Are vaginal and rectal pressures equivalent approximations of one another for the purpose of performing subtracted cystometry?

OBJECTIVE: To determine if rectal and vaginal pressures are clinically equivalent to one another for the purpose of calculating subtracted detrusor pressure during routine filling cystometry and pressure-flow voiding studies.
METHODS: A total of 140 consecutive filling and voiding cystometrograms were performed at separate sessions on 127 female patients undergoing routine clinical cystometry for a variety of clinical indications, usually urinary incontinence. In all cases, intravaginal as well as intrarectal pressures were measured simultaneously using microtip transducer pressure catheters, and two subtracted detrusor pressures were calculated throughout each study. Rectal and vaginal pressure measurements from the same patient were compared with the patient in the supine position with an empty bladder, in the erect position with a full bladder, and in the sitting position during voiding at the point of maximum urinary flow.
RESULTS: The mean pressures were similar in all cases. Although there was no statistical difference in the mean differences between the rectal and vaginal pressures in the supine-empty position (P = .5528), significant differences were noted between them in the erect-full and sitting-voiding positions (P = .0016 and P = .0033, respectively). Linear regression analysis of the data obtained in each position was carried out, plotting vaginal pressure on the x axis and rectal pressure on the y axis. The corresponding r values for each position were 0.431 for the supine-empty position, 0.547 for the erect-full position, and 0.478 for the sitting-voiding position, indicating poor correlation between pressures in individual patients. In nine patients (6.5%) with significant vaginal relaxation and large cystoceles, a steady rise in vaginal pressure was noted during bladder filling. In six patients (4.4%), one or more spontaneous vaginal contractions were noted during the course of the study, whereas in 68 (48.9%), spontaneous rectal contractions were present. Of the 68 cases where spontaneous rectal contractions were noted, these contractions faded away in 53 cases (77.9%) as the study progressed.
CONCLUSIONS: Rectal pressure and vaginal pressure are not the same during filling and voiding cystometry. Although they are reasonable approximations of each other for most qualitative clinical diagnostic purposes, potentially significant differences in subtracted detrusor pressure may occur, depending on which pressure is used as the approximation of intra-abdominal pressure. This may affect clinical management decisions in individual patients. The technique used for approximating abdominal pressure must be stated clearly in any report or publication dealing with subtracted cystometry.