Study design, materials and methods
This is a cross-sectional study. We reviewed Urodynamic studies performed from 2015 to 2021 in an academic hospital in Chile. Urodynamics were conducted following ICS good practices. Female adult patients with lower urinary tract symptoms were selected. Only patients who voided at least 150 ml in both free and invasive flowmetry were included. Patients with high-grade prolapse were excluded. Depending on the referral diagnosis and clinical suspicion patients were classified into 5 different groups (they could be part of more than one group): voiding dysfunction, mixed urinary incontinence, isolated stress urinary incontinence (SUI), isolated overactive bladder-urge incontinence (OAB) and prior anti-incontinence surgery. Each patient was asked to urinate with the same sensation they would do at home for a non-invasive uroflowmetry (measuring free maximum flow (fQMax). 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, a pressure-flow study (PFS) was carried out to evaluate invasive maximum flow (iQmax). Both Qmax measures were manually corrected. Comparison of both uroflowmetries (free and invasive) was conducted for the whole sample through Spearman's rank correlation coefficient and intraclass correlation coefficient (ICC).The difference between both Qmax (fQmax-iQmax=𝚫Qmax) was calculated. Then, 𝚫Qmax was assessed among the different groups to see if there were any significant associations using U Mann-Whitney. Numbers are shown with median and interquartile range and were evaluated with non-parametric tests using IBM SPSS version 23.0 and two-tailed p-values <0.05 was considered as statistically significant.
Results
891 urodynamic studies from female patients were reviewed and 407 patients met the inclusion criteria forming the analysis group. Overall, patients’ mean age was 53.9+-12 years, other socio demographic information is shown in table 1. Our population had a referral diagnosis of: voiding dysfunction (VD) (6.6%), mixed urinary incontinence (MUI) (61.7%), stress urinary incontinence (SUI) (24.6%), overactive bladder (OAB) (7.4%) and prior anti-incontinence surgery (13.3%). They had a median of 2 vaginal deliveries (IQR 3). 12.8% complained of vaginal bulge. They used a median of 3 pads/day (IQR 3) and their daily and nightly micturitions were 7(IQR 5) and 1 (IQR 2), respectively.
Free Qmax was higher (30 ml/sec) and correlated poorly (r: 0271, p<0.001) with the iQmax (17 ml/sec). The F-Test to assess reliability of the two measurement tools showed there was a systematic error when measuring Qmax (p<0.001). Intraclass correlation coefficient (ICC) showed that only 14.8% of the variability among the measurement tools corresponded to a random error, in other words most variability was explained by systematic error.
The comparison between both fQMax and iQmax is shown in Table 2. The whole group had a median of 12 ml/sec difference between both micturitions. This contrast behaved differently among the groups, showing that voiding dysfunction patients had a much lighter difference (only 5 ml/sec), whereas patients with SUI showed a deeper difference with 17 ml/sec.
Interpretation of results
There is a bad correlation between free and invasive Qmax(1,2) and the systematic error shown by these two tools makes them not interchangeable. Theories proposed to explain this big gap includes that a catheter may provoke: a) diminution of the cross-sectional area of the urethra; b) a poor urethral-pelvic floor relaxation; and c) a urethra-bladder reflex leading to a poor detrusor contraction(1-3). Our results let us hypothesize that different urethras react variably to instrumentalization. Low resistance bladder outlets like in SUI patients seem to be more affected by the presence of the catheter, while patients with a presumed impaired urethral relaxation, like in voiding dysfunction, seem to be affected to a lesser extent.