The PVR volume can be assessed by either direct catheterization or ultrasonography. Although sufficiently accurate for clinical purposes, ultrasonography measurements of PVR volume have a standard error of 15% to 20%. It is reasonable to confirm an elevated PVR volume detected on ultrasound with a catheterized volume (37). It is important to perform this test within 10 minutes of a void to avoid an artificially elevated result because of diuresis. It is agreed that a PVR level less than 50 mL is normal and greater than 200 mL is abnormal, but there is much debate about values in the midrange. Because many women are unable to void well during an anxiety-ridden first visit, it is helpful to recheck the PVR volume at a future visit before embarking on further diagnostic tests. The value of assessing bladder emptying in neurologically normal women who do not have pelvic organ prolapse or symptoms of voiding dysfunction has not been demonstrated.
Urodynamics At its most basic level, a urodynamic study is anything that provides objective evidence about lower urinary tract function (38). In this sense, measurement of a patient’s voided urine volume and catheterization to determine her PVR volume are urodynamic studies. A frequency/volume chart is also a valuable urodynamic study. Obtaining clinically valuable information does not always require the use of expensive, complex technology. After basic testing, further testing is recommended in the following circumstances: the diagnosis is uncertain (for example, because of major discrepancies between the history, the voiding diary, and symptom scales); surgery is being considered; an elevated PVR volume, a neurologic condition that may complicate treatment (such as multiple sclerosis), marked pelvic organ prolapse, or numerous prior surgical attempts at correction. Bladder and kidney imaging should be considered if the patient has hematuria in the absence of an infection. Current urodynamic definitions are summarized in Table 26.6.
To assess voiding function, urodynamic testing usually begins with uroﬂowmetry, a study in which the volume of urine voided is plotted over time. Flow time, peak ﬂow rate, and time to peak ﬂow usually increase as the voided volume increases.
Cystometry is done to assess bladder and urethral function during bladder filling. Simple (or single-channel) cystometry is performed when bladder pressure only is measured during filling. Because the bladder is an intra-abdominal organ, the pressure recorded in the bladder is a combination of several other pressures, most notably the pressure created by the activity of the detrusor muscle itself and the pressure exerted on the bladder by the weight of the surrounding intra-abdominal contents (e.g., uterus, intestines, straining, or exertion). For this reason, the technique of complex (also called multichannel or subtracted) cystometry is used to try to approximate the actual pressure exerted in the bladder by the activity of the detrusor muscle alone. The detrusor pressure (Pdet) is obtained by measuring
Table 26.6 Urodynamic Definitions
I. Bladder sensation
Table 26.6 (Continued )
From Abrams P, Cardozo L, Fall M, et al. The standardization of terminology of lower urinary tract function: report from the Standardization Sub-committee of the International Continence Society. Neurourol Urodyn 2002;21:167–178, with permission.
total intravesical pressure (Pves) with a bladder pressure catheter, approximating intra-abdominal pressure (Pabd) with a rectal or vaginal catheter, and then electronically subtracting the latter from the former:
Pdet = Pves − Pabd.