BJUI
Safer urethral catheters: how study of catheter
balloon pressure and force can guide design
BJU INTERNATIONAL
Alex K. Wu*, Sarah D. Blaschko*, Maurice Garcia*, Jack W. McAninch* and
David S. Aaronson*†
*Departments of Urology, University of California, San Francisco, CA, and †Kaiser Permanente, Oakland, CA, USA
Accepted for publication 21 April 2011
OBJECTIVES
• To better define urethral catheter balloon
pressures and extraction forces during
traumatic placement and removal of
urethral catheters.
• To help guide design for safer urethral
catheters.
MATERIALS AND METHODS
• Measurements of balloon pressure were
made upon filling within the urethra vs the
bladder.
• Extraction forces were measured upon
removal of a catheter with a filled balloon
from the bladder.
• Models for the bladder and urethra
included an ex vivo model (funnel, ‘bladder’,
attached to a 30 F tube, ‘urethra’) and
fresh human male cadavers.
• The mean (SEM) balloon pressures and
extraction forces were calculated.
RESULTS
• In the ex vivo model, the mean (SEM)
pressures upon filling the balloon with
INTRODUCTION
The urethral catheter is ubiquitous in
medical care. A urethral catheter is inserted
in nearly a quarter of all hospitalized
patients [1]. Unlike other medical devices, all
levels of healthcare professionals from
doctors to nursing trainees place urinary
catheters [2,3]. National healthcare efforts
have focused on reducing the incidence of
catheter-related UTIs, deemed a ‘never event’
by the Centers for Medicare and Medicaid
Services [4]. However, catheter-related
iatrogenic injuries receive little attention
despite the fact that an estimated 0.3% of
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What’s known on the subject? and What does the study add?
The urethal catheter is a ubiquitous device that has not been modified or improved for
safety in the last 20 years, although it can be associated with significant patient harm.
This study utilizes force and pressure measurements of the urethral catheter in order to
aid in future safety modifications.
10 mL were on average three-times higher
within the ex vivo ‘urethra’ (177 [6] kPa) vs
‘bladder’ (59 [2] kPa) across multiple
catheter types.
• In the human cadaver, the mean balloon
pressure was 1.9-times higher within the
urethra (139 [11] kPa) vs bladder
(68 [4] kPa).
• Balloon pressure increased non-linearly
during intraurethral filling of both
models, resulting in either balloon rupture
(silicone catheters) or ‘ballooning’ of the
neck of the balloon filling port (latex
catheters).
• Removal of a filled balloon per the ex
vivo model ‘urethra’ and cadaveric urethra,
similarly required increasing force with
greater balloon fill volumes (e.g.
9.34 [0.44] N for 5 mL vs 41.37 [8.01] N for
10 mL balloon volume).
CONCLUSIONS
hospital patients suffer catheter placement
injuries [5]. Additionally, accidental removal
of an urethral catheter with a filled balloon
is estimated to occur in 5% of intensive care
unit (ICU) patients [6]. Iatrogenic urethral
catheter injury represents a significant
safety issue that warrants further
investigation as it can cause pain, severe
bleeding, urethral stricture, and need for
surgical intervention.
catheter placement and removal. Significant
urethral injury can occur when the catheter
balloon is inappropriately filled in the
relatively non-distensible urethra [7,8] or
when the filled catheter balloon is
accidentally pulled out of the bladder [6].
Iatrogenic catheter-related urethral injury
occurs by various mechanisms. To identify
ways to prevent catheter-related injuries, we
focused on the two main mechanisms:
• Iatrogenic complications from improper
urethral catheter use is common.
• Catheter balloon pressures and manual
extraction forces associated with urethral
injury are significantly greater than those
found with normal use.
• The differences in pressure and force
may be incorporated into a safer urethral
catheter design, which may significantly
reduce iatrogenic urethral injury associated
with catheterization.
KEYWORDS
urethral catheter, urethral trauma,
safety, iatrogenic injury, biomedical device
design
The force used to place urethral catheters
has been studied in an ex vivo model [9],
but, to our knowledge, no study has shown
force and pressure measurements associated
with urethral catheter injury. We
hypothesize that urethral catheter balloon
pressures are greatly increased when the
balloon is filled within the urethra vs the
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URETHRAL CATHETER BALLOON PRESSURE AND FORCE
FIG. 1 The 5 mL balloon of this latex catheter is
filled with 10 mL of water while placed in a
non-distensible 30 F tube. Note how the balloon
port has expanded due to the high pressures within
the retention balloon system.
bladder. Also, we hypothesize that
withdrawal of a filled catheter balloon from
the bladder into the urethra requires
significant force. Our overarching aim is to
design a safer urethral catheter to prevent
unnecessary patient harm.
MATERIALS AND METHODS
INTRA-URETHRAL FILLING STUDIES USING
AN EX VIVO URETHRAL MODEL
Studies were conducted using a mock
urinary bladder neck and urethra, using a
rigid plastic funnel connected to a well
lubricated 30 F semi-rigid tube. We used
16 F latex catheters with 5 mL retention
balloons (Allegiance, USA; recommended
fill volume: 10 mL), 16 F silicone catheters
with 5 mL retention balloons (Rochester
Medical, USA; recommended fill volume
10 mL), and 22 F latex catheters with 30 mL
retention balloons (Allegiance, USA;
recommended fill volume 35 mL). Pressure
within the balloon was measured using a
pressure gauge (Marsh Pressure Gauge,
Marshall Instruments, Inc., Anaheim, CA,
USA) attached to the catheter balloon
filling-port. Pressure measurements were
recorded initially and at 10 s upon filling
the balloon within the ‘urethra’ and the
‘bladder’ using 0, 5, 7, and 10 mL of water.
The 10-s measurement was added after
the catheter system was noted to require
time to equilibrate. Care was taken to
exclude air from the retention balloon
system. All studies were repeated in
triplicate; result are reported as the mean
(SEM).
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INTRA-URETHRAL FILLING STUDIES:
CADAVER MODEL
Lubricated 16 F latex catheters with 5 mL
balloons were inserted per urethra of three
fresh male cadavers in standard fashion [10].
Sterile water was used to fill the catheter
balloon with 0, 5, and 10 mL in the bladder
and at defined locations within the urethra,
including the prostatic urethra, the bulbar
urethra, and the pendulous urethra. The
location was estimated as the catheter was
withdrawn past the bladder neck. Pressure
within the balloon was recorded when the
balloon was filled with each amount of
water as described above. The same catheter
was used sequentially with deflation of the
balloon before movement to the next
segment of the urethra. If balloon port
expansion occurred from high balloon port
pressures, then a new catheter was used.
Additional measurements were taken with
the catheter balloon filled with 0–5 mL of
water in the fossa navicularis to replicate
the pressure effects during retrograde
urethrography. Pressure was measured upon
initial filling of the balloon and 10 s
afterward to account for tissue relaxation
and system equilibration. At 10 s after
balloon filling, no further changes in balloon
pressure were noted (data not shown). The
experiments were repeated in a total of
three cadavers. Individual cadavers were not
used for multiple experiments due to
concern that the urethral tissue would be
significantly altered by each experiment.
FORCED REMOVAL OF A URETHRAL
CATHETER (BALLOON FILLED) PER URETHRA:
EX-VIVO MODEL
Studies using our ex vivo model (described
above) were performed. Latex and silicone
16 F catheters (5 mL balloon) were filled
with 5, 7, 10 and 20 mL of sterile water.
Latex 22 F (30 mL balloon) catheters
were filled with 5, 10, 35, and 55 mL of
sterile water. The largest volumes instilled
represent the common practice of balloon
over-filling to reduce postoperative
haematuria after surgical procedures such
as TURP. A force meter (Accu-Scale &
Systems Inc, Melrose, MA, USA) was used to
measure the force required to remove the
catheter from the ‘bladder’ with the balloon
filled at varying amounts. All studies were
repeated in triplicate with mean (SEM)
reported.
FORCED REMOVAL OF A URETHRAL
CATHETER (BALLOON FILLED) PER URETHRA:
CADAVER MODEL
Lubricated 16 F latex catheters were placed
per urethra in three fresh male cadavers
using standard technique. The catheter
balloon was filled with 0, 5, 7, and 10 mL of
sterile water. After filling to the designated
amount, the catheter was forcibly removed.
Experimental measurements were performed
as describe above for studies using the ex
vivo model. After force measurements were
obtained for removal with 10 mL of fill, the
cadaver was not used again out of concern
that the bladder neck would be significantly
altered by catheter balloon removal.
Experimentation was repeated in three
cadavers. All cadavers were obtained by the
University of California, San Francisco,
Orthopedic Surgery department by voluntary
donation for the purpose of medical training
and research.
RESULTS
PRESSURE ASSOCIATED WITH FILLING THE
BALLOON IN THE URETHRA
The catheter balloons, free of external
constriction, had good compliance. For
all catheters (varying by material type
and balloon volume), the mean (SEM)
balloon pressure within the ‘bladder’
was 59 (2) kPa. Balloon pressures in the
‘bladder’ model, using a 16 F latex catheter,
increased from 63 (2) kPa to 67 (3) kPa
with 5 mL to 20 mL of fluid, respectively.
In the ex vivo model, silicone catheters and
the larger latex catheters also showed
relatively stable balloon pressures with
increasing fill volumes within the model
bladder (Table 1).
The mean (SEM) balloon pressure within the
‘urethra’ was 177 (6) kPa, about three-times
higher, than in the mock bladder model
across catheter types with 10 mL of fill. At
balloon fill volumes of 7 mL, within the
‘urethra’, the neck of the balloon filling port
expanded preferentially (Fig. 1). Furthermore,
balloon pressure did not increase in a linear
fashion with filling volumes >7 mL because
the neck of the balloon port shunted the
excess pressure. Using the 16 F latex
catheter, maximal pressure was achieved
when the balloon was filled with 5 mL
[mean (SEM) 182 (21) kPa]; when the balloon
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TABLE 1 Balloon pressure in the ex vivo model at various states of filling and using different catheter types. The percentage change in pressure from the
bladder to the urethra model is shown in the rightmost column
Balloon filling
volume, mL
0
5
7
10*
20
0
5
7
10*
20
0
5
10
35*
55
16 F latex 5 mL balloon
16 F silicone 5 mL balloon
22 F latex 30 mL balloon
Mean (SEM) pressure
in bladder model, kPa
0 (0)
63 (2)
62 (2)
63 (3)
67 (3)
0 (0)
47 (9)
48 (9)
50 (10)
53 (6)
0 (0)
73 (7)
68 (3)
60 (0)
57 (3)
Mean (SEM) pressure
in urethra model, kPa
0 (0)
182 (21)
183 (17)
173 (15)
167 (9)
0 (0)
207 (30)
210 (24)
Balloon burst
Balloon burst
0 (0)
212 (9)
177 (12)
137 (9)
137 (9)
% Change
0
287
297
274
250
0
443
435
NA
NA
0
289
265
228
241
*Manufacturer-recommended balloon filling volume; NA, not applicable; 1 kPa = 0.0099 atm = 0.15 psi = 7.5 mmHg.
TABLE 2 Balloon pressures upon filling a 5 mL balloon of a latex catheter in different locations (prostatic, bulbar, and penile urethra) of the cadaver model.
The balloon was filled with either 5 or 10 mL sterile water. Pressure was measured both immediately after filling and after 10 s, to allow system equilibration.
The percentage change in pressure from the bladder after 10 s is shown in the third and rightmost column
Bladder
Prostatic urethra
Bulbar urethra
Penile urethra
5 mL
Mean (SEM)
immediate
pressure, kPa
95 (3)
130 (38)
147 (15)
165 (9)
Mean (SEM)
pressure after
10 s, kPa
65 (8)
110 (21)
120 (25)
132 (2)
% change
from bladder
after 10 s
NA
169
185
204
10 mL
Mean (SEM)
immediate
pressure, kPa
97 (3)
178 (38)
180 (15)
183 (9)
Mean (SEM)
pressure after
10 s, kPa
68 (4)
112 (19)
140 (21)
150 (16)
% change
from bladder
after 10 s
NA
164
205
220
NA, not applicable.
was filled with 20 mL of fluid, the mean
maximal pressure was only 167 (9) kPa.
Balloon filling with greater volumes than
the threshold pressure level resulted in
continued balloon port expansion, not
increasing intra-balloon pressure. Similar
results were found with 22 F latex catheters
(30 mL balloon), although the threshold fill
volume at which we noted balloon port
expansion was 10 mL. Silicone catheters did
not perform in a similar fashion: as the
balloon was filled within the urethra, the
balloon port did not expand to serve as a
‘pop-off’ valve. Rather, when the balloon
was filled with >7–10 mL of water, the
balloon ruptured within the model ‘urethra’
(Table 1).
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Identical experiments in the male cadavers
yielded similar results. Balloon pressures
within the urethra were nearly twice as high
as within the bladder (Table 2), and, in all
measurements (within the bladder and in
various locations within the urethra),
balloon pressure was always highest
immediately upon filling, and slightly lower
10 s after filling, presumably due to
relaxation of the cadaveric tissue and
balloon port neck expansion. The mean
balloon filling pressure was highest within
the penile urethra (≈2.2-times that within
the bladder), followed by the bulbar
(≈2.0-times that within the bladder) and
prostatic (≈1.6-times that within the
bladder) urethra with 10 mL (Table 2).
When the balloon was filled within the fossa
navicularis, filling with only 3 mL of fluid
resulted in a balloon pressure increase of
145 kPa, which is comparable with that
achieved with three-fold greater filling
volume with the balloon in the bulbar
urethra, 140 kPa. Expansion of the balloon
port was seen in all trials using a latex
catheter with balloon filling within the
urethra.
FORCE REQUIRED TO WITHDRAW A
CATHETER FROM THE BLADDER
Experiments using both ex vivo models and
fresh male cadavers confirmed that at
higher balloon fill volumes, greater manual
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URETHRAL CATHETER BALLOON PRESSURE AND FORCE
TABLE 3 Traction force required to dislodge catheter from ex vivo ‘bladder’ and cadaver. Data are
shown for various degrees of balloon filling volume, and, using catheters made of different materials
(silicone, latex) and of different diameters (16 F and 22 F)
16 F latex catheter with 5 mL balloon
16 F silicone catheter with 5 mL balloon
22 F latex catheter with 30 mL balloon
16 F latex catheter with 5 mL balloon in cadaver model
Balloon filling
volume, mL
0
5
7
10*
20
0
5
7
10*
20
0
5
10
35*
55
0
5
7
10*
Mean (SEM)
force, N
0
9.34 (0.44)
25.80 (4.45)
41.37 (8.01)
>89†
0
4.45
15.57 (1.78)
33.36 (1.78)
>44.5‡
0
6.67 (1.33)
16.46 (1.78)
>89†
>89†
0
20.02 (2.22)
24.46 (2.22)
45.82 (4.00)
*Manufacturer-recommended balloon filling volume; †Exceeded experimental limit, catheter could
not be removed; ‡Balloon burst at this force.
traction force is required to withdraw the
catheter from the bladder (Table 3). In the ex
vivo ‘bladder/urethra’ model removal of a 16
Fr latex catheter required an average of
9.34 (0.44) N and 41.37 (8.01) N traction
force when the balloon was filled with 5 mL
and 10 mL fluid volume, respectively.
Over-filling of the 5 mL balloon with 20 mL
fluid required significantly greater (>89 N)
force, exceeding the measurement threshold
of our measurement apparatus. Similar
results were noted using a 22 F latex
catheter (30 mL balloon) filled with >35 mL
fluid as a manual traction force >89 N was
required to withdraw the filled balloon per
urethra. Silicone catheters also required
greater manual traction force to withdraw
the balloon per urethra at higher filling
volumes. At balloon volumes >20 mL water,
the silicone balloon invariably burst with a
force >44.5 N.
Incorrect insertion or removal can result in
significant patient harm. While incorrect
insertion or partial removal can be readily
identified [11], the trauma at this point has
already occurred. Unfortunately, incidents of
catheter balloon filling within the urethra
and accidental catheter removal of an
indwelling catheter with the balloon filled
continue to be reported in the patient care
setting. Although the exact incidence of
these problems has not been well studied,
two key factors suggest the incidence of
these safety-related issues will continue: i.
Catheters are used by such a diverse group
of health professionals that rigorous
training is a practical challenge; ii. Few
modifications to catheter design have
occurred within the last 20 years, and such
efforts have been primarily limited to
mechanisms to reduce catheter-related UTIs
[12].
DISCUSSION
Surprisingly, no attention has been placed
on how to make catheters safer to insert
and use. We sought to define the physical
parameters whereby iatrogenic injuries from
urethral catheters occur to help guide
design of a safer device. We found that
Indwelling urethral catheters are
commonplace in healthcare settings, and
they are inserted into patients by health
professionals at all levels of training.
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balloon pressures, when the catheter balloon
was filled appropriately within the bladder,
varied little and were less than half that
seen when filled in the urethra in both the
ex vivo and cadaveric model. Interestingly,
we discovered that a threshold balloon
pressure exists for latex, but not silicone
catheters, at which point the neck of the
balloon port served as an unrecognised
pressure-release valve mechanism (albeit set
at a level that does not prevent patient
harm). The less compliant silicone catheters
do not demonstrate this pressure release
mechanism, as they burst beyond the
threshold balloon pressure level. Finally, we
found balloon pressures vary in different
portions of the urethra with higher balloon
pressures as the balloon was filled more
distally. For instance, at the fossa navicularis,
balloon pressures were similar to those
found within the prostatic urethra with
much lower balloon filling volumes (3 vs
10 mL). This may help explain the frequent
expression of patient discomfort during
retrograde urethrography where the
urethral catheter is filled in the fossa
navicularis.
The second portion of the present study
showed in both models that increasing force
was required to dislodge a urethral catheter
from the bladder into the urethra as the
balloon fill volume increased. This trend was
noted in all catheter sizes and types tested.
Interestingly, catheters composed of silicone
were the only type to burst their balloon
upon forceful removal from the bladder,
which reflects the less compliant properties
of silicone compared with latex.
It is imperative that healthcare professionals
performing urethral catheterization receive
appropriate training [2]. A recent study of a
training intervention on urethral catheter
placement at a single institution
significantly reduced the incidence of male
urethral trauma [5]. Yet, despite this
intervention iatrogenic injuries still occurred.
Designing a safer urethral catheter that
facilitates healthcare professionals in the
placement of the catheter within the
bladder would reduce unnecessary patient
harm and warrants further exploration. We
hypothesize that a catheter that has a
thinner wall at the neck of the balloon port,
serving as a designed ‘pop-off’ pressure
valve, might provide manufacturers the
unique opportunity to inexpensively add a
catheter safety mechanism. This mechanism
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would serve to notify the health professional
placing the catheter of the incorrect
location of the catheter balloon within the
urethra through visual and tactile feedback
by expansion of the balloon port. This
‘second balloon’ would also serve to prevent
harmful high intra-urethral pressures
thereby avoiding urethral rupture, bleeding,
pain, and increased duration of
hospitalization.
The present study also addresses the
common occurrence of accidental
dislodgement of a catheter with a filled
balloon from the bladder. An estimated one
in twenty ICU patients suffers this event
during their hospitalization [6]. The present
study raises the question of whether or not
to over-fill the catheter balloon to make it
more difficult for a patient at higher risk for
urethral self-injury to apply sufficient force
to dislodge their catheter. However, one
caveat is that a larger balloon may cause
more urethral damage if a patient was still
able to apply sufficient extraction force to
remove their catheter.
Additionally, the present findings suggest
that given the propensity of silicone
balloons to burst at high pressures or forces,
these catheters might not be ideal for
patients with altered mental status who are
at higher risk of pulling out their catheters.
Notwithstanding, intravesical bursting of the
catheter balloon may lead to retained
catheter fragments with bleeding and
trauma from the burst itself necessitating
cystoscopic intervention [13].
due to the wider and shortened female
urethra. The number of repetitions
performed in ex vivo experimentation may
be considered a limitation, but we feel that
the use of highly standardized, high-grade
medical catheters negates the need for
additional testing. Finally, balloon pressures
serve as a surrogate measure of urethral
trauma. Certainly, as evidenced by the
non-zero filling pressures within the bladder,
balloon pressures are related to both the
baseline compliance of the catheter balloon
material and the amount of pressure exerted
on and from surrounding tissue.
Filling of urethral catheter retention
balloons within the urethra results in two or
more times greater balloon pressures than
when filled within the bladder. Catheters
with larger balloon filling volumes require
greater force to remove from the bladder
when filled completely. The differences in
balloon pressure found with filling within
the urethra vs the bladder, and the force
required to remove a catheter with a filled
balloon from the bladder, could potentially
be exploited to design a safer urethral
catheter thereby preventing unnecessary
patient harm.
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CONFLICT OF INTEREST
None declared.
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Study limitations include the inability to test
all catheter types and sizes in the cadaveric
model, and the lack of in vivo data, which
would not be ethical. However, ex vivo
studies provided us with inferential data as
they trended with our findings in the fresh
cadavers. We acknowledge that actual
pressure and force measurements will
change in a living human with perfused
tissue, but feel that the data trends and
catheter deformation findings would remain
constant. We did not design the present
study for the female population because
catheter related injuries are far less common
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Correspondence: Alex K. Wu, Department of
Urology, University of California, San
Francisco, San Francisco, CA 94143-0320,
USA.
e-mail: wuak@urology.ucsf.edu
Abbreviation: ICU, intensive care unit.
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