Hypothesis / aims of study
The elasticity of the bladder wall can be acutely decreased through repeated filling and passive emptying through the process of strain-induced stress softening (strain softening) [1-2]. Furthermore, this strain softening is reversed though active voiding, resulting in increased elasticity during the next fill [1-2]. This reversible strain softening has been termed dynamic elasticity and has been quantified in individuals with overactive bladder during clinical urodynamics [2]. The aim of this study was to test the hypothesis that dynamic elasticity is present in an isolated perfused pig bladder model [3] and quantify dynamic elasticity by means of the same methodology used in comparative-fill urodynamics in humans [2].
Study design, materials and methods
Pig bladders with the vascular tree and a portion of the aorta were harvested from a local abattoir immediately after slaughter and the vascular system was flushed with Krebs‐Henseleit buffer. After transport to the lab in cold buffer, the superior vesical arteries were cannulated and perfused with oxygenated physiologic-temperature Krebs‐Henseleit buffer at 4 mL/min. The urethra was catheterized to allow infusion, monitoring of intravesical pressure and bladder emptying (Fig 1 A). Bladders underwent a comparative-fill urodynamics protocol involving an initial fill plus three comparative fills (Fill 1-Fill 3) (Fig 1 B). All fills were to a volume of 250 mL, which was assumed to be 50% of a 500 mL bladder. These fills were followed by either passive emptying through syringe aspiration or by an active voiding contraction induced through vesical perfusion of high potassium buffer. Following the initial fill, an active void was induced to empty the bladder and reverse any strain softening. After the active void, filling pressure was recorded throughout Fill 1 (baseline, after active void), and then the bladder was passively emptied to prevent reversal of any strain softening due to Fill 1. After this passive emptying, filling pressure was recorded throughout Fill 2 (after strain softening). Next, the bladder was actively voided to reverse any strain softening, and then filling pressure was recorded throughout Fill 3 (after active void). To quantify dynamic elasticity, the average pressure throughout each fill was calculated and the change in average pressure between fills was divided by the change in percent capacity throughout filling [2].
Interpretation of results
The results of this study support the hypothesis that dynamic elasticity is present in an isolated perfused pig bladder model. Furthermore, the results demonstrate the quantification dynamic elasticity in the pig bladders by means of the same methodology used in a comparative-fill urodynamics protocol in individuals with overactive bladder [2].
Concluding message
Regulation of dynamic elasticity would affect bladder wall tension during filling, and consequently affect the stimulation of tension-sensitive nerves responsible for the sensation of bladder fullness. As a result, a defect in the regulation of dynamic elasticity would be expected to alter sensation and could contribute to overactive bladder. The presence of acute dynamic elasticity in the isolated pig bladder model will allow for more detailed investigations of this bladder material property. Factors such as incomplete voiding, non-voiding contractions, and bladder ischemia that could affect dynamic elasticity could be analyzed individually in the pig bladder model, leading to a better understanding of this material property and its effects on bladder biomechanics. Improved knowledge of the role of dynamic elasticity in bladder function and the mechanisms responsible for this property could have diagnostic and therapeutic implications in the management of bladder pathology.