PASSIVE URETHRAL RESISTANCE TO DILATION IN HEALTHY WOMEN - AN EXPERIMENTAL SIMULATION OF URINE INGRESSION IN THE RESTING URETHRA

The dynamic urethral pressure response to a simulated urine ingression was studied at the bladder neck, in the high pressure zone, and in th e distal urethra in 10 healthy female volunteers. The pressure respons e was characterised by a steep pressure increase simulataneous with th e urethral dilation, followed by a decay during the next seconds until a new equilibrium pressure was reached. The pressure decay could be d escribed by a double exponential function in the form P-t = P-equ + P( alpha)e(-t/tau alpha) + P(beta)e(-t/tau beta) where P-t represents the pressure at the time t, P-equ represents the pressure at equilibrium, P-alpha and P-beta express the decline in pressure, and tau(alpha) an d tau(beta) are time constants. The size of the pressure response prov ed highly dependent on velocity and size of dilation as well as urethr al site of measurement, with the maximum values in the high pressure z one. The time constants, on the other hand, were uninfluenced by these factors. The pressure response represents an integrated stress respon se from the surrounding tissues which may reflect the visco-elastic pr operties of the structures involved. The findings indicate that striat ed muscle fibres are of dominating significance for the pressure respo nse, and the varying size of the response along the urethra is in acco rdance with the localization of the horseshoe-shaped rhabdosphincter, which quantitatively is the dominating circularly arranged structure a round the female urethra. Functionally, the stress response will oppos e any dilation, and increasingly with rising size or velocity of dilat ion. But even though this reaction seems to represent a forceful mecha nism prepared instantaneously to assist in securing continence, it is not energy consuming, and consequently is a very economic additive to the urethral closure mechanism. (C) 1995 Wiley-Liss, Inc.