The pathophysiology of increasing postvoid residual (PVR) volume is not generally well understood, and interactions with bladder outlet obstruction and detrusor contraction strength are complex. Due to anatomic differences between men and women their voiding dynamics differ significantly. Studies on PVR pathophysiology in women are very scarce. Additionally, previous studies designed to better understand the voiding phase in women have excluded patients with abdominal straining (1, 2), in order to avoid its possible effect on urinary flow rate. Simple voiding indexes such as the bladder outlet obstruction index (BOOI), urethral resistance and projected isovolumetric pressure 1 (PIP1) have been used in women. Furthermore, PIP1 have been shown to be comparable to the more complex parameter k of the Valentini-Besson-Nelson (VBN) model. In men, “relative bladder outlet resistance” (defined as a ratio of bladder outlet resistance and detrusor strength index) seems to better predict an increased PVR volume than detrusor contraction strength or bladder outlet obstruction indexes alone (3). The aims of this study are: a) to compare voiding parameters in women with and without increased PVR volume, b) to correlate these parameters with PVR volume and PVR percentage and c) to describe their ability to predict increased PVR volume.

This is a retrospective analysis of a prospectively built database of female urodynamic studies (UDS). In a five year period, 826 consecutive women without diseases affecting the spinal cord underwent conventional UDS following the ICS guidelines. Patients who presented an increase ≥ 10 cm H2O over baseline in abdominal pressure at maximum flow rate were excluded, leaving 487 women for analysis (59%). In this group of women, 90 patients with increased PVR volume (≥ 50 mL) in pressure-flow study were selected. Thirty five patients were excluded due to inconsistencies between pressure-flow study and free uroflowmetry (20 for not having increased PVR volume after uroflowmetry and 15 for having an evident lower maximum flow rate in the pressure-flow study), leaving 55 women to evaluate (cases). These patients were matched with the same number of patients without increased PVR volume, controlled for age, presence of urodynamic stress urinary incontinence and pre-voiding bladder volume, without having into account any of their voiding parameters (controls). Of all patients, only 4 had neurological diseases affecting the lower urinary tract: 4 previous cerebrovascular accidents (2 in each group, at least 6 years before). No patient received medications active on the lower urinary tract. The following data was considered: age, LUTS (slow urinary stream, straining to void, intermittent urinary stream and feeling of incomplete bladder emptying), presence of urodynamic SUI and detrusor overactivity, pre-voiding bladder volume, voided volume, PVR volume, PVR percentage (PVR volume / pre-voiding bladder volume x 100), pdet,open, Qmax, pdetQmax and pdetmax. The following simple voiding indexes were calculated: two bladder outlet obstruction indexes (bladder outlet obstruction index, BOOI: pdetQmax – 2 Qmax; urethral resistance: pdetQmax / Qmax2), one detrusor contraction strength index (projected isovolumetric pressure 1, PIP1: pdetQmax + Qmax) (2), and two “relative bladder outlet obstruction” indexes (BOOI / PIP1 and Urethral resistance / PIP1). Student's t test or Wilcoxon's ranksum test (quantitative variables) and χ2 test or Fisher's exact test (qualitative variables) were used to compare voiding parameters in women with and without increased PVR. Linear regression analysis was applied to correlate simple voiding indexes with PVR volume and PVR percentage. The area under the receiver operating characteristic (ROC) curve was calculated to describe increased PVR volume prediction ability of these indexes. Statistical analysis was processed with Stata, version 15.1 program (Stata- Corp 2017) and statistical significance was defined as P < 0.05. All p values were two sided.

One-hundred ten women aged 65.9 ± 13.7 (range 20 - 87) were analyzed. Table 1 shows the comparison of some clinical and urodynamic characteristics between women with and without increased PVR volume. All voiding parameters were significantly different between women with and without increased PVR volume, with the exception of PIP1. Table 2A shows the correlation between simple voiding indexes and PVR volume and PVR percentage using linear regression analysis. The best correlation was found using BOOI. PIP1 did not correlate with PVR volume or PVR percentage, and “relative bladder outlet obstruction” indexes did not improve the correlations. Table 2B shows increased PVR volume prediction ability of simple voiding indexes using areas under ROC curves. BOOI and urethral resistance had good ability to predict increased PVR volume, but PIP1 did not. “Relative bladder outlet obstruction” parameters did not improve the prediction of increased PVR volume.

By using linear regression analysis and measurement of areas under ROC curves, we have found that PVR volume and PVR percentage correlates with bladder outlet obstruction and not with detrusor contraction strength parameters in women who voided without abdominal straining. These results differ from those previously found in men, in which “relative bladder outlet resistance” have shown to better predict increased PVR volume than detrusor contraction strength or bladder outlet obstruction indexes alone (3). Unlike other investigations in women, this study has the strength of eliminating the effect of abdominal straining during voiding (that can increase urinary flow rate), and to control for several confounding variables. a) age; b) presence of stress urinary incontinence (that  decreases urethral resistance) and c) pre-voiding bladder volume (increasing bladder filling volume raises latent detrusor power, within certain ranges). To our knowledge no previous studies have done this before. These results support the use of bladder outlet obstruction indexes to predict PVR volume in women. Further studies controlling for confounding variables of voiding parameters are required to understand the pathophysiology of increased PVR volume in women.

This matched case control study of women who voided without abdominal straining showed that PVR volume and PVR percentage correlates with bladder outlet obstruction and not with detrusor contraction strength parameters.

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