It is recommended by the ICS that urodynamics is performed using water-filled catheters as it is an accurate and established system to measure pressure [1].  However water-filled urodynamic traces can be difficult to analyse when artefacts occur from movement and knocking of the catheters.  This increases the risk of a patient being diagnosed incorrectly based on their urodynamic test.  The presence of noise has been used to question the usefulness of the technology in the clinical setting.  We have therefore designed a system that can reduce this artefactual noise, and present the results of a bench test to demonstrate its efficacy.

A reference pressure tube was closely attached to the main pressure line in a water-filled urodynamics system (Mediplus 9115, twin extension set). This was then attached to two pressure transducers on a Laborie Aquarius TT urodynamic machine. Any movement or knocking on the tubes therefore produced similar artefacts generated on both lines.  As a result the artefacts will be automatically removed from the urodynamic trace. The aim was to measure by how much the noise on the main pressure line could be reduced by subtracting the noise also picked up on the reference line.  To test this theory, the main pressure line and the reference line were dropped together repeatedly from a set height. The raw data on the main pressure line was then compared to the data produced by subtracting the reference line data. To analyse the results the area under the curve of each artefact produced was calculated for the main pressure line with and without the reference line subtraction.

The main pressure line and the reference line were dropped together from a height of 30cm, 30 times.  Figure 1 shows the raw data on the main pressure line compared to the data produced by subtracting the reference line.  The result was a substantially cleaner signal. To begin analysis on the visualised improvement of the artefacts the raw data from the main pressure line was generated. For each artefact the pressure changes per 0.1 second above and below resting pressure where collated to give the area under the curve. The average area under the curve for artefacts produced on the main pressure line alone was 75.89 units. The same analysis was performed on the artefacts corrected by the reference line. The artefact was significantly reduced by subtracting the reference line reading as the area under the curve was only 26.08 units. The percentage difference between the area under the noise artefact was calculated as a 63% difference (SD 17.5%), showing a measurable reduction in noise artefact.

We have demonstrated that subtracting a reference line signal that contains similar noise artefact to the pressure line results in less noise and a clearer pressure signal.  It will now be necessary to test this method in the clinical setting, to investigate whether real patient pressure signals are affected by the subtraction method. 
An additional benefit of this method is that the water-filled transducers do not need the height adjusting when the patient changes position, as the reference line compensates for this vertical height difference automatically [2].  The need to adjust transducer height has been presented as another disadvantage of a water-filled system, and the adjustment is sometimes not made, resulting in less quality assurance of the measured signal.  Using a reference line will thus doubly improve the signal quality of water-filled urodynamic systems.

This bench study showed artefacts on a water-filled urodynamics trace can be significantly reduced using a reference line and automatic subtraction.  We now plan further research confirm this finding with a larger data set and use during a clinical urodynamic study.

Good urodynamic practices: Uroflowmetry, filling cystometry and pressure-flow studies. Werner Schafer et al, Neurology and Urodynamics, 21:261-274 (2002).
Automatic reference height correction for a water filled urodynamic system. Bacon et al, S35, ePoster station 10, abstract 681 (2017).