Pressure at residual volume: a useful adjunct to standard fill cystometry

Determining the role of the bladder in incontinence and/or upper urinary tract deterioration involves the cystometrographic evaluation of detrusor compliance. Management decisions based on information provided by standard fill water cystometry assume that data reflect the physiological state of the patient but this is not always true. We report the use of pressure at residual volume as a physiological measurement to aid in the interpretation of data obtained during standard fill urodynamics.

All patients presenting to our urodynamic suite for standard cystometry between June 1994 and December 1995 underwent measurement of bladder pressure at insertion of the urodynamic catheter. Residual volume and pressure measured before draining this urine, that is pressure at residual volume, were recorded. The bladder was then filled at a rate equal to or less than 10% per minute of known or predicted capacity. When a volume equal to the residual volume had been instilled, filling pressure was recorded and the remainder of the standard urodynamic evaluation was completed. Uncooperative patients as well as those with an active infection or residual volume less than 10 ml. were excluded from further analysis.

In 152 children pressure at residual volume was compared to filling pressure and the difference was analyzed. Filling pressure was higher than pressure at residual volume in the majority of cases. In 59 children there was a positive change (filling pressure greater than pressure at residual volume) of 5 cm. water or greater and in 13 there was a negative change (pressure at residual volume greater than filling pressure) of 5 cm. water or less. When only cases in which pressure at residual volume was greater than 5 cm. water were analyzed, 46 and 33% had a difference between filling pressure and pressure at residual volume of 5 cm. water or greater and 10 cm. water or greater, respectively.

Pressure at residual volume is physiological, simple to determine and may provide additional information regarding storage characteristics of the bladder. In the context of the clinical history differences between pressure at residual volume and filling pressure may help to interpret standard fill cystometry data.