A quantitative comparison between free uroflow variables and urodynamic data, and the effect of the size of urodynamic catheters on its interpretation

OBJECTIVE: To assess the effect of the urodynamic catheter on the urinary flow rate and residual volume in various urodynamic diagnoses, and compare the outcome when using a smaller catheter, as the effect of this catheter on free uroflow variables is mostly studied in patients with bladder outlet obstruction (BOO) and little is known about its effect in other urodynamic diagnoses. PATIENTS AND METHODS: In all, 319 men undergoing a pressure-flow study (PFS) with a 5 F filling and 5 F measuring bladder catheter were subdivided into three groups based on a urodynamic diagnosis, i.e. normal PFS (group 1), BOO (group 2) and detrusor underactivity (DU, group 3). Another group (4) comprised 61 patients who had a PFS with the filling catheter removed before the voiding phase. The effect of the catheters on the maximum urinary flow rate (Q(max)) and the postvoid residual volume (PVR) was analysed statistically and compared among the groups. We also compared the free-flow variables with the clinical and urodynamic variables. RESULTS: Groups 1–3 (with two catheters) had a significantly lower Q(max) and higher PVR than those voiding with one catheter (group 4). The reduction in Q(max) was highest in group 3 (41.9%) and least in group 2 (21%). Group 4 showed no significant change in Q(max) in cases with BOO and a normal PFS but a significant decline in those with DU (19.6%). The PVR was positively associated with the bladder capacity and negatively with detrusor contractility, but no association with a urodynamic diagnosis of BOO or any specific symptom. CONCLUSION: Detrusor contractility was the strongest predictor of the obstructive effect caused by the catheter. This study justifies the use of a single 5 F catheter at the time of voiding, although that can also cause a reduction in flow in patients with DU.