Bladder Pressure Assessment Technique

There are several forms of cystometry. A simple cystometry can be performed with a catheter and a syringe or manometer held 15 cm above the patient’s symphysis pubis. The advantage of this is that it is inexpensive and readily available; however, exact measurements of bladder pressure and determination of compliance are not possible with this method.

Single-channel cystometry involves recording intravesical pressure during filling with a single catheter. A major disadvantage of the single-channel technique is that increases in intra-abdominal pressure (IAP) may result in elevated detrusor pressure readings.

Multichannel cystometry is performed with the use of a second catheter to measure IAP. The catheter may be placed in the vagina or, more commonly, the rectum.

In patients who void volitionally and are not on clean intermittent catheterization, urinalysis is recommended prior to urodynamic evaluation, as well as urine culture and treatment if urinalysis findings are positive.

In patients with an indwelling catheter and/or who are on clean intermittent catheterization, a urinalysis is obtained. If significant pyuria is present and/or the patient has symptoms of a urinary tract infection, a urine culture should be obtained.

Research is ongoing with implantable bladder pressure monitors. ^{[2, 3]}

 Various techniques/kits have been used to measure bladder pressure, including manometry, “homemade” ICU systems, and commercially available IAP monitoring devices.

The steps below are part of a urodynamic study, and bladder pressure is being measured during all phases of the study to assess for compliance and detrusor overactivity during filling, bladder outlet obstruction during voiding, and decreased contractility during voiding.

Patients who void volitionally are asked to arrive with a full bladder. A bladder scan is used to document the bladder volume. If the patient has at least 150 mL in his/her bladder and has the desire to void, a uroflow is obtained. The patient is then catheterized with a urodynamic catheter, typically a 10F or smaller, via sterile technique with lidocaine topical jelly, and the postvoid residual is recorded. A rectal catheter is then placed.

Electromyography (EMG) electrodes, often patch electrodes (they are more comfortable and easier to use), are applied to the perineum. The transducers are equalized according to the recommendations of the individual urodynamic machine/company.

A cough test is performed to ensure that the catheters are functional.

The bladder is filled with room-temperature sterile saline (or contrast material if videourodynamic evaluation is being performed). The physiologic flow rate is used (see definition), since cold water and rapid filling can provoke involuntary detrusor contractions. Furthermore, supraphysiologic filling rates can result in loss of compliance.

The procedure may be performed with the patient sitting, semi-erect, or standing.

The catheters are calibrated so that zero corresponds to atmospheric pressure. If detrusor overactivity is noted at the onset of bladder filling, the rate is decreased.

The patient is asked to comment when he/she feels the first urge to void and whether he/she has any sensation of urgency or is leaking during filling. When the patient feels that he/she has reached capacity, inflow is stopped, and the patient is permitted to void. The patient then voids into the uroflow meter, and the flow pattern and EMG activity are recorded. The maximum detrusor pressure during flow, as well as the maximum flow rate, is often assessed.

A pressure flow study is generated by plotting the various bladder pressures and corresponding flow rates during voiding. Once voiding is complete, the bladder is emptied, and, if present, the postvoid residual is recorded.

Manometry is one of the original methods used to measure bladder pressure via a Foley catheter. ^[21] The formal technique involves placement of a manometry tube between the Foley catheter and the drainage bag. A priming volume of fluid must be infused into the bladder to ensure adequate volume to fill the Foley and the manometry tube until equilibrium is reached. The tube must be vented to ambient air pressure. In addition, it requires proper positioning of the zero point, the angle of the manometer, and avoidance of Foley kinking for accuracy.

A limitation of this technique is the need for repeat breaking of the closed system (Foley-drainage bag seal) every time a pressure reading is to be obtained. The vented manometry tube must also be reassembled, which takes time and introduces the risk of infection. In addition, the pressure measurement is in centimeters of water and must be converted to mm Hg (divided by 1.36) to follow current recommendations for management of intra-abdominal compartment syndrome (IACS), which increases the risk of possible miscalculations.

Various homemade systems have been described. ^{[4, 22, 23, 24]} One such system involves placement of a Foley catheter into the bladder and drainage of the bladder. The catheter is then connected to an infusion system, which consists of a needle, intravenous tubing, syringe, pressure transducer, stopcocks, and saline.

First, the needle is inserted into the urine sampling port of the Foley catheter. The transducer is then connected to a monitor and zeroed. The drain tubing is then clamped, and the bladder is infused with saline. The pressure within the system must be allowed to equilibrate before the mean bladder pressure on the monitor is recorded. The system is then broken down and the drain tubing unclamped until the next pressure reading.

An alternative system utilizes a 3-way Foley catheter. Instead of inserting a needle into the urine sampling port, the Y-extension of a pediatric feeding tube is connected to the irrigation port of a 3-way catheter, and fluid infusion/pressure transduction is performed through this lumen.

Potential problems related to such homemade IAP kits include the following ^[21] :

• Having the necessary supplies available and the staff knowledgeable in the setup during the acute time of need

• Lack of quality control

• Lack of standardization

• Lack of data reproducibility

• Time consumption

• Concerns with sterility and resultant catheter-associated urinary tract infections related to breaks in the closed system

Manufacturers have developed simple, easy-to-use systems that allow for frequent IAP monitoring, enabling early intervention, when appropriate. Such “preassembled kits significantly reduce setup time, reduce the margin for monitoring error by maintaining consistency of setup, and reduce the risk of system contamination during assembly.” ^[24]

The commercially available IAP devices require the patient to be placed fully supine in order to ensure consistent measurements. ^{[25, 26]} Even a small elevation in head position or body position can alter the pressure readings. ^[27] The patient must have no abdominal contraction present, with the transducer placed at the level of the iliac crest at the midaxillary line or at the level of the symphysis pubis, depending on the kit.

All of the preassembled kit devices are endorsed by the World Society of the Abdominal Compartment Syndrome, but AbViser is the only one validated to provide reproducible results. ^{[14, 27, 28]}

Two other commercial kits are also available for IAP monitoring: the Bard IAP monitoring device and the UnoMeter Abdo-Pressure kit.

The Bard IAP Monitoring Device maintains a closed urinary drainage system to prevent catheter-associated urinary tract infections. ^[29]  It also contains a valve port that allows saline infusions through the sampling port on the catheter and still allows for obtaining urine for samples.

The UnoMeter Abdo-Pressure kit is a manometer-type device that does not require reassembly. ^[30]  It requires only a single priming at setup with a needle-free injection/sampling port (unless air is found in the tube) and measures in mm Hg for ease with use of the IACS management recommendations and so decreases the risk of calculation error. However, it still requires opening to air with a clamp.

AbViser

Sterile saline is hung on an intravenous pole. The saline bag tubing spike is placed into the saline bag. A pressure cuff around the saline bag is unnecessary. ^[14]

The AbViser is primed by aspirating and compressing the syringe to flush all of the air from the infusing tubing until saline runs out of the AbViser autovalve and all of the bubbles are out of the tubing. The end of the urine catheter is aseptically prepared and then placed over the barbed connector of the AbViser. The drain tubing is connected to the AbViser valve.

The catheter connection is ensured, the area dried, and a strip of tape wrapped around the end of the catheter to keep the connection snug.

The pressure transducer is attached to the patient or to an intravenous pole at the level of the iliac crest in the midaxillary line. After zeroing the transducer, ensure that the stopcock is open to the patient and the transducer.

To measure the bladder pressure, the patient must be supine. The plunger is retracted until 20 mL (in adults) of fluid is within the syringe. The syringe plunger is compressed within 10 seconds, infusing the fluid into the bladder.

To record IAP, allow the system to equilibrate and then note the pressure reading on the monitor at the end of expiration. The IAP reading lasts for 1-3 minutes, and the valve will then automatically open and the pressure reading will decrease to zero.

After the pressure is recorded, one should confirm that the autovalve has opened and urine is draining normally. The valve will remain open until the next volume of fluid is infused.

Bard intra-abdominal pressure monitoring device

The Bard IAP monitoring device technique is similar to that of the Abviser; however, the clamp handle is manually adjusted to measure IAP and to drain. ^[29]

Approximately 25 mL of saline is infused into the bladder over 20-30 seconds; after the system equilibrates for 30-60 seconds, the pressure reading on the monitor at the end of expiration is recorded. The clamp handle is then turned to drain.

The UnoMeter Abdo-Pressure kit

The UnoMeter Abdo-Pressure kit is a closed sterile circuit that connects between the patient’s Foley catheter and the urine collection device. ^[30]

The Foley manometer is placed between the catheter and the drainage device and is primed with 20 mL of sterile saline through its needle-free injection/sampling port. The “0 mm Hg” mark of the manometer tube is placed at the symphysis pubis, and the filter is elevated vertically above the patient. The clamp is opened, and the end-expiration value for the bladder pressure of pvesical is read when the meniscus has stabilized. After reading, close the clamp and place the Foley manometer in its drainage position.

Abdominal compartment syndrome (ACS) is defined as a sustained IAP (steady-state pressure concealed within the abdominal cavity) of more than 20 mm Hg (with or without an abdominal perfusion pressure < 60 mm Hg) that is associated with new organ dysfunction/failure. Primary ACS refers to ACS associated with injury or disease in the abdominopelvic region that frequently requires early surgical or interventional radiological intervention. Secondary ACS refers to ACS that does not originate from the abdominopelvic region. Recurrent ACS is defined as ACS that redevelops following previous surgical or medical treatment of primary or secondary ACS. ^[6]

Intra-abdominal hypertension (IAH) is defined as a sustained or repeated pathological elevation in IAP (≥12 mm Hg).

In patients at risk for developing ACS, the clinical examination is unreliable for determining the presence or absence of an elevated IAP. ^{[4, 22, 31]}  Bladder pressure monitoring, therefore, is used as a surrogate for IAP monitoring. IAP must be measured with an objective, reliable, reproducible method that can be performed at intervals frequent enough to detect increases in IAP, allowing interventions to occur prior to ACS onset. ^[21]

Critically ill patients can develop a capillary “leak” (permeability) that results in intravascular fluid extravasation into extravascular tissue. This typically occurs during the initial 12-36 hours of critical illness when fluid resuscitation is ongoing.

One of the primary sites of fluid accumulation is within intra-abdominal tissue, which includes the bowel wall and mesentery. As the extravascular fluid accumulates in the abdominal cavity, the abdomen accommodates by expanding. However, once the abdominal wall compliance threshold is reached, further fluid accumulation results in increases in IAP. Sustained IAPs of 12 mm Hg or more significantly affect the cardiovascular, pulmonary, gastrointestinal, renal, and nervous systems and is referred to as IAH. When the IAP increases beyond 20 mm Hg and organ failure begins to develop, the patient has advanced to ACS and requires immediate surgical intervention. ^[32]

The most reliable method for monitoring IAP is via pressure transduction through a catheter within the peritoneal cavity. Less-invasive options include pressure transduction through a tube placed in the stomach, bladder, or rectum, ^{[4, 22]} Obeid et al found bladder pressure to be the most technically reliable and to most closely reflect pressure within the intraperitoneal cavity. ^[32]  Bladder pressures obtained through a Foley catheter correlate strongly with the IAP and are considered the criterion standard method of monitoring IAP by the International Consensus Committee – The World Society of Abdominal Compartment Syndrome. ^{[5, 6, 33]}