URETHROVESICAL PRESSURE DYNAMICS

The anatomical separation of sphincteric elements and
supportive structures is mirrored in the functional
separation of urethral closure pressure and pressure
transmission. The relationship between resting urethral
pressure, pressure transmission, and the pressure needed
to cause leakage of urine are central to understanding
urinary continence. These relationships have been
described in what we have called the ‘pressuregram’
The constrictive effect of the urethral sphincter deforms
the wall of the urethra so as to maintain
urethral pressure above bladder pressure, and this
pressure differential keeps urine in the bladder at
rest. For example, if bladder pressure is 10 cmH2O while
urethral pressure is 60 cmH2O, a closure pressure of
50 cmH2O prevents urine from moving from the bladder
through the urethra.

Bladder pressure often increases by 200 cmH2O or
more during a cough, and leakage of urine would occur
unless urethral pressure also increases. The efficiency of
this pressure transmission is expressed as a percentage.
A pressure transmission of 100% means, for example,
that during a 200 cmH2O increase in bladder pressure
(from 10 cmH2O to 210 cmH2O), the urethral pressure
would also increase by 200 cmH2O (from 60 to
260 cmH2O).

The pressure transmission is less than 100% for
incontinent women. For example, abdominal pressure
may increase by 200 cmH2O while urethral pressure
may only increase by 140 cmH2O, for a pressure transmission
of 70%.
If a woman starts with a urethral pressure of
30 cmH2O, resting bladder pressure of 10 cmH2O and
her pressure transmission is 70%, then with a cough
pressure of 100 cmH2O her bladder pressure would
increase to 110 cmH2O while urethral pressure
would increase to just 100 cmH2O and leakage of urine
would occur.

example 4 shows the same elements, but
with a higher urethral closure pressure; and similarly
example 5 shows what happens with a weaker cough.
According to this conceptual framework, resting
pressure and pressure transmission are the two key continence
variables. What factors determine these two
phenomena? How are they altered to cause incontinence?
Although the pressuregram concept is useful for
understanding the role of resting pressure and pressure
transmission, it has not been possible to reliably make
these measurements because of the rapid movement
of the urethra relative to the urodynamic transducer
during a cough.