Despite the impression by some clinicians that they can examine a patient and predict whether they have an elevated abdominal pressure, the fact is clinical judgment for this disorder is no better than the flip of a coin.  Several studies have confirmed that even in the hands of a staff level academic surgeon, abdominal exam is completely unreliable at determining the presence or absence of an elevated intra-abdominal pressure.[1, 2] These authors conclude that due to the inaccuracies of physical exam findings, intra-abdominal pressures must be measured by an objective, reliable, reproducible method at an interval that is frequent enough to detect rising pressure and allow interventions to occur prior to the onset of the highly mortal abdominal compartment syndrome.

Since clinical exam is inaccurate, early detection of increasing intra-abdominal pressure requires a reliable, reproducible method of measuring it.  To date, the most reliable method is via pressure transduction through a catheter within the peritoneal cavity. Other less invasive options include pressure transduction through a tube placed in the stomach, bladder, or rectum.[1, 2] Of these options, Obeid et al found bladder pressure to most closely reflect intraperitoneal pressure and to be the most technically reliable.[3] Multiple other authors confirm Obeids’ findings that bladder pressures most closely tracks peritoneal pressures, whereas stomach pressures are less reliable.[4-7] Bladder pressures taken through a Foley catheter correlate very closely with pressures measured directly in the abdominal cavity and are now considered to be the gold standard method of monitoring intra-abdominal pressure by an international consensus committee – the world society of abdominal compartment syndrome.[8]

Methods to measure bladder pressure

Manometry 

One of the original methods described to measure bladder pressure via the Foley catheter is the manometry technique.[9-11] The formal method of manometry requires a manometry tube that is placed inline between the Foley and the drain tube.  A priming volume of fluid must be infused into the bladder to assure adequate volume to fill the Foley and the manometry tube until equilibrium is reached. It is an absolute requirement to vent this tube to ambient air pressure to avoid inaccuracies that will be introduced by an air-lock or siphon effect that can develop in the distal drain tube.[12] One must also carefully pay attention to where they hold the zero point, the angle of the manometer and avoidance of Foley kinking during the measurement.[12] Once the measurement is completed the tube is removed and the Foley is reconnected to the drain tube. Repeat measurements require breaking the system again and reassembling the vented manometry tube – a time consuming proposition. While this technique is accurate, there are significant disadvantages due to the need to recurrently open the system and the time requirements to obtain the pressure (leading to infrequent data acquisition). Another disadvantage is that the information is obtained in centimeters of water and must be converted to mm Hg (divide by 1.36) if one is using any of the current recommendations for intervention.

Manometry as is often currently practiced (dumping the urine back into the patient, holding the tube up and observing the height of the fluid column) is fraught with error and should not be used to obtain an intra-abdominal pressure measurement.[11, 12] This method introduces two major items that can lead to error: inadequate volume of infusion to fill the manometry column and siphon effect of the distal drain tube.[12] There must be a volume of infusion not less than 30 ml to ensure the large diameter foley drain tube can fill up to the  level of the true IAP in patients with any significant elevation of pressures – failure to have adequate volume may lead to a falsely low measurement of IAP.[12] Unless one pre-fills the system with saline, there may not be an adequate volume of urine in the drain tube to adequately fill the manometer. Another common source of error is the siphon effect. Lifting the drain tube causes urine to run distally as well as proximally back into the bladder. The distal fluid, if caught in a loop of the drain tube, will create a hydrodynamic siphon and “pull” the urine out of the bladder leading to a false elevation in the measured IAP.[12] Since the clinical situation in which these patients are having their IAP measured is always complex, these errors are often overlooked and will lead to misleading data. As with the traditional methods of manometry, additional errors can be introduced unless careful attention is paid to the zero point, the angle of the manometer and avoidance of Foley kinking during the measurement. For these reasons as well as potential infectious complications discussed below, simply lifting the urine drain tube and eyeballing the fluid column height should not be relied upon since it may lead to significant inaccuracies.

An additional concern with lifting the urinary drain tube and dumping large volumes of urine back into the patient is that of urinary tract infections.  Maki et al demonstrated manipulation of the drainage tube such that it rises above the level of the bladder (and dumps old urine back into the patients bladder) is the single best predictor of catheter associated urinary tract infection caused by handling the catheter (more predictive that violating the sampling port or maintaining a closed system).[13, 14] Maki et al feels that biofilm creep is the primary cause of CAUTI, but concludes that “Infections in which the biofilm does not play a pathogenetic role are probably caused by mass transport of intraluminal contaminants into the bladder by retrograde reflux of microbe-laden urine when a catheter or collection system is moved or manipulated.” In summary, manometry using homemade systems or done by simply lifting the drain tube and eyeballing the level of urine is fraught with risks of erroneous data acquisition unless a formal vented manometry tube is placed in line and carefully attention to detail occurs.  As currently practiced, it also raises significant concerns regarding an increased urinary tract infection rate.  Given other accurate and low risk options, this method should likely remain of historical interest but not be routinely applied in a modern ICU. 

Self made systems assembled in the ICU

Measuring bladder pressure with a “home made” system is well described in the medical literature [1, 2, 15-17]: A Foley catheter is placed and the bladder is drained.  An infusion system consisting of a needle, IV tubing, syringe, pressure transducer, stopcocks and saline bag are assembled and the needle is inserted into the urine sampling port of the Foley catheter. The transducer is attached to a monitor and zeroed. The drain tubing is then clamped and 50 to 100 ml of saline are infused into the bladder. After equilibration of the pressure within the system, the mean bladder pressure is noted on the monitor. Once noted, the system is disassembled and removed and the drain tubing is unclamped. A similar system can be assembled using a 3-way Foley catheter. Instead of a needle inserted into the sampling port, the Y-extension of a pediatric feeding tube is connected to the irrigation port of the 3-way catheter and fluid infusion/pressure transduction is conducted through this lumen.

Home Made IAP monitors are not adequate for broad application in ICU's - due to usability issues and reproducibility problems  this will not become a standard, routing method to measure IAP

Although ingenious for development of a concept and cutting edge research, the above processes have significant disadvantages when applied to routine ICU monitoring. First of all, both home made systems require the staff to collect up a number of scattered items and assemble them correctly, a hassle which may reduce the likelihood that the pressure is even measured until the disease  process is far progressed (i.e a compartment syndrome – which is a surgical disease, rather than intra-abdominal hypertension which is a medical problem). The former invades a sterile space with a needle or Luer attachment (the urinary drain system) every time the procedure is done (every 1 to 4 hours), while the later requires replacement of a 2-way catheter with a 3-way catheter. Both methods risk variations in assembly and measurement making inter-observer variations in pressure measurement a concern.[18] This variation should not be taken lightly – it is well recognized by experts in the field and elimination of variation is a cornerstone to monitoring fluid filled systems in the ICU.[18-21] Finally – all these homemade systems suffer from the concept of lack of “usability”.  Usability relates to the evaluation of human-technological interfaces to assess how easy a technology is to use. It evaluate efficacy – i.e. does the technology work when used properly – and it evaluates satisfaction – i.e. did the user find the technology easy to use. Even a good idea – an ingenious idea – that is difficult or un-satisfying to use (like home made IAP monitoring systems) will never find its way into routine practice – regardless of outcomes data since humans will simply not adopt the idea because of lack of “usability”. The point here is that due some of the usability issues surrounding homemade IAP monitoring, very few medical institutions have adopted it routinely as a screening and monitoring tool – resulting in frequent delays is diagnosis or complete failures to diagnose intra-abdominal hypertension.

Many potential problems exist with homemade IAP kits

Commercially available intra-abdominal pressure monitoring devices

In an effort to simplify and standardize bladder pressure measurement manufacturers have developed systems that are simple and easy to use, allowing frequent data acquisition so early interventions can be implemented.  The following devices are currently on the market.

AbViser Intra-abdominal pressure monitoring device

The AbViser, (Wolfe Tory Medical, Salt Lake City Utah –www.wolfetory.com) comes as a kit containing a pre-assembled intra-abdominal pressure monitoring system and pressure transducer. The kit adapts to any Foley catheter and interfaces with any ICU monitor. This allows the kit to be immediately integrated into an ICU without the need to change current catheters, transducers or cabling. With a simple injection of saline the AbViser valve automatically occludes the Foley drain tube and sterile saline is infused into the bladder. Intra-abdominal pressure measurement can be made in under 30 seconds. After a pressure measurement is obtained the valve automatically opens and the fluid drains out into the collection bag. The process, which requires very little time to conduct, can be repeated any time a pressure measurement is necessary. Once the device is attached, no further exposure of the sterile urinary system is necessary so the risk of introducing infection or exposing nursing staff to body fluids is avoided.[22]  Because no needles are used, there is no risk to the patient or the nurse from a needle.  Finally, as a pre-assembled system, the risk of variations in assembly and measurement make inter-observer variations in pressure measurement unlikely as demonstrated in a number of studies.[23-25]

Foley-manometer

The Foley Manometer (Holtech Medical - www.holtech-medical.com) comes as a kit containing a preassembled manometer that attaches in-line with the Foley drain systems. This product has solved the problems with inaccuracies due to the siphon effect by placing a vented port in the manometer so the distal drain tube can be equilibrated to ambient air pressure.  Due to recent improvement in the product, it now infuses a fairly small volume of urine into the patient, resulting in a risk of CAUTI similar to that seen at baseline ICU patients.[26]

Speigelberg IAP monitor and CiMON monitor

Available in Europe are two catheters that can be inserted through the nostril into the stomach where they measure gastric pressure as a surrogate for intra-abdominal pressure. They interface with a separate monitor, allowing continuous data that may be useful in the extremely complex patient in whom continuous data might be warranted.

Summary:

1. Kirkpatrick, A.W., et al., Is clinical examination an accurate indicator of raised intra-abdominal pressure in critically injured patients? Can J Surg, 2000. 43(3): p. 207-11.

2. Sugrue, M., et al., Clinical examination is an inaccurate predictor of intraabdominal pressure. World J Surg, 2002. 26(12): p. 1428-31.

3. Obeid, F., et al., Increases in intra-abdominal pressure affect pulmonary compliance. Arch Surg, 1995. 130(5): p. 544-7; discussion 547-8.

4. Iberti, T.J., C.E. Lieber, and E. Benjamin, Determination of intra-abdominal pressure using a transurethral bladder catheter: clinical validation of the technique. Anesthesiology, 1989. 70(1): p. 47-50.

5. Suominen, et al., Comparison of direct and intravesical measurement of intraabdominal pressure in children. J Pediatr Surg, 2006. 41(8): p. 1381-5.

6. Davis, et al., Comparison of indirect methods of measuring intra-abdominal pressure in children. Intensive Care Med, 2005. 31(3): p. 471-475.

7. Risin, E., et al., A new technique of direct intra-abdominal pressure measurement: a preliminary study. Am J Surg, 2006. 191(2): p. 235-7.

8. Malbrain, et al., Results from the International Conference of Experts on Intra-abdominal Hypertension and Abdominal Compartment Syndrome. I. Definitions. Intensive Care Med, 2006. 32(11): p. 1722-32.

9. Kron, I.L., A simple technique to accurately determine intra-abdominal pressure. Crit Care Med, 1989. 17(7): p. 714-5.

10. Harrahill, M., Intra-abdominal pressure monitoring. J Emerg Nurs, 1998. 24(5): p. 465-6.

11. Sedrak, M., K. Major, and M. Wilson, Simple fluid-column manometry to monitor for the development of abdominal compartment syndrome. Contemporary Surgery, 2002. 58(5): p. 227-229.

12. Wolfe, T.R. and E.J. Kimball, Bladder pressure measurement with manometry: Infusion volumes and human error significantly affect IAP measurement accuracy in a controlled IAP/IAH model. ANZ J Surg, 2005. 75: p. A1-A24 (abstract).

13. Maki, D.G., V. Knasinki, and P.A. Tambyah, Risk factors for catheter associated urinary track infections: a prospective study showing the minimal effects of catheter care violations on the risk of CAUTI. Inf Control Hospital Epidemiol, 2000. 21: p. 165 (abstract).

14. Maki, D.G. and P.A. Tambyah, Engineering out the risk of infection with urinary catheters. Emerging infectious diseases, 2001. 7(2): p. 1-6.

15. Cheatham, M.L. and K. Safcsak, Intraabdominal pressure: a revised method for measurement. J Am Coll Surg, 1998. 186(5): p. 594-5.

16. Fritsch, D.E. and R.A. Steinmann, Managing trauma patients with abdominal compartment syndrome. Crit Care Nurse, 2000. 20(6): p. 48-58.

17. Kron, I.L., P.K. Harman, and S.P. Nolan, The measurement of intra-abdominal pressure as a criterion for abdominal re-exploration. Ann Surg, 1984. 199(1): p. 28-30.

18. Sugrue, M., Intra-abdominal pressure: time for clinical practice guidelines? Intensive Care Med, 2002. 28(4): p. 389-91.

19. Fusco, M.A., R.S. Martin, and M.C. Chang, Estimation of intra-abdominal pressure by bladder pressure measurement: validity and methodology. J Trauma, 2001. 50(2): p. 297-302.

20. Malbrain, M.L.N.G., Different techniques to measure intra-abdominal pressure (IAP): time for a critical re-appraisal. Intensive Care Med, 2004. 30(3): p. 357-71.

21. Darovic, G.O. and J.P. Zbilut, Fluid-filled monitoring systems, in Hemodynamic Monitoring: Invasive and noninvasive clinical application, G.O. Darovic, Editor. 2002, W.B. Saunders company: Philadelphia, Pennsylvania. p. 113-131.

22. Ejike, J.C., K. Bahjri, and M. Mathur, What is the normal intra-abdominal pressure in critically ill children and how should we measure it? Crit Care Med, 2008. 36(7): p. 2157-62.

23. Kimball, E.J., et al., Reproducibility of bladder pressure measurements in critically ill patients. Intensive Care Med, 2007. 33(7): p. 1195-8.

24. Shuster, M., et al., Reliability of bladder pressure measurements taken in the supine position with a 30 degree head of bed elevation. Acta Clinica Belgica, 2009. 64(3): p. 263 (abstract 58).

25. Cheatham, M.L., et al., The impact of body position on intra-abdominal pressure measurement: a multicenter analysis. Crit Care Med, 2009. 37(7): p. 2187-90.

26. Malbrain, M., et al., Intra-abdominal pressure measurement with the Foleymanometer does not increase urinary tract infection. Acta Clinica Belgica, 2009. 64(3): p. 274 (Abstract 119).

27. Wolfe, T.R. and E.J. Kimball, Intra and Inter-observer variability does not occur with a new intra-abdominal pressure monitoring kit. ANZ J. Surg, 2005. 75: p. A1-A2, Abstract 3.