Practical Interpretation and Application of Urodynamic Findings

Filling and storage


Coarse sensation


Contractions (involuntary detrusor)


Cystometric capacity



Complete emptying

Clinical obstruction

  • in the filling phase, the “C’s” consist of contractions (involuntary), compliance, continence, capacity and coarse sensation.

  • in the emptying phase contractility, complete emptying, coordination and clinical obstruction are evaluated.

The “C’s” are not specific for all types urinary dysfunction nor all urodynamic abnormalities. Nevertheless, by organizing and interpreting a study within this framework, it provides an organizing thread from which to formulate a diagnosis and begin to assemble a management plan.

Of course all PFUD tracings should be interpreted in the context of the patient’s history, physical examination and other relevant studies. Additionally, reproducing the patient’s symptoms or at least notating whether this was achieved during the study is also important in order to properly interpret the tracing and any abnormalities seen. Notwithstanding these limitations, it remains that a systematic and organized approach to interpretation of the PFUD tracing is likely to yield the most useful and complete set of data and optimize clinical care and outcomes.

Simply reviewing a UDS tracing is not sufficient to generate an accurate interpretation. The filling and voiding phases of the study are dynamic processes that are influenced by patient understanding of testing instructions (i.e. waiting for permission to void), and artifact (i.e. movement of uroflow detector during the test). Therefore, it is important that the person interpreting the UDS tracing is involved with the actual UDS study as knowledge of the testing environment will help differentiate artifacts from true findings.

Filling and Storage Phase

The filling phase starts with the initiation of instillation of saline (or contrast if a video urodynamic study is being performed) and ends with the instruction to void or “permission to void”. Prior to giving permission to void the provider performing the UDS needs to ensure that all questions regarding the filling and storage phase have been addressed. Once permission to void has been given, the emptying phase begins. It is helpful to have a recent voiding diary available prior to the UDS. The voiding diary will help assess how the UDS tracing reflects their voided volumes in a non-clinical environment.

Coarse Sensation

The sensation of bladder filling experienced by the patient is variable but absence of normal sensation, or delayed sensation of bladder may be indicative of neurological abnormalities. Furthermore, hypersensitivity, lack of sensation during detrusor overactivity, sensation of extreme pain, or low bladder capacity overall due to sensation of fullness may be indicative of other lower urinary tract pathology.

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 coarse sensation can be delayed in patients with poorly controlled diabetes and HIV. Sensation can be absent in patients with spinal cord injuries. Hypersensitivity at low volumes may be indicative of interstitial cystitis (Painful bladder syndrome), UTI or other disorders.

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 several events in the study [1]:

  1. 1.

    first sensation of bladder filling (during filling cystometry, the sensation when he/she first becomes aware of bladder filling)


  2. 2.

    first desire to void (the feeling, during filling cystometry, that the patient would desire to pass urine and the next convenient moment, but voiding can be delayed if necessary),


  3. 3.

    strong desire to void (during filling cystometry, as a persistent desire to void without the fear of leakage),


  4. 4.

    maximum cystometric capacity (in patients with normal sensation, is the volume at which the patient feels he/she can no longer delay micturition (has a strong desire to void)).


  5. 5.

    Urgency (during filling cystometry, the sudden compelling desire to void) at any time during the UDS.


Filling sensations are very subjective and as such there are not a universally accepted set of normative values 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 [17]. 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. Overly warm, cold, 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 mathematically 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 [22]. 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” [12]. A DLPP of greater than 40 is considered deleterious to the upper tracts [13]. However, in certain individuals; a DLPP of less than 40 may also put the upper tracts at risk (Fig. 3.1).


Figure 3.1
Decreased compliance. Note the change in detrusor pressure of 6–46 cm H2O during instillation of 135 ml of fluid volume. Change in Pdet is 46–6 = 40 cm H2O with a change in volume of 135 mL. Compliance = (ΔVolume/ΔPdet) = 135 mL/40 cm = 3.375 mL/cm H2O

Pelvic radiation, denervation from radical pelvic surgery, neurogenic bladder and chronically indwelling Foley catheters are common etiologies of abnormal bladder compliance. Patients, who have abnormal compliance with a chronic indwelling Foley, should be converted to a short period of CIC to allow for bladder cycling if feasible. 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. It is important to recognize that an artifactual decrease in the Pabd (P2) transducer can misinterpreted as decreased compliance, but in fact this is due to artifact or repositioning of the abdominal pressure transducer during the study (Fig. 3.2).


Figure 3.2
The apparent rise in Pdet is artifactual and secondary to a change in position and signal drop out of Pabd which can be mistaken for decreased compliance

Contractions, Involuntary (Detrusor Overactivity)

Detrusor overactivity (DO) is defined as a urodynamic observation characterized by involuntary detrusor contractions (IDC) 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 Pdet tracing along with a similar wave like form on Pves in the absence of “permission to void”. Additionally, the interpreter must ensure that there is not drop out from the rectal/abdominal catheter (Pabd) 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. 3.3). DO which occurs at cystometric capacity and results in bladder emptying is referred to as “terminal detrusor overactivity”. An after-contraction (Fig. 3.4) is a large amplitude rise in Pdet 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 Pdet to be considered DO, the definitive interpretation of low amplitude DO (less than 5 cm H2O) requires a high quality UDS study [1].


Figure 3.3
Detrusor overactivity with associated urinary urge incontinence. Note while there is some artifact from Pabd, the waveform of Pves correlates to Pdet. In both sequences there is an involuntary detrusor contraction (IDC) followed by involuntary flow of urine. During the second IDC the patient is give permission to void (3rd mark). It is important to notate events in real time as this tracing could be mistaken for a normal voiding pattern if patient were given permission to void prior to increased detrusor pressure

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Nov 21, 2017 | Posted by in UROLOGY | Comments Off on Practical Interpretation and Application of Urodynamic Findings

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