When air filled catheters (AFCs) were being assessed, studies have compared simultaneous water filled catheter (WFC) and AFC readings in both bladder and abdomen.  These studies have so far shown that differences exist between these readings, giving rise to cautions being published about AFC use. A valid question raised as a result is to ask how variable the WFC readings themselves are, as to date these have been assumed to be more reliable as a comparator.  There is therefore a need to compare simultaneous pressure readings from two WFCs simultaneously in a patient.

During urodynamics, only one pressure line is used to measure each channel.  The bladder is catheterised with another line, but this is used to fill the bladder and is not normally used for pressure measurement.  However, the normal practice in our centre is to measure urethral pressure profiles (UPPs) using the Brown Wickham water perfused method[1].  At the start of this study, both the UPP line and the pressure measurement line are within the bladder lumen, and we can use these two concurrent pressure readings to compare pressure measurement between the two WFCs.  In order to assess a variety of pressure readings, rather than just resting pressures, these UPP tests would ideally contain other features, such as detrusor overactive (DO) pressure waves, so that comparison can be made with the previous AFC/WFC comparisons.

Our database of UPP tests during urodynamic studies was examined to extract simultaneous pressure readings from the vesical line and the UPP line.  All urodynamic tests were done with WFCs according to International Continence Society guidelines, using Aquarius urodynamic machines (Laborie, Canada). The Brown Wickham method for UPP measurement was carried out using a syringe driver running at 120 ml/h (i.e. 2 ml/min) through a pressure dome connected via manometer tubing to an 8 Fr UPP catheter (Mediplus, UK).  Since this results in a slight pressure increase on the UPP transducer due to the pumping of water through the system, the difference between the two pressure readings before and during each event was examined, rather than the absolute values.  Analysing the difference in pressures also removes any extra hydrostatic pressure influence from the transducer height.
Traces were included in the analysis if the quality of pressure transmission on the lines was proven to be good by an initial cough test, the level of acceptability being that the smaller cough spike was greater than 70% of the amplitude of the larger spike[2].  Both male and female patients of all ages and all clinical histories were included.  If the traces contained two UPP readings, as is normal for our centre, the bladder pressure between the two readings was also used for data.  Where the traces contained pressure rises while reading from the bladder, the peaks of these rises were also recorded and analysed. The agreement between the two WFCs was assessed using a Bland-Altman plot. The mean differences between catheters at rest and during pressure rises were assessed with a paired t-test.

30 UPP traces were initially selected (7 male, 23 female), and the average age of this cohort was 63.4 years (range 30-89 years). 1 trace was removed from further analysis due to poor quality. Figure 1 shows a Bland-Altman plot assessing the difference in bladder pressure measurement between the two water perfused catheters during pressure rises. A total number of 29 pressure rises were included. Reasons for the pressure rises included detrusor overactivity and Valsalva manoeuvres. A paired t-test was performed to compare the mean difference in pressure readings at rest compared to during pressure rises. There was no significant difference in pressure readings between the two groups (P=0.7936, S.D ± 2.8).

This study shows that there is good agreement in pressure readings between two WFCs. We can therefore be confident that the presence of two catheters within the bladder does not interfere with pressure readings, as has been previously suggested[3]. 1 data points lies outside the limits of agreement - this was a high pressure Valsalva and therefore the difference between measurements is likely to be greater than at lower pressure readings. Although this study has shown that there is no significant difference, it should be investigated further whether the agreement between the catheters reduces at higher pressures.

This study supports the continued use of water filled catheters when performing urodynamics, as recommended by the International Continence Society.

Brown M, Wickham J. The urethral pressure profile. British Journal of Urology. 1969;41(2):211-217.
Hogan S, Gammie A, Abrams P. Urodynamic features and artefacts. Neurourology and Urodynamics. 2012;31(7):1104-1117.
Digesu G, Derpapas A, Robshaw P, Vijaya G, Hendricken C, Khullar V. Are the measurements of water-filled and air-charged catheters the same in urodynamics?. International Urogynecology Journal. 2013;25(1):123-130.