Introduction to Urodynamics
The term urodynamics means observation of the changing function of the lower urinary tract over time. Specifically, urodynamic studies are a number of interactive diagnostic tests that can be used to obtain functional information about bladder filling, urine storage, and voiding.
To understand the fundamental value of urodynamics, one should realize that the female bladder responds similarly to a variety of pathologies. Symptoms do not always reflect accurately the physiologic state of the bladder. For example, a patient may feel that her bladder is full when it is nearly empty, or that her bladder is contracting when it is not. The validity of any urodynamic diagnosis is linked to the patient’s symptoms and the reproduction of these symptoms during the testing session. To obtain the most accurate, clinically relevant interpretation of urodynamic studies, the urodynamicist should clearly understand lower urinary tract function and correlate urodynamic data with other clinical information. Ideally, the urodynamicist should be the physician who takes the history, reviews the voiding diary, performs the physical examination, interprets other tests, explains the diagnosis, and develops a reasonable management plan.
Results of urodynamic investigations should be recorded in a way that can be communicated among physicians and other health care personnel. For this reason, the recommendations detailed in the standardization reports of the International Continence Society (ICS) should be followed (see Appendix).
Normal Bladder Physiology
A prerequisite to performing and interpreting a urodynamic study is having a full understanding of normal lower urinary tract function. During filling, a normal adult bladder should act as a low-pressure reservoir. It has the inherent ability of responding to filling (when done at a physiologic rate) with an almost imperceptible change in intravesical and detrusor pressure. This high compliance of the bladder is due primarily to its elastic and viscoelastic properties. During filling, the bladder outlet should be closed and remain closed even during increases in intraabdominal pressure. There is a gradual increase in urethral pressure during bladder filling contributed to in part by the striated sphincter of the urethra and perhaps also by smooth sphincteric elements as well. This can be detected by measuring urethral pressure but also can be seen on electromyography (EMG). The normal opening vesical pressure in an adult female bladder should be in the range of 2 to 6 cm H 2 O, with it only increasing to approximately 10 cm H 2 O at maximum capacity.
Bladder filling should be associated with appropriate sensation. Most women will feel a first sensation of bladder filling between 75 and 150 mL, feel an urge to void between 300 and 400 mL, and have a maximum capacity or strong desire to void between 400 and 700 mL. Although many factors are involved in the initiation of micturition in adults, it is intravesical pressure that creates the sensation of distention that is primarily responsible for the initiation of a normal voluntarily induced emptying of the bladder. This also involves inhibition of the striated sphincter of the urethra and an inhibition of all aspects of any spinal sympathetic reflexes evoked during filling. A decrease in outlet resistance occurs with adaptive shaping and funneling of the relaxed bladder outlet leading to efficient voiding. Figure 10.1 demonstrates a storage and micturition reflex in a neurologically intact female.
Functional Classification of Lower Urinary Tract Dysfunction
For one to be able to formulate the appropriate questions to be answered by a urodynamic study, the clinician must have a clear understanding of the possible or potential causes of symptoms. The functional classification of lower urinary tract dysfunction as proposed and popularized by is a simple, clinically relevant way of classifying lower urinary tract dysfunction. This is discussed in detail in Chapter 8 . Wein proposed that functional abnormalities of the lower urinary tract can be broadly divided into failure of the bladder to adequately store urine or failure of the bladder to adequately empty. Failure of the bladder to store urine results from bladder overactivity (involuntary contractions or decreased compliance), decreased outlet resistance, heightened or altered sensation, or a combination of these various abnormalities. Failure of the bladder to empty results from decreased bladder contractility, increased outlet resistance, or a combination of both. Finally, a combined dysfunction of failure to store and empty is also potentially feasible.
To use a functional classification appropriately, there must be an understanding of the physiology of urine storage and voiding as well as knowledge regarding the various disease states that can cause lower urinary tract dysfunction. The advantages of a simple classification such as this are that it helps to clarify treatment options for a given patient. Simply determining whether there is a problem with storage, emptying, or both as well as assessing bladder and urethral (outlet) function can commonly lead to a correct urodynamic diagnosis with institution of appropriate therapy. The clinician should always focus on the urodynamic finding that is most likely to explain a particular symptom; and more importantly how a particular finding may ultimately affect the patient and her subsequent treatment. See Table 10.1 for a variety of disease states categorized using this classification.
| I. Failure to Store |
|
| II. Failure to Empty |
|
Role of Urodynamics in Clinical Practice
Although urodynamic testing has been frequently used in the evaluation of women with lower urinary tract dysfunction, level 1, evidence-based “indications” for its use are surprisingly few. The clinical question that must always be asked is, will the results of a urodynamic evaluation either alter the outcome of the proposed management plan or be able to identify a potentially life-threatening situation (e.g., neurogenic voiding dysfunction). Urodynamic studies should be considered in clinical situations in which the testing is felt to be needed to make an accurate diagnosis or to determine the impact of a disease state that has the potential to cause irreversible damage to the urinary tract, such as spinal cord injury. summarized the role of urodynamics in clinical practice by listing the following situations in which urodynamic studies should be considered.
- 1.
To identify or rule out factors contributing to lower urinary tract dysfunction and assess their relative importance.
- 2.
To obtain information about other aspects of lower urinary tract dysfunction.
- 3.
To predict the consequences of lower urinary tract dysfunction on the upper tract.
- 4.
To predict the outcome including undesirable side effects of a contemplated treatment.
- 5.
To confirm the effects of an intervention or understand the mode of action of a particular type of treatment (especially a new one).
- 6.
To understand the reasons for failure of previous treatments for symptoms or for lower urinary tract function in general.
Urodynamic Equipment
The (ICS) has developed recommendations for the minimum requirements for the equipment used to conduct urodynamic studies. These include three measurement channels, two for pressure and one for flow, a display on either a printer or a monitor, and a method of secure storage of the recorded pressures ( ). The infused volume and voided volume may be recorded graphically or numerically. There also must be a method of event recording to mark information about sensation, leakage, and additional comments during the study. Cystometry transducers are devices that convert an applied pressure into an electrical signal. Pressure to be measured may be transmitted to the transducer using an open-ended or sealed catheter filled with liquid or gas or directly when the transducer is small and mounted on the tip of the catheter (microtip transducer). The transducer is normally calibrated against atmospheric pressure with a zero reference level being the superior edge of the symphysis pubis. The customary unit of measurement for pressure during urodynamics is cm H 2 O. The standard catheter for routine urodynamics is a transurethral double lumen catheter ( ). The urethral catheters should be as small as possible but not too small as to dampen pressure transmission or limit the desired filling rate. The smallest available is a six French double lumen catheter this allows the fill and void sequence to be repeated without recategorization.
More recently, air charged-catheters have become popular for pressure measurement (T-Doc, Wenonah, NJ). These catheters have a miniature air-filled balloons placed circumferentially around a polyethylene catheter. The catheters are disposable, single use. Practical advantages over fluid filled pressure lines revolve around the fact that there is no hydrostatic pressure effect to account for, so there is no need to position anything at the level of the symphysis pubis or to flush the system through to exclude air that is essential when using a fluid-filled system. With that said, many urodynamic “standard measurements” have been determined using fluid-filled systems. Although there is comparative evidence for the use of air-charged catheters to measure urethral pressure, to date there are no studies comparing air charged catheters to fluid-filled catheters for measuring intravesical and intraabdominal pressure.
Although it is beyond the scope of this chapter to discuss the various commercially available urodynamic systems, most are computer-based digital systems that allow for easy data storage and postprocessing of the study. Available systems can range in price from several thousand dollars to more than $100,000 for a video urodynamic system (see Chapter 11 ). Despite constant nuances in systems and software, the clinician performing the study remains the most important constant in data collection and interpretation. In certain clinical settings, a simple three-channel system ( Fig. 10.2 ) may be adequate, whereas large referral centers that evaluate complex cases or have a high percentage of patients with neurogenic bladder may require a more sophisticated system with the potential for video assessment ( Fig. 10.3 ). Recently urodynamic systems that are wireless and use Bluetooth technology have become available (Goby, Laborie; Williston, VT). These systems have the advantage of allowing more patient flexibility and privacy as the catheters are not tethered to the urodynamic machine, because a “wireless transmission” (up to 30 m) brings the signal from the transducer to the urodynamic machine.
Performing a Urodynamic Evaluation
Urodynamic procedures should be performed with a clear indication and with a specific question or questions that hopefully can be answered by the study. The procedure needs to be performed interactively with the patient and should include continuous and careful observation of the collected data. Artifacts are best corrected as they occur because they are often difficult to correct after the study is completed. It is valuable for the urodynamicist to have a clear understanding of the patient’s symptoms, practical experience with the equipment being used, and an understanding of quality control, with the ultimate goal of determining whether the questions that were attempted to be answered have been appropriately addressed. Obviously a prerequisite to this is a good understanding of pertinent anatomy and physiology of the lower urinary tract as well as biomechanics and physics of the urodynamic study. The test should be explained to the patients in advance. Although complications are rare, certain morbidities such as urinary retention, hematuria, urinary tract infection, and pain can occur.
Based on the American Urological Association (AUA) “Best Practice Policy Statement” assuming the preprocedure urine is negative for a urinary tract infection, preprocedure antibiotic prophylaxis is not indicated except in high-risk patients ( Table 10.2 ). The antimicrobial agents of choice are fluoroquinolones or trimethoprim/sulfamethoxazole. In patients who are allergic to both medications, the alternatives include a combination of aminoglycosides and ampicillin, first- or second-generation cephalosporin, or amoxicillin/clavulanate. If preprocedure urinary tract infection is diagnosed, the condition should be treated before the procedure. The American Heart Association no longer recommends the use of antibiotic prophylaxis before genitourinary procedures solely to prevent infective endocarditis. For patients who have had previous orthopedic surgery, there is no indication for antibiotic prophylaxis if the urine culture is negative and patients are not high risk.
| Advanced age |
| Anatomic anomalies of the urinary tract |
| Poor nutritional status |
| Smoking |
| Chronic corticosteroid use |
| Immunodeficiency |
| Externalized catheters |
| Colonized endogenous/exogenous material |
| Distant coexistent infection |
| Prolonged hospitalization |
Because micturition is normally a private act, the study setting should be quiet to provide as little distraction as possible. There should be a limited amount of observers as possible to minimize patient embarrassment.
A list of problems or questions that should be solved or answered by the urodynamic studies should be made before any testing is performed. A good practice is to decide on the questions before starting the study and then potentially design the study to answer those questions with the idea of customizing every study to a specific patient. It is also important to explain to the patient exactly why the tests are being performed and how the results may affect the treatment plan that will be proposed. The following is a brief description of the various tests that can be performed during a urodynamic evaluation.
- 1.
Postvoid residual (PVR) urine volume. This is an objective assessment of how well a woman empties her bladder. It can be performed by ultrasound or bladder scan or direct catheterization. An elevation of the PVR indicates a problem with emptying but does not really provide any information regarding the etiology of the dysfunction. A persistently elevated PVR may prompt further testing. Although no specific number has been agreed on to define what a normal PVR is, most would agree that a PVR less than 50 mL or a PVR less than 80% of the total bladder volume would be considered normal.
- 2.
Uroflowmetry is a measurement of the rate of urine flow over time ( Fig. 10.4 ). It is also an assessment of bladder emptying. A normal uroflow is a bell-shaped curve ( Fig. 10.5 ). When the flow rate is reduced or the pattern is altered, this may indicate bladder underactivity or bladder outlet obstruction.
FIGURE 10.4
Special commode and flowmeter used for spontaneous uroflowmetry.
FIGURE 10.5
Graphic representation of normal uroflow curve.
- 3.
Cystometrogram (CMG). Filling cystometry is the method by which the pressure/volume relationship of the bladder is measured during bladder filling. The test starts when filling commences and ends when the patient and urodynamicist decide that permission to void has been given. Although single-channel cystometry has been described, it is recommended that subtracted CMG be performed using the total vesical pressure and abdominal pressure by catheter placements in the bladder and rectum or vagina to calculate the true detrusor pressure, which is the vesical pressure minus the abdominal pressure. Also, leak point pressure measurements are obtained during filling cystometry.
- 4.
EMG is the study of electronic potentials produced by the depolarization of muscle membranes. In the case of urodynamics, EMG measurement is usually performed with patch electrodes, placed in close proximity to the striated sphincter muscles of the perineum. The test is mostly used to assure appropriate coordination between the pelvic floor muscles and lower urinary tract. EMG activity is measured during both filling and voiding.
- 5.
Urethral pressure profilometry (UPP). These are measurements of the intraluminal pressure along the length of the urethra. The urethral pressure is defined as the fluid pressure needed to just open a closed urethra. The UPP is obtained by withdrawal of a pressure sensor catheter along the length of the urethra.
- 6.
Pressure flow studies of voiding are the method by which the relationship between pressure in the bladder and urine flow rate is measured during bladder emptying. Detrusor pressure is measured with a simultaneous measurement of flow. The voiding phase starts when permission to void is given or when uncontrollable voiding begins and ends when the patient considers voiding has finished.
Figure 10.6 demonstrates a typical setup for a multichannel urodynamic assessment. The technique of performing a full urodynamic assessment in our laboratory is as follows:
- 1.
The patient presents with a symptomatically full bladder. She voids spontaneously in a uroflow chair. A PVR urine volume is obtained via a transurethral catheter.
- 2.
With the patient in the supine position on a urodynamic chair, the abdominal catheter is placed into the vagina and taped to the inside of the leg. If the patient has severe vaginal prolapse or has undergone previous vaginal surgery resulting in a narrowed vagina, the catheter is placed into the rectum. A dual microtransducer catheter with a filling port is then placed into the bladder. The patient is moved to a sitting position and the catheter secured to a mechanical puller (if urethral pressure studies are anticipated) or to the inside of the leg, so that the proximal transducer is near the mid-urethra (area of maximum urethral closure pressure).
- 3.
After the catheters are appropriately placed, the subtraction is checked by asking the patient to cough. Cough-induced pressure spikes should be seen on the intravesical pressure ( P ves ), intraabdominal pressure ( P abd ), and urethral pressure ( P ure ) channels, but not on the true detrusor pressure channel. If there is an inappropriate deflection on detrusor pressure ( P det ), it is usually secondary to inaccurate placement of the vaginal (or rectal) catheter. If repositioning the catheter does not correct the problem, all connections and calibration techniques should be rechecked.
- 4.
Bladder filling is begun. First sensation, first desire to void, and strong desire to void are recorded. Throughout the filling portion of the examination, the patient is asked to perform provocative activities, such as coughing and straining. The external urethral meatus is constantly observed for any involuntary urine loss. The first set of provocative maneuvers should be performed at a volume between 150 and 250 mL. If the patient has urodynamic stress incontinence leak point pressures can be obtained. Any abnormal rise in true detrusor pressure is noted. If the patient’s symptoms are reproduced during filling, then the test can be completed in the sitting position. If they are not, the patient should be asked to stand and repeat provocative maneuvers in an attempt to reproduce her symptoms.
- 5.
At the completion of filling, urethral pressure and flow studies can be performed, if indicated.
Figure 10.7 illustrates an example of filling and voiding urethrocystometry. No urodynamic abnormalities are noted in this study.
Interpretation of Specific Tests
Uroflowmetry
Uroflowmetry is noninvasive, inexpensive and is best used as a screening test for patients who may have voiding dysfunction. Many clinicians feel that uroflowmetry should routinely precede all other urodynamic studies. It is easy to perform and quickly provides data on both storage and emptying symptoms. The study should be conducted with as much privacy as possible with patients being asked to void when they feel they have a normal desire. Addition of a PVR measurement usually by ultrasound adds value to this study. Normal voiding includes a detrusor muscle contraction, coordinated bladder outlet relaxation, low voiding pressure, and a smooth arc-shaped flow curve ( ). It should be noted that uroflowmetry alone is insufficient to diagnose bladder outlet obstruction because it cannot distinguish true obstruction from poor detrusor contractility. Because small voided volumes affect the curved shape only voided volumes of at least 150 mL are interpretable ( Fig. 10.8 ). Because women have very short urethras with minimal outlet resistance, generally speaking the only factors influencing female outflow are the strength of the detrusor muscle, the urethral resistance, and the degree of relaxation of the sphincter mechanism. The most common conditions associated with increase in outlet resistance are previous anti-incontinence surgery or significant anatomic distortion usually from pelvic organ prolapse.
Urine flow may be described in terms of flow rate and flow pattern and may be continuous or intermittent. Flow rate ( Q ) is defined as the volume of fluid expelled via the urethra per unit time and is expressed in milliliters per second (mL/sec). Certain information is necessary in interpreting the tracing, including the volume voided, the environment and position in which the patient passed urine, whether the bladder filled naturally or by a catheter, and whether diuresis was stimulated by fluid or diuretics. If filling was by catheter, the type of fluid used should be stated; it should also be stated whether the flow study was part of another investigation. Maximum flow rate ( Q max ) is the maximum measured value of the flow rate after correction for artifacts. Voided volume is the total volume expelled via the urethra.
Flow time ( Q time ) is the time over which measurable flow occurs. Average flow rate ( Q ave ) is voided volume divided by flow time. The average flow should be interpreted with caution if flow is interrupted or if there is a terminal dribble. Time to maximum flow is the elapsed time from onset of flow to maximum flow.
Most experts agree that one can consider a study normal if the patient voids at least 200 mL over 15 to 20 s, and it is recorded as a smooth single curve with a maximum flow rate greater than 20 mL/sec ( Fig. 10.5 ). Maximum flow rates of less than 15 mL/sec, with a voided volume greater than 200 mL, are generally considered abnormal. However, because flow rate is determined by the relationship between detrusor force and urethral resistance, and because these factors may vary considerably and still produce adequate bladder emptying, a precise definition of a normal or a low flow rate cannot be made. The maximum urinary flow rate (MUFR) and average urinary flow rate are the two most important uroflowmetry parameters.
The clinical usefulness of uroflowmetry has been hampered by the lack of absolute values defining normal limits. These normal limits would need to be over a wide range of voided volumes ideally in the form of nomograms. performed uroflowmetry on 249 female volunteers between ages 16 and 63. Uroflowmetry was performed on each woman in a completely private environment, and a second uroflow study was obtained in 46 of those women. The MUFR and the average urinary flow rate of the first voids were compared with respective voided volumes. By using statistical formulations of both voided volumes and urine flow rates, relationships between the two variables were obtained. This allowed the construction of nomograms that are shown in Figure 10.9 . The previously described nomograms refer to free flowmetry voids and are not applicable to flow rates obtained during a pressure-flow study, because all urethral catheters can be expected to have the effect of decreasing urine flow rates for the equivalent voided volume. In 1999, Haylen et al. completed a further study of 250 symptomatic women who were consecutive referrals for urogynecologic assessment, which included urodynamics. Flow data for these women were converted to centiles from the Liverpool nomograms. They noted a decreased flow rate in symptomatic women, in general, and specifically in women with genital prolapse. In contrast to asymptomatic women, there is a decline in flow rates with age, and final urodynamic diagnosis shows that women with various combinations of stress incontinence, overactive bladder, or voiding dysfunction have flow rates that are significantly different than those of asymptomatic women.
Curve patterns refer to the configuration of the uroflowmetric curve. Continuous flow that shows a rapidly increasing flow rate that reaches the maximum within one third of the total voiding time is usually considered normal ( Fig. 10.10 A ). Figure 10.10 B demonstrates what has been termed a superflow pattern in which there is a very rapid acceleration to a high MUFR.
Flow is considered intermittent when the flow rate drops and subsequently increases. Intermittent flow rates are described as multiple-peak patterns when there is a downward deflection of the flow rate that does not reach 2 mL/sec ( Fig. 10.10 C ). When the downward deflection of the flow rate reaches 2 mL/sec or less ( Fig. 10.10 D ), an interrupted pattern occurs. Obstructed voiding patterns are much less common in women than in men and usually produce a low, flat tracing ( Fig. 10.10 E ).
Abnormal uroflowmetric parameters can occur secondarily to factors that affect detrusor contractility, urethral resistance, or both. Also inappropriate contraction of pelvic floor muscles can affect flow rates. Detrusor contractility can be affected by neuropathic lesions, pharmacologic manipulation, intrinsic detrusor muscle or bladder wall dysfunction, or psychogenic inhibition. Urethral resistance can be altered by tissue trophic changes producing atrophy or fibrosis, drug effects such as α-adrenergic stimulators or blockers, neuropathic striated muscle contraction, pain or fear, and urethral axis distortion secondary to severe pelvic organ prolapse. Outlet obstruction secondary to an intraurethral lesion or stricture is exceedingly rare in women. Extraurethral lesions, such as vaginal masses or cysts, or a large enterocele or rectocele may externally compress the urethra, resulting in obstructed voiding. Inability to relax pelvic floor muscles appropriately can cause prolonged flow times. This may occur after a traumatic delivery or pelvic floor reconstructive surgery as well as any disease state that causes vaginal pain. Detrusor-external sphincter dyssynergia is a condition in which there is lack of coordination between the detrusor muscle and the external striated urethral sphincter. This leads to obstructed voiding and is always secondary to a neurologic lesion, most classically high spinal cord trauma (see Chapter 11 ). In the nervous and anxious but neurologically intact patient, the urethra may be closed by pelvic floor contracture. Urethral closure may be due to contraction of the intraurethral striated muscle or a contraction of the pelvic floor musculature. This condition has been termed dysfunctional voiding.
If uroflow measures are normal in patients complaining of symptoms of voiding difficulty, further investigation is unnecessary, but abnormal flow rates may require further urodynamic testing.
Uroflowmetry is a simple urodynamic investigation that is useful as a preliminary screening test to distinguish patients who need more extensive studies from those who do not. Uroflowmetry is also an integral part of a full urodynamic study performed for more complex problems.
Filling Cystometry
Cystometry is the urodynamic study used to describe the pressure/volume relationship of the bladder. One should also collect information on bladder sensation during filling. Normal bladder sensation is usually judged at three defined points: a first sensation of bladder filling, a first desire to void, and a strong desire to void ( Fig. 10.11 ). Although specific parameters are not given for these various changes in sensation, increased bladder sensation is defined as an early first sensation of bladder filling or an early strong desire to void that occurs at what is felt to be a low bladder volume and ultimately persists, while reduced bladder sensation is a diminished sensation through bladder filling. Detrusor pressure during filling should be very low with no involuntary contractions. Detrusor overactivity (DO) is defined as involuntary detrusor contractions during filling that may be spontaneous or provoked ( Fig. 10.12 ). DO is classified as either neurogenic when there is a relevant neurologic condition or idiopathic when there is no identifiable cause (see Appendix). Bladder compliance describes the relationship between the change in bladder volume and change in detrusor pressure. Compliance is calculated by dividing the volume change Δ V by the change in detrusor pressure Δ P det during that change in bladder volume. It is expressed in mL/cm H 2 O. Although it is difficult to define what normal compliance is in terms of mL/cm H 2 O, several authors have shown the mean values for compliance in healthy subjects range from 46 to 124 mL/cm H 2 O ( ). Because of these wide-ranging values, it is more practical from a clinical standpoint to use absolute pressure instead of a compliance number. For example, it has been shown that urine storage pressure greater than 40 cmH 2 O can be associated with harmful effects to the upper tract ( ). Also, depending on the clinical scenario, compliance in terms of mL/cm H 2 O can mean different things. However, in general, prolonged storage at high pressures can lead to upper tract deterioration.
When there appears to be an abnormal rise in true detrusor pressure, the urodynamicist must determine if this rise is due to poor bladder compliance ( Fig. 10.13 ) or best classified as DO ( Fig. 10.14 ). Abnormal bladder compliance is going to be more common in certain subsets of patients, including women with neurogenic disease, a history of radiation or multiple bladder surgeries, multiple bladder tumor resections, a defunctionalized bladder after long-term anuria, long-term indwelling catheterization, recurrent bladder infections, or with long-standing bladder outlet obstruction. In patients with abnormal bladder compliance during filling cystometry, there will be a steep rise in detrusor pressure that is usually parallel to bladder filling. Abnormal compliance is related to filling; DO is not. Most of the time if you stop infusing fluid the detrusor pressure will plateau in cases of low compliance but continues to rise toward the peak with DO ( Figs. 10.11 and 10.12 ). With continuation of filling, assuming there is no “pop-off” affect as noted by leakage from the urethra or vesicoureteral reflux (see Chapter 11 ), the detrusor pressure will continue to rise in low compliance but a detrusor contraction will usually end, resulting in a significant drop in detrusor pressure ultimately returning to the baseline pressure before the DO.
