Dynamics of the upper urinary tract: II. The effect of variations of peristaltic frequency and bladder pressure on pyeloureteral pressure/flow relations

Mixed uncertainty analysis on pumping by peristaltic hearts using Dempster-Shafer theory

In this paper, we introduce the numerical strategy for mixed uncertainty propagation based on probability and Dempster-Shafer theories, and apply it to the computational model of peristalsis in a heart-pumping system. Specifically, the stochastic uncertainty in the system is represented with random variables while epistemic uncertainty is represented using non-probabilistic uncertain variables with belief functions. The mixed uncertainty is propagated through the system, resulting in the uncertainty in the chosen quantities of interest (QoI, such as flow volume, cost of transport and work). With the introduced numerical method, the uncertainty in the statistics of QoIs will be represented using belief functions. With three representative probability distributions consistent with the belief structure, global sensitivity analysis has also been implemented to identify important uncertain factors and the results have been compared between different peristalsis models. To reduce the computational cost, physics constrained generalized polynomial chaos method is adopted to construct cheaper surrogates as approximations for the full simulation.

CFD investigation of multiple peristaltic waves in a 3D unobstructed ureter

Ureters are essential components of the urinary system and play a crucial role in the transportation of urine from the kidneys to the bladder. In the current study, a three-dimensional ureter is modelled. A series of peristaltic waves are made to travel on the ureter wall to analyse and measure parameter effects such as pressure, velocity, gradient pressure, and wall shear at different time steps. The flow dynamics in the ureters are thoroughly analysed using the commercially available ANSYS-CFX software. The maximum pressure is found in the triple wave at the ureteropelvic junction and maximum velocity is observed in the single and double wave motion due to the contraction produced by the peristalsis motion. The pressure gradient is maximum at the inlet of the ureter during the single bolus motion. The contraction produces a high jet of velocity due to neck formation and also helps in urine trapping in the form of a bolus, which leads to the formation of reverse flow. Due to the reduction in area, shear stress builds on the ureter wall. The high shear stress may rupture the junctions in the ureter.

Fluid mechanical modeling of the upper urinary tract

The upper urinary tract (UUT) consists of kidneys and ureters, and is an integral part of the human urogenital system. Yet malfunctioning and complications of the UUT can happen at all stages of life, attributed to reasons such as congenital anomalies, urinary tract infections, urolithiasis and urothelial cancers, all of which require urological interventions and significantly compromise patients' quality of life. Therefore, many models have been developed to address the relevant scientific and clinical challenges of the UUT. Of all approaches, fluid mechanical modeling serves a pivotal role and various methods have been employed to develop physiologically meaningful models. In this article, we provide an overview on the historical evolution of fluid mechanical models of UUT that utilize theoretical, computational, and experimental approaches. Descriptions of the physiological functionality of each component are also given and the mechanical characterizations associated with the UUT are provided. As such, it is our aim to offer a brief summary of the current knowledge of the subject, and provide a comprehensive introduction for engineers, scientists, and clinicians who are interested in the field of fluid mechanical modeling of UUT. This article is categorized under: Cancer > Biomedical Engineering Infectious Diseases > Biomedical Engineering Reproductive System Diseases > Biomedical Engineering.

In Situ and Intraoperative Detection of the Ureter Injury Using a Highly Sensitive Piezoresistive Sensor with a Tunable Porous Structure

Iatrogenic ureteral injury, as a commonly encountered problem in gynecologic, colorectal, and pelvic surgeries, is known to be difficult to detect in situ and in real-time. Consequently, this injury may be left untreated, thereby leading to serious complications such as infections, renal failure, or even death. Here, high-performance tubular porous pressure sensors were proposed to identify the ureter in situ intraoperatively. The electrical conductivity, mechanical compressibility, and sensor sensitivity can be tuned by changing the pore structure of porous conductive composites. A low percolation threshold of 0.33 vol % was achieved due to the segregated conductive network by pores. Pores also lead to a low effective Young's modulus and high compressibility of the composites and thus result in a high sensitivity of 448.2 kPa^-1 of sensors, which is consistent with the results of COMSOL simulation. Self-mounted on the tip of forceps, the sensors can monitor tube pressures with different frequencies and amplitudes, as demonstrated using an artificial pump system. The sensors can also differentiate ureter pulses from aorta pulses of a Bama minipig in situ and in real-time. This work provides a facile, cost-effective, and nondestructive method to identify the ureter intraoperatively, which cannot be effectively achieved by traditional methods.

Uncertainty quantification reveals the physical constraints on pumping by peristaltic hearts

Most biological functional systems are complex, and this complexity is a fundamental driver of diversity. Because input parameters interact in complex ways, a holistic understanding of functional systems is key to understanding how natural selection produces diversity. We present uncertainty quantification (UQ) as a quantitative analysis tool on computational models to study the interplay of complex systems and diversity. We investigate peristaltic pumping in a racetrack circulatory system using a computational model and analyse the impact of three input parameters (Womersley number, compression frequency, compression ratio) on flow and the energetic costs of circulation. We employed two models of peristalsis (one that allows elastic interactions between the heart tube and fluid and one that does not), to investigate the role of elastic interactions on model output. A computationally cheaper surrogate of the input parameter space was created with generalized polynomial chaos expansion to save computational resources. Sobol indices were then calculated based on the generalized polynomial chaos expansion and model output. We found that all flow metrics were highly sensitive to changes in compression ratio and insensitive to Womersley number and compression frequency, consistent across models of peristalsis. Elastic interactions changed the patterns of parameter sensitivity for energetic costs between the two models, revealing that elastic interactions are probably a key physical metric of peristalsis. The UQ analysis created two hypotheses regarding diversity: favouring high flow rates (where compression ratio is large and highly conserved) and minimizing energetic costs (which avoids combinations of high compression ratios, high frequencies and low Womersley numbers).

Detection of Vesicoureteral Reflux Using Electrical Impedance Tomography

OBJECTIVE

The purpose of this study is to detect vesicoureteral reflux (VUR) noninvasively using an electrical impedance tomography (EIT). VUR is characterized by the backflow of urine from the bladder to the kidneys.

METHODS

Using porcine models, small quantities of a solution mimicking the electrical properties of urine were infused into each ureter. EIT measurements were taken before, during and after the infusion using electrodes positioned around the abdomen. The collected data from 116 experiments were then processed and time-difference images reconstructed. Objective VUR detection was determined through statistical analysis of the mean change in the voltage signals and EIT image pixel intensities.

RESULTS

Unilateral VUR was successfully detected in 94.83% of all mean voltage signals and in over 98.28% of the reconstructed images. The images showed strong visual contrast between the region of interest and the background.

CONCLUSION

In animal models, EIT has the capability to detect reflux in the kidneys with high accuracy. The results show promise for EIT to be used for screening of VUR in children.

SIGNIFICANCE

VUR is the most common congenital urinary tract abnormality in children. The condition predisposes children to urinary tract infections and kidney damage. The current gold standard diagnostic test, a voiding cystourethrogram, is invasive and uses ionizing radiation; therefore, there is a need for new tools for identifying VUR in children. This study presents a noninvasive method to detect VUR in animal models, illustrating the potential for EIT as a screening tool in clinical scenarios.

Fluid-structure interaction simulation of ureter with vesicoureteral reflux and primary obstructed megaureter

Two common abnormalities in ureters include primary refluxing megaureter (PRM) and primary obstructed megaureter (POM). The aim of this study was to represent the numerical simulation of the urine flow at the end of the ureter with vesicoureteral reflux (VUR) and POM during peristalsis. Methodologically, the peristalsis in the ureter wall was created using Gaussian distribution. Fluid-structure interaction (FSI) was applied to simulate urine-elastic wall interactions; and governing equations were solved using the arbitrary Lagrangian-Eulerian method. Theories such as wall elasticity, Newtonian fluid, and incompressible Navier-Stokes equations were used. Velocity fields, viscous stresses and volumetric outflow rate profiles were obtained through the simulation of the ureter with VUR and POM during peristalsis. In addition, the effect of urine viscosity on flow rate was investigated. When the bladder pressure increased, VUR occurred because of the ureterovesical junction (UVJ) dysfunction, leading to high stresses on the wall. In the POM, the outflow rate was ultimately zero, and stresses on the wall were severe in the obstructed section. Comparing the results demonstrated that the peristalsis leads to even further dilation of the prestenosis portion. It was also observed that the reflux occurs in the ureter with VUR when the bladder pressure is high. Additionally, the urine velocity during the peristalsis was higher than the non-peristaltic ureter.

Three-Dimensional Numerical Simulations of Peristaltic Contractions in Obstructed Ureter Flows

Ureteral peristalsis can be considered as a series of waves on the ureteral wall, which transfers the urine along the ureter toward the bladder. The stones that form in the kidney and migrate to the ureter can create a substantial health problem due to the pain caused by interaction of the ureteral walls and stones during the peristaltic motion. Three-dimensional (3D) computational fluid dynamics (CFD) simulations were carried out using the commercial code ansys fluent to solve for the peristaltic movement of the ureter, with and without stones. The effect of stone size was considered through the investigation of varying obstructions of 5%, 15%, and 35% for fixed spherical stone shape. Also, an understanding of the effect of stone shape was obtained through separate CFD calculations of the peristaltic ureter with three different types of stones, a sphere, a cube, and a star, all at a fixed obstruction percentage of 15%. Velocity vectors, mass flow rates, pressure gradients, and wall shear stresses were analyzed along one bolus of urine during peristalsis of the ureteral wall to study the various effects. It was found that the increase in obstruction increased the backflow, pressure gradients, and wall shear stresses proximal to the stone. On the other hand, with regard to the stone shape study, while the cube-shaped stones resulted in the largest backflow, the star-shaped stone showed highest pressure gradient magnitudes. Interestingly, the change in stone shape did not have a significant effect on the wall shear stress at the obstruction level studied here.

A biomechanical simulation of ureteral flow during peristalsis using intraluminal morphometric data

Reflux nephropathy and vesicoureteral reflux are two of the most important abnormalities in the upper urinary system in which toxins and bacteria from the bladder infect the ureter and the kidney and initiate renal scar formation. A quantitative analysis that characterizes urine flow will further help our understanding of the ureter and also assist in the design of flow aided devices such as valves and stents to correct reflux situations. Here, A numerical simulation with fluid-structure interactions (FSI) using arbitrary Lagrangian-Eulerian (ALE) formulation and adaptive mesh procedure was introduced and solved to perform ureteral flow analysis. Incompressible Navier-Stokes equations were utilized as the governing equations of fluid domain. Ureteral in-vivo morphometric data during peristalsis were used to construct the presented model. A nonlinear material model was used to exhibit ureteral wall mechanical properties. Direct coupling method was used to solve the solid, fluid and interface equations simultaneously. Results showed that recirculation regions formed against the jet flow, neighboring the bolus peak. Through wave propagation, separation occurred behind the moving bolus on the wall and ureteropelvic reflux began from that location and extended upstream to the ureteral inlet. The maximum luminal pressure consistently occurred behind the urine bolus during peristalsis. The measured magnitude of maximum volumetric flow rate resulted from isolated bolus transportation was 0.92 ml/min. In conclusion; due to presence of fluid inertial forces during peristalsis, the function of ureteropelvic junction in prevention of reflux is significant, especially at the beginning of peristaltic wave propagation. Moreover, modeling of ureteral function using imaging data will be valuable and it may help physicians to diagnose and cure the abnormalities.

A structural basis for sustained bacterial adhesion: biomechanical properties of CFA/I pili

Enterotoxigenic Escherichia coli (ETEC) are a major cause of diarrheal disease worldwide. Adhesion pili (or fimbriae), such as the CFA/I (colonization factor antigen I) organelles that enable ETEC to attach efficiently to the host intestinal tract epithelium, are critical virulence factors for initiation of infection. We characterized the intrinsic biomechanical properties and kinetics of individual CFA/I pili at the single-organelle level, demonstrating that weak external forces (7.5 pN) are sufficient to unwind the intact helical filament of this prototypical ETEC pilus and that it quickly regains its original structure when the force is removed. While the general relationship between exertion of force and an increase in the filament length for CFA/I pili associated with diarrheal disease is analogous to that of P pili and type 1 pili, associated with urinary tract and other infections, the biomechanical properties of these different pili differ in key quantitative details. Unique features of CFA/I pili, including the significantly lower force required for unwinding, the higher extension speed at which the pili enter a dynamic range of unwinding, and the appearance of sudden force drops during unwinding, can be attributed to morphological features of CFA/I pili including weak layer-to-layer interactions between subunits on adjacent turns of the helix and the approximately horizontal orientation of pilin subunits with respect to the filament axis. Our results indicate that ETEC CFA/I pili are flexible organelles optimized to withstand harsh motion without breaking, resulting in continued attachment to the intestinal epithelium by the pathogenic bacteria that express these pili.

A mathematical simulation of the ureter: effects of the model parameters on ureteral pressure/flow relations

Ureteral peristaltic mechanism facilitates urine transport from the kidney to the bladder. Numerical analysis of the peristaltic flow in the ureter aims to further our understanding of the reflux phenomenon and other ureteral abnormalities. Fluid-structure interaction (FSI) plays an important role in accuracy of this approach and the arbitrary Lagrangian-Eulerian (ALE) formulation is a strong method to analyze the coupled fluid-structure interaction between the compliant wall and the surrounding fluid. This formulation, however, was not used in previous studies of peristalsis in living organisms. In the present investigation, a numerical simulation is introduced and solved through ALE formulation to perform the ureteral flow and stress analysis. The incompressible Navier-Stokes equations are used as the governing equations for the fluid, and a linear elastic model is utilized for the compliant wall. The wall stimulation is modeled by nonlinear contact analysis using a rigid contact surface since an appropriate model for simulation of ureteral peristalsis needs to contain cell-to-cell wall stimulation. In contrast to previous studies, the wall displacements are not predetermined in the presented model of this finite-length compliant tube, neither the peristalsis needs to be periodic. Moreover, the temporal changes of ureteral wall intraluminal shear stress during peristalsis are included in our study. Iterative computing of two-way coupling is used to solve the governing equations. Two phases of nonperistaltic and peristaltic transport of urine in the ureter are discussed. Results are obtained following an analysis of the effects of the ureteral wall compliance, the pressure difference between the ureteral inlet and outlet, the maximum height of the contraction wave, the contraction wave velocity, and the number of contraction waves on the ureteral outlet flow. The results indicate that the proximal part of the ureter is prone to a higher shear stress during peristalsis compared with its middle and distal parts. It is also shown that the peristalsis is more efficient as the maximum height of the contraction wave increases. Finally, it is concluded that improper function of ureteropelvic junction results in the passage of part of urine back flow even in the case of slow start-up of the peristaltic contraction wave.

Fast uncoiling kinetics of F1C pili expressed by uropathogenic Escherichia coli are revealed on a single pilus level using force-measuring optical tweezers

Uropathogenic Escherichia coli (UPEC) express various kinds of organelles, so-called pili or fimbriae, that mediate adhesion to host tissue in the urinary tract through specific receptor-adhesin interactions. The biomechanical properties of these pili have been considered important for the ability of bacteria to withstand shear forces from rinsing urine flows. Force-measuring optical tweezers have been used to characterize individual organelles of F1C type expressed by UPEC bacteria with respect to such properties. Qualitatively, the force-versus-elongation response was found to be similar to that of other types of helix-like pili expressed by UPEC, i.e., type 1, P, and S, with force-induced elongation in three regions, one of which represents the important uncoiling mechanism of the helix-like quaternary structure. Quantitatively, the steady-state uncoiling force was assessed as 26.4 ±1.4 pN, which is similar to those of other pili (which range from 21 pN for S(I) to 30 pN for type 1). The corner velocity for dynamic response (1,400 nm/s) was found to be larger than those of the other pili (400-700 nm/s for S and P pili, and 6 nm/s for type 1). The kinetics were found to be faster, with a thermal opening rate of 17 Hz, a few times higher than S and P pili, and three orders of magnitude higher than type 1. These data suggest that F1C pili are, like P and S pili, evolutionarily selected to primarily withstand the conditions expressed in the upper urinary tract.

Multipili attachment of bacteria with helixlike pili exposed to stress

A number of biomechanical properties of various types of pili expressed by Escherichia coli, predominantly their force-versus-elongation behavior, have previously been assessed in detail on a single pilus level. In vivo, however, bacteria bind in general to host cells by a multitude of pili, which presumably provides them with adhesion properties that differs from those of single pili. Based upon the previously assessed biomechanical properties of individual pili, this work presents a theoretical analysis of the adhesion properties of multipili-attaching bacteria expressing helixlike pili exposed to an external force. Expressions for the adhesion lifetime of dual- and multipili-attaching bacteria are derived and their validity is verified by Monte Carlo simulations. It is demonstrated that the adhesion lifetime of a multipili-binding bacterium depends to a large degree on the cooperativity of the attaching pili, which, in turn, depends strongly on their internal biomechanical properties, in particular their helixlike structure and its ability to elongate, which, in turn, depends on the intrinsic properties of the bonds, e.g., their lengths and activation energies. It is shown, for example, that a decrease in the length of a layer-to-layer bond in the rod of P pili, expressed by E. coli, by 50% leads to a decrease in the adhesion lifetime of a bacterium attaching by ten pili and exposed to a force of 500 pN by three orders of magnitude. The results indicate moreover that the intrinsic properties of the rod for this particular type of pili are optimized for multipili attachment under a broad range of external forces and presumably also to its in vivo environment. For example, P pili seems to be optimized to withstand a force exposure during approximately 3 s, which correspond to the time it takes for a bolus to pass a bacterium attached to the ureteral wall. Even though the results presented in this work apply quantitatively to one type of pilus, they are assumed to apply qualitatively to all helixlike pili systems expressing slip bonds.

A numerical simulation of peristaltic motion in the ureter using fluid structure interactions

An axisymmetric model with fluid-structure interactions (FSI) is introduced and solved to perform ureter flow and stress analysis. The Navier-Stokes equations are solved for the fluid and a linear elastic model for ureter is used. The finite element equations for both the structure and the fluid were solved by the Newton-Raphson iterative method. Our results indicated that shear stresses were high around the throat of moving contracted wall. The pressure gradient magnitude along the ureter wall and the symmetry line had the maximum value around the throat of moving contracted wall which decreased as the peristalsis propagates toward the bladder. The flow rate at the ureter outlet at the end of the peristaltic motion was about 650 mm3/s. During propagation of the peristalsis toward the bladder, the inlet backward flow region was limited to the areas near symmetry line but the inner ureter backward flow regions extended to the whole ureter contraction part. The backward flow was vanished after 1.5 seconds of peristalsis propagation start up and after that time the urine flow was forward in the whole ureter length, so reflux is more probable to be present at the beginning of the wall peristaltic motion.

Intraluminal pressure changes in vivo in the middle and distal pig ureter during propagation of a peristaltic wave

OBJECTIVES

To establish the characteristics of mechanical activity during ureteral peristalsis and unidirectional bolus transport, pressure changes in the middle and distal (juxtavesical and ureterovesical junction) porcine ureter were quantified in vivo.

METHODS

Five female New Yorkshire pigs (50 to 60 kg) were studied under halothane anesthesia. The endoscopic approach was used to position an 8-channel 6 F perfusion catheter under direct vision into the distal ureter by way of the orifice. Ureteral activity was studied in two separate sessions at 1-week intervals. The pressure, propagation velocity, and length of the peristaltic waves were analyzed.

RESULTS

The average maximal pressure in a not previously instrumented ureter amounted to 35.7 +/- 1.2 cm H(2)O in the mid-ureter, and decreased to 19.4 +/- 1.3 cm H(2)O in the juxtavesical ureter (P < 0.001) and further to 7.2 +/- 1.0 cm H(2)O (P < 0.001) in the submucosal segment. The propagation velocity of the peristaltic wave through the ureter was 2.1 +/- 1.3 cm/s. The length of the pressure peak was 5.9 +/- 1.6 cm.

CONCLUSIONS

A ureteral peristaltic contraction wave travels at approximately 2 cm/s and is approximately 6 cm long. It is responsible for the unidirectional transport of a urinary bolus and itself acts as an "active" antireflux mechanism. The maximal pressure in the lumen of the ureter decreased from proximally to distally, but remains sufficiently high at the ureterovesical junction to prevent retrograde urine leakage when the ureter empties its urinary bolus into the bladder and the orifice is open.

Videomicroscopic imaging of ureteral peristaltic function in rats during cystometry

Videomicroscopic imaging of the upper urinary tract was performed in 26 female anesthetized rats during bladder filling and micturition. Recordings were made of the pressure of the renal pelvis through a nephrostomy and visualization of the dynamics of the ureteral bolus. Peristaltic velocity, frequency, bolus length, and direction urine bolus propagation were derived on the basis of image processing using indigo carmine for contrast. In addition, nonstop cystometrograms were performed at an infusion rate of 0.22 ml/min characterizing bladder filling and micturition reflexes. Using this setup the pharmacological response of the upper and lower urinary tract dynamics to intravenous oxybutynin and LY274614 was evaluated and compared to observations made with placebo time controls. The data, obtained from the time controls, indicate that there is a significant time-dependent influence on the upper urinary function caused by the experimental methodology in the frequency of ureteral peristalsis and length of the bolus. Oxybutynin produced a significant increase in the length of the but not in the velocity of the bolus. LY274614 depressed pelvic pressure and ureteral frequency and increased bolus length. It is concluded that videomicroscopic imaging, in association with nonstop cystometry, provides a unique method to investigate the pharmacological effects of centrally and peripherally acting drugs on the upper and lower urinary tract function without mechanical manipulation of the ureter.

Cystometry

Cystometry provides crucial information on which therapy for voiding dysfunction is predicated. The technique of cystometry can be altered to address specific clinical questions; however, the goal of the study is to reproduce the clinical situation being investigated. Specific areas remain to be clarified, including the estimation and interpretation of compliance and the utility of standard versus natural filling methods.

Progress in urodynamic research on the upper urinary tract: implications for practical urology

The development of new surgical techniques for bladder substitution and continent urinary diversion has extended interest in urodynamics of the upper urinary tract. From a subdiscipline attracting mainly scientists and bioengineers, renal pelvic kinetics and ureteral peristalsis have evolved as important factors in routine clinical urology. The observed changes in peristaltic pattern during high diuresis, obstruction and urinary reflux have influenced management of stone disease and neurogenic bladder. The demonstration that high intravesical pressure is reflected to the kidney not only when the ureteric orifice is incompetent, but also during high diuresis, established the necessity for low pressures in neobladders. Much further clarification of urinary transport from the renal tubules to the bladder should be achievable by refined techniques of fluoroscopy, isotopic renography and manometry.

The sensitivity of pressure specific bladder volume versus total bladder capacity as a measure of bladder storage dysfunction

Assessment of bladder storage function requires an accurate measure of bladder capacity and pressure. Pressure specific bladder volume is the volume that a bladder can accommodate at a specific pressure. A total of 21 consecutive children with neurogenic bladders who were candidates for bladder augmentation based on standard clinical criteria (upper urinary tract deterioration, incontinence and infection) was studied to determine the efficacy of pressure specific bladder volume as a measure of bladder dysfunction. Urodynamic indexes were compared to previously established nomograms. All 21 patients had bladder volumes at pressures of 30 cm. water or less, which decreased below the 5th percentile as determined by the nomogram. In 7 patients (33%) normal total bladder capacity was achieved at the expense of elevated storage pressures. Pressure specific bladder volume provides a better measure of bladder storage function than total bladder capacity because it relates volume to intravesical pressure, does not rely on a subjective end point to bladder filling, and is objective and reproducible.

The effect of urinary flow and bladder fullness on renal pelvic pressure in a rat model

We describe an in vivo animal model used to study the interactions of urinary flow, and bladder pressure and fullness on renal pelvic pressure. These parameters were examined in 17 nonrefluxing Sprague-Dawley rats. At urinary flow rates less than 14 cc/kg. per hour and bladders less than 60% full, renal pelvic pressures were below 9 cm. water but at urinary flow rates more than 30 cc/kg. per hour renal pelvic pressure increased above 10 cm. water when the bladder was only 20% full. At all urinary flow rates examined renal pelvic pressure increased to more than 20 cm. water as the bladder approached 100% fullness. To quantitate the combined effects of these changes in renal pelvic pressure and urinary flow on the renal pelvis a renal pelvic work index (renal pelvic pressure times urinary flow rate) was defined. Using this index the magnitude of the change between low urinary flows with an empty bladder and high urinary flows with a full bladder can be observed. The results of these studies in this model might be applicable to high urinary flow states or bladders that fail to empty completely.