Intraabdominal pressure monitoring techniques
What is wrong with physical exam alone?
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, 33] 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 (www.wsacs.org).[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, 13] 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, 13] 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.[13] 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).[14, 15] 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 (Harrahill Technique[10], etc) 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, 16-18]: 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 saline is 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 new concept 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 intraabdominal 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). This hassle and delay in care will add an enormous cost to the patients care due to prolonged tissue ischemia and potential need for operative intervention that might be avoided with earlier monitoring done with self assembled kits. Furthermore, the former method invades a sterile space with a needle or Luer attachment (the urinary drain system) every time the procedure is done (every few hours ideally), 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.[19, 20] The potential for variable data should not be taken lightly – elimination of data variability is a cornerstone to monitoring fluid filled systems in the ICU.[19, 21-23]
Another issue not well recognized by most clinicians is that the Foley catheter drain tube usually contains a measurable pressure that can easily be confused as an intraabdominal pressure. This occurs due to the "air lock" commonly found in drain tubes.[30] Should that airlock result in a meniscus height difference between the distal and proximal urine/air levels it will lead to a pressure difference that can be accidentally measured with the homemade system. Typical pressures seen from this phenomenon are 8-12 mm Hg pressure. Erroneously measuring this pressure by turning a stopcock wrong or clamping wrong etc is quite easily done since the result seen seems to be reasonable for IAP. This of course would be felt to be a relatively mild increase in IAP and no therapy would be initiated - delaying appropriate care if the actual pressure is higher. This air-lock phenomenon may account for the well described change of IAP pressure at change of shift when a new person comes on and finds IAP to be totally different from the prior shift.
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 evaluates 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 unsatisfying 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 that is used early and often to ensure diagnosis of mild IAH before it progresses – resulting in frequent delays is diagnosis or complete failures to diagnose intra-abdominal hypertension.
Discussion related in homemade kit use by hospitalists - click here
Femoral Vein Pressure as correlate to IAP
Occasionally an article appears that mentions femoral CVP as an accurate correlate for IAP. This concept has undergone rigorous investigation by a multinational, multicenter trial which was published in 2011. [31, 32] The authors found a large variation in the femoral CVP compared to bladder pressure and concluded: "Femoral Venous pressure cannot be used as a reliable surrogate measure of IAP." They do suggest that anyone with an high femoral CVP should have IAP monitored however, because though the correlation is not accurate enough to predict definite IAH, it is suggestive and concerning.
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.
Anyone who is sophisticated enough and interested enough in
the topic to have accessed this web site should recognized the need
to implement a commercial product to standardize and simplify their
ICU practice of monitoring IAP.[20]
By introducing a prepackaged device you should be able to
improve compliance and increase the frequency of IAP measurements –
hopefully doing so early in the high risk patient so you can
introduce the interventions discussed on this web site (click here for intervention section). While the “cost” of materials
for a prepackaged device are more than those of homemade products,
the return on investment is seen by less nursing time invested in
measurement, higher quality data to assist in critical decision
making, and earlier detection of IAH allowing
earlier intervention to prevent ACS. Furthermore,
commercially available kits, specifically the AbViser, are the only
devices validated to provide reproducible data both for the same
nurse and for other providers (i.e. both intra and interobserver
variability is eliminated).[28, 29] All practitioner who have used
home-made systems often enough realize they can get errors in data -
sometimes at the worse possible moments when the patient is unstable
- and usually due to operator error. Finally, numerous recent studies
have noted decreased time of ventilator (and less VAP), quicker ICU
turn around, fewer surgical interventions with earlier surgical
recovery when required – all resulting in significant cost savings
that dwarf any relatively small costs of a monitoring device.[20,
24-27]
Summary:
Although
commercially available products do cost more than a home-made device
they offer several major advantages: Ease and speed of data
acquisition leading to early and frequent monitoring of IAP (i.e.
they solve the usability conundrum that exists with home-made
systems), standardized reproducible measurements, manufacturing
quality control and sterility.
The first advantage is the usability issue related to speed
and ease of data acquisition.
Many ICU’s do not even measure IAP due to the difficulties in
assembling their own monitoring device. Those that do measure IAP
tend to do so only late in the disease process to confirm that the
patient has abdominal compartment syndrome – a very progressed form
of abdominal hypertension that is a surgical emergency. This delay
in obtaining IAP measurements prevents the clinicians from detecting
the problem early when it is still amenable to urgent medical
interventions that can prevent the abdominal compartment syndrome
from developing. Pre-assembled devices are able to provide data in
seconds, completely eliminating the delays and hassles surrounding
homemade products and allowing the clinician to obtain frequent data
points and trend patients’ intra-abdominal pressure. The second
problem with homemade devices is the problems with data
reproducibility, quality control and sterility.
Because the devices are
constructed in the ICU with no quality control oversight and no
testing that is mandatory for pre-assembled products, there is a
high chance of errors in assembly and even greater chance of errors
in data acquisition by different nurses with varying degrees of
experience obtaining this data. They are also not constructed using
sterile techniques. Pre-assembled sterile devices standardize pressure
measurements and are proven to provide reproducible data between
many different nurses.[28] This allows the clinician to trust and
rely on the data provided.
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