As mentioned previously, urinary continence is provided by the muscular action of the external sphincter. The pressure applied by the ES is routinely measure to give an indication of the patency of the musculature or innervation.2 methods are available.
Each has its merits.
Water filled catheter
The method of Brown & Wickham is used extensively. Simply passing a fluid filled catheter down the urethra would not measure the urethral closure pressure. This is because the walls of the urethra would occlude the holes.
In the diagram we can see the cases where the applied pressure in the catheter is (A) less than the urethral wall pressure - holes sealed. (B) greater than the urethral wall pressure - fluid escapes. (C) equal to the urethral wall pressure - fluid neither escapes nor is the wall distorted.
We can measure Pf at the distal end of the catheter so we need only find some means of ascertaining when Pw = Pf. Experiment shows that if a small flow rate is maintained through the holes then the pressure measured is very close to the urethral wall pressure. The error in the measurement depends on a number of parameters, but is typically 1 cm H2Oper ml min-1.This technique relies upon the fluid filled system, being able to respond sufficiently quickly to any rise in pressure in the urethra. As the urethral pressure rises temporarily, the catheter will be occluded. The pressure measured will begin to rise, but it will not read the correct pressure until the infusion system manages to inflate the fluid filled catheter to the urethral pressure. Of course by this time, the moving catheter may have passed the high pressure region and so a low reading may be the result. This will depend on the 3 variables
There is a maximum rate of rise in pressure which the system can record. This is the rate when the side holes of the catheter are blocked and is determined by 1&2 above.
Qinf can not be made too large as that would increase the error so the ideal system would have a low compliance catheter system and a low withdrawal speed
It is only rises in pressure which are not recorded faithfully as when the urethral pressure falls, the water can escape quickly into the urethra.
A very simple technique exists to check this. If the holes are occluded suddenly the pressure will rise at its maximum rate. This can be measured using the standard recording protocol. According to Brown et al (Clin. Phys. Physiol.Meas. 1980 4 255) a figure of 200 cm H2O per second is adequate at a profile withdrawal rate of 1 cm per second. At a profile withdrawal rate of 2cm per second the response would need to be 400 cm H2O per second and so on.
A normal female urethral pressure profile is shown in . The traces are, from the bottom urethral pressure, intravesical pressure - flat - stable bladder-urethral closure pressure.
In the measurement of urethral pressure profiles it is essential that a double lumen catheter be used. It has been demonstrated that if the bladder is unstable at the time of measurement, the urethral pressure profile is much changed. The intravesical pressure is therefore measured simultaneously to exclude instability. This has a secondary benefit with urethral stress tests enabling dynamic closure pressure measurements to be performed.
The effective pressure maintaining continence is not the urethral pressure, but the so-called closure pressure ( the urethral pressure - intravesical pressure) If the intravesical pressure ever exceeds the urethral pressure the possibility of leakage obviously exists. In the static case this is rare, but once the patient applies fast transient pressures to the bladder from coughing or exercise the situation becomes quite complex. The cough or stress will cause a spike of pressure in the bladder, but should also be transmitted to the urethra so that the closure pressure stays greater than zero. In some individuals however, childbirth, obesity, or previous surgery may have damaged the urethra such that the pressure transmitted to it is less than to the bladder. In this case the closure pressure may go negative and leakage is possible. These patients are said to suffer stress incontinence.
It is difficult clinically to differentiategenuine stress incontinence (GSI) from detrusor instability secondary to stress and this is one of the main uses of urodynamics.
Figure 19 shows negative closure pressure in a patient with stress incontinence. Because the negative spike is quite large it overlaps the positive spike of the intravesical pressure. Great care must be taken in the interpretation of these traces. The frequencies making up the cough pulse far exceed the response of the fluid filled system. However some authors (Hanzal etal Br J Urol 1991 68 369) claim up to 93% sensitivity and 82% specificity for genuine stress incontinence GSI. Accurate study of stress response is possible with catheter tip transducers (frequency response 30Hz).
Alternatively stress incontinence is diagnosed solely from the presence of leakage at stress while monitoring detrusor pressure to exclude unstable bladder.