Hypothesis / aims of study
During urodynamic studies, we previously demonstrated that filling and passive emptying of the bladder results in a reduction of intravesical pressure, a material behavior termed strain softening. However, unlike a latex balloon in which strain softening is not fully reversible, active voiding restores the pressure back to baseline. We termed this reversible strain softening “dynamic elasticity”[1]. Because passive emptying of the bladder requires catheter placement to facilitate bladder drainage, it is an invasive technique. We hypothesize that strain softening can be produced in a completely non-invasive manner through the use of a bladder external compression protocol. Therefore, the aim of this study was to determine if strain softening produced by passive emptying is equivalent to strain softening produced by repeated external compressions in a working perfused isolated pig bladder. In doing so, we aim to take an essential step toward the development of a novel, non-invasive technique (repeat external bladder compressions) to reduce intravesical pressure and possibly urinary urgency.
Study design, materials and methods
Adult male and female porcine bladders were procured from local abattoirs immediately after slaughter. Using a previously designed ex vivo functional porcine model, the bladder was perfused with Krebs-Henseleit buffer at a flow rate of 4 ml/min [2]. The bladder was maintained at physiological temperature (37-42°C) using a heat lamp and a humidifier throughout the experiment. Each bladder was filled to 250 ml then allowed to reach equilibrium pressure over 5 minutes (P1f). Study 1 – passive emptying phase: The bladder was then filled from 250 ml to 500 ml in order to find the peak pressure (Pref) and the equilibrium pressure after 5 minutes. Next, the bladder was passively emptied via syringe aspiration to a volume of 250 ml and intravesical pressure was recorded (P2f). Active voiding of the remaining volume was induced with a potassium enriched solution, KPSS, to reset any strain softening that had occurred (Fig. 1A). Study 2 –compression phase: The bladder was filled to 250 ml and intravesical pressure was recorded (P1c). Then the bladder was allowed to reach equilibrium. The bladder was then isovolumetrically compressed using external compression to reach the intravesical pressure recorded at a volume of 500 ml (Pref) during Study 1 of the protocol. This external pressure was held for 15 seconds and released for 15 seconds to allow the bladder to rest between compressions. This cyclic loading was repeated 5 times. Afterwards, the bladder was given 5 minutes to reach equilibrium pressure (P2c) (Fig. 1B). The pressures after each 5-minute wait period were recorded, particularly the pressures after equilibration at 250 ml during Study 1 (P1f), the highest pressure reached at 500 ml during Study 1 (Pref) and the pressure after equilibration at 250 ml following syringe aspiration (P2f). P1f and P2f pressures were compared to P1c and P2c, which were the corresponding pressures, measured during Study 2 while compressing to Pref obtained during Study 1.
Interpretation of results
Although to a lesser degree, the bladders in study 2 undergoing application of compression external pressure showed a similar decrease in intravesical pressure as those in study 1 undergoing passive voiding. This suggests strain softening occurs in the bladders undergoing isovolumetric compression.