It is important to begin the study with an empty bladder. Thus, most often patients are catheterized prior to the start of the study. This will help ensure that the infused volumes at which sensations are recorded are accurate. It is also important to ensure that the recorded infused amount accurately reflects the actual infused amount. Such calibrations should be done regularly and periodically as routine maintenance of the urodynamic equipment. Bladder course sensation can be delayed in patients with poorly controlled diabetes and HIV. Sensation can be absent in patients with spinal cord injuries.

Patients should be informed of the study objectives prior to beginning testing and this is especially relevant when assessing sensation. They should be prompted to inform the person performing the study of:

Filling sensation is very subjective and as such there is not a universally accepted normative value hence the term “coarse sensation” is utilized. Typical ranges are first sensation ~170–200 mL, first desire to void ~250 mL, strong desire to void ~400 mL, and maximum capacity ~480 mL [8]. Reviewing a recent voiding diary may be helpful. Sensation is affected by the placement of a catheter in the bladder which may cause irritation and/or pain which may be erroneously interpreted as a sensation to void. Cold or overly warmed or too rapidly infused fluid can also affect bladder sensation. When documenting the interpretation of the UDS, tracing coarse sensation is usually reported as absent, reduced, or increased [9].

Compliance reflects the passive viscoelastic properties of the bladder and is defined as the relationship between change in bladder volume and change in detrusor pressure [1]. Compliance is calculated by dividing the volume change of the bladder just prior to volitional micturition or the first involuntary bladder contraction by the detrusor pressure at that same point [1]. In a normally compliant bladder and in the absence of detrusor overactivity, the detrusor pressure should remain essentially unchanged during filling. Decreased bladder compliance is generally acknowledged as a risk factor for upper tract deterioration.

Despite the importance of this data point, there exists no universally accepted normative value. Compliance of less than 20 mL/cm H₂O is commonly used as the threshold below which is considered abnormal [10]. Occasionally, a prolonged involuntary bladder contraction (detrusor overactivity or DO) can be confused with true abnormal compliance. One way to differentiate between these is to stop infusing fluid and observe for a few minutes. Typically, pressures will return to baseline after a few minutes with DO, whereas pressures will remain high in abnormal compliance. Video urodynamics/VCUG can be helpful as high-grade reflux and large bladder diverticulum can act as a “pop-off” masking underlying abnormal compliance.

Testing of the detrusor leak point pressure (DLPP) in patients with abnormal compliance can be helpful in risk assessment of future upper tract deterioration. DLPP is defined as “lowest value of the detrusor pressure at which leakage is observed in the absence of abdominal strain or detrusor contraction” [11]. A DLPP of greater than 40 is considered deleterious to the upper tracts [12]. However, in certain individuals, a DLPP of less than 40 may also put the upper tracts at risk (Fig. 2.2).

Pelvic radiation, denervation from radical pelvic surgery, neurogenic bladder, and indwelling Foley are common etiologies of abnormal bladder compliance. Patients who have abnormal compliance with a recent indwelling Foley, if feasible, should be converted to a short period of CIC to allow for bladder cycling. Often, in these patients without a high suspicion of true poor compliance, normal compliance will be noted after a short period of CIC and/or bladder cycling. When documenting the interpretation of the UDS tracing, compliance is usually reported as normal or abnormal or can be listed as a calculated value as noted previously.

Detrusor overactivity (DO) is defined as a urodynamic observation characterized by involuntary detrusor contractions during the filling phase which may be spontaneous or provoked. If there is a relevant neurologic lesion, it is deemed neurogenic DO. If there is no relevant neurologic lesion, it is deemed idiopathic DO [1]. It is important to ensure than any suspected detrusor overactivity is in fact accurate and not artifact. True detrusor overactivity is noted as a wavelike form on the P _det tracing along with a similar wavelike form on P _ves in the absence of “permission to void.” Additionally, the interpreter must ensure that there is no dropout from the rectal/abdominal catheter (P _abd) that may artificially simulate a rise in detrusor pressure.

Often, patients will report an unintended or sudden urge to urinate which may or may not correlate with an IDC. It is key for the interpreter of the UDS tracing to be involved in the study as this helps identify artifact from true detrusor overactivity and can confirm if the DO replicates the patients presenting symptoms. Additionally, DO can be “stress induced” by strain or cough, so it is important to be aware of potential precipitating events both during the study and at home.

When documenting the interpretation of the UDS tracing, detrusor contractions during the filling phase are usually reported as absent (“stable filling”), present and suppressible, present with resulting detrusor overactivity incontinence , or terminal DO (DO-related incontinence resulting in emptying of the bladder) (Fig. 2.3). DO, which occurs at cystometric capacity and results in bladder emptying, is referred to as “terminal detrusor overactivity.” An after contraction is a large amplitude rise in P _det occurring after the cessation of voiding. The clinical significance of this finding is unclear as it may represent catheter artifact or a true abnormality. While there is no defined high/low limit of rise in P _det to be considered DO, the definitive interpretation of low-amplitude DO (less than 5 cm H₂O) requires a high-quality UDS study [1].

Cystometric capacity is the volume in which “patients with normal sensation can no longer delay micturition” [1]. Cystometric capacity should not be confused with functional bladder capacity which is obtained from a voiding diary in conjunction with a post void residual. Cystometric capacity is typically less than the functional bladder capacity. There is no universally defined normal cystometric capacity, but typical values range from 370 to 540 mL ± 100 cm³ [13] (Fig. 2.4). Of note, the provider performing the UDS should ensure the patient is not experiencing an involuntary detrusor contraction which is generating the sensation such that they cannot delay micturition.

The filling rate of the bladder can also affect the cystometric capacity. Generally, a filling rate of 50–70 mL/min is used in adults [14]. This filling range allows for the test to be completed in a reasonable amount of time yet minimizes the artifacts related to overly rapid bladder filling [15]. A voiding diary suggestive of large/small bladder capacity can assist in determining if a faster/slower fill rate is more appropriate. When documenting the interpretation of the UDS tracing, cystometric capacity is usually reported in cm³ or mL .