An Analysis of Detrusor Dynamics and Urethral Flow in the Canine Urinary Tract, under Obstructive and Non-Obstructive Conditions

Benign prostatic hyperplasia (BPH) is typically a disease of men aged 40 years, and above. By the age of 50 years, approximately 20% of men are affected, while by the age of 75 years, approximately 75% of men are affected. Given this incidence, and the aging population, BPH has become a major health care issue. The pathology of BPH is induced by a hyperplastic prostate occluding the bladder outlet. How an outlet obstruction impacts both the detrusor function and urethral flow must, therefore, be fully understood before an optimal diagnosis or treatment of BPH can be formulated. This study uses a canine model to evaluate how the passive urethral resistance, the detrusor internal work (W, = VAp) and the detrusor external work (W, = pQdt) respond to a changing outlet obstruction. The experimental data is also used to define a urethral pressure-area relation, and to characterize urethral flows in terms of a Reynolds number and an entry length. The detrusor function, outlet resistance and urethral flows of five female canines were assessed under non-obstructive (phase I) and obstructive (phase II) conditions. All urodynamic analyses were performed on a surgically exposed urinary tract. Solid state pressure transducers were used to measure intravesical and distal urethral pressures. An ultrasonic flowmeter was used to obtain an instantaneous measurement of the urethral flowrate. An electrical stimulation (30V, 50Hz, 0. msec and 10sec duration) of the pelvic nerves was used to induce bladder contractions. All outlet obstructions were modeled using an inflatable sphincter cuff. During the phase II studies, an outlet was assumed nonobstructed at a 0% cuff volume and completely occluded at a 100% cuff volume. The mean outlet resistances of the phase I studies ranged between 0.07-1.7 cmH2Osml ' 2 . As outlet obstruction was increased, the normalized internal work, normalized external work and normalized outlet resistance (all normalized with respect to the corresponding mean phase I result) remained approximately constant until a threshold cuff volume 60% for the work parameters, and 75% for the resistance was exceeded. As obstruction was increased beyond the respective thresholds, the normalized outlet resistance and the normalized internal work increased from their (non-obstructed) unity references to 400 and 4 respectively. The normalized external work decreased from unity to zero. At cuff volumes below 60%, the magnitudes of the internal work and the external work remained approximately equal. At volumes exceeding 60%, however, the internal work increased while the external work decreased. The concept of the internal work as an elastic energy that is fully recovered during voiding thus appears valid only at low outlet obstructions. As the normalized resistance was increased from unity to 60, the normalized internal work increased exponentially, while the normalized external work decreased exponentially. At normalized resistances exceeding 100, both normalized work parameters were plateaued at their maximum and minimum values of 4 and 0 respectively. Given this exponential correlation, at low outlet obstructions, the internal and external work appear more sensitive to changes in the outlet obstruction than the outlet resistance. Under non-obstructive conditions, the peak Reynolds numbers R,P and entry length to diameter ratios li/D obtained ranged between 500 <R,P< 1500, and 30 < /D< 90. The non-obstructed urethral diameters D computed ranged between 1.5mm < D <2.5mm, while the urethral lengths I used for the urodynamic assessments ranged between 75mm < I < 95mm. The non-obstructed phase I flows were thus typically laminar entry length flows. The peak Reynolds numbers for the obstructed phase II studies also remained within the boundaries of laminar flow.