PARAMETERS OF DETRUSOR CONTRACTILITY .1. THE EFFECTS OF ELECTRICAL-STIMULATION, HYSTERESIS, AND BLADDER VOLUME IN AN INVITRO WHOLE RABBIT BLADDER MODEL

This study represents the first of a five phase project which was designed to develop parameters to assess detrusor contractility and urethral obstruction. The first phase was designed to 1) determine the optimal volume at which to measure various parameters of detrusor contractility and 2) determine the effects of hysteresis on isovolumetric detrusor pressure (Piso). To this end, the bladders of five male white New Zealand rabbits were isolated and placed in a bath chamber. Intravesical pressure and the weight of the voided urine were monitored by a pressure transducer and electronic balance, respectively. Bladder capacity ranged between 41 ml and 53 ml (mean 47 +/- 2.34). In the first phase of this experiment, Piso was determined at 1, 2.5, 5, 10, and 20 ml. Piso was always found at a volume of 2.5 ml (20.7 +/- 2.86). There was a 13% decrease in Piso at 1 and 5 ml, a 35% decrease at 10 ml, and a 49% decrease at 20 ml. This was significant at P < 0.05. In the second part of the experiment, the effect of different methods of filling on the magnitude of subsequent isometric contractions was examined. It was found that preceding strain (hysteresis) had no effect on subsequent contractions. In addition, the average Piso for each volume remained the same regardless of the method of filling. In conclusion, these data suggest that the effect of hysteresis and method of bladder filling on Piso was negligible. In addition, as one would expect from the geometric configuration of the bladder, maximum detrusor pressure occurred at relatively low bladder volumes, i.e., less than 5% capacity. Further research is in progress to determine which parameters are important in delineating and predicting bladder performance.