System identification of bladder hydrodynamics

The bladders of six minipigs, three normal and three obstructed, were subjected to a fixed stochastic volume perturbation about a constant volume level, and evoked pressure changes were measured. The hydrodynamic stiffness transfer functions relating volume and pressure were calculated and parameterized by a second-order lumped parameter model having inertial, viscous and elastic terms. Estimates of the elastic constant (K) increased linearly with tonic level in both normal and obstructed animals. The rate of increase was substantially greater in the obstructed animals than in the normals.