This study aimed to compare pressure measurements made simultaneously via air and water during urodynamic studies (UDS), by repurposing the air-charged catheter (ACC) filling channel to measure pressure as a water-filled catheter. To assess variability due to the use of two different channels, the same comparison was made in a control group where dual water pressure measurements were made in water filled catheters (WFC). The diagnostic implications of any pressure measurement differences were also assessed. We believe this to be the first study comparing pressures measured via both air and water simultaneously in one catheter that is independent from the manufacturer of T-DOC ACCs.

Male patients, over the age of 16, with lower urinary tract symptoms who had been referred for urodynamic investigation were recruited prospectively in a single centre. Patients were split into two groups: one group using ACCs (comparing air and water pressure measurements), and the other group using WFCs (comparing dual water pressure measurements). All urodynamic tests were performed by the first author and according to ICS Good Urodynamic Practice (GUP) guidelines [1].

In both groups the filling channel of the catheters was repurposed to measure pressure via water (the water reference pressure) during the voiding phase. The filling channel of the bladder catheter was connected to a three-way tap to enable switching between the saline pump for filling and the transducer for pressure measurement during voiding. This meant dual pressures in the bladder were only measured during voiding. Dual pressures in the rectum were measured throughout both filling and voiding.

The water-filled rectal catheter had an open fingercot covering the measurement and filling ports to prevent faecal blocking and adhere to ICS GUP guidelines [1]. This was not possible for the rectal catheter in the ACC group due to potential interference with the measurement balloon. It was acknowledged that this could affect the water reference pressure measurement in the rectum. Therefore, in order to try and prevent faecal blocking, the ACC filling channel was flushed prior to voiding. Additionally, all abdominal pressure traces were screened by two independent assessors for validity in order to exclude those with poor pressure response indicative of blockage. This exercise was blinded with regards to intra-vesical and detrusor pressure, and to group allocation, to try and minimise bias.

In each catheter, the measurement channel pressure (air or water) was compared with the filling channel water reference pressure at maximum flow using paired- sample t-tests, correlation (Spearman’s rank analysis), and Bland-Altman analyses. The root mean square difference (RMSD) between the measurement channel and filling channel was calculated for intra-abdominal (Pabd), intra-vesical (Pves), and detrusor (Pdet) pressures throughout detrusor contraction. Differences in diagnoses between the paired pressure measurements were assessed by plotting the detrusor pressure at maximum flow (PdetQmax) for each patient on a LINPURR/ICS nomogram.

Seventy-five patients were recruited to the study. Fifteen patients were excluded, leaving sixty studies for analysis (30 patients in the ACC group, and 30 patients in the WFC group). Patients were excluded for the following reasons: unable to void (5), unable to catheterise (4), water-filled rectal catheter not covered by finger cot in error (2), excluded following Pabd validity assessment (2), tap not turned to measure pressure via water prior to voiding in error (1), bladder catheter expelled (1).

All paired pressure measurements at maximum flow were strongly correlated (rho=>0.97) apart from air and water in ACCs measuring Pabd (rho=0.79). The mean difference (measuring channel minus filling channel water reference pressure) of the Pabd measurement in ACCs was 5.7 cmH2O with a standard deviation of 7.8 cmH2O (p=0.0003), compared to 0.6 cmH2O with a standard deviation of 2.3 cmH2O (p=0.1) in WFCs. The mean difference of the Pves measurement in ACCs was -1.6 cmH2O with a standard deviation of 4.8 cmH2O (p=0.09), compared to 1.3 cmH2O with a standard deviation of 2.0 cmH2O (p=0.002) in WFCs.

Figure 1 displays example urodynamic traces during the voiding phase with the smallest and largest Pdet RMSD in each group. In the ACC group, the mean RMSD between the measuring (air) and filling (water) channels during the voiding detrusor contraction for Pabd, Pves and Pdet was 7.4, 4.8 and 9.3 cmH2O respectively. In the WFC group this was 2.0, 1.3 and 2.7 cmH2O.

Figure 2 shows PdetQmax plotted against Qmax for each patient on overlaid ICS and LINPURR nomograms. Two measurements are plotted for each patient, one using PdetQmax from the measuring channel pressure and the other using PdetQmax from the filling channel pressure. There are 10 diagnoses that cross one obstruction boundary between the paired air and water pressure measurements in ACCs, and six that cross a bladder contractility boundary. This compares to only two patients crossing an obstruction boundary between the paired water and water pressure measurements in WFCs, and one patient crossing a contractility boundary.

The main finding of this study was the significantly higher Pabd measured via air compared to water in the rectum. This could be attributed to a difference between the heights of the ACC tip and the external WFC transducer. If the ACC tip is lower than the transducer, which is feasible since the distal rectum tilts backwards, the pressure measured via air will be higher. Studies comparing ACCs and WFCs in females using separate ACCs and WFCs made similar findings [2] [3]. However, this does not address the large range of pressure differences measured in ACCs which was approximately ±20 cmH2O.

The standard deviation of the intravesical pressure difference was also much larger in ACCs compared to WFCs. The observed differences, which ranged between approximately ±10 cmH2O may result from height differences between the WFC external transducer and ACC tip. In most cases a higher Pves was measured via water than air, which may reflect a tendency of the more rigid T-DOC ACC to point upwards in the bladder, above the external transducer.

Figure 2 illustrates the diagnostic implications of using air instead of water to measure pressures during UDS. There are 10 diagnoses that cross an obstruction boundary in the ACC group, and six that cross a bladder contractility boundary. These differences in diagnoses mean that ACCs and WFCs cannot be used interchangeably because of the effects the different technology could have on diagnosis, and therefore the clinical management of patients. Consideration should be given to whether revision of normal values and nomograms for use with ACCs would allow the technology to be used more reliably in male patients undergoing UDS.

ACCs cannot be used interchangeably with WFCs for UDS in male patients due to the impact of pressure differences on urodynamic diagnoses. The largest contribution to this effect is differences between air and water measurement of Pabd. Small diagnostic differences were also observed with dual water pressure measurements in water-filled catheters.

Rosier, P., Schaefer, W., Lose, G., Goldman, H., Guralnick, M., Eustice, S., Dickinson, T., Hashim, H. 2017. International Continence Society Good Urodynamic Practices and Terms 2016: Urodynamics, uroflowmetry, cystometry, and pressure-flow study. Neurourology and Urodynamics, 36(5):1243-60.
Digesu, A., Derpapas, A., Robshaw, P., Vijaya, G., Hendricken, C., Khullar, V. 2014. Are the measurements of water- filled and air-charged catheters the same in urodynamics? International Urogynecology Journal, 25(1):123-30.
Gammie, A., Abrams, P., Bevan, W., Ellis-Jones, J., Gray, J., Hassine, A., Williams, J., Hashim, H. 2016. Simultaneous in vivo comparison of water-filled and air-filled pressure measurement catheters: Implications for good urodynamic practice. Neurourology and Urodynamics, 35(8):926-33.

Continence 2S2 (2022) 100256
DOI: 10.1016/j.cont.2022.100256