Measurement of bladder contractility is an important part of the assessment of bladder function. Bladder function is dependent on its ability to generate sufficient pressure to overcome any outflow tract resistance. When a bladder is unable to do so, it is termed detrusor underactivity (DU)1. It may manifest clinically as underactive bladder, experienced as a range of lower urinary tract symptoms, including reduced urinary flow rate and the feeling of incomplete emptying due to increased post-void residual volumes2. 
Several urodynamic parameters have been proposed to estimate bladder contractility, including bladder contractility index (BCI), projected isovolumetric pressure (PIP, or PIP1 in women) and Watts Factor at maximum flow (WFQmax). While BCI is used in men, the diagnosis in women is challenging, as often only low pressures are produced to initiate voiding, given typically low bladder outlet resistance. 
t20-80, the time for the pressure to rise from 20% to 80% of its maximum value during the isovolumetric phase of detrusor contraction (i.e., before flow is initiated) was proposed as a correlate of true detrusor contractility, and named the Detrusor Contractility Parameter, or DCP. This parameter was proposed due to its good correlation with vCE, the maximum velocity of muscle element shortening, also derived from the isovolumetric phase of bladder contraction. However, the calculation of vCE is complex.

Hypothesis / aims of study
We propose a simple way of calculating vCE using an Excel spreadsheet and information readily available from the urodynamics trace. DCP, PIP1, WFQmax and BCI were then tested for correlation with vCE.

One hundred consecutive pressure-flow traces from 100 female patients were used to estimate t20-80¬ and for vCE calculations. Inclusion criteria were patients undergoing routine or video urodynamics, with voided volumes >100ml. Exclusion criteria included: evidence of bladder diverticulum, previous neobladder or bladder augmentation and traces with poor quality subtraction during voiding or voided vesical or rectal lines. 

All traces were obtained using urodynamic equipment (Aquarius software version 12, Laborie Medical Systems, Mississauga, Canada) at a single, tertiary referral hospital. Tests were conducted as per International Continence Society Good Urodynamic Practices. Water-filled lines were used; intravesical pressure was recorded using a 1.1mm diameter epidural catheter, alongside an 8Ch filling catheter which was removed before voiding. Abdominal pressures were measured with a rectal catheter with slit balloon. Sodium meglumine diatrizoate (Urografin 150, Schering AG, Germany) or 0.9% (w/v) saline at room temperature were used as bladder filling agents at a speed of 30-50ml/min. 

Calculation of DCP
DCP was calculated directly from pressure-flow traces. DCP, or t¬¬20-80, was defined as the time interval between pdet rising from 20% to 80% of the value when flow starts. The pressures (cmH2O) at t0, the start of detrusor contraction, and t100, the start of flow, were recorded. The pressure at 20% and 80% of the pressure rise from t0 to t100 were calculated. The t20 and t80, i.e. times at 20% and 80% pressure, respectively, were recorded. DCP was calculated as t20 subtracted from t80.

Calculation of vCE
vCE was calculated as follows: export data (10 Hz) from the urodynamic machine and load into Excel. Arrange data into columns for time, flow and pdet, then generate new columns for smoothed pdet (averaged over 0.4 s), and (dp/dt)/p. Insert X-Y scatter graph for upper 2/3 of pressure values between t0 to t100 and add log trend line. vCE, the vertical axis crossing of this trend line, is approximated by the constant in the equation (i.e. vertical axis value when p=1) of the trend curve. 

Quality assurance
To ensure good quality only data was used, parts of the traces with the following artefacts were excluded or adjusted: 
o	Artefacts were present in some traces due to abdominal contraction or straining. Changes in pves were used as an alternative in 4 traces.
o	Ensure t100 is at the point of maximal pressure before flow starts - a drop in pressure after t100 reflects either flow that is not registered or a bladder shape change. 
o	Pressure rises obscured by poor quality pressure transmission were excluded.
o	Poor curve fitting due to noise on pressure signals was taken to be significant when the R2 value of the curve fit was less than 0.4.
Correlation of vCE and DCP
The resultant vCE calculations were correlated with DCP, PIP1, BCI and WFQmax to test the validity of these methods for estimation of bladder contractility. 

Statistical Analysis
Data are reported as medians (25, 75% interquartiles). Associations between different variables are tested with a Pearson product moment correlation coefficient (the standard function for trendlines in Excel).

The urodynamic traces of 100 consecutive women were assessed. Of these, 80 and 20 had standard and video urodynamics respectively. Table 1 is a summary of patient values. Following quality assessment and removal of unusable traces due to artefact or poor quality, correlation analysis showed t20-80 had the best correlation with vCE (0.41), and thus may be a valid measure of detrusor contractility. This suggests that t20-80 values of greater than 2 seconds are found in the majority of slower contracting detrusors. The R2 value for the trend curve is 0.41. (Figure 1). Analysis revealed poor correlation with PIP1 (0.00), BCI (0.00) and WFQmax (0.01).

The measurement of bladder contractility is an important part of urodynamics, in particular for patients with voiding symptoms such as high post void residual volumes or poor stream. By accurately diagnosing patients with poor contractility, operations on the bladder outlet, such as stress incontinence surgery may be avoided, as this may result in poorer patient outcomes.
The maximum velocity of muscle contraction, vCE, is thought to be the best way to assess true detrusor contractility. Our results echo that of previous studies which have shown that t20-80 is significantly associated with vCE¬ and may be used as a surrogate marker3. However, calculation of vCE to date has always been complex. We have devised an easy way to do this using the Excel spreadsheet software. 
We propose that manufacturers should allow the user to mark t0 and t100 and the best part of the pdet curve for the vCE analysis. This would allow for automated analysis of vCE. In patients where vCE cannot be determined, due either to noisy signals, straining or poor-quality transmission, t20-80 may be a useful surrogate indicator for detrusor contractility.

We have provided a simple way of calculating the maximum contraction velocity of the detrusor, vCE, using standard Excel software and information available from the urodynamic trace. Validity of DCP is confirmed by correlation analysis with vCE.

Chapple CR, Osman NI, Birder L, Dmochowski R, Drake MJ, van Koeveringe G, et al. Terminology report from the International Continence Society (ICS) Working Group on Underactive Bladder (UAB). Neurourology and Urodynamics. 2018;37(8).
Osman N, Mangera A, Hillary C, Inman R, Chapple CR. The underactive bladder: detection and diagnosis. F1000Research. 2016;5
Gammie A, Kitney D, Drake M, Abrams P, Fry C. The calculation and comparison of the Detrusor Contractility Parameter and Watts Factor. Neurourology and Urodynamics. 2018;37(8).