Figure 3: Diagram illustrating principles of recording in cystometry. Bladder pressure and rectal pressure are recorded via catheters and also subtracted, electronically, to measure the pressure generated by the bladder itself, the detrusor pressure. Voided volume is measured, typically by a load cell, and flow rate determined electronically by differentiation (If flow rate is measured directly, volume is determined by integration.) The bladder is filled by infusion through an additional catheter (or additional lumen in the bladder pressure catheter). Traditionally data was plotted on a chart recorder but is now recorded to computer disc and subsequently printed for hard copy.