Investigation of urethral flow and cross-sectional area using video-urodynamics

An accurate mathematical model predicting urethral cross-sectional area from urodynamic data could be useful for diagnosis of voiding dysfunction. The purpose of this project was to use fluoroscopy images of the female urethra during voiding to verify a shape assumption for the urethra and a model of laminar flow in the urethra. Maximum flow rate and simultaneous bladder and detrusor pressures during voiding were used independently in a formula based on Bernoulli´s Law to predict the minimum cross-sectional area of the urethra. Cross-sectional area of the urethra was also calculated from fluoroscopic data assuming a circular cross-section. Nine adult female subjects were tested using videourodynamics. Maximum flow rate during voiding was 14.4±3.0 ml/sec. Bladder, abdominal, and detrusor pressure simultaneous with maximum flow rate were 63±7, 29±6, and 33±6 cm H ₂O, respectively. The minimum cross-sectional area of the urethra, predicted from pressure and flow was 4.4±1.0 mm² using bladder pressure and 6.6±1.6 mm² using detrusor pressure. The minimum and maximum cross-sectional areas of the urethra from fluoroscopy data were 7.9±1.9 and 93.1±31.0 mm², respectively. Percentage error of the predictions of area from pressure and flow were 39±6% using bladder pressure and 33±7% using detrusor pressure. Detrusor pressure gives a more accurate prediction of minimum cross-sectional area than bladder pressure, although the predictions were not significantly different. Future work will be aimed at improving the accuracy of both measurements and mathematical models