Estimation of urethral resistance by curve fitting in the pressure-flow plot. Theory and experience in normal men and men with benign prostatic hypertrophy.

The curvature of the urethral resistance relation was estimated in 21 normal men, in 32 prostatism patients, in 60 men with a diagnosis of benign prostatic hypertrophy (BPH) and obstruction, and in 55 men after treatment of BPH. The curvature was determined by curve fitting in the pressure-flow plot using the method of least squares. In all, 58% of the normal men and prostatism patients had a urethral resistance relation with the convexity directed upward, demonstrating a low-compliant type of distensibility. After transurethral resection (TUR), significantly more men had the convexity of the urethral resistance relation directed downward, showing that the distensibility was more of a constrictive type. This change in the elastic properties of the flow-controlling zone can be assessed only by using several curvatures to estimate the urethral resistance relation. Another advantage for the method of using different curvatures is that the effects of bladder contractility are minimized. However, should one use only one form of the urethral resistance relation, it is recommended that a linear relation be chosen.