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Foley drainage tubing configuration affects bladder pressure: a bench model study.

A bench model was created to measure and analyze pressures in a simulated bladder and an actual urine drainage system. Fluid-filled dependent (generally U-shaped) loops in the urine drainage tubing generated back-pressure (in units of cm [H.sub.2]0), directly related to the difference in fluid meniscus heights (in units of cm) across the dependent loop that interfered with emptying of the simulated bladder. If the results obtained with a simulated bladder occur in actual bladders with indwelling urinary catheters, retained urine volume (that can promote urinary tract infection) will increase with larger differences in meniscus heights across the dependent loop due to increased back-pressure. Dependent loops in urine drainage tubing should be avoided. If the dependent loops cannot be avoided or a configuration without dependent loops cannot be maintained, they should be routinely emptied of urine, especially if the bag-side meniscus is higher than the bladder-side meniscus.

Key Words: Dependent loop, bladder distention, drainage, catheter-associated urinary tract infection (CAUTI), Foley catheter, back-pressure.


In a recent surgery, one author of this study noted poor urine flow after kidney reperfusion in a liver-kidney transplant patient; however, after straightening the urine drainage tubing to empty the urine that had accumulated in the generally U-shaped dependent loop in the Foley drainage tube, the rate of urine outflow from the bladder appeared to increase. This sequence of events raised a simple question with clinical implications: could potentially harmful back-pressures sometimes exist in urine drainage systems that are considered as passive drains that reliably channel urine from the bladder to the urine collection bag? At a minimum, obstruction to urine outflow may cause patient discomfort and may also predispose a patient to a catheter-associated urinary tract infection (CAUTI). Informal experimentation with a commercial urine drainage system led to the hypothesis that significant back-pressures might arise in clinical practice, and consequently, to the bench experiments described herein.

Garcia et al. (2007) described a cessation of drainage in urine drainage systems with fluid-filled dependent loops due to backpressure created by fluid trapped in the loop. Dependent loops in bladder drainage tubing significantly increase the risk of CAUTI (Maki & Tambyah, 2001). As a measure to avoid CAUTI, Trautner and Darouiche (2004) and Kwak et al. (2010) recommend ensuring dependent drainage as appropriate management of Foley drainage systems. The urge to void generally occurs near a bladder volume of 150 mL, and an accumulating bladder volume is associated with physical discomfort (Sulzbach, 2002).

Materials and Methods


The primary purpose of the study was to determine the relationship between the back-pressure exerted on the simulated bladder and the difference in meniscus heights; the hypothesis was that the relationship would be linear.

Bench Model

A bench model (see Figure 1, a) of the urinary system was constructed and drained using a Foley catheter (Bardex I.C. 930116, 16 Fr, Bard Inc., Covington, GA), urine drainage tubing, and a urine collection bag. An infusion pump (Alaris PC 8015 Series, CareFusion, San Diego, CA) was used as a model for the kidneys and ureters, and a 500 mL soft IV fluid bag represented the bladder (Baxter, Deerfield, IL). The pump was connected to the bladder with an IV bag trochar via one of the IV bag's access ports. The Foley catheter was inserted in the other port and sealed to prevent leakage around it. The drainage tubing and vented collection bag (Bard Medical Division) were hung from a perforated board to simulate typical clinical orientations and height relationships of the various components (see Figure 1, a).

Instrumentation and Software

Pressures were measured using disposable invasive blood pressure transducers (Edwards Life Sciences, Irvine, CA) attached to a Component Monitoring System monitor and pressure modules (Model M1006A, Philips, Andover, MA) at locations shown in Figure 1, a. Data were captured at approximately two-second intervals. The pressure sampling line for the bladoder pressure was fluid-filled, and the sampling line for the air space in the drainage tubing was air-filled. Fluid meniscus heights were measured relative to the floor using a metal yardstick with centimeter gradations (AE141, Swanson Tool Co., Frankfort, IL).


After zeroing the pressure transducers, each repetition consisted of two phases. Phase 1 is analogous to the initial placement of a catheter and ends when fluid first enters the collection bag; it occurs with the deepest possible loop (see Figure 2, "Start"). Phase 2 consists of measurements with a sequence of progressively more shallow loops, allowing collection of data on multiple dependent loop depths. The loop depths were chosen to mimic the spectrum that is observed clinically with different patient positions and tubing routing.

For each measurement of pressures and meniscus heights, we allowed the distal (down stream) bag side meniscus to crest and allow fluid to drip into the collection bag as is observed in clinical practice when there is a dependent loop present. Pressures and heights of the fluid menisci were recorded. Measurements were done at the end of Phase 1 and for each of the seven successive configurations in Phase 2.

Phase 1.

* Clamp the Foley catheter 1 cm distal to the bladder and arrange the tubing in the first configuration (see Figure 2, "Start") having the deepest possible loop.

* Transfer approximately 420 mL of tap water with yellow food coloring into the bladder.

* Start the data collection software and unclamp the Foley catheter.

* Start the infusion pump at 900 mL/hour to simulate urine production. This rate (900 mL/hour) was chosen for efficiency because there is little resistance in the system. The simulated "urine" flow rate is not intended to be a clinical one.

* When the downstream meniscus crests and fluid enters the collection bag (see Figure 2, "Configuration 1"), record pressures and meniscus heights as described above.

Phase 2.

Each of the remaining seven predetermined drainage bag positions (see Figure 2, "Configurations 2 to 8") proceeded as follows:

* Advance the collection bag to the new position to reduce the depth of the loop and pour out some fluid into the urine collection bag.

* Wait for the return of a crested configuration with stable pressures, and record meniscus heights and pressures as described above.

A typical pressure versus time profile from this protocol is shown in Figure 3.


Filling of the Drainage Tubing: Evolution and Locations of the Fluid Menisci

The urine drainage tubing is initially empty (see Figure 1, a). However, gravity causes fluid to accumulate at the bottom of the generally U-shaped dependent loop in the urine drainage tubing (see Figure 1, b). Once the fluid level rises sufficiently to occlude the drainage tubing, the air in the bladder-side portion of the tubing is trapped between the fluid-filled Foley catheter and the bladder-side meniscus. Because air is essentially incompressible at pressures present in the system, the bladder-side meniscus is almost stationary, and fluid is forced to climb the bag-side portion of the dependent loop (see Figure 1, c and d). Advancement of the fluid is driven by the pressure of the air in the bladder-side limb, which ultimately must be provided via the bladder.


During each repetition of the experiment, the bladder (primed with 420 mL of fluid) is initially able to freely drain, but as fluid is forced to climb the ascending limb of the dependent loop, bladder pressure increases, peaking as the fluid reaches the crest at the collection bag. After cresting, new fluid added to the system displaces fluid into the collection bag. Figure 3 shows a typical pressure versus time profile. Start and stop times from the laboratory notes, pressures captured by our recording system, and meniscus elevations measured with a metal meter ruler were used to plot the relationships between the air space pressure (Figure 4; [R.sup.2] = 0.998) and the bladder pressure ([R.sup.2] = 0.999), respectively, versus the vertical distance from the bladder-side meniscus to the bag-side meniscus in the drainage tubing. Meniscus heights were measured at the center of the drainage tubing during time periods emphasized (in gold) in Figure 3.


The primary objective was to mimic the spectrum of what is observed in the clinical setting to study the relationship between the geometry of the drainage tubing dependent loop and the bladder pressure required to move urine into the collection bag. The worst-case clinical scenario is to hang the urine collection bag directly adjacent to the Foley catheter, creating the deepest dependent loop and allowing the maximum possible difference in meniscus elevation; this configuration results in a bladder pressure of 40.8 cm [H.sub.2]O (30 mmHg).

Understanding the physics of urine-filled dependent loops facilitates visual monitoring of the pressures in a urine drainage system. Preventing the formation of loops, or at least minimizing their depth, minimizes the outflow pressure thresholds that need to be overcome for urine to flow into the collection bag, thereby minimizing 1) bladder distention, 2) a static fluid urine column in the drainage tubing, and 3) the associated risk of the urine accumulated in the dependent loop flowing back into the bladder when the patient is moved. All of these elements may be associated with CAUTI and also a sense of bladder discomfort, and are avoidable by ensuring the drainage tubing is free of dependent loops. The pressure exerted upon the bladder is directly shown (in cm [H.sub.2]O) by the difference in elevation between the easily observed fluid menisci. As a rule, any time the elevation of the bag-side meniscus is higher than the patient-side meniscus, there is always unintended back-pressure on the bladder.


As a practical, actionable outcome of this study, when a urine-filled dependent loop is present and the bag-side meniscus is higher than the bladder-side meniscus, a nurse can visually estimate the back-pressure (in units of cm [H.sub.2]O) that is impeding bladder emptying by measuring or estimating the difference in meniscus heights in units of cm. Assuming the density of urine is similar to water (a reasonable assumption), if differences of 5, 10, 15, 20, 25 cm in meniscus heights are observed, then the back-pressure preventing voiding of the bladder via the indwelling urinary catheter are, respectively, 5, 10, 15, 20, and 25 cm [H.sub.2]O. For a given patient with a given bladder compliance curve (cystometrogram), a higher difference in meniscus height will result in a larger retained urine volume, which can predispose to CAUTI.

Based on this bench study, dependent loops should be avoided when setting up a urine drainage system. If the excess urine drainage tubing will not stay in a configuration without dependent loops (the excess drainage tubing keeps falling and re-forming a dependent loop), the urine that subsequently accumulates in the dependent loop should be regularly drained. If the fluid level in the hanging drainage tubing is lower on the side of the patient than the collection bag side, then back-pressure will prevent the bladder from emptying completely. The patient may then feel a need to urinate because of the retained urine volume and may be inherently more susceptible to a bladder infection. If the difference in urine levels is high, the retained urine volume is likely to be large. After draining the dependent loop, the loop may refill due to the retained urine volume in the bladder and need to be drained again. Draining the urine collected in the hanging tubing completely and repositioning the tubing as necessary to prevent formation of a hanging loop should be a priority to maintain patient comfort and prevent CAUTI.

Research Summary


Urinary retention and bladder distention can lead to urinary tract infection. The excess drainage tubing in a urine drainage system will droop and form a generally U-shaped loop (a dependent loop) where urine will collect. If the urine is allowed to collect, the meniscus on the bag side will rise higher than the bladder-side meniscus, indicating back-pressure that impedes bladder emptying.


To determine if the back-pressure impeding bladder emptying is linearly proportional to the difference in meniscus heights across a fluid-filled dependent loop.


An instrumented bench model was built to measure and analyze the pressures in a simulated bladder drained via an indwelling urinary catheter connected to a urine drainage and collection system.


The back-pressure (in units of cm [H.sub.2]O) impeding bladder emptying was found to be linearly related ([R.sup.2] = 0.998) to the difference in meniscus heights (in units of cm), meaning that a taller difference in meniscus heights indicates a larger back-pressure opposing bladder emptying, resulting in larger retained urine volume.


Assuming that results with a simulated bladder apply to actual bladders drained with indwelling urinary catheters, a) clinicians should avoid dependent loops in the drainage tubing, b) if dependent loops are found or cannot be avoided because a configuration without dependent loops cannot be maintained because the excess drainage tubing keeps falling down and re-forming a dependent loop, drain the urine that collects in the dependent loops frequently.

Level of Evidence - VI (Polit & Beck, 2012)

doi: 10.7257/1053-816X.2014.34.1.33

Funding/Support: This study was carried out at the University of Florida Center for Safety Simulation & Advanced Learning Technologies (CSSALT), and was supported by institutional and departmental sources. Part of this material was presented in poster form at the American Society of Anesthesiologists Annual Meeting, October 15-19, 2011, Chicago, IL.

Acknowledgements: We would like to thank Dr. Robert Williams (deceased) and Mrs. Rosita Williams for their generous support of the Center for Safety Simulation, & Advanced Learning Technologies, which made this study possible.


Garcia, M.M., Gulati, S., Liepmann, D., Stackhouse, G.B, Greene, K., & Stoller, M.L. (2007). Traditional Foley drainage systems - Do they drain the bladder? Journal of Urology 177(1), 203-207.

Kwak, Y.G., Lee, S.O., Kim, H.Y., Kim, Y.K., Park, E.S., Jin, H.Y. .... Korean Nosocomia] Infections Surveillance System (KONIS). (2010). KONIS. Risk factors for device-associated infection related to organisational characteristics of intensive care units: Findings from the Korean Nosocomial Infections Surveillance System. Journal of Hospital Infection 75(3), 195-199.

Maki, D.G., & Tambyah, P.A. (2001). Engineering out the risk for infection with urinary catheters. Emerging Infectious Diseases 7(2), 342-347.

Polit, D.F., & Beck, C.T. (2012). Nursing research: Generating and assessing evidence for nursing practice (9th ed.). Philadelphia: Lippincott, Williams & Wilkins.

Sulzbach, L.M. (2002). Ask the experts. Critical Care Nurses 22(3), 84-87.

Trautner, B.W., & Darouiche, R.O. (2004). Catheter-associated infections: Pathogenesis affects prevention. Archives of Internal Medicine 164(8), 842-850.

Wilhelm K. Schwab, PhO, was an Engineer with a career-long interest in understanding how things work and how to make them work better. He worked with the University of Florida Clinical and Translational Research Informatics Program (CTRIP), Gainesville, FL. He passed away on September 7, 2012.

David E. Lizdas, BSME, is a Mechanical Engineer, the Center for Safety Simulation, & Advanced Learning Technologies, Department of Anesthesiology, University of Florida College of Medicine, Gainesville, FL.

Nikolaus Gravenstein, MD, is an Anesthesiologist, the Department of Anesthesiology, University of Florida College of Medicine, and the Center for Safety Simulation, & Advanced Learning Technologies, Department of Anesthesiology, University of Florida College of Medicine, Gainesville, FL.

Samsun Lampotang, PhO, is an Engineer and Professor of Anesthesiology, the Department of Anesthesiology, University of Florida College of Medicine, and the Center for Safety Simulation, & Advanced Learning Technologies, Department of Anesthesiology, University of Florida College of Medicine, Gainesville, FL.
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Title Annotation:Research
Author:Schwab, Wilhelm K.; Lizdas, David E.; Gravenstein, Nikolaus; Lampotang, Samsun
Publication:Urologic Nursing
Article Type:Clinical report
Date:Jan 1, 2014
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