This study aimed  to compare the parameters of  free and invasive uroflowmetry in patients with overactive bladder and to assess the correlation of both uroflowmetries among different definitions of detrusor underactivity (DU) and bladder outlet obstruction (BOO).

This is a cross-sectional study. Urodynamic studies performed from 2015 to 2020 in an academic hospital in Chile were reviewed. Urodynamics were conducted following ICS good practices.  Female adult patients with overactive bladder syndrome were selected. Patients with previous incontinence or radical pelvic surgery, use of anticholinergic treatment, high-grade pelvic organ prolapse (higher than stage II), vesical cancer, urinary tract calculi, neurogenic bladder, and painful bladder syndrome were excluded. Subjects without results of flow-pressure study or those with a free voided volume less than 150 ml were also excluded. Each patient was asked to urinate with the same sensation they would do at home for a non-invasive uroflowmetry, and immediately after that the catheters for the invasive study were installed measuring the residual volume and calculating the Bladder voiding efficiency (BVE). The cystometry was conducted using a 6 Fr double lumen catheter for bladder filling and measurement of intravesical pressure, and a rectal balloon catheter was installed to measure abdominal pressure. Warm saline solution was infused at a rate between 10-50 ml/min. Finally, pressure-flow study (PFS) was carried out evaluating maximum flow (Qmax), detrusor pressure at maximum flow (Pdet@Qmax) and again BVE. Comparison of both uroflowmetries (free and invasive) was conducted for the whole sample through Spearman's rank correlation coefficient and intraclass correlation coefficient (ICC). Two definitions of DU were applied, (I) Abarbanel1: Pdet@Qmax<30 cmH2O and PFS Qmax<10 mL/s) and (II) Gammie2: Pdet@Qmax<20 cmH2O, Qmax<15 mL/s and BVE%<90. We also compared two cut-offs for the likelihood of BOO according to the Solomon-Greenwell3 female BOO index (fBOOI). Likely obstructed if BOOIf was higher than 5, and obstruction almost certain if BOOIf was higher than 18. Age and parameters of the free flow were compared in each of these four groups using U Mann-Whitney to assess differences between the group of patients that met the criteria and those who did not.  Data was processed using IBM SPSS version 23.0 and two tailed p-values <0.05 was considered as statistically significant.

1266 urodynamic studies were reviewed and 195 patients met the inclusion criteria forming the analysis group. Overall, patients’ mean age was 54.2 years, 32,3% had hypertension, 11,3% had diabetes, 90,8% had mixed urinary incontinence, and 39% complained of at least one voiding symptom. They had a median of 2 vaginal deliveries (IQR 0-2). 10.3% complained of vaginal bulge. They used a median of 3 pads/day (IQR 2-4) and their daily and nightly micturitions were 8 (IQR 6-10) and 2 (IQR 1-3), respectively.  The comparison between both the free and invasive flowmetry (PFS) is shown in Table 1. Although the voided volume was lower for the free uroflowmetry, Qmax was higher and correlated poorly (r: 0.331, p<0.001) with that of the PFS. Similarly, BVE  had a poor correlation between the two studies (r: 0.291, p<0.001). The F-Test to assess reliability of the two measurement tools showed there was a systematic error when measuring Qmax and BVE (p<0.001). Intraclass correlation coefficient (ICC) showed that only 5,3% and 18,6% of the variability among the measurement tools corresponded to a random error for Qmax and BVE, respectively. Thus, most of the variation was explained by the systematic error between the two measurement tools (free and invasive flowmetry). Comparison for each of the four different groups is depicted in Table 2. 8 and 14 patients met the DU criteria (Gammie and Abarbanel, respectively), while 60 and 35 patients met the BOO criteria (fBOOi >5 and >18, respectively). Only Abarbanel’s definition demonstrated a significant difference among the features of the free flow, whereas the rest of the definitions failed to do so.

Not only both Qmax and BVE correlate poorly between both free and invasive flowmetry, but also these two tools, which try to assess micturition parameters, showed a systematic error, making them not interchangeable. In other words, invasive flowmetry might be a faulty instrument to qualify a patient micturition if free uroflowmetry is considered closer to patients’ daily life. Moreover, after applying different criteria to define DU/BOO using PFS data, subjects that met the criteria showed similar free flowmetries compared to the non DU/BOO individuals. Hence, a patient with a poor invasive uroflow might have a rather normal free flow, and considering PFS parameters exclusively could account for an overdiagnosis of voiding dysfunction.

The classic method of invasive uroflowmetry (pressure-flow study) might not be representative of the physiological micturition. Current definitions of either detrusor underactivity or bladder-outlet obstruction relying on PFS entirely could lead to overdiagnosis of these pathologic phenomena.

Abarbanel J, Marcus E-L. Impaired detrusor contractility in community-dwelling elderly presenting with lower urinary tract symptoms. Urology. 2007;69(3):436-440. doi:10.1016/j.urology.2006.11.019
Gammie A, Kaper M, Dorrepaal C, Kos T, Abrams P. Signs and Symptoms of Detrusor Underactivity: An Analysis of Clinical Presentation and Urodynamic Tests From a Large Group of Patients Undergoing Pressure Flow Studies. Eur Urol. 2016;69(2):361-369. doi:10.1016/j.eururo.2015.08.014
Solomon E, Yasmin H, Duffy M, Rashid T, Akinluyi E, Greenwell TJ. Developing and validating a new nomogram for diagnosing bladder outlet obstruction in women. Neurourol Urodyn. 2018;37(1):368-378. doi:10.1002/nau.23307