Predicting urethral area from video-urodynamics in women with voiding dysfunction

Aims: Mathematical models are useful for developing predictive parameters for characterizing the biomechanics of voiding dysfunction. The goal of this project was to test a one-dimensional steady flow model used to predict the minimum cross-sectional urethral area from urodynamic data. Methods: Nine adult female subjects underwent video-urodynamic testing. By using Bernoulli's formula and the Torricelli theorem, the minimum urethral area was predicted from pressure and flow rate at the moment of maximum flow rate during voiding. This prediction was compared with the minimum cross-sectional area of the urethra, which was calculated from minimum urethral diameter as measured from fluoroscopy, assuming a circular cross-section. Results: The maximum flow rate during voiding was 14.4 +/- 3.0 mL/sec. Mean bladder, abdominal, and detrusor pressures simultaneous with maximum flow rate were 63 +/- 7,29 +/- 6, and 33 +/- 6 cm H2O, respectively. Mean minimum cross-sectional area of the urethra from fluoroscopy was 8.0 +/- 2.0 mm(2). Mean minimum cross-sectional area of the urethra predicted by the mathematical model was 5.0 +/- 1.0 mm(2) using bladder pressure and 7.0 +/- 2.0 mm(2) using detrusor pressure. There were no significant differences between the three cross-sectional area measures. However, when area predictions were expressed as percentage of fluoroscopic measurements, the estimate from detrusor pressure (97 +/- 13%) was significantly larger than the estimate from bladder pressure (69 +/- 7%). Conclusions: A steady flow model is accurate enough on average to describe urine flow in the urethra. However, it may not be sufficiently accurate to aid in diagnosis of individuals.