Urodynamic studies (UDS) are an integral part of assessing voiding symptoms in both genders. Its settings and components are points of debate for possible effects on results that sometimes vary from preliminary diagnoses, particularly the urethral or vesical catheters used filling and voiding cystometry and their effect on uroflowmetry (UFM) parameters. We aim to study the possible effects of the inserted urethral catheter on UFM and voiding cystometry parameters during the voiding cystometry on patients undergoing UDS assessment.

We prospectively enrolled 150 patients undergoing UDS for established voiding symptoms from January 2016 to March 2018. Exclusion criteria were a pre-UDS voided volume < 150mls and/or inability to void during UDS with catheter in place. Biometric data and clinical history were collected. Free UFM preceded the UDS. Double-lumen 7F air-charged urethral catheter was inserted to measure intra-vesical pressure and for filling and voiding cystometry. A single consultant urologist analyzed the UFM and UDS strips. Parameters of free UFM were compared with that of voiding cystometry using t-test.

The study included 105 eligible patients, the mean age of which was 55 years and 54% were females. Urge urinary incontinence was the most frequent form of leak (n=28) and 69% of strips showed detrusor overactivity. Statistically significant better results were observed between pre-UDS UFM and pressure-flow UFM for the mean values for the maximum flow rate (Qmax) (+4.33ml/s, p<0.001), average flow rate (+1.95ml/s, p<0.05), voiding time (-16.6s, p<0.001), and time to Qmax (-6.6s, p<0.001), but not for post-void residual volume (PVR) (12.2mls, p=0.16) and percentage PVR of cystometric capacity (0.55%, p=0.7).  Time to Qmax becomes insignificant when compared among males only (p=0.2), while all PVR assessments become significant among females only (p<0.05). Analysis was repeated including those who voided 120mls or more (n=120) on pre-UDS UFM and yielded results of similar significance.

This study is among a few prospective trials designed particularly to unveil and address the possible effects that the urethral or vesical catheter has on UFM parameters during a conventional UDS study with voiding cystometry. The presence of the catheter has been hypothesized to create an unnormal representation of a patient’s voiding patterns, from irritation to both obstructing or facilitating bladder emptying on a spectrum of catheter sizes. Our study shows that the urethral catheter significantly and negatively influences the Qmax in UDS studies in both males and females, which would subsequently affect calculation of indices for bladder outlet obstruction and contractility and possibly miscategorize patients. The performance of a free-flow pre-UDS UFM allows for the establishment of a more natural representation of the patient’s voiding ability and bladder emptying, which can then serve as a point of comparison to support or nullify the UFM findings during the subsequent UDS with the urethral catheter in place.
We initially employed a minimum voided volume cut-off point of 150 ml, but due to the lack of consensus on this cut-off value we performed a second analysis to include patients who had voided 120 mls or more, and despite this smaller voided amount, the effects of the urethral catheter on UFM parameters remained significant. The limitations of our study are the small number of patients after subcategorizing by gender.

The insertion of a urethral catheter has a significant effect on UFM parameters in patients undergoing UDS, including Qmax, voiding time, and in females the residual volume. Pre-UDS UFM studies are important to identify such effects and establishing a baseline for comparison so that such effects may not influence the final diagnosis. More studies and consensuses are needed to standardize protocols that eliminate the impact of the effect of the urethral catheter on UDS parameters.