Hypothesis / aims of study
The infusion rate may influence the parameters from resting urethral pressure profile (UPP) measurement using a water-filled catheter. The study aims to compare the effect of the infusion rates of 0.5 ml/min and 1 ml/min (60ml/h), which is a default infusion rate according to MMS Solar user’s manual, on UPP parameters when assessing female urethral condition.
Study design, materials and methods
Before an anti-incontinence surgery adult women with stress urinary incontinence (SUI) were recommended for urodynamic studies, before which an informed written consent was obtained from patients as a routine. The written consent announced that the data in the urodynamic examination could be used in further analyses for clinical research purposes. And an Institutional Review Board Approval was not needed for the study. The technique, definitions and units of the urodynamic measurement conformed to the standards proposed jointly by the ICS and the International Urogynecological Association .
In a brief, following Pressure-flow study (PFS) UPP was performed at a half-full bladder with a patient in a semi-lithosupine position, using MMS Solar urodynamic equipment with external pressure transducers. Pressure channels were zeroed to the atmosphere with water transducer holder at symphysis pubis level. The UPP was performed with a F6 2-lumen water-filled catheter, of which one lumen for measuring the bladder pressure, and the other lumen for measuring the urethra pressure. Using a MMS automatic catheter puller, the catheter mounted was pulled out at the rate of 1 mm/s, which was used to calculate the length of the urethra. All the patients had two tests of UPP at an infusion rate of 0.5 ml/min and 1 ml/min respectively. The data were analyzed using MedCalc statistical software version 15.2.2 (MedCalc Software bvba, Ostend, Belgium; http://www.medcalc.org; 2015). For each UPP parameter, the bias (mean of difference) and limits of agreement (mean±1.96 x standard deviation) between two tests were analyzed using Bland and Altman plot. Reproducibility of the parameters was determined using within case coefficient of variation (CV).
UPP parameters from 121 SUI patients with a mean age of 56.49±12.01 yr (range: 31–90 yr) were analyzed. Initial analysis showed that except maximum urethral closure pressure (MUCP) and proximal area to peak (PAP), significant differences were found between two tests (P<0.05) among all other UPP parameters. Functional length of the urethra (FUL) and profile length（PL）had lower CV values and were relatively better than the other parameters in reproducibility. Bland–Altman plots showed that the limits of agreement for all UPP parameters were too wide for the two infusion rates to be used interchangeable, although those for FUL and PL were relatively narrow. When the UPP parameters were further analyzed through the stratification of the patients as low (lower than 40cmH2O), medium (between 40 and 60cmH2O) and high (higher than 60cmH2O) urethral pressure group by the value of MUCP, In all UPP parameters apart from FUL, PL, and LtoP (Length to Peak), statistical differences existed between all groups (P<=0.05) except that between low and medium group in DAP (Distal Area to Peak) parameter (P =0.098) (Table). CV values became smaller in some parameters and the limits of agreements were still not acceptable for most parameters. The lower the MUCP was, the less wide the agreement ranges were. (Fig.) For all the UPP parameters the agreement of the two infusion rates was not acceptable in a clinical view.
Interpretation of results
PFS is now used widely and accepted as an international standard in the diagnosis of Bladder Outlet Obstruction (BOO) for men with BPH, but the criteria, especially for women, have yet to be determined . When a patient has a compromised bladder function and could not produce urinary flow in a PFS, UPP are used as one of alternative methods to evaluate the possibility of BOO. Resting UPP has long been utilized in the evaluation of urethral resistance . Inherent pitfalls of UPP have been drawbacks in its application, and accordingly its use in the assessment of BOO has been in dispute. Nevertheless, resting UPP is a routine preoperative test in many urodynamic laboratories, including ours. But the test is vulnerable to physiologic and technical artifacts that must be minimized to produce technically accurate and clinically meaningful results. To compensate the inherent deficiency, it is recommended that at least two tests of UPP measurement are applied before corresponding parameters are reported correctly. From our experience parameters from a repeated UPP are usually as the same as those from the previous one, which is summarized in another paper. Another situation is that UPP curves or parameters are not easy to be obtained due to damaged urethra in women with SUI or irritable urethra, but a changed infusion rate may overcome the condition. Previous study has shown that UPP parameters are consistent irrespective of infusion rates (within a limit) .
All the patients in the present study were with SUI. It showed that the reproducibility of UPP parameters was poor. The MUCP might not be influenced by infusion rates, which is contrary to an early study that showed MUCP was reduced when the infusion rate was less than 2ml/min . It is interesting to find that the urethra length parameters, which are associated with the puller rates and showed a better reproducibility in the study, might be influenced at different infusion rates. The changed infusion rates at some limits might influence the sensitivity of the UPP parameters. Further analysis showed that in lower urethral pressure group the agreement of the tests became better.