Bladder pressure is measured because the bladder is filled to
capacity with a salt solution (normal saline) at a rate of 10 to 100ml/minute, with
the patient lying down.
The study is usually performed using a urinary catheter
passed through the urethra into the bladder.
The catheter contains two
channels. One channel is used for filling, and pressure can be recorded through
the other. A 'volume versus pressure' graph, which is called a cystometrogram
(CMG), is produced.
The cystometrogram is basically performed to evaluate the
compliance and stability of the detrusor muscle. Eighty five per cent of all
incontinence occurs in women, and three quarters of those suffer with stress
incontinence (ie leakage in the absence of over activity).
Compliance is simply the elastic property of the detrusor
muscles. An evaluation of compliance is an evaluation of the ability of the
bladder to 'stretch' to 'normal' capacity, while maintaining low
pressures.
Stability is evaluated by observing the detrusor while
filling the bladder to normal capacity. The evaluation determines the presence
or absence of detrusor overactivity (or instability).
Vesical pressure is the pressure that's measured inside the
bladder, with a catheter that's specifically designed for pressure monitoring
in the urinary tract.
This is a combination of the pressure being exerted on
the bladder by the abdominal contents, the weight or pressure of any urine in
the bladder and the force that the detrusor muscle is exerting on that fluid.
The pressure in an empty bladder is usually called resting pressure, which changes with position.
The normal bladder resting pressures vary between 8 and
40cm of water (ie the pressure exerted at the bottom of a column of water 40cm
high), depending upon the particular patient and position during
study.
Abdominal pressure is measured by placing a special catheter
either in the rectum or the vagina. Abdominal pressure information is
significant because the bladder is contained in the floor of the abdominal
cavity, and it's important to isolate pressures and activities occurring in the
bladder itself.
The detrusor pressure is a subtracted pressure that's
calculated by subtracting the abdominal pressure from the vesical pressure. In
doing so, artefacts from abdominal straining, gas and the weight of the
abdominal contents are removed from the information being processed from the
catheter in the bladder, thereby representing the actual activities taking
place in the bladder during the CMG.
A bladder with normal compliance will demonstrate no greater
than 15cm water increase in detrusor pressure because it progresses from empty to
capacity during a CMG.
When the bladder is properly positioned in the abdominal
cavity, both it and the bladder neck are above the
pelvic floor muscles. With
aging, or after childbirth, the female pelvic floor can relax, causing the base
of the bladder and the bladder neck to fall below the pelvic floor.
For a patient to remain dry, the pressures in the urethra
must remain greater than the pressure in the bladder, during
filling.
The average urethral closure pressure for a female is 60cmH20
and for a male it is 80cmH20.
