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 the distal urethra in 10 healthy female volunteers. The pressure response was characterised by a steep pressure increase simulataneous with the urethral dilation, followed by a decay during the next seconds until a new equilibrium pressure was reached. The pressure decay could be described by a double exponential function in the form P t = P equ + P α e ‐t/τα + P β c t/τβ , where P t represents the pressure at the time t, P equ represents the pressure at equilibrium, P α and P β express the decline in pressure, and τ α and τ β are time constants. The size of the pressure response proved highly dependent on velocity and size of dilation as well as urethral site of measurement, with the maximum values in the high pressure zone. The time constants, on the other hand, were uninfluenced by these factors. The pressure response represents an integrated stress response from the surrounding tissues which may reflect the visco‐elastic properties of the structures involved. The findings indicate that striated muscle fibres are of dominating significance for the pressure response, and the varying size of the response along the urethra is in accordance with the localization of the horseshoe‐shaped rhabdosphincter, which quantitatively is the dominating circularly arranged structure around the female urethra. Functionally, the stress response will oppose any dilation, and increasingly with rising size or velocity of dilation. But even though this reaction seems to represent a forceful mechanism prepared instantaneously to assist in securing continence, it is not energy consuming, and consequently is a very economic additive to the urethral closure mechanism. © 1995 Wiley‐Liss, Inc.