Determination of yield pressures: a method for measuring anal sphincter competence.

Resting pressures recorded from the anal sphincter by the open-tip method seem to reflect the last pressure to which the catheter tip was exposed before it entered the sphincter, presumable because sphincter tissues "seal" the recording orifice and thereby "trap" pressure within the recording system. By injecting or infusing small increments of fluid into the system, one can measure a physiologically meaningful pressure--the pressure required to break the "seal." For the resting sphincter, this pressure has been termed the resting yield pressure; for the maximally tightened sphincter, the augmented yield pressure. By determining yield pressures before and during active contraction of the sphincter the involuntary and voluntary components of sphincter function can be separately assessed. Measurement of yield pressures can separate sphincters judged competent or incompetent on clinical grounds. Injection of microliter quantities of fluid into the recording catheter whose tip is in the sphincter causes a marked rise in pressure. Although "bleeders" or constant slow infusions of fluid do not affect pressures recorded from within a cavity, they do significantly alter pressures recorded from a sphincter zone.