Elsevier

Journal of Biomechanics

Volume 61, 16 August 2017, Pages 232-241
Journal of Biomechanics

Quantification of bladder wall biomechanics during urodynamics: A methodologic investigation using ultrasound

https://doi.org/10.1016/j.jbiomech.2017.07.028Get rights and content

Abstract

Overactive bladder is often characterized by biomechanical changes in the bladder wall, but there is no established method to measure these changes in vivo. The goal of this study was to develop a novel method to determine detrusor wall biomechanical parameters during urodynamics through the incorporation of transabdominal ultrasound imaging. Individuals with overactive bladder (OAB) underwent ultrasound imaging during filling. The fill rate was 10% of the cystometric capacity per minute as determined by an initial fill. Transabdominal ultrasound images were captured in the midsagittal and transverse planes at 1 min intervals. Using image data and Pves, detrusor wall tension, stress, and compliance were calculated. From each cross  sectional image, luminal and wall areas along with inner perimeters were measured. In the sagittal and transverse planes, wall tension was calculated as Pves  luminal area, wall stress as tension/wall area, and strain as the change in perimeter normalized to the perimeter at 10% capacity. Elastic modulus was calculated as stress/strain in the medial–lateral and cranial-caudal directions. Patient-reported fullness sensation was continuously recorded. Data from five individuals with OAB showed that detrusor wall tension, volume, and strain had the highest correlations to continuous bladder sensation of all quantities measured. This study demonstrates how detrusor wall tension, stress, strain, and elastic modulus can be quantified by adding ultrasound imaging to standard urodynamics. This technique may be useful in diagnosing and better understanding the biomechanics involved in OAB and other bladder disorders.

Introduction

Overactive bladder (OAB) is a chronic filling-phase condition that adversely affects an estimated 17% of men and 30% of women (Coyne et al., 2013), but there is limited understanding of how bladder wall biomechanics influences OAB. Multi-channel urodynamics is considered the gold standard for evaluating all forms of voiding dysfunction, including OAB. However, pressure generally increases little during bladder filling (Fig. 1) (Frenkl et al., 2011) and does not reflect detrusor wall tension nor predict urgency. Because increased detrusor wall tension is likely a key factor in OAB pathophysiology in some patients (Drake et al., 2005), true filling phase physiology cannot be evaluated during standard clinical urodynamics and improved diagnostics are necessary.

Tension-sensitive afferent nerves in the bladder wall convey fullness sensation to the brain (Morrison, 1999), while bladder compliance affects the tension sensor load during filling (De Wachter et al., 2012). Bladder compliance has been shown to depend on shape (Damaser and Lehman, 1995). Bladder tension and stress have been estimated in animals by assuming a spherical shape (Le Feber et al., 2004, Watanabe et al., 1981), but bladder shape in humans can be highly variable (Bih et al., 1998, Lotz et al., 2004). For any volume of fluid, the shape with the smallest surface area is a sphere, so for a bladder that fills with a somewhat spherical shape when unconstrained, conditions making the bladder less spherical increase surface area and strain on the tension sensor. Laplace’s law states that in thin-walled vessels, such as the bladder, wall tension is proportional to the product of pressure and radius, and therefore, an increase in radius alone, with little or no change in pressure is sufficient to increase wall tension significantly (Watanabe et al., 1981).

The International Continence Society (ICS) defines compliance as the ratio of total volume change to pressure change (Abrams et al., 2002), but this can vary greatly depending on which initial and final points are used for volumes and pressures (Smith et al., 2012). This study demonstrates a method to use transabdominal ultrasound to gather bladder geometric information during urodynamics. This information along with pressure was used to compute novel urodynamic metrics of bladder filling mechanics including wall tension, stress, strain, and elastic modulus. These data are correlated with continuous patient-reported real-time sensation recorded throughout filling using a novel sensation meter (Nagle et al., 2016). These novel metrics will facilitate important new insights into the pathophysiology of bladder disorders. In particular, quantifying tension may be important in sub-characterizing a new form of bladder-wall-tension-mediated OAB. Additionally, the imaging techniques could be used to develop non-invasive means of characterizing OAB.

Section snippets

Theory

Extending work by (Watanabe et al., 1981), the bladder was modeled as a container with a thin, elastic, homogeneous wall which was theoretically cut into two sections in the transverse (medial-lateral direction) and sagittal (cranial-caudal direction) planes (Fig. 2a). The force (F) perpendicular to the cut acting to separate these halves isF=PvesAlumenwhere Pves is vesical pressure and Alumen is the luminal area in the plane of the cut (Fig.2b). This force is equal to the opposing wall tension

Results

Demographic Data: Demographic data from five individuals with OAB were obtained, including sex, race, BMI, cystometric capacity, and ICS-defined compliance (Table 1). DO was diagnosed all five participants and transient rises in pressure tended to be associated with increases in bladder sensation (Colhoun et al., 2016 a). Average Pves during the initial fill and fill with ultrasound were 54.0 ± 11.4 and 42.7 ± 11.4 cmH2O. Image resolution ranged from 0.2 to 0.23 mm/pixel. Data from a representative

Discussion

This study presents a novel method to clinically investigate bladder wall biomechanics and quantify wall stress, perimeter strain, and more using geometric data from ultrasound images and urodynamic pressure data. This is the first study to quantify these biomechanical properties in humans, and represents a potential advance over standard urodynamics which provides little information about the true state of the detrusor wall.

In standard urodynamics, bladder compliance is measured as the change

Acknowledgements

Research funding for this study was provided by the Virginia Commonwealth University Presidential Research Quest Fund, the Dean’s Undergraduate Research Initiative, and NIH grant R01DK101719. The authors would like to thank Sandy Smith, Rachel Wilbur, Kimberly Bradley, Zachary Cullingsworth, Stefan Harris, and Paul Ratz for their technical contributions to this work.

Conflict of interest

The authors have no conflicts of interest associated with this manuscript.

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