Cystometry. In cystometry, fluid is infused into the bladder while intravesical and intraabdominal pressure is documented using urethral and vaginal or rectal catheters. Detrusor pressure is calculated by subtracting intraabdominal pressure from intravesical pressure.

A single-channel cystometrogram documents intravesical pressure as a function of the volume of fluid infused. Four phases are seen. The first three phases represent bladder filling. The first phase contains a sharp initial rise in pressure as fluid is first infused. The second phase, known as the tonus limb, features a smaller rise in pressure as additional fluid is infused, and it reflects accommodation of the elastic bladder wall. The third phase contains a more dramatic rise in pressure that occurs as the bladder wall becomes maximally distended. The fourth phase is the voiding phase, which occurs when the bladder has reached its maximum capacity. Throughout this process, patients are asked to comment on the sensation at first filling and when they experience both their first desire to void and a strong desire to void. The volumes at which patients experience these sensations are noted.