Detrusor underactivity (DU) increases with age. The International Continence Society definition of DU contains two different pathophysiological causes, namely weak detrusor contraction force and abnormally short detrusor contractions. Each of the incidence and prevalence of the condition is highly dependent on both the definition and diagnostic methods used. Urodynamic studies have attempted to define DU, but no precise definition is widely accepted. Widely used measurements of bladder contractility are bladder contractility index (BCI) and the Watts factor (WF). Impaired bladder contraction represent not only decreased the peak of WF, but also poorly sustained contractions. From this point of view, the maximum height of the resulting curve (Wmax) and its pattern should be discussed separately. In the present study, we compare several contemporary urodynamic criteria and parameters calculated with WF.

Thirty five patients with pre and post radical prostatectomy were evaluated. The urodynamic parameters included the maximum flow rate (Qmax), postvoid residual volume (PVR), detrusor pressure at maximum flow (Pdet at Qmax), Wmax, W (Qmax), BCI (bladder contractility index; Pdet Qmax + 5Qmax), rV (Wmax), Line (W), W80-W20 were examined. (Each definition was shown in figure)
(BCI; Pdet Qmax + 5Qmax, and parameters calculated with WF (rV: volume of fluid during emptying and was expressed in a relative volume (rV = 1; completely full bladder and rV = 0: volume of residual urine), Line (W); slope of a straight line fitted to the W function between, rV = 0.20 and rV = 0.80; rV (Wmax), relative bladder volume at which this maximum occurred; W80-W20, value of W at a relative volume rV = 0.80 minus its value at a relative volume rV = 0.20) (Figure).

After radical prostatectomy, Qmax increased significantly (pre: 13.6 ± 6.7, post: 18.0 ± 7.0 ml/min; P < 0.01), whereas PdetQmax and PVR decreased significantly (50.6 ± 21.0 and 29.9 ± 16.9 cmH2O, 51.3 ± 61.9 and 15.1 ± 28.6 ml; P < 0.05). BCI, Wmax and W (Qmax) did not change significantly after radical prostatectomy (123.7 ± 36.0 and 114.0 ± 19.9, 11.5 ± 2.8 and 11.1 ± 3.0 W/m2, 8.8 ± 2.6 and 9.7 ± 2.9 W/m2), but rV(Wmax), Line(W), W80 – W20 decreased significantly (0.47 ± 0.3 and 0.20 ± 0.20, −0.0016 ± 0.01 and −0.010 ± 0.01 W/m2/mL, −0.4 ± 2.0 and −2.1 ± 2.6 W/m2; P < 0.05).

Wmax and W (Qmax) represents the maximum power of bladder contraction at a particular point in time, whereas rV(Wmax) Line(W), W80 – W20 can be used to detect whole detrusor contraction pattern. The commonly used threshold values for the definition of DU are BCI < 100 or W max < 7 W/m2. In our current study, we couldn’t detect DU using these criteria. However, parameters calculated with WF detect subtle change or pattern of micturition.

The parameters calculated with Watts factor provides some insight into the diagnosis of DU.