The use of contrast media is essential for video urodynamics (VUDS), which results in the pressure flow study being performed with a fluid of different density and viscosity than urine or saline.  There have only been small studies reported at conferences investigating the effect of this difference on the urodynamic parameters [1,2].  

The voided liquid of a bladder filled with a mixture of contrast, urine and/or saline will first void the contrast, since this sits lower in the bladder due to its greater density.  We therefore hypothesised that the difference between unintubated free flow and catheterised pressure flow study may change between standard urodynamics and video urodynamics.  

We therefore examined the mean difference between free and intubated flow for male patients undergoing standard urodynamics (UDS), and compared that with the mean difference for those undergoing video urodynamics.

A retrospective analysis was conducted on urodynamic data from 1596 male patients who underwent both free flow and catheterized urodynamics. The following parameters were compared: Free flow Qmax, Free flow residual urine, Voiding CMG Qmax (intubated), and Voiding CMG residual urine. Paired t-tests were used to compare Qmax and residual urine volumes between free flow and catheterized conditions for both UDS and VUDS. To refine the analysis, we also compared individual Qmax differences between free flow and catheterized studies for both UDS and VUDS using a two-sample t-test. Statistical significance was set at p < 0.05.

The analysis revealed significant differences in Qmax and residual urine volumes:
•	For VUDS, the mean free flow Qmax was 13.2 ml/s (SD = 7.5), and the mean intubated Qmax was 11.5 ml/s (SD = 5.3), showing a significant reduction (p < 0.01).
•	Residual urine for VUDS was significantly higher during both free flow (mean = 85 ml, SD = 52.8) and intubated conditions (mean = 98.5 ml, SD = 64.2) compared to UDS (free flow residual urine = 66.26 ml, SD = 51.3, intubated residual urine = 58.8 ml, SD = 46.1) (p < 0.01 for both).
•	In UDS, the mean intubated Qmax was 10.6 ml/s (SD = 5.6), which was higher than the mean free flow Qmax (9.8 ml/s, SD = 7.5). The negative t-statistic (-3.49) suggests variations in the data, with some patients showing a reduction in Qmax during catheterization, but the mean Qmax in the intubated group is higher than in free flow (p < 0.01).
•	When comparing individual Qmax differences (free flow Qmax - intubated Qmax), the two-sample t-test yielded a t-statistic of 5.91 and a p-value of 4.29 × 10⁻⁹, indicating that the reduction in Qmax in VUDS is significantly greater than the increase in Qmax in UDS.

The results show that contrast may reduce Qmax during the voiding phase in VUDS, compared to free flow Qmax. Residual urine volumes were higher in VUDS compared to UDS during both free flow and intubated conditions. This suggests that contrast may also contribute to increased residual urine, potentially due to its higher viscosity. In UDS, while the catheterized Qmax was higher, there was still variation, and some patients showed a reduction in Qmax during catheterized studies, reflecting individual patient variability.

The use of contrast in VUDS resulted in a significant reduction in Qmax during catheterized studies compared to free flow, as well as higher residual urine volumes compared to UDS. These findings suggest that contrast media may influence urodynamic parameters in significant ways. Also, since Qmax depends on bladder volume, a separate analysis of volume will be required.
It is important to note that patients undergoing VUDS may have more complex medical issues, which could contribute to the reported results.

Does contrast medium affect the results of urodynamics?  Sullivan J et al, Proc ICS 1999, poster 331, p 98-99.
Urodynamic implications of the differences in the viscosity of saline, urine and Urografin. Proc ICS 1979, p211-215.