4.3.1 Flowmeter

There are two types of flowmeter that are commonly used: the rotating disc and the weight transducer. The rotating disc can be dismantled for cleaning. A stand-alone flowmeter may be used separately from cystometry.

4.3.2 Commode

A commode with a funnel is placed above the flowmeter to enable accurate aim of the flow on to the flowmeter (Figure 4.1).

Figure 4.1 A flowmeter and commode

4.4.1 Water-Filled System

The water-filled system is the current International Continence Society standard and is the most common method for cystometry [5]. External pressure transducers are mounted on the urodynamic equipment (Figure 4.2) and connected to the water-filled lines. The use of external transducers means that the system is prone to movement artefacts. The height of the transducers is adjusted when the patient’s position changes relative to them. Water-filled lines are prone to air bubbles collecting during the test. Lines should be flushed to expel air when there is damping, which would cause inaccurate measurement of pressure. The position of the syringes (superior, inferior or lateral) to the domes is unimportant as long as the principles of flushing through are followed.

Figure 4.2 External pressure transducers mounted on the equipment

4.4.1.1 Types of Water-Filled Catheters

A water-filled pressure catheter is placed in the bladder to measure intravesical pressure and a second catheter is inserted into either the rectum or the vagina to measure intra-abdominal pressure. Either single-lumen or double-lumen water-filled catheters can be used to measure intravesical pressure.

Single-Lumen Catheters

If a single-lumen catheter is chosen, a second catheter called the ‘filling catheter’ is inserted transurethrally alongside the vesical pressure catheter. The filling catheter is used to fill the bladder with saline and should be 8 Fr or less in diameter. The pressure catheter should be 4.5 Fr or less in diameter.

Double-Lumen Catheters

Double-lumen catheters have two lumens, with one lumen for filling with saline and one for measuring intravesical pressure. These are 6 or 8 Fr size and allow several filling and voiding cycles. Double-lumen catheters with smaller gauge are more prone to pump artefacts when faster filling speeds are used.

Abdominal (Rectal/Vaginal) Catheters

Several prototypes of abdominal lines (Figure 4.3) are available from manufacturers.

Figure 4.3 An abdominal catheter

4.4.2 Solid-State System

A solid-state catheter-tip transducer is not connected to an external pressure transducer because it has a transducer mounted at the tip of the catheter. This type of system is rarely used.

A filling catheter, similar to that used with a water-filled system, is inserted alongside the solid-tip catheter and is necessary to infuse water. Solid-tip vesical catheters (Figure 4.4) can be either a single transducer or a double transducer, which can be mounted to measure urethral pressure simultaneously.

Figure 4.4 Solid-tip vesical catheter

Solid-state catheters with a single-tip transducer inserted into the rectum and covered with a condom, sheath or gloved finger can be used to measure intra-abdominal pressure.

4.4.3 Air-Filled Catheters

Air-filled catheters are disposable and have a valve that allows zeroing after catheterisation, but before priming of the balloon. Although they are commercially available, their use has not yet been validated. Recent studies have shown that pressures measured with air-filled catheters are not interchangeable with those using water-filled systems. Care should be taken when comparing results [6].