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UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1 7
Phil Wuthier, BSN, RN, OCN, BS/MSME,
PE, is an Oncology Staff Nurse, Penrose
Hospital, Colorado Springs, CO.
Karen Sublett, MS, RN, ACNS-BC, OCN,
is an Oncology Clinical Nurse Specialist,
Penrose Hospital, Colorado Springs, CO.
Lance Riehl, BSN, RN, OCN, is a Charge
Nurse and Oncology Staff Nurse, Penrose
Hospital, Colorado Springs, CO.
Urinary Catheter Dependent
Loops as a Potential Contributing
Cause of Bacteriuria:
An Observational Study
Phil Wuthier, Karen Sublett, and Lance Riehl
Catheter-associated uri-
nary tract infections
(CAUTIs), and more
globally, catheter-asso-
ciated asymptomatic bacteriuria
remain a problematic issue in the
hospital setting. The financial
burden to institutions and the
personal burden of increased
morbidity to patients make the
understanding of the sources and
possible remedies of these infec-
tions of prime medical and nurs-
ing relevance. According to one
group of urologic care providers,
“In an era that has witnessed out-
standing technological advances
in medical practice, it is difficult
to understand why we are still
unable to perform the relatively
simple task of draining urine
from the bladder without produc-
ing infection and a range of asso-
ciated complications” (Feneley,
Kunin, & Strickler, 2012, p. 1748).
These authors further assert that
it is no longer acceptable to sim-
ply manage symptoms caused by
an indwelling catheter – an in -
creas ed fundamental understand-
ing is needed to resolve the prob-
lems they introduce.
Literature Review
Contemporary studies affirm
that the pathogenesis of catheter-
ized bacteriuria remains poorly
understood by the microbiology
community (Barford & Coates,
2009). Certain isolated bench-
marks seem clearly established.
In a landmark study (Tambyah,
Halvorson, & Maki, 1999) evaluat-
ing 1,497 newly catheterized
patients, the researchers conclud-
ed that catheter associated infec-
tions can occur either by extralu-
minal (66% of cases) or intralu-
minal (34% of cases) means.
Extraluminal contaminations typ-
ically occurred either soon after
catheter insertion (mean 1.0 day,
SD = 0) (presumably due to non-
aseptic insertion in which bacte-
ria residing near the meatal ori-
fice are transported with the
catheter to the bladder) or after
6.3 days on average (SD = 5.0)
(presumably due to the develop-
ment of a biofilm growing in the
optimally warm, moist area
between the urethra and the out-
side of the catheter) (Tambyah et
al., 1999).
In stark contrast, the same
study identified that intraluminal
contaminations occurred within
the same general time frame after
catheter insertion (mean of 7.9
days, SD = 8.0), but involved trans-
mission through the full length of
Learning outcome, instructions for completing the evaluation, and statements of disclosure can be found on page 16.
SERIES/RESEARCH
© 2016 Society of Urologic Nurses and Associates
Wuthier, P., Sublett, K., & Riehl, L. (2016). Urinary catheter dependent loops as a
potential contributing cause of bacteriuria: An observational study. Urologic
Nursing, 36(1), 7-16. doi:10.7257/1053-816X.2016.36.1.7
Urologic studies suggest that urinary catheter dependent loops (tubing low
points) may be a contributing cause of bacteriuria and urinary tract infection
among catheterized patients. The means by which this type of contaminant trans-
mission occurs, however, remains poorly understood. An observational, cross-
sectional study was conducted to provide a foundational look at catheter depend-
ent loops and their possible role in catheter-acquired urinary tract infections, and
as a building block for further research.
Key Words: Catheter-associated urinary tract infection (CAUTI), bac ter i uria,
indwelling catheter, dependent loop, air lock, intraluminal
transmission, urinary retention, Froude number, Eotvos number.
Copyright 2016 Society of Urologic Nurses and Associates (SUNA) All rights reserved. No part of this document may be reproduced or transmitted in any form
without the written permission of the Society of Urologic Nurses and Associates. doi:10.7257/1053-816X.2016.36.1.7
8UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1
the drainage tube and catheter. It
was concluded that this means of
transmission re mained an impor-
tant source of contamination
despite that clos ed drainage sys-
tems were nearly universally uti-
lized. This study demonstrated
that the bacteria associated with
each transmission route were fun-
damentally different organisms.
Both routes were deemed impor-
tant in efforts to reduce CAUTI
(Tambyah et al., 1999).
Intraluminal Transmission
It is significant to note that
the intraluminal contamination
route was determined to be a
non-biofilm driven infection
route (Maki, 2001). As men-
tioned above, the contaminants
were transported within the
lumen of the system over a signif-
icantly greater distance in about
the same amount of time as extra-
luminal contamination. These
facts indicate a substantially dif-
ferent means of transmission. No
studies have been identified,
which definitively confirm the
method of intraluminal contami-
nation. The presence of primarily
exogenous bacteria in infections
identified to be intraluminal in
origin are consistent with con-
tamination from the hands of
health care workers (Shuman &
Chenoweth, 2010). While the
exact mechanism remains unde-
fined in scientific literature,
Hooten et al. (2010) note that
“both animal and human studies
have demonstrated that bacteria
that enter the drainage bag are
soon found in the bladder” (p.
634).
In a landmark study of 850
newly catheterized patients, risk
factors and nursing interventions
were evaluated as to their effect
on CAUTI. The researchers con-
cluded “the only catheter-care
violation predictive of an in -
creased risk for CAUTI was the
drainage tube sagging below the
level of the collection bag”
(Maki, Knaskinki & Tambyah,
2000, p. 165). This result points
to the critical issue of dependent
loops in catheterized patients
without formally identifying the
means of transmission. Pub lish -
ed technical writing to explain
why dependent loops are prob-
lematic in causing intraluminal
CAUTI is lacking. Retrograde
urine flow (Maki & Tambyah,
2001) and urinary retention
(Garcia et al., 2007) have been
offered as contributing factors. In
a recently published review of
the current understanding and
study-based directives on reduc-
ing CAUTI, Tambyah and Oon
(2012) state that “all guidelines
agree that ensuring dependent
drainage significantly reduced
the risk of CAUTI (the drainage
tubing should be below the level
of the patient’s bladder but above
the level of the collection bag)”
(p. 368). This conclusion affirms
the need to more effectively
understand the characteristics of
dependent loops.
Dependent Loop Pressures
A dependent loop can be
defined as a configuration of
catheter tubing where the drain -
age tubing dips below the entry
SERIES/RESEARCH
Research Summary
Introduction
Urologic studies suggest that urinary catheter depend-
ent loops (tubing low points) may be a contributing cause of
bacteriuria and urinary tract infection (UTI) among catheter-
ized patients. The means by which this type of contaminant
transmission occurs, however, remains poorly understood.
This study sought to investigate the nature of catheter tubing-
dependent loops and the prevalence of pressurized tubing,
which is known to impede urine flow from the patient bladder.
Objective
To determine the prevalence and nature of indwelling
catheter dependent loops within an inpatient hospital setting.
Methods
An observational, cross-sectional study was conducted
in which measurements were taken of indwelling catheter-
tubing arrangements for consenting patients. The study was
conducted along with recurring nursing compliance audits
on two specific days.
Results
Data were collected on 55 patients of the 78 patients
with urinary catheters at the time of audit. Tubing with a
dependent loop was found in 87.3% of cases. Of these
dependent loops, 28 cases (58.3%) existed with a stable
positive pressure in the tubing directly distal to the patient
(Type 1 dependent loop), and 14 cases (29.2%) were of
Type 2 nature. Four cases (8.3%) showed no pressure dif-
ferential at the time of examination. Two cases (4.2%)
showed unstable urine within the tubing. Evaluation of the
Type 1 dependent loops suggested that 9 cases displayed a
“potential air-lock” condition, and 3 cases displayed a “prob-
able air-lock”. Of the Type 2 configuration of dependent
loops, 14 cases indicated a “probable air-lock” condition.
Conclusions
Despite manufacturer literature and hospital teaching,
dependent loops in indwelling catheter tubing are quite
prevalent in the acute care setting as evidenced by this
study. Dependent loops may contribute to difficulties with
adequately draining urine and increase patient risk for
catheter-associated urinary tract infection (CAUTI). Nursing
staff, therefore, must be vigilant in managing dependent
loops in the clinical setting.
Level of Evidence – VI
(Polit & Beck, 2012)
Copyright 2016 Society of Urologic Nurses and Associates (SUNA) All rights reserved. No part of this document may be reproduced or transmitted in any form
without the written permission of the Society of Urologic Nurses and Associates. doi:10.7257/1053-816X.2016.36.1.7
UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1 9
point into the catheter bag. The
depth of this low point with
respect to the catheter bag is a
critical parameter as detailed
herein. As a patient moves in a
bed or chair, or as the bag is
moved from one location to
another, the characteristics of the
tubing and the fluid contained
therein naturally change. It is a
common clinical observation that
pressure builds and drops within
the tubing with movement and
with the release of urine. Schwab,
Lizdas, Gravenstein and Lampo -
tang (2014) demonstrated that the
difference in fluid levels reflect
pressure differences within the
tube. In other words, dependent
loops function exactly as a U-tube
manometer, where the differ-
ences in the height of the urine
levels about the low point show
the amount of pressure that
exists. These authors further
asserted that the pressures that
build within the tubing retard the
urine flow toward the bag.
Anecdotally, nurses and patients
have regularly observed this con-
clusion because those with an in-
dwelling catheter often complain
of the feeling of bladder pressure
and the need to urinate in spite of
the presence of the catheter.
Two-Phase Flow
In order to more fully under-
stand the impact of catheter
dependent loops, several engi-
neering concepts need to be
explored. For liquid flow
through a tube, a dimensionless
parameter called the Froude
number (FR) is used by engineers
to determine the liquid flow
velocity necessary to expel the
air from the high and low points
of the tubing. Table 1 defines the
calculation of FR and details how
typical flow rates generated by
the bladder are not sufficient to
expel air from the drainage
tubing. This conclusion is con -
sistent with clinical observations
that show the drainage tubing to
routinely be a mixture of two-
phase flow (urine and air).
Surface Tension-Dominated Flow
The Eotvos Number (EO) is
more suitable than FR to explain
flow characteristics through
small diameter tubing. Fluid flow
through tubes of small diameter,
such as exist for the catheter tip
or drainage tubing, is dominated
by surface tension effects. EO
represents a ratio of buoyancy
forces to surface ten sion forces,
and has been identi fied as a key
parameter in evaluating two-
phase flow in small diameter
tubes (see Table 1) (Funada,
Joseph, Maehara, & Yamashita,
2005). Table 2 shows calculations
of EO for both the catheter and
drainage tube, and confirms the
significance of surface tension
effects.
The fact that surface tension
effects are not negligible in the
drainage of urine presents itself
in several clinically observable
ways. First, it is a frequent
observation in the clinical setting
to see urine in the drainage
tubing seemingly defying gravity.
The surface tension effects are
evident with small sections of
SERIES
Table 1.
Dimensionless Parameter Summary
Froude
Number
(FR)
FR is calculated as the resultant of the liquid flow velocity divided by
the square root of the product of the pipe internal diameter and the
gravitational constant (Shosho & Ryan 2001).
FR > 1 is required to purge air from a horizontal pipeline (the most
restrictive condition).
For the typical 8mm internal diameter of catheter drainage tubing, the
velocity needed to purge air can be calculated to be 28 cm/s, or
50,700 ml/hour. when expressed as a flow rate. This far exceeds
urine flow rates generated by the human bladder.
Eotvos
Number
(EO)
EO is calculated as the product of the tube internal diameter
squared, the gravitational constant and the difference between the
liquid and gas densities, divided by the liquid surface tension
(Shosho & Ryan, 2001).
Surface tension effects become dominant for EO < 40.
EO > 3.37 is required for air bubbles to exist and move in a small
diameter tube. EO values below this critical value will not allow for air
to enter a urine filled tube without pressure applied.
Table 2.
Urine Eotvos Number Calculations
Urine Properties
Specific Gravity 1.005 1.010 1.015 1.020 1.025
Surface Tension
(dynes/cm) 67.0 64.25 61.5 58.75 56.0
Catheter: Internal Diameter = 2 mm
Eotvos Number 0.59 0.62 0.65 0.68 0.72
Drainage Tubing: Internal Diameter = 8 mm
Eotvos Number 9.41 9.86 10.35 10.89 11.48
Critical Diameter: Eotvos = 3.37
Internal Diameter (mm) 4.79 4.67 4.56 4.45 4.33
Note: The drainage tubing diameter is larger than the critical diameter, and the
catheter lumen diameter is smaller than the critical diameter.
Copyright 2016 Society of Urologic Nurses and Associates (SUNA) All rights reserved. No part of this document may be reproduced or transmitted in any form
without the written permission of the Society of Urologic Nurses and Associates. doi:10.7257/1053-816X.2016.36.1.7
10 UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1
urine independently suspended
in vertical sections of tubing.
This phenomena is “pseudo
stable” in that if one gently
disturbs the tubing, the sus -
pended urine will quickly re -
distribute into a more expected
configuration. This non-stable
fluid configuration was eval u -
ated for each catheter considered
in this study.
Further, the nature of the
surface tension dominant flow is
evident in how urine and air
move through the drainage tub -
ing. Commonly, urine proceeds
from the bladder in a small
thread or trickle fashion. Con -
versely, when drained at a
steeper angle, the urine flows
with large bullet-shaped air
bubbles moving up toward the
patient as urine drains around
the outskirts of the bubble
(commonly known as “Taylor” or
“Dumitrescu” bubbles). This
bubble-driven flow has been
studied for many years for a
variety of applications. In actual
modern engineering tubing
design, the problematic surface
tension dominated flow is
simply avoided by pumping such
a fluid instead of relying on a
gravity driven scenario. This
option is not readily available
with a simple urinary catheter
device.
Critical Eotvos Value
Of specific relevance in this
discussion is the existence of a
critical EO value below which an
air bubble will not rise in a
gravitational liquid field. It was
experimentally observed as early
as 1913 that there existed a
minimum tube diameter size
below that a “Taylor bubble”
would not rise due to surface
tension effects overcoming the
impact of gravity (Gibson, 1913).
More recent theoretical and
empirical studies have identified
the critical EO value of 3.37 (Bi &
Zhao, 2001). Using this critical
EO value, one can calculate the
critical internal tube diameter for
which an air bubble will not rise
within urine (see Table 2).
The calculations presented in
Table 2 highlight the fact that
under gravitational influence
alone, an air bubble will rise
through the drainage tubing but
will not progress upstream of the
indwelling catheter transition
junction where the internal
diameter decreases. This is con -
sistent with clinical observation
of in-situ urinary catheters in
which there is often a distinct air-
urine transition point near the
location where the internal
diameter decreases into the in -
dwelling catheter. The length of
the solid urine to the air-urine
interface was a parameter re -
corded in this study. Having the
internal diameter of the existing
indwelling catheter below the
critical EO value is a benefit to
restricting potentially con taminat -
ed retrograde flow (of either urine
or air) back into the bladder.
An important consequence
of this critical EO physical
phenomenon is that in order to
overcome this feature of surface
tension-dominated flow, a pres -
sure differential is required. In
the desired ante-grade flow,
modulated pressure from the
bladder forces fluid toward the
collection bag. Any distally ap -
plied pressure to the stable air-
urine boundary has the pro -
pensity to drive either air or
liquid back toward the bladder,
overcoming the critical EO estab -
lished barrier. However applied
(whether by mani pu lation of the
catheter bag, drainage tubing or
other means), potentially con -
taminated fluid or air can be
propelled back toward the blad -
der when a distal pressure exists
greater than that supplied by the
bladder at the time in question.
As discussed previous ly, the
existence of a dependent loop
allows for pressure differentials
(above or below atmospheric
pressures) to exist in the tubing,
enabling a potential means for
contaminant movement.
Two Types of Dependent Loops
Urine flow through a de -
pendent loop typically appears
in two distinct manners designat-
ed as Type 1 and Type 2 configu-
rations (see Figures 1 and 2). To
understand the significance of a
Type 1 dependent loop, an intro-
ductory analogy is helpful.
Figure 3 depicts a scenario with
an open reservoir positioned
with a buried pipeline progress-
ing down an undulating hillside
to an elevated watering hole in
the valley below. Although it
may seem somewhat counterin-
tuitive at first, when water is ini-
tially released from the reservoir,
the condition can exist where the
line will become air-locked, and
the water from the elevated reser-
voir will not flow downhill to the
watering hole. Of relevance is the
fact that a section of air will
become entrapped between the
high and low points of the sys-
tem. As flow continues, this air
will become pressurized, retard-
ing the forward flow and poten-
tially arresting all flow. This is an
important point to understand
because it pertains to the proper
functioning of indwelling cath -
eters. If the elevated discharge
(H) is greater than the height
between the tank and high point
(J), the pressure within the en -
trapped air column will increase
sufficiently to stop all flow from
the elevated reservoir (see Figure
3).
Type 1 Dependent Loop
The previous analogy pro-
vided a necessary backdrop for
understanding the Type 1 de -
pendent loop. As shown in
Figure 1, the bladder now re -
places the elevated reservoir. The
fact that the bladder can generate
its own pressure, combined with
the certainty of an air-urine inter-
face distal to the patient (see dis-
cussion of FR and EO), elimi-
nates the need for a high point as
described in Figure 3. Due to the
existence of the dependent loop,
the drainage tubing will pressur-
ize (as shown), retarding ante-
SERIES/RESEARCH
Copyright 2016 Society of Urologic Nurses and Associates (SUNA) All rights reserved. No part of this document may be reproduced or transmitted in any form
without the written permission of the Society of Urologic Nurses and Associates. doi:10.7257/1053-816X.2016.36.1.7
UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1 11
grade flow toward the catheter
bag. For this research study, let-
tered dimensions A through F
were used for easy measurement
of the in-situ tubing locations.
The locations of critical features
were determined from the let-
tered location shown to the level
of the patient room floor. The
urine levels, which were easily
measured, showed the magni-
tude of the pressure applied by
the bladder.
Again using the air-lock anal-
ogy of Figure 3, there exists a
dependent loop depth (dimen-
sion A-C, see Figure 1) that
exceeds the ability of the cathe -
terized bladder to overcome with
pressure. The problem of an air-
locked catheter condition is evi-
dent to many clinical nurses who
have heard the complaints of
catheterized patients feeling a
need to urinate. Moving the bag
or repositioning the tubing some-
times results in a flow of urine
and gives immediate relief for the
patient. Other times, the relief
takes hours. It is clear that a con-
straining dependent loop depth
exists where an air-locked condi-
tion will result. Our examination
of peer-reviewed medical, nurs-
ing, or other journals have not
produced other literature delin-
eating this critical value. There -
fore, based on clinical observa-
tion, it is contended that a
dependent loop greater than 18
cm (dimension A-C) cannot be
overcome by the typical bladder.
Expressed in an alternate way,
the bladder with a catheter in
place is unable to generate more
than 18 centimeters of water col-
umn (CMWC) pressure (or equiv-
alently, 7 inches of water column
pressure or 0.25 psig). Type 1
dependent loops of greater than
18 centimeters in depth will
become air-locked and require
nursing intervention or patient
movement to resume urine flow
toward the catheter bag.
Type 2 Dependent Loop
A Type 2 dependent loop is a
more infrequent clinical observa-
SERIES
Figure 1.
Type 1 Dependent Loop
Notes:
Lettered dimensions A through E represent the height of these locations with
respect to ground level.
Dimension B-D, as shown, is the pressure differential existing in the tubing.
Dimension A-C is a measure of the magnitude of the dependent loop.
The unit of measure of CMWC (centimeters of water column) is used to quantify
small pressure differences. The unit is based on the density of pure water. If one
were to simply measure the differences in height of two urine levels (e.g., dimen-
sion B-D, above), the error introduced would be simply within the range of the urine
specific gravity (1.002 to 1.025).
Figure 2.
Type 2 Dependent Loop
Note: Dimension D-B, as shown, is a measure of the negative pressure existing in
the tubing.
Bladder
Drainage Tubing
Drainage Bag
C
DB
E
A
Pressure Differential
Dimension “B-D”
Bladder
Drainage Tubing
Drainage Bag
C
D
B
E
A
Pressure Differential
Dimension “D-B”
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12 UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1
tion in which there is a reverse
pressurization to that described
above. In a Type 2 dependent
loop, the proximal liquid height
exceeds the distal height indi -
cating a lower pressure in the
section of air between the patient
and the dependent loop (see
Figure 2). Such cases can be
confirmed to have no air re -
striction nor pressurization of the
catheter bag, both of which
would alter the pressure equal -
ization about the dependent
loop. Rather, the situation exists
where the catheter tip openings
located within the bladder have
become temporarily blocked by
tissue. As the drainage tubing or
bag is moved, the restricted air
column expands, creating a
negative pressure. This negative
pressure further secures the
blocking tissue against the cathe -
ter holes. One source in dicates
that “negative pressure in the
catheter can suck the bladder
mucosa into the eye-holes of the
catheter causing hemorrhagic
pseudopolyps” (Barford &
Coates, 2009, p. 52).
Purpose of the Study
This research study sought to
provide a simple, foundational
look at catheter dependent loops
in the clinical setting. It was
designed to identify and quantify
physical characteristics of uri-
nary catheter drainage tubing for
catheterized hospital patients.
The fundamental research ques-
tion posed was, “On two given
audit days at the 244-bed hospi-
tal in question, what is the preva-
lence of indwelling catheter
dependent loops, and what char-
acteristics of urine within
dependent loops contribute to
developing air-locked tubing and
impeded urine flow?”
Methodology
This study was designed as a
cross-sectional, observational stu -
dy to be conducted at a large, full-
service, community-based hospi-
tal in the Western United States.
Prior to execution, the study’s
design and protocol were re -
viewed and approved by the hos-
pital’s Evidence-Based Practice
and Research Council and Nurs -
ing Directors/ Leadership Team,
and was formally reviewed and
approved by the facility’s Insti -
tutional Review Board (IRB). The
study was conducted as part
of recurring, planned Nursing
Quality and Patient Safety Council
CAUTI-Prevention Bundle Com -
pliance audits. Audits are routine-
ly executed throughout the year
in this setting to evaluate nursing
compliance with established hos-
pital standards. This audit team
routinely examines aspects of in-
situ urinary catheters, in cluding
but not limited to a) appropriate
support of the catheter bag off the
floor, b) use of a catheter secure-
ment device on the patient thigh,
c) existence of the tamper-evident
seal between catheter and
drainage tubing, d) existence of
any dependent loops, e) appro-
priate markings on the catheter
bag as to its insertion date, and f)
existence of a dedicated graduat-
ed cylinder for emptying. Data
were collected as the study inves-
tigators accompanied audit staff
from room to room on each of the
hospital’s inpatient units. All
patients present in the hospital
with indwelling cathe ters on
audit days who were 18 years of
age and older, non-pregnant, and
able to give formal written con-
sent were included in the study.
Data collection occurred on two
days separated by three months,
mid-week between the hours of
SERIES/RESEARCH
Figure 3.
Analogy: Reservoir on a Hill
Notes:
As depicted, “E” is the high point in the system, and “C” is the low point. Dimension
“J” is the distance from the reservoir water level to the high point, and is a direct
measure of the driving pressure of the system. Dimension “H” is a the height of the
elevated discharge above the low point.
As fluid is released from the reservoir and crests point “E,” the low point will fill. As
fluid encapsulates the low point, air becomes entrapped between “E” and “C.” As
more fluid is released and collects at “C,” the volume of the entrapped air is
reduced, resulting in it becoming pressurized. This pressure will be displayed by
varying water heights on each side of “C.”
Stable air lock will exist if dimension “H” exceeds dimension “J.” A more complicat-
ed terrain of multiple high points (as is often seen with the catheter tubing) will
increase the potential for air lock.
Point E
Entrapped Air
Point C H
J
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UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1 13
9:00 a.m. and 5:00 p.m. Both
audits were initiated without
prior knowledge of staff or floor
clinical managers.
The procedure for data collec-
tion involved proceeding to indi-
vidual hospital rooms where
patients had an indwelling cathe -
ter in place. Each patient with an
indwelling catheter was presented
with a brief team introduction and
description of the purpose of the
visit. The investigator presented
the formal consent document for
patient or legal representative
approval and obtained the neces-
sary signatures. All actions were
completed, with the aim of not
disturbing the in-situ catheter
placement prior to data collection.
A data collection sheet for
each catheter was used for re -
cording the physical dimension
of the drainage tubing and the
location of urine within the tub-
ing. Six dimensions (points A-F
on Figures 1, 2, and 4) of the
drainage tubing were measured
(with +/- 1 cm accuracy) by the
primary investigator and verbally
communicated to a second inves-
tigator for transcription onto the
approved data sheet. The amount
of urine present in the catheter
bag and the existence of a urine
meter were also recorded. A Type
1 dependent loop, Type 2
dependent loop, or other urine
configuration was recorded. The
data sheet provided space for
pictorial or descriptive anecdotes
as to any unique aspects of indi-
vidual observations. Data on the
stability of the specific urine con-
figuration within the tubing were
collected. This was determined
by gently tapping the drainage
tubing. Unstable conditions easi-
ly manifested by rapid changes
in the urine levels. The in -
dwelling catheter system was not
otherwise altered or disturbed by
the investigators. All catheters
were attached by standard 48
inches of drainage tubing to
overnight drainage bags. No
patient-specific information was
recorded on the data sheets as
required by the IRB approval.
Careful hand hygiene, nylon
examination gloves, and a single-
use, disposable measuring tape
were used for each patient to
minimize any potential for harm-
ful contamination introduction.
Findings/Results
The analysis of data from the
two audit days revealed results
from each day were similar in
nature. Table 3 shows a summary
of the sample population charac-
teristics. The combined results
indicated that 78 catheters (42
from day 1 and 36 from day 2)
were present at the time of the
audit. Data were collected on 55
patients (70.5%).
Dependent loops were re -
corded in 48 of the 55 cases
(87.3%). Table 4 shows a sum-
mary of the collected data for de -
pendent loop cases. In the de -
pendent loop cases, 28 (58.3%)
were of stable Type 1 configura-
tion, 14 (29.2%) were of stable
Type 2 configuration, 4 (8.3%)
had no pressure differential, and
2 (4.2%) had an unstable config-
uration. Of the unstable cases,
one originally presented as a
Type 1 and the other as a Type 2.
Thirty-four (61.8%) of the 55
catheters had a urine-air inter-
face downstream of the sample
port (see Figure 4, dimension
“F”), with an average length of
16.7 cm.
SERIES
Figure 4.
Air-Urine Interface Location
Notes:
Dimension “F” is the measured distance from the catheter sample port on the tran-
sitional fitting to the location of the air-urine interface.
The location of the air-urine interface depends on the pressure within the tubing,
the shape of the tubing profile, and specifically, the location of where the tubing
takes a first, strong, downward angle.
Table 3.
Sample Characteristics (N= 78)
Audit Day 1 Audit Day 2
Overall
Percentage
Patients evaluated 29 26 70.5%
Declined participation 3 2 6.4%
Unable to consent 6 8 18.0%
Off unit 4 0 5.1%
TOTAL 42 36
Air Space
Air-Urine Interface
Urine
Urinary Catheter
Transition Fitting
Drainage Tubing
F
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14 UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1
Type 1 dependent loop con-
figurations were recorded in 28
catheters, with an average meas-
ured pressure differential of 11.0
CMWC (see Table 4 for measured
parameters). From the study
data, a “probable air-lock” was
determined when the depth of
the dependent loop exceeded 18
centimeters and fluid was not
cresting into the bag (dimension
A > B). A “potential air-lock” was
labeled for similar dependent
loops but where the fluid at Point
A was cresting (A = B). This later
condition could occur when a
substantial amount of fluid exist-
ed at the low point in the tubing
prior to the application of pres-
sure from the bladder, which
might occur after the bag or tub-
ing is moved. The applied blad-
der pressure will force a small
amount of fluid into the bag, but
then air lock. As before, the
applied bladder pressure (B-D)
will be less than the 18 CMWC
maximum value. Using these cri-
teria, three study cases were
deemed to be a “probable air-
lock” case, and 9 cases were
deemed “potential air-lock.”
Type 2 dependent loop cases
were evaluated in a different
manner. Fourteen evaluated
cathe ters had a stable Type 2
dependent loop configuration.
The average vacuum amount (see
Figure 2, dimension D-B) was
measured to be 17.9 CMWC. As
described previously, one evalu-
ated catheter showed an unstable
Type 2 configuration. The ex -
istence of the dependent loop
ensured a section of entrapped
air, which when expanded,
caused the system to become air-
locked. The criteria used to
confirm the Type 2 “probable air-
lock” for the study involved
simply agitating the tubing to
ensure that the slight vacuum
pressure (dimension D-B) was
stable and not the result of a
transient surface tension driven
pseudo-state (described earlier).
Using this “stability criteria,” 14
catheters were evaluated to be
air-locked at the time of the eval-
uation.
Discussion
From the study results, depen -
dent loops were prevalent among
the indwelling catheters evaluat-
ed, with 42 (76.4% of the total)
showing an established pressure
differential across the tubing low
point. At the time of the evalua-
tion, 26 catheters (48.1% of the
total) demonstrated the charac-
teristic of being at least potential-
ly air-locked due to presence of
the dependent loop.
The Type 1 dependent loop
was the most common pressure
configuration. It was noteworthy
that in the evaluation of the 18
CMWC maximum bladder pres-
sure hypothesis, only 3 data
points did not follow the expect-
ed rule. Of these three points, the
applied pressures (dimension B-
D) were respectively 19, 21 and
27 CMWC. For the largest outlier
of 27 CMWC, it was noted that
the bag urine volume was 1,200
ml at the time of evaluation – sig-
nificantly above the 399 ml aver-
age volume. It seems reasonable
to disregard this value as an out-
lier with some other contributing
factors involved. Considering
that the field measurements were
taken with an expected accuracy
of +/- 1 cm, the other two outliers
are not significantly outside of
that expected.
Three cases existed where
the dependent loop depth was
exactly that of the expected max-
SERIES/RESEARCH
Table 4.
Summary Data
Measured Parameters
(cm unless otherwise noted)
All Data:
Dimension*
A-C
All Data:
Dimension*
E-C
Stable**
Type 1
B-D
Stable**
Type 2
D-B
All Data:
Dimension*
F
Catheter
Volume
(mL)
N48 48 28 14 34 54
Average 17.1 48.3 11.0 17.9 16.7 399.0
Median 16.5 50 10 13.5 12 275
Maximum 36 76 27 56 54 2000
Minimum 1 2 1 1 0 0
Std. Dev. 11.5 17.9 6.4 15.7 14.7 385.3
* Dimensions indicate measurement points along the catheter drainage tubing as pictured in Figures 1, 2, and 4.
** Cases with no pressure differential or unstable configuration excluded from columns B-D and D-B.
Notes: Dimension A-C represents the depth of the dependent loop; Dimension E-C is the height of catheter insertion point
(patient location) above the tubing low point; Dimension B-D or D-B is the displayed pressure differential existing in the tubing;
Dimension F is the measured distance from the catheter sample port on the transitional fitting to the location of the air-urine
interface.
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UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1 15
imum 18 cm depth. In these
cases, the observed pressure dif-
ferentials were 12, 13, and 17
CMWC, very close to the hypoth-
esized maximum.
Evaluating the cases of Type 1
potential or probable air lock inde-
pendently from the presumed
non-locking cases (ex cluding the
three outliers) re vealed an interest-
ing difference. Tabulating values
for the difference between the
actual dependent loop depth and
the applied bladder pressure
(namely, dimension [A-C] less
dimension [B-D]) for each case
showed that for those cases
deemed as “non air-locked” cases
the difference was on average 3.1
cm (SD = 2.1). The cases deemed
potential or probable air lock had
a difference of 15.6 cm (SD = 3.8).
The large difference in these
averages gives credence to the
hypothesis that there is limited
pressure differential the bladder
can provide (with a catheter in
place).
It is also notable that the air-
urine interface existed down-
stream of the transitional fitting
in 61.8% of the cases. The aver-
age length of 16.7 cm is a sub-
stantial distance. Any future
catheter design solution seeking
to vent the entrapped air must
consider this common opera-
tional condition. A vented transi-
tional fitting between the cathe -
ter and drainage tubing would
prove to be ineffective in more
than half of the considered
catheters at the captured moment
of the evaluation.
Limitations
This study was conducted on
two single days in a clinical set-
ting. As such, the sample size
was small. Anecdotally, floor
charge nurses reported cases
where catheters had previously
been removed early in the morn-
ing by physician orders or nurse-
driven protocol. Further studies
would benefit by having more
collection days, including week-
end situations and encompassing
various times of the day. Data
collection during normal sleep-
ing hours could possibly show a
prevalence of pressure bearing
dependent loops. However, it is
not believed that such additional
data would be substantially dif-
ferent than that presented herein.
Further, as an observational
study, cases where a potential
and probable air lock existed
could not be confirmed at the
time of observation. Future stud-
ies allowing a research team to
identify such cases, drain the
catheter bag without otherwise
altering the physical configura-
tion, and then observe for addi-
tional flow into the bag would
illuminate the results and poten-
tially confirm an air-locked state.
Study investigators did not
have access to patients’ personal
health information or medical
records; therefore, no correlation
could be made with cases of diag-
nosed CAUTI or bacteriuria. Data
on the French size of the
indwelling catheter were not col-
lected. Nine of the 55 patients
were evaluated in a sitting posi-
tion in a chair, wheelchair, or
recliner. Further studies could
more formally evaluate the effect
of each of these variables.
Conclusions
The present study results
give credence to the anecdotal
experience of many nurses, in -
cluding the authors, that depend-
ent loops are very common in the
acute care clinical setting, and in
spite of frequent education, are
often difficult to avoid. Future
studies and increased under-
standing of CAUTI pathogenesis
are needed to confirm the con-
clusions of previous works that
dependent loops may play a sig-
nificant role in intraluminal con-
tamination. If this is borne out,
then the task of minimizing this
route will prove formidable.
Findings of this study reflect
the results of Danek, Gravenstein,
Lizdas, and Lapotang’s (2015)
recently published exploration of
dependent loops in urinary
drainage systems. In their study,
Danek et al. (2015) identified
85% of hospitalized patients
with indwelling catheters had
dependent loops, comparable
with our findings of 87%. This
congruence highlights the wide
prevalence of catheter dependent
loops and opportunities for im -
proving urinary drainage systems
and nursing practice related to
indwelling catheters.
Further work in the evalua-
tion of dependent loops is need-
ed to carefully and systematical-
ly determine the critical depend-
ent loop depth, which will result
in air lock in a Type 1 dependent
loop. The hypothesis of an 18
CMWC maximum catheterized
bladder pressure limit asserted in
the preliminary discussion needs
confirmation and further refine-
ment.
Nursing Implications
Despite manufacturer’s liter-
ature and frequent procedural
reminders to nurses, catheter
dependent loops remain preva-
lent in the clinical environment.
It is reasonable to anticipate this
problem will persist even with
an increase in staff education and
awareness. In light of this, nurses
must maintain vigilance in car-
ing for patients with indwelling
catheters to prevent and remedy
catheter dependent loops. When
a dependent loop is found,
manipulation of the tubing to
allow urine to flow freely into the
drainage bag is key.
Eliminating even the small-
est amount of a dependent loop
would require taut tubing going
from the patient to the catheter
bag. Even if tolerated by the
patient, frequent repositioning in
a bed or chair, as required by best
nursing practice, would invari-
ably change the tubing configura-
tion. Complete and continual
elimination of all dependent
loops does not seem a realistic
goal. Rather, innovation in the
design of the indwelling catheter
is needed, which will maintain
the current closed system charac-
SERIES
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16 UROLOGIC NURSING / January-February 2016 / Volume 36 Number 1
teristic, allow for gentle disper-
sion of tubing pressure fluctua-
tions, accommodate the ever-
changing tubing profile, and per-
mit uninhibited flow of urine
from the catheter to the bag.
Future research regarding in -
dwelling catheters could focus
on evaluating methods of nursing
care delivery that reduce
dependent loops, as well as test
novel products and devices
designed to improve on the exist-
ing urinary catheter design. Until
the time that such a design is
realized, vigilance is required to
keep any dependent loop under
18 cm in depth.
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SERIES/RESEARCH
Instructions for Continuing Nursing Education Contact Hours
Urinary Catheter Dependent Loops as a Potential Contributing
Cause of Bacteriuria: An Observational Study
Deadline for Submission:
February 28, 2018
UNJ 1601
To Obtain CNE Contact Hours
1. For those wishing to obtain CNE contact hours, you must read the article
and complete the evaluation through SUNA’s Online Library. Complete
your evaluation online and print your CNE certificate immediately, or later.
Simply go to www.prolibraries.com/suna
2. Evaluations must be completed online by February 28, 2018. Upon
completion of the evaluation, a certificate for 1.4 contact hour(s) may be
printed.
Learning Outcome
After completing this learning activity, the learner will be able to examine
catheter dependent loops and their possible role in catheter-acquired urinary
tract infections.
The author(s), editor, editorial board, content
reviewers, and education director reported no
actual or potential conflict of interest in relation to
this continuing nursing education article.
This educational activity is provided by the
Society of Urologic Nurses and Associates
(SUNA).
SUNA is accredited as a provider of continuing
nursing education by the American Nurses
Credentialing Center’s Commission on Accredi -
tation.
SUNA is a provider approved by the California
Board of Registered Nursing, provider number
CEP 5556. Licensees in the state of California
must retain this certificate for four years after the
CNE activity is completed.
This article was reviewed and formatted for
contact hour credit by Rosemarie Marmion, MSN,
RN-BC, NE-BC, SUNA Education Director.
Articles in the SUNA Online Library are FREE for SUNA Members.
CNE Evaluation Fee – $15
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