallel. With the assumption of a spherical geometry of the bladder,
equations were also derived to obtain the bladder wall mechanical
properties, spring incremental constant (S), and muscle incremental
viscosity (b) as functions of bladder volume and the hydrodynamic
properties R and K. Integration of the incremental equation describing the
viscous component yields an expression that fits well the force-velocity
experimental data from bladder strips reported by others. This finding
suggests that muscle viscosity measured with the small-amplitude
oscillations and analyzed with the proper theoretical model may be related
to the force-velocity characteristics of the muscle. The equations
delivered here form the basis for analyzing the experimental data described
in the companion paper.
