Quantification of urethral resistance and bladder function during voiding, with special reference to the effects of prostate size reduction on urethral obstruction due to benign prostatic hyperplasia

Abstract

Abstract In pressure/flow studies of adult voiding the behaviour of the urethra can be distinguished from that of the bladder, and quantified separately, as follows. Different degrees of urethral obstruction can be quantified and ranked using a group‐specific resistance factor (URA) that is based empirically on the pressure/flow plots obtained in a large number of voidings of adult patients. Different detrusor contractions can be similarly quantified using a physiologically based measure (WF, abbreviated from power [W] factor) of the contraction strength. Both URA and WF can be calculated by computer from measurements of detrusor pressure, voiding flow rate, and the residual volume in the bladder after voiding. For other groups of patients, e.g., children, different group‐specific resistance factors are appropriate. In men with obstructed voiding due to benign prostatic hyperplasia (BPH), “chemical castration” with cyproterone acetate or buserelin caused a substantial, reversible reduction in the size of the prostate. The effect on the degree of urethral obstruction and the voiding bladder contraction has been studied using the above‐mentioned methods. There was an increase in peak urinary flow rate, a reduction in residual urine, and a decrease in daytime voiding frequency. Surprisingly, however, there was on average no decrease in urethral resistance but only an increase in the average detrusor contraction strength. In individual patients large increases and decreases of urethral resistance and detrusor contraction strength occurred, which largely cancelled each other out. These large changes masked the relatively modest urethral resistance decrease caused directly by prostate size reduction.