An evaluation of contractility parameters determined from isometric contractions and micturition studies

Quantifying whole bladder biomechanics using the novel pentaplanar reflected image macroscopy system

Optimal bladder compliance is essential to urinary bladder storage and voiding functions. Calculated as the change in filling volume per change in pressure, bladder compliance is used clinically to characterize changes in bladder wall biomechanical properties that associate with lower urinary tract dysfunction. But because this method calculates compliance without regard to wall structure or wall volume, it gives little insight into the mechanical properties of the bladder wall during filling. Thus, we developed Pentaplanar Reflected Image Macroscopy (PRIM): a novel ex vivo imaging method to accurately calculate bladder wall stress and stretch in real time during bladder filling. The PRIM system simultaneously records intravesical pressure, infused volume, and an image of the bladder in five distinct visual planes. Wall thickness and volume were then measured and used to calculate stress and stretch during filling. As predicted, wall stress was nonlinear; only when intravesical pressure exceeded ~ 15 mmHg did bladder wall stress rapidly increase with respect to stretch. This method of calculating compliance as stress vs stretch also showed that the mechanical properties of the bladder wall remain similar in bladders of varying capacity. This study demonstrates how wall tension, stress and stretch can be measured, quantified, and used to accurately define bladder wall biomechanics in terms of actual material properties and not pressure/volume changes. This method is especially useful for determining how changes in bladder biomechanics are altered in pathologies where profound bladder wall remodeling occurs, such as diabetes and spinal cord injury.

A computational analysis of the effect of supporting organs on predicted vesical pressure in stress urinary incontinence

Stress urinary incontinence (SUI) or urine leakage from urethra occurs due to an increase in abdominal pressure resulting from stress like a cough or jumping height. SUI is more frequent among post-menopausal women. In the absence of bladder contraction, vesical pressure exceeds urethral pressure leading to urine leakage. The main aim of this study is to utilize fluid-structure interaction techniques to model bladder and urethra computationally under an external pressure like sneezing. Both models have been developed with linear elastic properties for the bladder wall while the patient model has also been simulated utilizing the Mooney-Rivlin solid model. The results show a good agreement between the clinical data and the predicted values of the computational models, specifically the pressure at the center of the bladder. There is 1.3% difference between the predicted vesical pressure and the vesical pressure obtained from urodynamic tests. It can be concluded that the accuracy of the predicted pressure in the center of the bladder is significantly higher for the simulation assuming nonlinear material property (hyperelastic) for the bladder in comparison to the accuracy of the linear elastic model. The model is beneficial for exploring treatment solutions for SUI disorder. Graphical abstract ^{3D processing of bladder deformation during abdominal pressure of a the physiological model and b the pathological model (starting from left to right and up to down, consecutively)}.

Work Capacity of the Bladder During Voiding: A Novel Method to Evaluate Bladder Contractile Function and Bladder Outlet Obstruction

Background:

Work in voiding (WIV) of the bladder may be used to evaluate bladder status throughout urination rather than at a single time point. Few studies, however, have assessed WIV owing to the complexity of its calculations. We have developed a method of calculating work capacity of the bladder while voiding and analyzed the associations of bladder work parameters with bladder contractile function and bladder outlet obstruction (BOO).

Methods:

The study retrospectively evaluated 160 men and 23 women, aged >40 years and with a detrusor pressure at maximal flow rate (P_det Q_max) of ≥40 cmH₂O in men, who underwent urodynamic testing. The bladder power integration method was used to calculate WIV; WIV per second (WIV/t) and WIV per liter of urine voided (WIV/v) were also calculated. In men, the relationships between these work capacity parameters and P_det Q_max and Abrams-Griffiths (AG) number were determined using linear-by-linear association tests, and relationships between work capacity parameters and BOO grade were investigated using Spearman's association test.

Results:

The mean WIV was 1.15 ± 0.78 J and 1.30 ± 0.88 J, mean WIV/t was 22.95 ± 14.45 mW and 23.78 ± 17.02 mW, and mean WIV/v was 5.59 ± 2.32 J/L and 2.83 ± 1.87 J/L in men and women, respectively. In men, WIV/v showed significant positive associations with P_det Q_max (r = 0.845, P = 0.000), AG number (r = 0.814, P = 0.000), and Schafer class (r = 0.726, P = 0.000). Conversely, WIV and WIV/t showed no associations with P_det Q_max or AG number. In patients with BOO (Schafer class > II), WIV/v correlated positively with increasing BOO grade.

Conclusions:

WIV can be calculated from simple urodynamic parameters using the bladder power integration method. WIV/v may be a marker of BOO grade, and the bladder contractile function can be evaluated by WIV and WIV/t.

International Continence Society guidelines on urodynamic equipment performance

These guidelines provide benchmarks for the performance of urodynamic equipment, and have been developed by the International Continence Society to assist purchasing decisions, design requirements, and performance checks. The guidelines suggest ranges of specification for uroflowmetry, volume, pressure, and EMG measurement, along with recommendations for user interfaces and performance tests. Factors affecting measurement relating to the different technologies used are also described. Summary tables of essential and desirable features are included for ease of reference. It is emphasized that these guidelines can only contribute to good urodynamics if equipment is used properly, in accordance with good practice.

Regional biomechanical and histological characterisation of the passive porcine urinary bladder: Implications for augmentation and tissue engineering strategies

The aim of this study was to identify and quantify potential regional and directional variations in the quasistatic uniaxial mechanical properties of the passive urinary bladder wall. Overall, the lower body and trigone regions demonstrated the highest degree of directional anisotropy, whereas the ventral region demonstrated the least directional anisotropy. Significant regional anisotropy was found only along the apex-to-base direction. The dorsal and ventral regions demonstrated a significantly increased distensibility along the apex-to-base direction compared to the other bladder regions, whereas the trigone and lower body regions demonstrated the least distensibility. The trigone, lower body and lateral regions also demonstrated the highest tensile strength both at regional and directional levels. The study detected significant regional and directional anisotropy in the mechanical properties of the bladder and correlated this anisotropy to the distended and non-distended tissue histioarchitecture and whole organ mechanics. By elucidating the inhomogeneous nature of the bladder, the results from this study will aid the regional differentiation of bladder treatments in terms of partial bladder replacement with suitable natural or synthetic biomaterials, as well as the development of more realistic constitutive models of bladder wall biomechanics and improved computational simulations to predict deformations in the natural and augmented bladder.

Modeling physiology of the urinary tract

We review mathematical and physical models of physiology of the organs of the urinary tract and their functions of producing, excreting, and voiding urine. Models for urine concentration in the kidney, urine flow in the ureters, bladder filling and emptying, urethral function during micturition, pelvic floor muscles, and neural control are reviewed in the context of their application to the development of new diagnostic and therapeutic techniques. The focus of this review is on modeling of physiology and function at the tissue and organ level, as almost all research to date has been done in those areas. Although physiological models of the lower urinary tract are in their infancy, they have the long-term potential to improve our understanding of physiological mechanisms, as well as to provide environments for simulation or testing in silico of new therapies and techniques.

Identification of bladder wall layers by Raman spectroscopy

PURPOSE

We explored the applicability of Raman spectroscopy to in situ investigation of bladder wall tissue.

MATERIALS AND METHODS

Bladder wall tissue was obtained from a guinea pig model and frozen sections were used for Raman spectroscopic investigations. From each section 500 to 700 spectra were obtained in a 2-dimensional grid spanning the urothelium, lamina propria and muscle layer. The data set of spectra was subdivided into groups of similar spectra by a cluster analysis algorithm. With each group assigned a different color Raman maps of frozen sections were constructed based on group membership of measured spectra. These maps were then compared with histological and histochemical data obtained from hematoxylin and eosin and immunohistochemical staining for collagen I and III and for smooth muscle actin to correlate Raman spectral features with bladder wall structure and molecular composition.

RESULTS

Urothelium, lamina propria and muscle layers could be clearly distinguished based on Raman spectra. Lamina propria spectra were dominated by signal contributions of collagen and the smooth muscle layer showed strong signal contributions of actin. The urothelium had a relatively strong lipid signal contribution.

CONCLUSIONS

These results and the fact that Raman spectroscopy is rapidly evolving into a technology that can be applied in vivo by thin, flexible fiberoptic catheters indicate that prospects are good for in vivo analysis of the molecular composition of the normal and pathological bladder without biopsies.

The aging bladder--a significant but underestimated role in the development of lower urinary tract symptoms

There are three major areas in which bladder dysfunction causes considerable losses in quality of life: detrusor instability, impaired detrusor function and detrusor reaction on bladder outlet obstruction. Considerable changes in detrusor morphology, detrusor innervation and bladder metabolism are seen with aging and obstruction. This causes dysfunction and lower urinary tract symptoms. The relation between aging per se and external influence on the detrusor from diseases in the nervous system, in the vascular supply and in the lower urinary tract itself is poorly understood. To improve our ability to prevent and treat these conditions we need more insight in these pathophysiological conditions.

Changes in bladder contractility and compliance due to urethral obstruction: a longitudinal followup of guinea pigs

PURPOSE

We established the longitudinal changes in bladder contractility and compliance as a result of urethral obstruction using a guinea pig model.

MATERIALS AND METHODS

Obstruction was induced in guinea pigs by a silver ring around the urethra. Urodynamic studies were performed longitudinally in individual animals. Bladder contractility and compliance were calculated from the measured bladder pressure and urine flow rate.

RESULTS

Bladder contractility developed in distinct phases. It reached a maximum 200% increase after an average of 3.25 weeks concomitant with an almost 2-fold increase in urethral resistance, remained 150% to 200% increased during weeks 4 to 7 and then decreased to starting levels again, while urethral resistance remained almost 2-fold increased. Bladder compliance decreased by 80% during the first 3 weeks and continued to decrease to 5% of its original value after 10 to 11 weeks.

CONCLUSIONS

Our data indicate that as a result of obstruction bladder function passes through a specific sequence of stages, including first a compensatory increase in contractility, then a stabilization phase and finally a decompensation state. In contrast bladder compliance shows a continuous decrease. The data suggest that for assessing how far a bladder has deteriorated due to obstruction a combination of functional and structural data may be warranted.

Dependence of male voiding efficiency on age, bladder contractility and urethral resistance: development of a voiding efficiency nomogram

The influence of age, urethral resistance and bladder contractility on voiding efficiency was evaluated by pressure-flow studies in 138 men of a mean age of 60 years (range 18 to 86). From these studies the urethral resistance parameter was calculated and the maximum bladder contraction strength was determined. Premature fading of the bladder contraction was quantified by a bladder contraction strength decay factor. Voiding efficiency was expressed by the parameter of post-void residual urine volume as a percentage of the initial bladder volume. Multiple regression analysis showed that voiding efficiency depended significantly in descending order of importance on urethral resistance, maximum bladder contraction strength and bladder contraction strength decay factor. Patient age was not an independent factor. Maximum bladder contraction strength and bladder contraction strength decay factor were not correlated, suggesting that maximum bladder contraction strength and its decay constitute different properties of bladder contractile function. A voiding efficiency nomogram is proposed, making use of the values for maximum bladder contraction strength and urethral resistance in individual patients. Such a nomogram may have predictive value for the occurrence of acute retention but it must be tested prospectively.

The effect of urinary bladder shape on its mechanics during filling

Despite the normal variation in the shape of the urinary bladder, it has always been modeled as a sphere. We have investigated whether its steady-state pressure-volume relation would be significantly different if it were a spheroid. From pressure-volume curves of anesthetized dogs, we deduced stress-strain constitutive relationships for the bladder wall material. We then solved the equilibrium equations for prolate and oblate spheroids with these constitutive relationships and predicted stress, strain, and volume at 120 different transmural pressures and eight different eccentricities of both types of spheroids. The pressure-volume relation of the prolate spheroid never differed very much from that of a sphere. However, an oblate spheroid made of urinary bladder material is significantly more compliant than either a prolate spheroid or a sphere made of the same material. Applications include identification of the position of highest stress in the bladder wall, estimation of material properties of urinary bladders, and determination of the physiological signal indicating bladder fullness.

Contractility parameters of the guinea pig bladder in situ: similarity to human bladder contractility

The parameters P(isv) (active isovolumetric detrusor pressure) and Vmax (maximum shortening velocity), which characterize the contractility of the detrusor muscle, were determined in guinea pigs. To this end it was necessary to develop a method of measuring flow rates in these small animals. The values found were used to calculate the contractility parameter Wmax. Thirteen animals were used. The results found for P(isv) and Vmax were 43.0 +/- 3.7 cm. H2O and 20.2 +/- 3.7 mm. per second, respectively. The latter corresponded to about 0.38 muscle lengths per second, which is similar to values reported for bladder strips from other species. Previous work showed that in vitro P(isv) decreased with increasing bladder volume over a wide range of volumes. In vivo P(isv) seemed to be independent of bladder volume. This suggests that neurogenic stimulation intensifies as volume increases. Vmax also was independent of volume. Wmax appeared to be suitable for detecting differences in the contractility of the bladders of different animals. Values were not significantly different in isovolumetric and nonisovolumetric contractions. Normalized to the size of the bladder, the Wmax values indicated that the power generated by the guinea pig bladder is similar to the power generated by the human bladder.

Characterisation of detrusor contractile function in relation to old age

This report describes an age-related clinical study of detrusor muscle function using urodynamic data obtained from 1391 women and 324 men of different age groups who were free from overt neurological disease and diabetes. Isometric detrusor function during unstable activity was assessed by integrating the calculated force of unstable detrusor activity during filling of the bladder. Isotonic voiding activity was measured by calculating the velocity constant Q*, which reflects the speed of detrusor muscle shortening. In addition, the change in detrusor pressure on initiation of voiding and the post-micturition residual urine volume were recorded. Older patients of both sexes had higher residual urine volumes. Older women showed lower detrusor shortening velocities. There were no age-related differences in isometric detrusor function. Older women showed a tendency for the detrusor contraction to fail on initiation of voiding, whereas older men did not demonstrate this difference. Voiding dysfunction in late life was more marked amongst women and seemed to be related to a failure of isotonic detrusor function with no related changes in isometric function.

The Cholinergic and Purinergic Components of Detrusor Contractility in a Whole Rabbit Bladder Model

Whole rabbit bladders were suspended in a bath chamber and stimulated with ATP, bethanechol, electrical field stimulation, and bethanechol + ATP. Detrusor pressure and fluid expelled by the bladder were recorded, synchronized, and digitized. Detrusor work and power were calculated with a computer program. Maximum work was 61.4 +/- 28.7, 83.3 +/- 17.0, 85.0 +/- 15.0, 90.8 +/- 13.1 cm. H2O, ml. for ATP, bethanechol, electrical and bethanechol + ATP, respectively. Maximum power generated by ATP was 4.8 +/- 3.0 cm. H2O, ml./sec and was approximately 66% of that generated by bethanechol, and 50% of that generated by electrical stimulation, and bethanechol + ATP. ATP cannot empty the bladder with moderate outlet resistance while bethanechol and electrical stimulation can. Our results suggest that ATP is able to generate detrusor power and achieve work in bladder emptying. However, ATP generated power and work is considerably less than that of electrical stimulation or bethanechol alone. ATP mediated contraction is not inhibited by atropine or tetrodotoxin but is inhibited by P2 purinoceptor desensitization, suggesting a functional role of purine receptors on detrusor smooth muscle. Since ATP generated pressure is more rapid than with bethanechol alone, we support the hypothesis that ATP may be important in the initiation of micturition.

Estimation of the maximum contraction velocity of the urinary bladder from pressure and flow throughout micturition

The contractility of the urinary bladder can be adequately described in terms of the parameters P0 (isometric pressure) and Vmax (maximum contraction velocity). In about 12% of urodynamic evaluations of patients these clinically relevant parameters can be calculated from pressure and flow rate as measured during micturition. A method was developed of estimating Vmax for any micturition from these signals. The properties of this estimated contractility parameter were clinically tested and are discussed.

Stop-flow measurement of detrusor contractility in bladder outflow obstruction

Detrusor contractility was evaluated by measuring the increase in isometric pressure during a stop test in 25 men with benign prostatic hyperplasia. Before transurethral prostatic resection (TUR), a low pressure increase (delta P det iso) was associated with a large volume of residual urine. After TUR both the speed and the amplitude of isometric detrusor contractions during the stop test increased. A pre-operative low detrusor pressure increase was correlated with a great improvement in flow rate after TUR. The value of the stop-flow test was limited, however, since improvement in other urodynamic parameters could not be predicted.

Isolation and individual electrical stimulation of single smooth-muscle cells from the urinary bladder of the pig

In contrast to striated muscle, measurements on strips of smooth muscle cannot be uniquely interpreted in terms of an array of contractile units. Therefore scaling down to the single-cell level is necessary to gain detailed understanding of the contractile process in this type of muscle. The present study describes the development of a method for isolating contractile single smooth muscle cells from pig urinary bladders. Contractile responses evoked by individual electrical stimulation were used as a measure of cell quality during development of the method. Responses were evaluated by measuring latency, contraction and relaxation times, as indicated by visible length changes, and stored on-line in a computer. Initial length, relative shortening and shortening speed were determined by measuring cell lengths in previously timed still video frames using a computer-controlled crosshair device. Increase of stimulus pulse duration resulted in improved responses, indicating that the observed shortening represented a physiological contractile response. Ultimately this method of evaluation was applied to two sets of cell preparations obtained by two different methods, one using only collagenase digestion, the other using mechanical manipulation as well. Both sets showed two main patterns of response to electrical stimulation: a pattern of contraction upon stimulation followed by enhanced contraction when stimulation was switched off (CK), and a pattern of contraction upon stimulation followed by relaxation when the stimulus was switched off (CR). The set of preparations containing the highest percentage of CR cells was found to be superior (i.e. greater initial length, shorter latency and contraction times, increased shortening and higher shortening speed). The method of isolation used for this set gives a high yield of contractile cells available for experimental use over a long span of time.