In clinical practice, urodynamics are obtained to reproduce clinical symptoms while allowing for precise measurements in order to identify the underlying causes for the symptoms and to quantify the related pathophysiologic processes.  Urodynamics may confirm a diagnosis or result in a new diagnosis. A careful assessment of the patient’s history and physical examination helps determine the question(s) that one hopes to answer with urodynamic evaluation.
In an effort to provide some standardization to urodynamic studies, the International Continence Society has developed guidelines for good urodynamic practice for the measurement, quality control, and documentation of urodynamic studies in both clinical and research environments.  In addition, the report from the standardization subcommittee of the International Continence Society provides definitions for urodynamic observations.  Practitioners performing and interpreting urodynamic studies should be aware of such guidelines and terminology.
Urodynamic studies may be obtained for various lower urinary tract symptoms, including the evaluation of incontinence, overactive bladder symptoms, and urinary retention. Urodynamics may be obtained to guide initial treatment, to follow progress with treatment or to evaluate treatment failures. In patients with neurogenic bladder/sphincter dysfunction, urodynamics are often obtained to evaluate the risk for upper urinary tract damage in those with bladder/sphincter dysfunction.
Urodynamics should not be performed in the setting of a symptomatic urinary tract infection. Urodynamics should be obtained to assist in further management of a patient’s lower urinary tract symptoms.
Urodynamic study has several components, as follows: (1) uroflowmetry, (2) filling cystometry (cystometrogram, CMG), (3) pressure flow study of voiding, and (3) electromyelography (EMG).
An image depicting a cystometrogram can be seen below.
The choice of which components to perform depends on the patient’s symptoms and the urodynamic questions one seeks to answer. Having a bladder diary completed for 2 days before performing the urodynamic evaluation is helpful. The bladder diary serves to confirm the patient’s symptoms as well as provide some assistance with the urodynamic studies. For example, one can determine the average volume voided during the bladder diary, which will help prevent over-filling of the patient’s bladder during the cystometrogram.
Potential complications from an urodynamic evaluation include urethral trauma leading to hematuria, dysuria, or false passage. In addition, although infrequent, the risk of developing a urinary tract infection exists. Thus, placement of the urethral catheter during the study should be performed using sterile technique with a well-lubricated catheter. In patients with a history of recurrent urinary tract infections and those on clean intermittent catheterization, a urine dipstick should be checked prior to starting the urodynamic evaluation. If significant pyuria on the dipstick exists, then the procedure is not performed and the urine is sent for culture. Patients with chronic indwelling catheters should have urine cultures obtained and be treated just prior to urodynamics.
The anatomy of the bladder forms an extraperitoneal muscular urine reservoir that lies behind the pubic symphysis in the pelvis. A normal bladder functions through a complex coordination of musculoskeletal, neurologic, and psychological functions that allow filling and emptying of the bladder contents. The prime effector of continence is the synergic relaxation of detrusor muscles and contraction of the bladder neck and pelvic floor muscles. See the image below.
For more information about the relevant anatomy, see Bladder Anatomy.
Patient education/informed consent
The patient must be aware of what the urodynamic study entails and the information that will ideally be gained from the study. The physician should review the different components of the urodynamic evaluation and discuss with the patient the rationale for obtaining each component.
In addition, the patient should be aware that the procedure is interactive, and the patient should discuss his/her sensations during the study and whether or not his/her symptoms were reproduced during the study. If a free-flow uroflowmetry is to be obtained, the patient must be instructed to drink fluids prior to presenting for the study and to come to the testing area with a full bladder. The patient should be aware of the risks of the procedure, and, should he/she experience symptoms of a urinary tract infection after the procedure, that a urine culture be obtained. Formal consents are not routinely obtained for urodynamic studies, but this does not obviate the need for a discussion of what the study entails, what information the physician hopes to obtain from the study, and the risks of the procedure.
Various companies manufacture urodynamic equipment. The individual performing the urodynamic study must be aware of the technical considerations specific to the equipment being used and follow the procedural recommendations regarding calibration of the equipment.
For the measurement of intravesical pressure and for bladder filling, a transurethral double-lumen catheter is recommended. By using a double-lumen catheter, the fill/void sequence can be repeated without the need to recatheterize the patient. When using a 6-F double lumen catheter, the infusion rate may be limited to 20-30 mL/minute despite having the filling rate set at 60 mL/sec. This discrepancy may lead to a higher than actual volume infused being recorded. Thus, determining the maximum filling rate that can be achieved with the catheter(s) being used, as well as correcting or calibrating the indicated infused volume, is important. 
A rectal balloon catheter is used for the measurement of abdominal pressure (Pabd). A flaccid, air-free balloon catheter placed in the rectal ampulla allows for an adequate assessment of Pabd. The balloon serves to provide a small fluid volume at the catheter opening and avoids fecal blockage, which can affect pressure transmission to the transducer. The detrusor pressure is determined by subtracting the abdominal pressure from the measured vesical pressure (Pves). In women one may also place the catheter into the vagina. 
Schafer et al, in their development of comprehensive guidelines for good urodynamic practice, noted that the “minimum recommended requirements for an urodynamic system are:
- Three measurement channels, 2 for pressure and 1 for flow
- A display (on printer and/or monitor) and secure storage of 3 pressures (Pabd, Pves, Pdet) and flow (Q) as tracings against time
- Infused volume and voided volume shown graphically or numerically
- On-line display of pressures and flow, with adequate scale and resolution
- The ability to record information about sensation and additional comments (event recording)” 
Calibrating the equipment according to the specifications of the manufacturer is important. Pressure signal quality is important and should be ensured at the start of any urodynamic study. The good urodynamic practice guidelines “recommend the following 3 criteria as minimum recommendations for ensuring quality control of pressure recordings:
- Resting values for abdominal (Pabd), intravesical (Pves) and detrusor (Pdet) are in a typical range (pdet is zero or close to zero; pabd and pves are 5-20cm H20 in supine, 15-40 cm H20 in sitting and 30-50cm H20 in standing position when the standard followed is with the transducer zeros set to atmospheric pressure and the transducer located at the upper edge of the symphysis pubis.
- The abdominal and intravesical pressures are “live” with minor variations caused by breathing or talking being similar for both signals and not showing in pdet
- Coughs are used to ensure that the abdominal and intravesical pressure signals respond equally.
All negative pressures values, except when caused by rectal activity, need to be corrected immediately.” 
Urodynamic studies are typically performed without anesthesia. A topical anesthetic may be used for catheter placement. The position of the patient may vary depending on the ambulation status of the patient and the urodynamic questions to be answered, ie, for evaluation of stress urinary incontinence the patient is often in a standing position, whereas for nonambulatory neurogenic bladder patients, the studies are performed in a supine position.
Uroflowmetry is the measurement of urine flow over time. Uroflowmetry is ideally obtained prior to the start of a multi-component urodynamic study, prior to catheterization, a “free uroflowmetry.” One can thus compare this “free flow uroflowmetry” to that obtained during the voiding phase of filling cystometry.
Various parameters are assessed with uroflowmetry, as follows:
- The flow pattern is described as either continuous (without interruption) or intermittent (stop and start pattern). A continuous flow pattern may be smooth or fluctuating. The shape of the flow curve is determined by a variety of factors including detrusor contractility, abdominal straining during voiding, and bladder outlet obstruction. 
- The flow rate is the volume of urine voided per unit of time (mL/sec). The maximum flow rate (Qmax) and the average flow rate are assessed.
- The voided volume. A minimum volume of 150 mL is the generally accepted value for an accurate uroflow.
- Voiding time – the total duration of voiding, including interruptions
- Flow time – the time over which detectable urine flow is occurring
- Time to maximum flow – the time from start of the study to Qmax 
A postvoid residual determination, usually by bladder scan, is obtained upon completion of the uroflowmetry.
In patients with suspected dysfunctional voiding or neurogenic lower urinary tract dysfunction, simultaneous electromyography monitoring may be performed to determine if external sphincter dyssynergia (failure of the external sphincter to relax just prior to and throughout voiding).
In “free uroflowmetry,” the shape of the flow curve may suggest the underlying etiology of the lower urinary tract symptoms; however, uroflowmetry alone is not definitive. Factors that may affect uroflowmetry include the detrusor contractility, bladder outlet resistance, and bladder volume.
Technical considerations that may affect the reliability of the uroflowmetry include calibration of the machine, the concentration of the urine, and the presence of a funnel or collecting device as well as the flowmeter. 
Filling Cystometry and Pressure-Flow Study of Voiding
Cystometry is the method by which the pressure/volume relationship of the bladder is measured during bladder filling. The ideal study is an interactive one that is interactive with the patient and that reproduces the patient’s symptoms. A frequency volume chart prepared in advance as well as a “free-flow uroflowmetry” provides helpful information for the filling cystometry and voiding phase
The urodynamic equipment should be calibrated as per the manufacturer’s recommendations and the pressures zeroed at the start of the study.
The urodynamic catheter should be secured in place and the bladder completely drained prior to starting the study. The rectal catheter, used to measure Pabd, is placed through the anal canal into the rectum (10-15 cm). If the patient has a large amount of stool in the rectum, obtaining good readings will be difficult. The catheter should be removed, the stool evacuated, and the catheter replaced. The lines should be flushed to ensure there are no bubbles or leaks.
The patient is asked to cough. During a cough, a rapid rise in pressure in Pves and Pabd should be noted with a rapid fall. Filling at a rate of 10-60 mL/min, typically 50 mL/min is then begun. If the patient has detrusor overactivity, it may be necessary to perform the study supine with a lower filling rate. If low compliance occurs during filling, the filling should be stopped to about 2 minutes to allow the detrusor pressure to stabilize and then resume filling at a lower rate. If the filling is stopped or the rate lowered, an annotation marking this change should be made on the study. To ensure good quality control, the patient should be asked to cough periodically throughout the study. 
Assessments obtained during filling cystometry include the following:
- First sensation of bladder filling – When the patient first becomes aware of bladder filling
- First desire to void – The desire to void that would lead the patient to void at the next convenient moment, but could delay voiding if necessary
- Strong desire to void - Defined as a persistent desire to void without the fear of leakage
- Cystometric bladder capacity – The bladder volume at the end of the filling cystometry when the patent is given permission to void. In the absence of normal sensation, it is the bladder volume at which time the physician decides to stop filling, which may be related to high detrusor filling pressure, large volume infused, or bladder pain.
- Maximum cystometric capacity – In patients with normal sensation it is the volume at which the patient feels that he/she can no longer delay voiding.
- Bladder compliance – Describes the relationship between the change in bladder volume and the change in bladder pressure and is determined by dividing the volume change by the change in detrusor pressure and is expressed in mL/cm H2 0. The standard points used to measure compliance are at the start of bladder filling and at cystometric capacity or immediately prior to the start of any detrusor contraction that results in significant urine leakage (see the image below). 
Presence/absence detrusor overactivity
Cystometrogram (CMG) demonstrating poor bladder compliance. Detrusor pressure increases with bladder filling.
During filling cystometry, one should annotate the study if the patient experiences urgency ( a sudden compelling desire to void), bladder pain, or if detrusor overactivity is identified. Detrusor overactivity is the presence of involuntary detrusor contractions during the filling cystometry, which may be spontaneous or provoked. No defined detrusor pressure value for determination of an involuntary detrusor contraction exists, as this may vary with the quality of the urodynamic technique (see the image below). 
Additional measurements may be obtained in patients to assess urethral function. The 2 common methods are urethral pressure profile (UPP) and leak-point pressures (LPP). The urethral pressure profile is the continuous measurement of the fluid pressure needed to just open a closed urethra.  The urethral pressure profile is a graph indicating the intraluminal pressure along the length of the urethra.
The abdominal leak point pressure (ALPP) or Valsalva leak point pressure (VLPP) is the intravesical pressure at which urine leakage occurs owing to increased intraabdominal pressure in the absence of a detrusor contraction.
The AUA guideline recommends that clinicians perform repeat stress testing with the urethral catheter removed in patients suspected of having stress urinary incontinence who do not demonstrate this finding with the catheter in place during urodynamic testing. [4, 5] Over 50% of women with symptoms of stress urinary incontinence who do not demonstrate stress urinary incontinence with the urethral catheter in place do so when it is removed. [6, 7]
Furthermore, the AUA guideline also recommends in women with high-grade pelvic organ prolapse but without significant symptoms of stress urinary incontinence, clinicians should perform stress testing with reduction of the prolapse. [4, 5] This measure is often assessed in females with stress urinary incontinence.
The use of abdominal leak point pressure and urethral pressure profile in the evaluation of women with stress urinary incontinence, does not, however, appear to predict which patient will have the best outcome of surgical treatment for stress urinary incontinence. 
The detrusor leak point pressure (DLPP) is the lowest detrusor pressure at which urine leakage occurs in the absence of either a detrusor contraction or increased abdominal pressures.  The DLPP is often determined in patients with neurogenic bladders, and a DLPP greater than or equal to 40 cm H2 0 is associated with an increased risk of upper tract deterioration. 
EMG should be performed in combination with CMG with or without pressure flow study in patients with relevant neurologic disease at risk for neurogenic bladder or in patients with other neurologic disease and increased postvoid residual or urinary symptoms. 
Pressure flow study measures the relationship between pressure in the bladder and urine flow rate during bladder emptying.
Several assessments are obtained during the pressure flow study, including the following:
Opening pressure – The pressure recorded at the onset of urine flow.
Opening time – The time period from the initial increase in detrusor pressure to the onset of urine flow.
Maximum detrusor pressure – The maximum detrusor pressure recorded during the bladder contraction.
Pressure at maximum flow – The lowest detrusor pressure recorded at maximum measured flow rate.
Closing pressure is the detrusor pressure measured at the end of the measured flow.
Minimum voiding pressure is the minimum detrusor pressure measured at the end of the measured flow rate.
During the pressure flow study, the detrusor function is also evaluated. 
Normal detrusor function is defined as normal voiding achieved by a voluntarily initiated continuous detrusor contraction that leads to complete bladder emptying within a normal time span and is in the absence of outlet obstruction. For a given detrusor contraction, the magnitude of the recorded pressure depends on the degree of outlet resistance. 
Abnormal detrusor function can be categorized as detrusor underactivity and acontractile detrusor. Detrusor underactivity is defined as a detrusor contraction of decreased strength and/or duration, resulting in prolonged bladder emptying and/or the inability to empty the bladder during a normal void time. Whereas an acontractile bladder is one in which no detrusor contraction is demonstrated. 
The pressure flow study will often identify one of the following 3 voiding states:
- Low detrusor pressure and high flow rate consistent with no outlet obstruction
- High detrusor pressure and low flow rate consistent with outlet obstruction
- Low detrusor pressure and a low flow rate, in which case, decreased detrusor contractility and outlet obstruction cannot be assessed 
Urodynamic testing voiding pressure-flow analysis remains the criterion standard for the diagnosis of bladder outlet obstruction and the etiology of lower urinary tract symptoms. Pressure flow study may be useful in patients with urgency urinary incontinence after bladder outlet procedures to evaluate for bladder outlet obstruction. Pressure flow study is also indicated in patients with relevant neurologic disease with or without symptoms or in patients with other neurologic disease and increase postvoid residual or urinary symptoms. 
Furthermore, although the definition of obstruction may be different for men and women, the concept of the pressure-flow relation to diagnose obstruction is applicable to both genders.  In 1979, Abrams and Griffiths developed a simple nomogram for the diagnosis of bladder outlet obstruction in males, which plots voiding detrusor pressure (on the x axis) against flow rate (on the y axis; see image below). 
The nomogram is limited in its ability to differentiate between impaired detrusor contractility with and without bladder outlet obstruction.  The pressure-flow nomogram cannot be applied to women. However, the concept of a relatively high detrusor pressure and a relatively low flow rate when compared to normal is indicative of obstruction in women.
Blaivas and Groutz, in 2000, defined bladder outlet obstruction in women as a free Q max of less than 12 mL/sec combined with a Pdet/Qmax of more than 20 cm H2 0 in pressure-flow study, or obvious radiographic obstruction in the presence of a sustained detrusor contraction of more than 20 cm H2 0, or urinary retention or the inability to void with a transurethral catheter in place despite a sustained detrusor contraction of more than 20 cm H2 0. 
Nitti et al in 1999 highlighted the usefulness of simultaneous fluoroscopic imaging of the bladder outlet during voiding in women to help make the diagnosis of bladder outlet obstruction and noted that radiographic evidence of obstruction between the bladder outlet and distal urethra in the presence of a sustained detrusor contraction, without the application of a strict pressure-flow criteria, was suggestive of bladder outlet obstruction. 
Role of videourodynamics
The AUA guideline on urodynamics notes that when available, clinicians may perform fluoroscopy at the time of urodynamics (videourodynamics) in patients with relevant neurologic disease at risk for neurogenic bladder or in patients with other neurologic disease and elevated postvoid residual or urinary symptoms.  Furthermore, videourodynamics enables the diagnosis of bladder neck abnormalities in patients with neurogenic bladder. It also may help identify the etiology of neurogenic bladder with respect to the underlying neurologic disease. [4, 13, 14] The use of fluoroscopy also allows for evaluation of vesicoureteral reflux, evaluation of the location of bladder outlet obstruction, and assessment of other bladder anomalies such as bladder diverticulum and stones. It is also thought to provide a more accurate method to diagnose detrusor sphincter dyssynergia, detrusor bladder neck dyssynergia, primary bladder neck obstruction, and dysfunctional voiding. 
Urodynamics is a useful tool in the diagnosis of lower urinary tract symptoms in males and females. The performance of urodynamics requires a careful assessment of the patient’s symptoms, determination of questions one hopes to answer with the urodynamic evaluation, and the components of the study needed to answer such questions. Adherence to guidelines helps ensure a quality study and requires a familiarity with the equipment as well as standardization of urodynamic technique. As with any procedure, proper patient counseling is imperative and active participation of the patient in the study serves to enhance the results.