How Should the BCI and BOOI Index be Correctly Applied in Patients With Low‐Compliance Bladder?

Urodynamic studies (UDS), as a crucial assessment of lower urinary tract function, are widely used in urology, particularly in the diagnosis of urinary continence and female urological disorders [1]. In 1999, Professor Abrams proposed two important and straightforward indicators for evaluating voiding function: the bladder contractility index (BCI) and the bladder outlet obstruction index (BOOI) [2]. The BCI is calculated using the formula: BCI = Pdet.Qmax + 5 × Qmax. BCI less than 100 indicates reduced contractility; BCI between 100 and 150 indicates normal contractility; BCI greater than 150 indicates increased bladder contractility. The BOOI is calculated using the formula: BOOI = Pdet.Qmax − 2 × Qmax. BOOI less than 20 indicates no obstruction; BOOI between 20 and 40 suggests possible obstruction; BOOI greater than 40 indicates obstruction. Despite the clinical utility of these indices, our review of the literature revealed a lack of specific guidelines, such as good urodynamic practice (GUP) [1] or other studies, that clearly define the methods for collecting and analyzing the parameters Pdet.Qmax and Qmax in different clinical urodynamic scenarios. However, while reading the research published by Professor Andrew Gammie's team, we found a description in "Figure 4" regarding the adjustment of the synchronized flow rate curve to obtain an accurate Qmax [3]. In the same figure, we noticed a phenomenon where the detrusor pressure curve rises significantly before the initiation of the voiding phase. This could result from a low-compliance bladder or terminal detrusor overactivity (TDO). Regardless of which of these possibilities is the case, a certain level of detrusor pressure is already present before the physiological contraction during the voiding phase (Figure 1). In our clinical practice, similar doubts arise when calculating BCI and BOOI for such patients. For instance, in the urodynamic traces presented in Figure 2, Figure 2a shows a bladder with normal compliance. For such patients, the parameters Pdet.Qmax and Qmax can be easily obtained for calculating BCI and BOOI. However, Figure 2b illustrates a low-compliance bladder where the detrusor pressure before the voiding phase has already reached 42 cm H2O. When the detrusor contracts during the voiding phase, the Pdet.Qmax is approximately 115 cm H2O. This raises an important question: when calculating BCI and BOOI for such cases, should the Pdet.Qmax value be adjusted by subtracting the elevated detrusor pressure caused by reduced compliance before voiding? Specifically, should the true Pdet.Qmax be considered as 115–42 cm H2O? (Figure 2). We would like to discuss with the authors and colleagues whether, for urodynamic traces of low-compliance bladders, the Pdet.Qmax value used in the calculation of BCI and BOOI should account for the increase in detrusor pressure caused by compliance-related factors before the voiding phase. Study concept and design: All authors. Drafting of the manuscript: All authors. The authors declare no conflicts of interest.