Routine urodynamic studies normally involve an initial uroflowmetry prior to performing a pressure flow study (PFS). It is hypothesised that pressure flow and free flow results are different secondary to the presence of a catheter. This is supported by the literature that examines both mechanical and physiological reasons for this difference (1,2). The aim of our study was to determine the magnitude of the difference in the uroflow parameters between free flow and PFS.

As part of the Robotic and Open Surgery for Prostate Cancer: A Prospective, Multi-centre, Comparative Study of Functional and Oncological Outcomes (ROSE) study,  41 patients were prospectively recruited. Patients underwent urodynamics prior to a Robotic Assisted Radical Prostatectomy if their surgery was scheduled 4 weeks or more after initial assessment. Uroflowmetry and urodynamic parameters were collated and tabulated. Statistical analysis was performed using SPSS.

Using the Bladder Outlet Obstruction Index (BOOI), 13 (32%) patients were obstructed, 10 (24%) were equivocal and 18 (44%) patients were unobstructed.  The Qmax for uroflowmetry and during PFS are shown in table 1.

The most significant finding is the catheterised Qmax as part of the PFS was reduced compared to the free flow. This difference was 3.0 ml/sec (p = 0.02) in the obstructed and 2.1 ml/sec (p = 0.03) in the equivocal group. The voided volume of the unobstructed and equivocal groups was 199ml (p = 0.02) and 220ml (p = 0.04) greater during the PFS than during the initial uroflowmetry. The mean PVR measured after uroflowmetry was lower than the PVR after the PFS in all 3 groups. The mean difference in PVR was 35ml in the obstructed group, 76ml in the unobstructed group and 44ml in the equivocal group.

There is a significant difference in the Qmax, voided volume and PVRs during uroflowmetry compared to the intubated flow parameters during PFS. Clinically, the difference is most significant in the obstructed group’s Qmax which was 3.0ml/s (25%) less in the intubated group. This difference in flow rate could significantly alter the interpretation of a urodynamic study and bladder outflow obstruction index (BOOI).  If it is not possible to obtain a flow rate from the (intubated) voiding phase of urodynamics, the free flow should be used with caution in calculating the BOOI.

The free flow measured when catheterised during the voiding phase of urodynamics is significantly slower compared to the patient's free flow for patients who were found to be obstructed on urodynamics.

Zhu B, Jiang H, Li Y. Impact of urethral catheterization on uroflow during pressure-flow study. J Int Med Res. 2016; 44(5):1034-39.
Valentini F, Robain G, Hennebelle D, Nelson P. Decreased maximum flow rate during intubated flow is not only due to urethral catheter in situ. Int Urogynecol J. 2012;24(3):461-67.