Effects of vesical and perfusion pressure on perfusate flow, and flow on vesical pressure, in the isolated perfused working pig bladder reveal a potential mechanism for the regulation of detrusor compliance

Bladder Compliance: How We Define It and Why It Is Important

Bladder compliance is the relationship between detrusor pressure and bladder storage volume. We discuss the definition of compliance, how it may be accurately measured, and its clinical relevance. Specifically, we discuss the association between low compliance and upper urinary tract deterioration. We discuss medical and surgical therapies that have been demonstrated to improve compliance and reduce upper tract risk. Finally, we propose a model, which not only considers compliance but also differential pressure between the bladder and ureters, and how this may also be an accurate predictor of upper tract deterioration. We call for further investigation to test this model.

A porcine bladder model of pre-clinical urodynamics demonstrates increased afferent nerve activity during filling

INTRODUCTION AND OBJECTIVES

Urodynamics are the accepted gold standard for the evaluation of multiple forms of voiding dysfunction. However, the tests are expensive, invasive, poorly reproducible, and often prone to artifacts. Therefore, there is a pressing need to develop next-generation urodynamics. The purpose of this study was to develop a novel ex vivo porcine bladder urodynamics model with afferent pelvic nerve signaling that can be used as a preclinical surrogate for bladder sensation.

METHODS

Porcine bladders including the ureters and vascular supply were harvested from local abattoirs using an established protocol in both male and female animals. Ex vivo bladder perfusion was performed using physiologic MOPS (3-(N-morpholino) propanesulfonic acid) buffer solution. The pelvic nerve adjacent to the bladder was grasped with micro-hook electrodes and electroneurogram (ENG) signals recorded at 20 kHz. Bladders were filled with saline at a nonphysiologic rate (100 mL/min) to a volume of 1 L using standard urodynamics equipment to simultaneously record intravesical pressure. ENG amplitude was calculated as the area under the curve for each minute, and ENG firing rate was calculated as number of spikes (above baseline threshold) per minute. At the conclusion of the experiment, representative nerve samples were removed and processed for nerve histology by a pathologist (hematoxylin and eosin and S100 stains).

RESULTS

A total of 10 pig bladders were used, and nerve histology confirmed the presence of nerve in all adequately processed samples. Vesical pressure, ENG firing rate, and ENG amplitude all increased as a function of filling. During filling tertiles (low fill: min 1-3, med fill: min 4-6, and high fill: min 7-10), normalized pressures were 0.22 ± 0.04, 0.38 ± 0.05, and 0.72 ± 0.07 (cmH2O). Similarly, normalized ENG firing rates were 0.08 ± 0.03, 0.31 ± 0.06, and 0.43 ± 0.04 spikes/minute, respectively, and normalized nerve amplitudes were 0.11 ± 0.06, 0.39 ± 0.06, and 0.56 ± 0.14) μV, respectively. Strong relationships between average normalized pressure values and averaged normalized ENG firing rate (r2  = 0.66) and average normalized ENG amplitude (r2  = 0.8) were identified.

CONCLUSIONS

The ex vivo perfused porcine bladder can be used as a preclinical model for the development of next-generation urodynamics technologies. Importantly, the model includes a reproducible method to measure afferent nerve activity that directly correlates with intravesical pressure during filling and could potentially be used as a surrogate measure of bladder sensation.

Review of Animal Models to Study Urinary Bladder Function

Simple Summary

The treatment of urinary bladder dysfunction requires the knowledge of bladder function, which involves physiology, pathology, and even psychology. Several animal models are available to study a variety of bladder disorders. These models include animals from rodents, such as mice and rats, to nonhuman primates, such as rabbits, felines, canines, pigs, and mini pigs. This review adapted animal models to study bladder function according to facility, priority, and disease.

Abstract

The urinary bladder (UB) serves as a storage and elimination organ for urine. UB dysfunction can cause multiple symptoms of failure to store urine or empty the bladder, e.g., incontinence, frequent urination, and urinary retention. Treatment of these symptoms requires knowledge on bladder function, which involves physiology, pathology, and even psychology. There is no ideal animal model for the study of UB function to understand and treat associated disorders, as the complexity in humans differs from that of other species. However, several animal models are available to study a variety of other bladder disorders. Such models include animals from rodents to nonhuman primates, such as mice, rats, rabbits, felines, canines, pigs, and mini pigs. For incontinence, vaginal distention might mimic birth trauma and can be measured based on leak point pressure. Using peripheral and central models, inflammation, bladder outlet obstruction, and genetic models facilitated the study of overactive bladder. However, the larger the animal model, the more difficult the study is, due to the associated animal ethics issues, laboratory facility, and budget. This review aims at facilitating adapted animal models to study bladder function according to facility, priority, and disease.

Regulation of bladder dynamic elasticity: a novel method to increase bladder capacity and reduce pressure using pulsatile external compressive exercises in a porcine model

PURPOSE

Dynamic elasticity is a biomechanical property of the bladder in which muscle compliance can be acutely adjusted through passive stretches and reversed with active contractions. The aim of this study was to determine if manipulating dynamic elasticity using external compression could be used as a novel method to acutely increase bladder capacity and reduce bladder pressure in a porcine model.

METHODS

Ex vivo experiment: bladders underwent continuous or pulsatile compression after establishing a reference pressure at bladder capacity. Bladders were then filled back to the reference pressure to determine if capacity could be acutely increased. In-vivo experiments: bladders underwent five cycles of pulsatile external compression with ultrasound confirmation. Pre and post-compression pressures were measured, and pressure was measured again 10 min post-compression.

RESULTS

Ex vivo experiment: pulsatile compression demonstrated increased bladder capacity by 16% (p = 0.01). Continuous compression demonstrated increased capacity by 9% (p < 0.03). Comparison of pulsatile to continuous compression showed that the pulsatile method was superior (p = 0.03). In-vivo experiments: pulsatile external compression reduced bladder pressure by 19% (p < 0.00001) with a return to baseline 10 min post-compression.

CONCLUSIONS

These results suggest that regulation of bladder dynamic elasticity achieved with external compression can acutely decrease bladder pressure and increase bladder capacity. Pulsatile compression was found to be more effective as compared to continuous compression. These results highlight the clinical potential for use of non-invasive pulsatile compression as a therapeutic technique to increase bladder capacity, decrease bladder pressure, and reduce the symptoms of urinary urgency.

A preliminary study of bilateral color mapping of pig bladder vasculature demonstrates potential for acute hemi-ischemic events

Background

Chronic ischemia is a known risk factor for the development of lower urinary tract symptoms (LUTS) and bladder hypocontractility. Less is known, however, about the impact of acute ischemia. Classic teaching suggests that collateral circulation is robust in the bladder and, therefore, loss of a single source of blood flow should have no deleterious effect. This study aims to provide visual evidence that segmental vascular supply is critical for maintaining adequate perfusion to the bladder.

Methods

Ex vivo pig bladders were cannulated bilaterally in the superior vesical arteries and perfused using contrasting red and green dye. Images were collected at each step of the perfusion dyeing process and these images were analyzed using a custom program to calculate the average hue of each side. Statistical analysis was performed using Student’s t-test.

Results

The two halves being perfused by separate arteries showed a statistically significant difference when compared (P<0.05) on both the outer wall (n=9) and in the mucosal layer (n=4). On the outside wall, the average normalized hue of the green halves was 27.5°±14.3°, while the average normalized hue of the red halves was −58.7°±3.1°. In the mucosa, the average normalized hue of the green halves was 34.5°±17.4°, while the average normalized hue of the red halves was −51.5°±3.5°.

Conclusions

This study identified a novel color mapping method to study pig bladder vascular supply. The results demonstrated a lack of collateral blood flow, highlighting the possibly of acute hemi-ischemic event. However, further research in the effect of acute ischemia on bladder function is necessary.

Phases of decompensation during acute ischemia demonstrated in an ex vivo porcine bladder model

Background

The aim of this project was to develop an ex-vivo porcine bladder model to test the effects of increasing durations of acute ischemia on detrusor function.

Methods

Porcine bladders were perfused through bilateral vesical arteries at physiologic flow (4 mL/min) and filled through a urethral catheter. Intravesical pressures were continuously recorded using standard urodynamics equipment. Bladder contractions, with simulated voiding, were induced by arterial infusion of KCl at 250 mL. Total, passive, and active pressures were recorded for each contraction and data were normalized to the control fill. Bladders underwent the following perfusion protocol by adjusting the arterial flow rates: Equilibration (4 mL/min), control (4 mL/min), partial ischemia (2 mL/min), global ischemia (0 mL/min) and reperfusion (4 mL/min). Perfusion periods were held for 15 min for one group and 30 min for another group of bladders.

Results

Porcine bladders (N=19) including 8 (15 min group) and 11 (30 min group) were used. With 15 min ischemia, passive pressure increased 39% (P=0.03) and the active pressure decreased 23% (P=0.002). Total pressure remained constant, identifying a compensated phase. Values returned to baseline with reperfusion. With 30 min ischemia, passive pressure remained unchanged. However, there was a decrease in total pressure 34% (P<0.001) and active pressure 61% (P<0.001), which incompletely recovered to baseline values, identifying a decompensated phase with incomplete recovery upon reperfusion.

Conclusion

In the porcine bladder, 15 min ischemia resulted in a compensated phase and 30 min ischemia resulted in a decompensated phase of detrusor function. This study provides mechanistic insight into the natural history of ischemia-mediated voiding dysfunction.

Bladder attack: transient bladder ischemia leads to a reversible decrease in detrusor compliance

Background

The deleterious effects of chronic ischemia on bladder function have been extensively studied; however, evaluation and characterization of the effects of acute ischemia and hypoxia are lacking. The present study examined pig and human detrusor smooth muscle (DSM) strips, in combination with an isolated perfused working pig bladder model to evaluate the relationship between transient ischemia and bladder function.

Methods

Organ bath and myographic studies were performed using pig and human DSM strips exposed to starvation/hypoxia conditions. Analogous conditions were then recreated in the ex vivo bladder preparation. Filled bladders were then treated with intravascular carbachol to induce contraction and subsequent void. An intravesical transducer continuously monitored changes in bladder pressure, while a tissue pO₂ monitor analyzed changes in oxygenation.

Results

After 120 min in starved/hypoxic conditions, both pig and human DSM strips demonstrated significantly increased resting tone, with a greater than two-fold increase in force over control. This was effectively blocked with atropine. DSM strips also demonstrated significantly weaker contractions; however, contractile force was nearly recovered following 15-min exposure to replete/oxygenated buffer. In the ex vivo bladder preparation, filling under ischemic conditions yielded a 225% increase in end-fill vesical pressures (P_ves) compared to controls. End-fill P_ves returned to baseline with reperfusion during a subsequent filling cycle.

Conclusions

Transient ischemia/hypoxia leads to an acute increase in tone in both DSM strips and ex vivo pig bladder. Remarkably, the effect is reversible with re-perfusion and may be blocked with anticholinergics, suggesting a relationship between acute ischemia and increased local acetylcholine release.

An external compress-release protocol induces dynamic elasticity in the porcine bladder: A novel technique for the treatment of overactive bladder?

INTRODUCTION

Dynamic elasticity is an acutely regulated bladder material property through which filling and passive emptying produce strain softening, and active voiding restores baseline pressure. The aim of this study was to test the hypothesis that strain softening produced by filling-passive emptying is equivalent to that produced by compression-release in a porcine bladder model.

METHODS/MATERIALS

Latex balloons and ex vivo perfused pig bladders were used for a series of alternating fill-passive emptying ("Fill") and external compress-release ("Press") protocols. For the Fill protocol balloons/bladders were (1) filled to defined volumes (prestrain softening), (2) filled to capacity to strain soften (reference), and (3) passively emptied to the original volume (poststrain softening). For the Press protocol, balloons/bladders were (1) filled to defined volumes (prestrain softening), (2) externally compressed to reference pressure and then released for five cycles (poststrain softening). After each protocol, bladders were voided with high-KCl buffer to induce "active" voiding.

RESULTS

In both balloons and porcine bladder, both the Fill and Press protocols produced significant strain softening (P < 0.05) and poststrain softening pressures were not different for Fill and Press protocols (P > 0.05), indicating a similar degree of strain softening with both methods.

CONCLUSIONS

Repeated external compression can induce bladder strain softening similar to filling and passive emptying. This technique may represent a means to acutely regulate bladder compliance and potentially be used as a mechanical treatment for urinary urgency.

Automated quantification of low amplitude rhythmic contractions (LARC) during real-world urodynamics identifies a potential detrusor overactivity subgroup

Objectives

Detrusor overactivity (DO) is characterized by non-voiding detrusor smooth muscle contractions during the bladder filling phase and often contributes to overactive bladder. In some patients DO is observed as isolated or sporadic contractions, while in others DO is manifested as low amplitude rhythmic contractions (LARC). The aim of this study was to develop an objective method to quantify LARC frequencies and amplitudes in urodynamic studies (UDS) and identify a subgroup DO of patients with LARC.

Methods

An automated Fast Fourier Transform (FFT) algorithm was developed to analyze a 205-second region of interest of retrospectively collected “real-world” UDS ending 30 seconds before voiding. The algorithm was designed to identify the three largest rhythmic amplitude peaks in vesical pressure (P_ves) in the 1.75–6 cycle/minute frequency range. These peak P_ves amplitudes were analyzed to determine whether they were 1) significant (above baseline P_ves activity) and 2) independent (distinct from any in abdominal pressure (P_abd) rhythm).

Results

95 UDS met criteria for inclusion and were analyzed with the FFT algorithm. During a blinded visual analysis, a neurourologist/urodynamicist identified 52/95 (55%) patients as having DO. The FFT algorithm identified significant and independent (S&I) LARC in 14/52 (27%) patients with DO and 0/43 patients (0%) without DO, resulting in 100% specificity and a significant association (Fischer’s exact test, p<0.0001). The average slowest S&I LARC frequency in this DO subgroup was 3.20±0.34 cycles/min with an amplitude of 8.40±1.30 cm-H₂O. This algorithm can analyze individual UDS in under 5 seconds, allowing real-time interpretation.

Conclusions

An FFT algorithm can be applied to “real-world” UDS to automatically characterize the frequency and amplitude of underlying LARC. This algorithm identified a potential subgroup of DO patients with LARC.

Potential vascular mechanisms in an ex vivo functional pig bladder model

AIMS

Chronic ischemia is a recognized factor in the pathophysiology of underactive bladder (UAB). Although relative ischemia (ie, low blood flow) is known to occur during filling, little is known regarding the pathophysiology that leads to UAB. Therefore, we developed an ex vivo functional porcine model to investigate the role of transient ischemia and whether autoregulation, a mechanism that maintains tissue oxygenation in certain vital organs, also exists in the bladder.

METHODS

Using bladders from slaughtered pigs, we prepared an isolated perfused model where we studied the effects of bladder perfusion flow rate on perfusion pressure and tissue oxygenation during the filling phase. Bladders were perfused at an initial flow rate of 20 mL/min and then clamped in a sequentially decreasing stepwise manner down to no flow and back to the initial flow rate.

RESULTS

We found a linear relationship between flow rate and perfusion pressure until the flow rate decreased below 5 mL/min at which point the vascular resistance decreased; however, tissue pO₂ remained stable after an initial decline.

CONCLUSIONS

These findings suggest that there may be an intrinsic autoregulatory mechanism in the bladder that allows it to undergo cyclic episodes of relative ischemia during its normal function. Factors that overcome this mechanism such as complete or chronic ischemia may be critical in the progression to detrusor underactivity and thereby highlight the importance of intervention during the early phases of this disease process.