Cystometrogram is the graphic recording of intravesical pressure at increasing bladder volumes, obtained by cystometry.
Cystometry is the procedure of filling an emptied, catheterised bladder with water and recording intravesical pressure at each bladder volume to study the bladder's volume-pressure relationship.
Bladder Filling and Plasticity
Unless the bladder is filled, urine accumulates in it without much rise in intravesical pressure, because bladder wall smooth muscle exhibits plasticity.
Due to plasticity, tension produced by stretching the bladder wall is not maintained, so the bladder volume-pressure relationship is best studied by cystometry.