Aims of Study
To show practical graphical and mathematical methods for quantifying detrusor compliance from cystometrography (cmg) plots of detrusor pressure versus volume infused.

Methods
The graphical method is based on cmg curve, where detrusor pressure is plotted against infused bladder volume. The technique consists of laying a parallel ruler from the origin of the plot parallel to the averaged detrusor e.g. relaxed pressure line (excluding the uninhibited contractions) and reading the pressure intercept at the VCap,Norm, which is obtained from estimating graphs or equations (1) as a function of age, sex, height and weight. This reading is divided by 6cmw and the resulting value is called quantifying dimensionless number Nwahl (2). Mathematically the same result is achieved by taking the volumes and pressures at any two points from the averaged pressure plot line and calculating Nwahl by [(P2 - P1)/6] / [(V2-V1)/(VCap,Norm)]. Rather than trying to make an estimate of artifacts and the averaged "relaxed" detrusor pressure line, a better method for calculating Nwahl is to use the automated system developed for this purpose (3,4) . But the method described here is useful for checking and understanding the automated results. This was applied to 20 cmgs of various diagnoses [frequency (1), day-time or mixed wetting (6), Hinman bladder (1), bladder extrophy (1), urethral valve (1), menincomyelocele (mmc, 10 of which 2 augmented)]. The tested patients were of different ages and sizes.

Results
Three patients, one with frequency and two with wetting problems, had completely normal cmg with Nwahl ranging 0.4 - 1.4. The bladder compliance was varying considerably between the wetting patients, Nwahl value being in average 2.5 (range 0.4-7.5). Three patients with abnormal, non-neurogenic bladders presented cmgs with minimal detrusor instability but poor compliance (Nwahl ranging 4.3-9.6). All neurogenic bladders were hyperreflexive, but their compliance was varying from normal for younger patients to highly abnormal for the older children.

Conclusions
Given a cmg plot of pressure versus volume, this methodology yields a result which is standardized to a patient's age and size. By these methods the results are not much affected by such factors as short test time due to strong urge or overactivity of the detrusor, both distorting the test results when using the usual equation of [(V2-V1)/(p2-p1)] between two points on cmg. Thus, this calculation is convenient for comparing compliance between patients and between serial tests for the same patient. Consequently, this method is a valuable tool when choosing a right treatment or assessing a treatment efficacy and timing for a patient.

References:
1. Estimation of bladder capacity and glomerular filtration rate from birth on. Submitted to Neurourology and Urodynamics, March 2000.
2. Quantification of bladder compliance by a dimensionless number. Submitted to Neurourology and Urodynamics, February 2000.
3. Quantification of detrusor compliance and contractility, Society for Urological Engineering, AUA annual meeting, 1998.
4. Quantification of detrusor compliance and contractility submitted to Neurourology and Urodynamics, March 2000.
5. A prototype pediatric cystometric apparatus of improved accuracy. Submitted J Endourology, February, 2000. .