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 resistan ce, detrusor internal work, and detrusor external work are independent measures of the voiding process. Passive urethral resistance, detruso r internal work, detrusor external work, and detrusor total work of 5 canines were determined under nonobstructive and obstructive outlet co nditions. All urodynamic analyses were performed on a surgically expos ed urinary tract. Solid-state pressure transducers were used to measur e the intravesical and distal urethral pressures, while an ultrasonic flow meter was used to obtain a simultaneous measure of urinary flow r ate. Detrusor contractions were induced using bilateral electrical sti mulation of the pelvic nerves. Varying degrees of outlet obstruction w ere created using an inflatable sphincter cuff secured around the prox imal urethra. Urethral resistance, internal work, and total work incre ased with increasing obstruction, while external work decreased with i ncreasing obstruction. Internal work was linearly and negatively corre lated to external work. At low degrees of obstruction, internal and ex ternal work changed more rapidly than passive urethral resistance per unit change in obstruction. As obstruction was increased, the change i n work parameters per unit change in obstruction decreased, while the change in passive urethral resistance per unit change in obstruction i ncreased. Our results indicate that at low degrees of outlet obstructi on (cuff volume <60% of isometric cuff volume), detrusor internal and external work are more sensitive to changes in obstruction than passiv e urethral resistance. It therefore appears prudent to use these work parameters with a passive urethral resistance relation (PURR) when eva luating early BPH and also when monitoring its progression. The data a lso show that a detrusor wilt acutely respond to a change in outlet ob struction by modulating its total work output. The mechanisms responsi ble for this response are presently under investigation. (C) 1996 Wile y-Liss, Inc.