Simple cystometry

In simple cystometry only the intra-vesical (total bladder) pressure is measured. This technique is not accurate because it assumes that the detrusor pressure approximates to the intra-vesical pressure. A significant proportion of the pressure measured in the bladder emanates from intra-abdominal structures and not from the detrusor muscle itself; since the bladder is subject to changes in intra-abdominal pressure as are all intra-abdominal organs. This technique may therefore lead to inaccurate diagnosis and is rarely performed.

Subtraction cystometry

Subtraction cystometry involves measurement of both the intra-vesical and intra-abdominal pressure simultaneously. Real time electronic subtraction of the intra-abdominal pressure from the intravesical pressure enables the detrusor pressure component of the intravesical pressure to be isolated and analysed. During this technique the bladder is artificially filled with fluid to simulate the storage phase. This method allows the accurate determination of detrusor pressure and does not involve any radiation. It is used in urodynamic units worldwide.

Videocystometrography (VCMG; also known as videocystometry and video urodynamics)

By combining subtraction cystometry with contrast media bladder filling and radiological screening it is possible to visualize the lower urinary tract during the storage and voiding phases. This results in the gold standard urodynamic investigation (Figure 4.1).

Figure 4.1 Normal videocystometrogram during voiding. The intra-abdominal and intra-vesical pressures are excluded from this trace. The top left schematic shows the detrusor muscle contracting during voiding and the bladder outlet opening. The top right schematic shows the appearances during voiding of the bladder and urethra using radiological screening.

Radiological screening provides valuable additional anatomical information on:

This information, along with the accompanying pressure/flow traces can be recorded allowing subsequent review and discussion.

Ambulatory urodynamics (AUM)

This is a highly specialized form of subtraction cystometry, which contrary to other pressure/flow cystometry techniques allows the bladder to fill naturally via the kidneys and is performed over a greater length of time. AUM will be discussed further at the end of this chapter.

Intra-vesical

During cystometry the total bladder (intra-vesical) pressure is measured by inserting a catheter into the bladder. The catheter is usually passed via the urethra using lubricant or anaesthetic gel. In certain circumstances the catheter can be inserted via the suprapubic route. It should be ensured that the catheter is fixed securely and is in no danger of falling out. If taped to the skin, the fixation should be as near to the meatus as possible; and in men there should be no obstruction to the urethra on the under surface of the penis.

Intra-abdominal

The intra-abdominal pressure is also measured so that a detrusor pressure can be calculated using a computer. The measured intra-vesical pressure is composed of two components: the pressure created by the detrusor muscle, and also the pressure transmitted to the bladder from the external intra-abdominal pressure. By subtracting the intra-abdominal pressure from the intra-vesical pressure the pressure generated intrinsically within the bladder, i.e. from the detrusor muscle, is isolated. By and large the detrusor pressure is of greatest interest to the urodynamacist. The intra-abdominal pressure is usually measured by inserting a catheter into the rectum; however insertion into the upper vagina or into a stoma is also possible. The catheter usually has a flaccid, air-free balloon on the rectal end. The purpose of the balloon is to maintain a small fluid volume at the catheter opening and to avoid faecal blockage. The balloon should only be filled to 10–20% of its unstretched capacity and overfilling is a common mistake causing misleading measurements; often a slit is cut in the balloon to prevent overfilling and a tamponade effect. The rectal catheter should be inserted 10 cm above the anal verge using lubricant and taped securely as close to the anal verge as possible.

External fluid charged pressure transducers

The ICS currently recommends use of external fluid pressure transducers due to their accuracy and inherent characteristics allowing easier use with regard to zero pressure and reference height (Figure 4.3). Urodynamics was originally developed using water filled systems and standardization has therefore developed from an understanding of the characteristics of fluid filled systems. The transducers are fixed externally on a stand and are connected via fluid filled lines to the pressure recording catheters. Fluid transmits the measured pressures directly to the external transducer and any interruption to the transmission of pressure waves may lead to artefacts. Common artefacts include an air bubble anywhere in the tubing between the tip of the catheter and the transducer; air is compressible unlike fluid and therefore a transmitted pressure wave will preferentially compress the air prior to reaching the transducer. The use of some of the pressure in compressing the air will result in a lesser wave being transmitted to the transducer resulting in ‘dampening’ of the trace (Figure 4.4). This is a common problem and even a miniscule air bubble can cause detectable dampening in the highly sensitive transducers. It can usually be rectified by flushing the air bubble out of the system. A fluid leak or a kink in the tubing will also have similar consequences, so it should be ensured that the system is water tight during the setup procedure and that there are no kinks anywhere along the tubing. Pressure sensing is point sensitive and therefore essentially unidirectional within the fluid filled system, albeit the pressure measurements are not so dependent on the orientation of the catheter within the bladder.

Figure 4.3 External fluid transducers. Left shown with disposable casing often called the ‘dome’ which is attached to the fluid containing tubing. Right shows the transducer without a ‘dome’.

Figure 4.4 Dampening due to air bubble. Upper diagram shows normal pressure transmission. Lower trace shows an air bubble being compressed and therefore ‘absorbing’ some of the pressure wave leading to reduced transmission and a lower baseline and lower deflection on the trace.

Catheter mounted transducers

These transducers are mounted on the end of special (microtip) catheters. They do not require a fluid filled system and the external proximal end of the catheter is connected directly to the electronic recording system, thus making them simpler to setup than fluid filled systems (Figure 4.5). A reference height does not need to be setup for these systems and they are not affected by movement artefacts. They are therefore advantageous in certain situations such as ambulatory urodynamics, but for conventional cystometry they are not recommended by the ICS. The main disadvantage of this system is that the pressure readings are directional depending on which way the transducer is facing or what the transducer is lying against. Also they do need to be calibrated regularly before use. In addition the catheters are expensive and must be thoroughly cleaned before and after each usage and must be handled with care. Reference height unlike external fluid catheters is at the level of the internal transducer (see later). These catheters should be pre-soaked prior to usage to allow a small amount of water absorption into the coating on the sensing area. Cleaning/disinfecting using a liquid based method immediately prior to usage is sufficient pre-soaking. If not cleaned by a fluid method immediately prior to usage then approximately 20 minutes of pre-soaking is required if the catheter is used regularly; if the catheter has not be used for some weeks then pre-soaking for greater than 1 hour is required.

Figure 4.5 (a) Microtip (solid state) catheter. Figure of a dual intra-vesical and urethral measuring microtip catheter with transducer visible distally and further urethral transducer visible a few centimetres more proximal. (b) Transducers on a microtip (solid state) catheter. Close-up of distal end of a rectal microtip catheter (top) and an intra-vesical microtip catheter (bottom); with arrows showing the transducers.

Air charged, pressure sensing technology

This is the newest transducer system. The system consists of reusable cables with built in transducers, and disposable catheters with a tiny balloon affixed on the distal end of the catheter (Figure 4.6). After the disposable catheter is inserted into the patient and connected to the permanent cable, the transducer is used to ‘charge’ the catheter by injecting a micro-volume of air into the catheter balloon. This creates a closed, pressure sensing system for accurate, in vivo recording of bladder, abdominal, and urethral pressures. This system is gaining popularity due to its simpler and quicker setup than fluid charged systems. In addition pressure sensing is not unidirectional and the catheters are disposable unlike with microtip catheters. The pressure measurement using the pressure sensing balloons is truly circumferential using this system unlike fluid filled and microtip systems and therefore may be of particular benefit when measuring urethral pressures, as the total pressure generated by the cylindrical urethral wall is measured. In addition these transducers are easily zeroed and similarly to microtip catheters the reference height does not need to be set externally but is taken at the point of the internal balloon. Dampening due to a change in the transmission medium (such as occurs when an air bubble is trapped in a fluid filled system) is not a problem in air charged pressure sensing catheters. The pressure measurements are also not influenced by any movements of the catheter, thus decreasing artefacts; this characteristic is ideal for ambulatory urodynamics. This technique does however need to be further studied before it can replace fluid filled catheters as the standard technique.

Figure 4.6 Air charged, pressure sensing technology. A disposable air charged catheter with integral balloon attached to the permanent cable with built in transducer.

Dual lumen

These are recommended by the ICS, since a single, dual lumen catheter can be used for both the intra-vesical pressure measurement and also for bladder filling (Figure 4.8). The thinnest possible catheter should be used to limit artefacts during voiding due to obstruction of the urethral lumen. However if too thin it may lead to excessive dampening of the pressure transmission and may also limit the pump rate. The smallest currently available size is 6 Fr, however such a small size is thought to limit the pump rate. In view of this a slightly larger size i.e. 8 Fr is probably preferable.

Figure 4.8 A dual lumen intra-vesical catheter. Used for both pressure measurement (blue channel) and bladder filling (transparent channel) with a single catheter.

Single lumen

If used it requires two separate catheters to be inserted into the bladder which is less convenient, but possibly has the advantage that the larger filling tube can be removed prior to voiding. However, this advantage is countered by the need to reinsert the filling line if a second fill/void cycle needs to be performed.

A note should be made of the type, size and number of catheters utilized during an investigation.

External fluid filled system

The reference height is the level at which the transducers must be placed so that all urodynamic pressures have the same hydrostatic component. To ensure standardization the ICS has defined the reference height as the superior edge of the symphysis pubis. The transducers are usually mounted on an adjustable stand and should be moved so they are level with the symphysis pubis prior to commencing the study (Figure 4.10).

Figure 4.10 Position of external fluid filled transducers. These should be set at the level of the pubic symphysis to allow standardization and comparison. Increasing the height of the external transducer lowers the measured pressure whereas lowering the height increases the measured pressure. The position of the catheter in the organ has no effect on the measured pressure.

Microtip and air filled systems

For microtip transducers the reference height is the transducer itself; for air filled transducers the reference height is at the position of the internal balloon. Using these systems it is therefore difficult to be certain of the position of the reference height or to ensure that the reference height of the intra-abdominal and the intra-vesical lines are equal, unless screening can show the position of the catheters (Figure 4.11). In addition alterations in the position of the patient (i.e. supine to standing) can cause significant differences in the relative positions of the intra-vesical and intra-abdominal transducers/balloons. In the supine position the rectal line is likely to be lower than the intra-vesical line, when standing the rectal line may be higher than the intra-vesical line. In practical terms these differences in height are unlikely to result in a significant effect on results.

Figure 4.11 Position of microtip transducer or air filled balloon. The position of the internal transducer or balloon within the bladder alters the measured pressure. If the position is lower, then a higher pressure will be measured due to the extra fluid column above the transducer/balloon: a high position will therefore have a lower pressure.