Natural pressure-volume curves and conventional cystometry.

Conventional cystometry using high non-physiological filling rates creates an artificial condition with continuous pressure rise. Controlled Slow Cystometry (CSC), using a defined rate-unit calculated after body weight (hour-diuresis unit, HD), has enabled systematic studies of bladder reaction to natural filling. Physiological filling rate is up to 15 HD. At low physiological filling rate (< 2.4 HD in cats) the pressure-volume curve is flat. Pressure rise is rate-dependent, not volume-dependent and can occur at any time during filling, as the need of the body to excrete urine at high rate is not coordinated with bladder filling. Thus, in an individual cat and human there are numerous variations in natural pressure-volume curves.