Stop test or pressure-flow study? Measuring detrusor contractility in older females.

Abstract

Impaired detrusor contractility is common in older adults. One aspect, detrusor contraction strength during voiding, can be measured by the isovolumetric detrusor pressure attained if flow is interrupted mechanically (a stop test). Because interruption is awkward in practice, however, simple indices or nomograms based on measurements made during uninterrupted voiding are an appealing alternative. We investigated whether such methods, originally developed for males, might be applicable in female subjects, and attempted to identify a single best method. We compared stop-test isovolumetric pressures with estimates based on pressure-flow studies in a group of elderly women suffering from urge incontinence. Measurements were made pre- and post-treatment with placebo or oxybutynin, allowing investigation of test-retest reliability and responsiveness to small changes of contractility. Existing methods of estimating detrusor contraction strength from pressure-flow studies, including the Schäfer contractility nomogram and the projected isovolumetric pressure PIP, greatly overestimate the isovolumetric pressure in these female patients. A simple modification provides a more reliable estimate, PIP(1), equal to p(det.Qmax) + Q(max) (with pressure in cmH(2)O and Q(max) in ml/sec). Typically PIP(1) ranges from 30 to 75 cmH(2)O in this population of elderly urge-incontinent women. PIP(1), however, is less responsive to a small change in contraction strength than the isovolumetric pressure measured by mechanical interruption. The parameter PIP(1) is simple to calculate from a standard pressure-flow study and may be useful for clinical assessment of detrusor contraction strength in older females. For research, however, a mechanical stop test still remains the most reliable and responsive method. The Schäfer contractility nomogram and related parameters such as DECO and BCI are not suitable for use in older women.