Estimation of contractility parameters for the urinary bladder

The contractility of (urinary bladder) smooth muscle can be described in terms of a relation between its contraction force and its shortening velocity. To a sufficient degree of accuracy this relation is approximated by the hyperbolic Hill equation, originally derived for striated muscle. Allowing for a constant “curvature” of the relation, this equation is completely determined by the parameters Po (isometric pressure) and vmax (maximum contraction velocity). Micturition results from the interaction of bladder contractility with the resistance of the urethra. If variations in urethral resistance occur, a plot of bladder wall shortening, calculated from flow rate, as a function of detrusor pressure during micturition traces part of the contractility relation. In the present study 241 micturitions out of a population of 2,073 measurements taken from a mixed group of patients, i.e., 12% of pressure‐flow studies, could thus be fitted with a Hill curve, yielding the parameters Po and vmax. A method was developed for estimating vmax from pressure and flow measured throughout any micturition. When applied to a test population it was found that the thus estimated contraction velocity called vest correlated significantly with the “true” contraction velocity vmax (Spearman's rank correlation coefficient 0.77, significant at the 0.1% level). Vest was not significantly different in groups of obstructed versus unobstructed patients, but it was found to be significantly lower in unobstructed patients that voided with significant residual volume, indicating that this parameter is not biased by urethral resistance and represents (one aspect of) the detrusor contractility.