Introduction & Objective :
Urodynamic studies have been used to demonstrate that filling and passive emptying results in a reduction of intravesical pressure, a material property termed strain softening. Active voiding reverses strain softening representing acute dynamic elasticity. Passive bladder emptying requires an invasive catheter. This study tested the hypothesis that strain softening can be produced non-invasively using an external compress-release protocol. The aim was to determine if strain softening produced by filling-passive emptying is equivalent to strain softening produced by repeated external compress-release cycles in an isolated, working pig bladder.
A perfused ex vivo functional porcine bladder model was filled to 250 ml and allowed 5 min to reach equilibrium pressure (P1f, f=filling). Then, effect of filling to 500 ml and passively emptying was compared to compressing and releasing the bladder. The bladder volume was increased to 500 ml, and peak pressure (Pref) was measured. Next, it was passively emptied to 250 ml via syringe aspiration and pressure was recorded (P2f). Active voiding was induced with potassium enriched solution to reset strain softening. The bladder was filled to 250 ml and allowed to equilibrate, where pressure (P1c, c=compression) was recorded, and the bladder was isovolumetrically compressed to Pref. The external pressure was held for 15s and released for 15s for 5 cycles. The 5 min equilibrium pressure after release (P2c) was noted.
Ten bladders were studied (n = 10). Strain softening occurred due to the filling-passive emptying (P2f < P1f, t-test, p < 0.05) and due to isovolumetrical compression with external pressure (P2c < P1c, t-test, p < 0.05). The pressure after filling-emptying was not statistically different from teh pressure after compression-release (P2c vs P2f, t-test, p > 0.05), suggesting a similar degree of strain softening was induced by each method.
Bladders undergoing compression-release showed a similar decrease in intravesical pressure compared to filling-passive emptying, indicating strain softening occurs via isovolumetric compression. Increasing bladder pressure through both filling and compression results in measureable strain softening. Repeated external bladder compression represents a potential means to lower intravesical pressure via strain softening, a novel, non-invasive technique with potential diagnostic and therapeutic clinical applications.