Good Urodynamics Practices are outlined in Part 1 of this book but are briefly reviewed here in relation to cystometric capacity measurement. Initial bladder volume and detrusor pressure should be zero at the start of an urodynamic study. This is achieved by emptying the bladder to completion with a catheter prior to placing the sensor tipped urodynamic catheter. In addition, vesical pressure and abdominal pressure should be equalized to establish a subtracted detrusor pressure of zero. Bladder filling begins at a rate determined by the urodynamicist, usually between 10 and 100 ml s/min. This rate may be held constant throughout the study or may be altered in response to patient parameters. The patient will also make urine at a rate determined by their hydration status and underlying renal function contributing to the volume in the bladder during the study. Given the short duration of time over which an urodynamic study is typically performed this latter contribution is often ignored. There is data to suggest however that natural filling during cystometry should be accounted for since it may occur at a rate of 6.1 ml/min, increasing bladder capacity 14 % [7]. Other studies have suggested a rate of 1.4 ml/min increasing capacity 12.1 % [8]. Instructions given to patients regarding hydration and the “need to come with a full bladder” may influence the rate of urine production during the urodynamics test. Specifically, when patients are informed that they must give a urine sample or perform a non-invasive uroflow on arrival they may drink excessively before testing [7]. In addition, prolonged set up and testing time can further increase the contribution of natural filling to infused volume, impacting the accuracy of capacity measures [8].

Sensation during filling cystometry is reported by the patient and documented on the urodynamics tracing. Once a normal desire to void is reported by the patient, cystometric capacity is documented. Next, the patient may be instructed to report when he or she can no longer delay micturition and this volume is recorded as maximum cystometric capacity. The capacity measures obtained should be compared to the voided volumes noted on the frequency–volume chart to ensure consistency and reliability of the urodynamic test measures. Artifact secondary to the non-physiologic fill rate and abnormal situation of being observed during a normal private process can impact measurements obtained. Repeat fill cycles of the bladder may be needed and are encouraged to accurately interpret bladder capacity. Abrams has correlated patient reported sensation during urodynamic testing with maximum cystometric capacity. He found that first sensation of bladder filling correlated with roughly 50 % of maximum capacity while normal desire to void and strong desire to void correlated with 75 and 90 %, respectively [9].

In situations where abnormal bladder function exists, cystometric capacity may be reached before the patient can report a desire to void. For example, in patients with impaired sensation, cystometric capacity is the volume at which bladder filling was stopped. There can be several reasons for stopping and this should be well annotated on the urodynamics tracing. With impaired compliance, high detrusor pressures may prompt the urodynamicist to stop filling; in other situations, pain, involuntary voiding, or a maximum predetermine filling volume may prompt discontinuation. Detrusor overactivity or impaired compliance may lead to leakage of urine before the patient reports sensation of fullness; in these situations, cystometric capacity is measured as the volume at which leakage began. In the setting of sphincteric incompetence, capacity measures may be artificially low due to passive leakage across the sphincter. Occluding the urethral outlet with a catheter balloon can increase capacity measures. Maximum cystometric capacity is only defined for patients with normal bladder sensation and therefore generally only cystometric capacity is reported in this population.

Normal cystometric capacity should be interpreted as a wide range of acceptable values. Reference ranges for normal cystometric capacity in adults have not been universally agreed upon primarily because the question still remains on how well this reflects pragmatic functional bladder capacity. Normal cystometric capacity is generally defined as 300–550 ml with larger values obtained in men compared to women [10]. These reference values represent mean values obtained on a sample of only 28 men and 10 women with no history of urologic disease. Mean age was only 24 years, which is hardly comparable to the population of patients generally undergoing urodynamic testing. In another study of 32 highly selected, middle aged volunteers without urologic disease, mean cystometric capacity was 586 ± 193 ml; this is much higher than several previous reports [11].

Abnormal cystometric capacity can be either too small or too large. Small capacity may result from involuntary bladder contractions, bladder pain, impaired compliance, or bladder oversensitivity. Yoon and Swift [12] defined abnormally small maximum cystometric capacity as less than 300 ml but advised using caution in interpreting low cystometric capacity as abnormal in the setting of a normal functional bladder capacity on frequency–volume charts. Large capacity may result from poor or absent bladder sensation during filling or neurologic bladder dysfunction. Large bladder capacity, outside of the range of normal, is generally not interpreted to be pathologic in the absence of neurologic disease. Artifact, as noted above, from catheter placement, filling rate, media temperature, media type, patient positioning, provocative measures such as coughing, patient discomfort or patient anxiety may impact reliability of this measure [13, 14]. For example, during cystometry a patient is generally positioned in either the seated or the standing position for the duration of the test. When completing a 24-h frequency–volume chart a patient generally changes body positions throughout the day and night with the largest voided volumes generally recorded after sleeping in the recumbent position. The impact of patient position on the variability in daytime voided volumes has not been reported. Faster filling rates during cystometry have been shown to produce lower maximum cystometric capacity measures [13]. Room temperature filling media which is much cooler than body temperature urine has been shown to result in lower maximum cystometric capacity measures as well [14]. The non-physiologic fill rate used during urodynamic testing may impact bladder sensations as well as bladder compliance. Prior studies have shown that supra-physiologic fill rates used during urodynamic testing produce an increase in bladder wall pressure and a decrease in bladder compliance [15]. It is unclear if bladder wall pressure influences bladder sensation and ultimately diminishes maximum cystometric capacity.

Frequency–volume charts and bladder diaries are felt to be more accurate representations of true bladder capacity since these measures document the patients voiding patterns in a more natural environment than cystometry during urodynamic testing. However, the act of keeping the journal may influence voiding patterns making the patient more in tune with their bladder sensations. Diokno et al. [16] reported that moderate correlation exists between functional bladder capacity and cystometric capacity thus establishing validity of the cystometric bladder capacity measure (r = 0.493, p < 0.01). Further studies on women with incontinence confirmed correlation between the two measures but showed a statistically significant difference in volumes obtained, with a smaller volume recorded at the time of cystometry. Thus they interpreted the data as displaying a weak clinical relationship between the two measures [12]. Yoon and Swift report poor positive predictive value (51.4 %) of cystometry in detecting abnormalities in bladder capacity but good negative predictive value (84.0 %). Thus, normal bladder capacity during cystometry correlates strongly with normal functional bladder capacity.

Radiologic imaging of the bladder and urethra during urodynamic testing can be performed with synchronous fluoroscopy termed videocystourethrography. This is generally reserved for patients with more complex clinical presentations or those with neurologic dysfunction. Concomitant imaging of the bladder and urethra allows direct observation of the effects of bladder events. During bladder filling and at capacity, an assessment of bladder shape, position with respect to the pubic symphysis, conformation of the bladder neck, diverticuli and vesicoureteral reflux (VUR) is undertaken (Fig. 11.2a, b). Leakage of urine can be detected at earlier time points and at smaller volumes using fluoroscopy than the standard uroflow sensor.