In vivo evaluation of visco-elasticity in a biological tube. Part 1. Stress relaxation in the female urethra studied by cross-sectional area and pressure measurements.

The urethral response to a sudden forced dilatation was studied by a mathematical analysis of the pressure response in ten healthy women. A total of 60 dilatations, using various sizes and velocities of deformation, were performed in the high-pressure zone. The decay in pressure during relaxation proved to follow an exponential equation of the following form: Y = Z + C alpha e-t/tau alpha + C beta e-t/tau beta, where Z is the equilibrium pressure, C alpha and C beta are pressure decay, and T alpha and T beta are time constants. The time constants were unaffected by the circumstances of dilatation, whereas all the other parameters were correlated to size or velocity of dilatation, or both. The time constants showed a fairly high reproducibility when repeated after one weak. The method is presumed to characterise the tissue composition of the periluminal tissue layers and may prove useful in the evaluation of the normal urethral sphincter function. Furthermore, it may prove of value in the elucidation of the pathophysiology of stress urinary incontinence.