One
in
three
men
will
develop
lower
urinary
tract
symptoms
(LUTS)
in
later
life
[1].
These
symptoms
can
be
attributed
either
to
bladder
outflow
obstruction
or
to
a
weak
bladder
contraction.
While
conventional
cystometry
can
in
principle
separate
the
two
groups
[2],
such
measurements
are
expensive,
time
consuming,
uncomfortable
and
have
some
morbidity
[3].
Therefore,
prostatectomy
is
often
undertaken
on
the
basis
of
symptoms
and
flow-rate
only.
A
quick,
non-invasive
means
for
measuring
voiding
function
would
be
of
considerable
benefit
to
such
patients
[4].
We
are
investigating
the
use
of
an
inflatable
penile
cuff
to
obstruct
flow
progressively
during
emptying
of
the
bladder.
We
make
continuous
recordings
of
cuff
pressure
and
urine
flow
rate,
which
give
characteristic
pressure-flow
graphs
for
normal
and
obstructed
subjects
(fig
1).
The
flow
typically
maintains
a
fixed
value
to
a
knee-point
pressure,
beyond
which
it
falls
steadily
to
zero.
We
believe
this
characteristic
conveys
important
information
about
the
function
of
the
bladder
and
outlet.
The
aim
of
this
study
was
to
determine
the
precise
relevance
of
each
feature
of
the
pressure-flow
characteristic,
using
a
simple
experimental
model
of
the
bladder-outlet-cuff
system.
Methods
The
bladder
was
represented
by
a
fixed
head
of
120cm
water,
while
the
urethra
was
represented
by
sections
of
soft,
thin-walled
latex
tube.
The
constriction
at
the
prostatic
urethra
was
modelled
by
a
tube
of
3.8mm
diameter,
while
the
penile
urethra
was
9.5mm
in
diameter.
In
vivo,
the
penile
cuff
applies
a
compressive
obstruction
to
the
penile
urethra
[5].
The
enlarged
prostate
may
have
a
similar
compressive
effect
on
the
prostatic
urethra
[6].
In
our
model,
the
prostatic
and
penile
‘urethras’
passed
through
separate
airtight
boxes.
A
pressure
(ppros)
was
applied
to
the
first
box
to
simulate
the
prostatic
opening
pressure,
while
a
pressure
(pcuff)
applied
to
the
second
box
corresponded
to
the
penile
cuff
pressure.
Fig
1
Flow
rate
vs
pcuff
in
male
subjects
Fig
2
Flow
rate
vs
pcuff
in
a
hydrodynamic
model
Results
Fig
2
shows
the
relation
of
flow
rate
with
cuff
pressure
(pcuff),
for
a
range
of
prostatic
opening
pressures
(ppros)
in
the
model,
and
exhibits
the
following
features.
First,
and
irrespective
of
the
value
of
ppros,
flow
stops
completely
only
when
the
simulated
cuff
pressure
(pcuff)
exceeds
the
simulated
bladder
pressure
of
120cm
water.
Second,
flow
is
relatively
constant
while
pcuff
is
below
a
knee-point
pressure
approximately
equal
to
ppros,
the
pressure
applied
at
the
prostatic
obstruction.
Third,
when
pcuff
exceeds
the
knee
pressure,
a
graph
of
[flow
rate]2
versus
pcuff
gives
a
straight
line
whose
slope
is
related
to
the
minimum
diameter
of
the
urethral
obstruction,
here
3.8mm.
These
observations
are
supported
by
the
underlying
hydrodynamic
theory
[6,7].
The
similarities
between
results
in
the
model
and
in
humans
are
notable.
By
analogy
with
the
model,
we
have
therefore
raised
the
following
hypotheses
for
human
subjects:
-
Only
when
penile
cuff
pressure
exceeds
bladder
pressure
will
urine
flow
be
stopped
completely;
-
The
knee-point
pressure
at
which
flow
begins
to
decrease
corresponds
to
the
prostatic
opening
pressure
or
opening
pressure
of
the
flow-controlling
zone
[6,7];
-
Beyond
the
knee pressure, the flow/pressure relation can be used to estimate the minimum
urethral
diameter.
Conclusion
We
believe
our
findings
may
be
of
significant
benefit
in
understanding
and
managing
patients
with
outflow
disorders,
particularly
the
role
of
the
putative
flow-controlling
zone.
We
now
plan
to
test
our
hypotheses
formally
in
a
large
group
of
both
symptomatic
and
asymptomatic
subjects.
References
1.
The
incidence
of
benign
prostatic
obstruction.
J
Urol
1968;
99:
639-645.
2.
Outcome
of
elective
prostatectomy.
Br
Med
J
1989;
299:
762-767.
3.
The
effect
of
prostatectomy
on
symptom
reduction
and
quality
of
life.
Br
J
Urol
1995;
77:
233-247.
4.
Towards
a
noninvasive
urodynamic
diagnosis
of
infravesical
obstruction.
Br
J
Urol
1999;
84:
195-203.
5.
A
new
method
for
non-invasive
assessment
of
voiding
pressure
?
Assessment
of
penile
cuff
occlusion.
Neurourol
Urodyn
1999;
18:
256-257.
6.
Urodynamics:
the
mechanics
and
hydrodynamics
of
the
lower
urinary
tract.
Medical
Physics
Handbooks
4.
Bristol:
Adam
Hilger
Ltd
1980.
7.
Urethral
resistance?
Urodynamic
concepts
of
physiological
and
pathological
bladder
outlet
function
during
voiding.
Neurourol
Urodyn
1985;
4:
161-201.
