Detrusor internal and external work in relation to passive urethral resistance in a canine model of the lower urinary tract.

This study was conducted to evaluate whether passive urethral resistance, detrusor internal work, and detrusor external work are independent measures of the voiding process. Passive urethral resistance, detrusor internal work, detrusor external work, and detrusor total work of 5 canines were determined under nonobstructive and obstructive outlet conditions. All urodynamic analyses were performed on a surgically exposed urinary tract. Solid-state pressure transducers were used to measure the intravesical and distal urethral pressures, while an ultrasonic flow meter was used to obtain a simultaneous measure of urinary flow rate. Detrusor contractions were induced using bilateral electrical stimulation of the pelvic nerves. Varying degrees of outlet obstruction were created using an inflatable sphincter cuff secured around the proximal urethra. Urethral resistance, internal work, and total work increased with increasing obstruction, while external work decreased with increasing obstruction. Internal work was linearly and negatively correlated to external work. At low degrees of obstruction, internal and external work changed more rapidly than passive urethral resistance per unit change in obstruction. As obstruction was increased, the change in work parameters per unit change in obstruction decreased, while the change in passive urethral resistance per unit change in obstruction increased. Our results indicate that at low degrees of outlet obstruction (cuff volume < 60% of isometric cuff volume), detrusor internal and external work are more sensitive to changes in obstruction than passive urethral resistance. It therefore appears prudent to use these work parameters with a passive urethral resistance relation (PURR) when evaluating early BPH and also when monitoring its progression. The data also show that a detrusor will acutely respond to a change in outlet obstruction by modulating its total work output. The mechanisms responsible for this response are presently under investigation.