Normal bladder physiology

A prerequisite to performing and interpreting a urodynamic study is having a full understanding of normal LUT function (see Chapter 3 ). 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 ( P _det ). 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. Fig. 11.1 demonstrates a storage and micturition reflex in a neurologically intact female.

A , Typical storage reflex in a neurologically intact woman; note increased electromyographic ( EMG ) activity during filling, coughing, straining, and an uninhibited detrusor contraction. This indicates an intact synergistic pelvic floor response. B , Typical micturition reflex in a neurologically intact woman. Note complete loss of EMG activity simultaneous with an increase in detrusor pressure at initiation of voiding. IDC , Involuntary detrusor contraction; P _abd , intraabdominal pressure; P _det , detrusor pressure; P _ure , urethral pressure; P _ves , intravesical pressure.

Functional classification of lower urinary tract dysfunction

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 LUT dysfunction as proposed and popularized by is a simple, clinically relevant way of classifying LUT dysfunction. Wein proposed that functional abnormalities of the LUT 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 (sphincter deficiency), 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 LUT dysfunction. The advantage of a simple classification such as this is 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 Box 11.1 for a variety of disease states categorized using this classification.

Role of urodynamics in clinical practice

Although urodynamic testing has been frequently used in the evaluation of women with LUT dysfunction, level 1 evidence–based “indications” for its use are surprisingly few. The clinical question that must always be asked is whether the results of a urodynamic evaluation will 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, to assist in making an appropriate treatment plan, 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 LUT dysfunction and assess their relative importance.

  
  
• 2.
  

  To obtain information about other aspects of LUT dysfunction.

  
  
• 3.
  

  To predict the consequences of LUT dysfunction on the upper urinary 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 LUT function in general.

Uroflowmetry

Uroflowmetry is noninvasive and 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. The addition of a PVR measurement, usually by ultrasound, adds value to this study. Normal voiding includes 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 BOO, 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. 11.8 ). 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 antiincontinence surgery or significant anatomic distortion, usually from pelvic organ prolapse.

Uroflow curve from a void of only 90 mL. Data obtained are not useful, because the voided volume was less than 150 mL.

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/s). 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 is the time over which measurable flow occurs. Average flow rate 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 a study can be considered normal if the patient voids at least 200 mL over 15 to 20 seconds and if it is recorded as a smooth single curve with a Q _max greater than 20 mL/s ( Fig. 11.5 ). Q _max values of less than 15 mL/s 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 years. 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, which are shown in Fig. 11.9 . 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 consecutively referred for urogynecologic assessment, which included urodynamics. Flow data for these women were converted to centiles from the Liverpool nomograms. The authors 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 (OAB), or voiding dysfunction have flow rates that are significantly different than those of asymptomatic women.

A , Liverpool maximum flow rate nomogram for women; curves are centiles. B , Liverpool average flow rate nomogram for women; curves are centiles.

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. 11.10 A). Fig. 11.10 B demonstrates what has been termed a superflow pattern, in which there is a very rapid acceleration to a high MUFR.

Graphic representation of various uroflow patterns. A , Normal curve; voided volume of 145 mL; maximum urinary flow rate (MUFR) 23 mL/s. B , Superflow pattern; voided volume of 330 mL; MUFR 49 mL/s. C , Intermittent flow rate with multiple peak pattern. This tracing is characteristic of voluntary urethral sphincter activity. D , Intermittent flow rate with interrupted pattern. This is a characteristic pattern seen when abdominal pressure is used to expel urine. E , Reduced urine flow rate secondary to detrusor outflow obstruction.

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/s ( Fig. 11.10 C). When the downward deflection of the flow rate reaches 2 mL/s or less ( Fig. 11.10 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. 11.10 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 alpha-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, or as part of any disease state that causes vaginal pain. Detrusor-external (or striated) 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. In the nervous and anxious but neurologically intact patient, the urethra may be closed by pelvic floor contracture. Urethral closure may be attributed to contraction of the intraurethral striated muscle or 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. 11.11 ). 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, whereas reduced bladder sensation is a diminished sensation through bladder filling. P _det 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. 11.12 and Video 11.2 ). DO is classified as either neurogenic (when there is a relevant neurologic condition) or idiopathic (when there is no identifiable cause). Bladder compliance describes the relationship between the change in bladder volume and change in P _det . Compliance is calculated by dividing the volume change by the change in 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 that 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 cm H ₂ O can be associated with harmful effects to the upper tract ( ). In general, prolonged storage at high pressures can lead to upper tract deterioration.

Filling cystometry; first sensation, fullness, and maximum capacity are noted. Detrusor remained stable during filling, and no stress incontinence was demonstrated.

Filling cystometry in a patient with detrusor overactivity; note that two uninhibited bladder contractions occurred during the filling portion of the test.

When there appears to be an abnormal rise in true P _det , the urodynamicist must determine if this rise is caused by poor bladder compliance ( Fig. 11.13 ) or best classified as DO ( Fig. 11.14 ). 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 long-standing BOO. In patients with abnormal bladder compliance during filling cystometry, there will be a steep rise in P _det 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 P _det will plateau in cases of low compliance but continue to rise toward the peak with DO ( Fig. 11.11 and 11.12 ). With continuation of filling, assuming there is no “pop-off” effect, as noted by leakage from the urethra or vesicoureteral reflux, the P _det will continue to rise in low compliance, but a detrusor contraction will usually end, resulting in a significant drop in P _det ultimately returning to the baseline pressure before the DO (Videos 11.2 and 11.3 ).

Multichannel filling cystometry in a patient with poor bladder compliance. Note steady rise in true detrusor pressure with plateaus when filling stops and increases when filling is resumed.

Multichannel filling cystometry in a patient with detrusor overactivity. Note that detrusor pressure returns to baseline after completion of contraction.

The condition of urodynamic stress incontinence is defined as involuntary loss of urine simultaneous with a rise in abdominal pressure in the absence of any rise in true P _det ( Fig. 11.15 ; Video 11.4 ). This diagnosis is made during filling cystometry. Provocation in the form of coughing, straining, and heel bouncing in the sitting (and, if necessary, standing) position should be performed at different volumes throughout the testing session. Once the diagnosis is made, Valsalva and cough leak point pressure measurements can be obtained if felt to be clinically indicated.

Filling cystometry demonstrating the condition of urodynamic stress incontinence. Urine leakage occurred simultaneous with a cough (rise in intraabdominal pressure) in the absence of any rise in true detrusor pressure.

Leak point pressures

There are two distinct types of leak point pressures that can be measured in an incontinent patient. The first is an abdominal leak point pressure sometimes termed either a Valsalva leak point pressure (VLPP) or cough leak point pressure; the second is a detrusor leak point pressure (DLPP). These pressures are independent of each other and conceptually measure completely different things. An abdominal leak point pressure, whether measured during cough or Valsalva, is a measure of the intrinsic sphincter strength of the urethra or its ability to resist leakage with increases in abdominal pressure. It is specifically defined as the intravesical pressure at which urine leakage occurs because of an increased abdominal pressure in the absence of a detrusor contraction. Thus, it is an objective assessment of the amount of abdominal pressure required to overcome outlet resistance and create leakage. This test is applicable to patients with urodynamic stress incontinence. Theoretically, the lower the abdominal leak point pressure, the weaker the sphincter. There is no normal abdominal leak point pressure, because patients without stress incontinence will not leak at any volume or physiologic abdominal pressure. The pressure should be a measure of the total (absolute) abdominal pressure required to cause leakage, not the change in pressure. The reading is taken from the vesicle channels, because it is usually a cleaner channel to obtain the pressure, assuming there are no involuntary contractions of the detrusor. A Valsalva effort is the most reliable method of provocation; however, if the patient cannot Valsalva on command, coughing can be used ( Figs. 11.16 and 11.17 ; Video 11.5 ).

A , Graphic representation of Valsalva leak point pressure measurement at bladder volumes of 150 and 300 mL. Leak point pressures tend to decrease with increased bladder volume. B , Graphic representation of cough leak point pressure at bladder volumes of 150 and 300 mL. Note the difficulty involved in isolating the cough that generates the minimal amount of abdominal pressure required to produce leakage.

Filling cystometry. Valsalva leak point pressure of 92 cm H ₂ O is noted in sitting position at a volume of 187 mL.

The second type of leak point pressure is the DLPP, which is a measure of P _det in patients with decreased bladder compliance. It is defined as the lowest P _det at which urine leakage occurs in the absence of either a detrusor contraction or an increase in abdominal pressure ( Video 11.6 ). From a clinical perspective, a DLPP is most useful in patients with lower motor neuron disease causing decentralization, and rarely in nonneurogenic patients with abnormal bladder compliance, which can occur after multiple bladder surgeries, radiation therapy, tuberculosis cystitis, and other rare situations or conditions. In general, the higher the DLPP, the more likely the upper urinary tract could be damaged because of intravesical pressure being transferred to the ureters and the kidneys. showed that DLPPs greater than 40 cm H ₂ O would result in hydronephrosis or vesicoureteral reflux in 85% of myelodysplastic patients.

Urethral pressure profilometry

Urethral pressure profilometry (UPP) indicates the intraluminal pressure along the length of the urethra with the bladder at rest. The maximum urethral pressure is the maximum pressure of the measured profile. The maximum urethral closure pressure is the difference between the maximum urethral pressure and the intravesical pressure ( Fig. 11.18 ). The functional urethral length (FUL) is the length of the urethra along which the urethral pressure exceeds the intravesical pressure, and the anatomic urethral length is the total length of the urethra. The pressure transmission ratio is the increment in urethral pressure on stress as a percentage of the simultaneously reported increment in vesical pressure.

Technique of static urethral pressure profilometry with simultaneous measurement of bladder pressure. Study begins with both microtransducers in the bladder ( top ). As the catheter is mechanically withdrawn through the urethra ( middle and bottom ), urethral and bladder pressures are recorded.

UPP involves the withdrawal of a catheter through the urethra at a constant rate of speed. Pressure is measured along the entire urethral length, as well as in the bladder. UPP can be performed using open or balloon catheters perfused with liquid or gas, catheter-mounted (microtip) transducers, or fiberoptic catheters, as well as air-charged catheters ( Fig. 11.18 ).

Normal values for maximal urethral closure pressure (MUCP) and FUL vary widely among women; however, most continent women will have an FUL of approximately 3 cm and an MUCP between 40 and 60 cm H ₂ O. Although many authors have used an MUCP of less than 20 cm H ₂ O to define intrinsic sphincter deficiency (ISD) in women with urodynamic stress urinary incontinence (SUI), this definition has many of the same problems that hamper the definition and diagnosis of ISD.

In 2002, the ICS standardization subcommittee concluded that the clinical utility of UPP is unclear ( ), stating that there are no urethral pressure measurements that:

• 1.
  

  Discriminate urethral incompetence from other disorders.

  
  
• 2.
  

  Provide a measure of the severity of the condition.

  
  
• 3.
  

  Provide a reliable indicator to surgical success.

Electromyography

EMG studies have become popular components of a urodynamic assessment. Muscle depolarization is detected by an electrode placed in or near a muscle. The test can be done with needle or surface electrodes; however, surface electrodes with self-adhesive skin patches that are applied over the skin of the anal sphincter are the most commonly used during a urodynamic assessment. The surface electrodes have a significant advantage over needle electrodes regarding patient convenience and comfort, as well as ease of doing the test. Although using a concentric needle electrode is a superior technique for obtaining a signal source of EMG activity of the striated external urethral sphincter muscle, the information obtained by the perianal surface electrodes will suffice in the evaluation of most women with LUT dysfunction. In a neurologically intact individual, EMG activity should increase during filling and provocation to prevent any leakage. Normal voiding requires external sphincter relaxation followed by contraction of the detrusor. Failure of the sphincter to relax or stay completely relaxed during micturition is abnormal. Thus, normally, EMG activity decreases before a voluntary bladder contraction; however, it is not abnormal for EMG activity to increase with an involuntary contraction as part of a guarding reflex to inhibit the detrusor contraction. Some question the clinical utility of routine EMG during urodynamic performance, and others argue that it remains a good objective method to assure appropriate synergistic responses during filling, voiding, and provocation.

Pressure-flow studies

Because uroflowmetry can provide only limited information, pressure-flow studies represent a natural progression. However, if uroflowmetry is normal, the information obtained in a pressure-flow study is largely unnecessary. In contrast, in those with voiding symptoms and an abnormal MUFR and/or peak flow velocity, pressure-flow studies are necessary to determine the cause. Flow rate depends on both the outlet resistance and the contractile properties of the detrusor, as well as the volume of fluid in the bladder. A low flow rate may be associated with a high voiding pressure or a below-normal voiding pressure. Similarly, the finding of a normal flow rate does not exclude BOO, because normal flow may be maintained by a high voiding pressure.

Some women have normal flow rates in the absence of a detrusor contraction. This may be because sphincteric relaxation, either alone or assisted by increased intraabdominal pressure from straining, is sufficient to produce a normal flow rate. Pressure-flow studies are essential for a complete functional classification of LUT disorders and an objective assessment of the basis of a patient’s voiding dysfunction.

These studies are usually performed after a cystometric evaluation. In most patients, it is clear when bladder filling should be stopped. However, if the patient has little sensation, it is important to use the functional bladder capacity from the frequency–volume chart as a guide to cystometric capacity. At this point, if a separate filling catheter is being used, it is removed; the intravesical catheter is left in place. An intravaginal or intrarectal catheter records intraabdominal pressure to assess whether the patient uses a Valsalva maneuver to void and to electronically derive true P _det . Thus, these studies involve the monitoring of abdominal, intravesical, and true P _det synchronously with flow ( Fig. 11.19 and 11.20 ). External sphincter EMG activity and urethral pressure also may be measured. To ensure that proper pressure transmission is occurring, the patient should be asked to cough before being allowed to void. With the patient in a sitting position, she is then instructed to void to completion if possible. It is important during the voiding phase to respect the patient’s privacy. Few women are able to void in the presence of others, so it may be necessary for the practitioner to leave the room for her to initiate voiding. Video 11.7 demonstrates a voiding mechanism with an appropriate detrusor contraction, and Video 11.8 demonstrates a voiding mechanism with significant Valsalva effort.

Pressure-flow study in a patient who voids with urethral relaxation and bladder contraction. Note minimal Valsalva effort.

Pressure-flow study in a patient who has voiding dysfunction after suburethral sling procedure. The study is consistent with outlet obstruction in that the flow is reduced and detrusor pressure is elevated.

Pressure-flow studies are invasive because the patient is asked to void around catheters. It is important to appreciate the limitations of pressure-flow studies, as well as the differences between the patient’s performance during urodynamic testing and her normal voiding. This is best judged by asking the patient and by comparing the noninstrumented urine flow rate with the flow rate obtained from the pressure-flow study.

Voiding in a urodynamic laboratory can be affected by a variety of factors. It is estimated that approximately 30% of women who void without problems at home are unable to void on command in the urodynamic laboratory. This is hardly surprising, because they are surrounded by complex equipment, have catheters in their bladder and vagina or rectum, and are usually being observed by strangers.

Fast filling or overfilling of the bladder may make normal voiding difficult. Studies that compared ambulatory urodynamics (natural bladder filling) with conventional urodynamics show that voiding pressures are higher with natural filling. This implies that the detrusor may be incompletely stimulated, partially inhibited, or mechanically less efficient if it is overfilled or filled too fast.

During the voiding phase, the detrusor muscle may be normal, acontractile, or underactive. Normal voiding is usually achieved by a voluntarily initiated continuous detrusor contraction that is sustained and can be suppressed. An underactive detrusor during micturition implies that the detrusor contraction is of inadequate magnitude or duration (or both) to effect bladder emptying within a normal time span.

During voiding, urethral function may be normal or abnormal. A normal urethra opens during voiding and is continuously relaxed to allow the bladder to be emptied at a normal pressure. Abnormal urethral function may be secondary to urethral overactivity or a mechanical obstruction, such as a urethral stricture or tumor. BOO, regardless of the cause, is characterized by increased P _det and reduced urine flow rate, seen by observing the synchronous values of flow rate and P _det during voiding ( Video 11.9 ). In mechanical obstruction, which is rare in females, the voiding pressures are constantly elevated. If the obstruction is caused by urethral overactivity, the voiding pressures may fluctuate. Urethral overactivity is characterized by the urethra contracting during voiding or the urethra failing to relax. In detrusor external sphincter dyssynergia (DESD), the patient’s phasic contractions of the intrinsic urethral striated muscle are simultaneous with the detrusor contraction. This produces a very high voiding pressure and an interrupted flow. The urodynamic characteristic of this type of urethral overactivity is a fall in flow rate accompanied by a rising P _det that then falls when the urethra relaxes, leading to a resumption of urine flow. Another form of urethral overactivity is called dysfunctional voiding. This is most commonly seen in children who are neurologically normal but complain of urinary incontinence or recurrent infections. The interrupted flow in these children is caused by pelvic floor overactivity rather than contractions of the intrinsic striated muscle.

Depending on age, menopausal status, total voided volume, and the presence or absence of LUT dysfunction, women void by any combination of a detrusor contraction, abdominal straining, and urethral relaxation.

It is difficult to ascertain whether abdominal straining that occurs during a pressure-flow study is real or artifactually induced by the surroundings and presence of indwelling catheters. The patient should always be asked to void normally and in as relaxed a way as possible. If the patient has an acontractile detrusor, voiding can be achieved only by straining. If the detrusor contracts during voiding, but the patient also strains, then the tracing is more difficult to interpret. It is also difficult to understand precisely what effect straining has on urine flow. In patients without obstruction, straining increases flow, but it does not produce the same increase in flow as that achieved by a P _det rise of the same magnitude (Video 11.8 ).

Although pressure-flow studies are an established and accepted urodynamic modality, what constitutes a normal voiding mechanism is poorly understood, as is the normal range for P _det during voiding in women. Most of the previously published literature has been derived from male subjects in whom pressures are abnormally high secondary to outflow obstruction.

Because there is no highly prevalent condition such as benign prostatic hypertrophy in women that causes female obstruction, it has been very difficult to establish nomograms. Because voiding dynamics differ in women, nomograms derived for men cannot be used in women. As mentioned, anatomic differences allow women to void by simply relaxing their pelvic floor, or by increasing abdominal pressure by straining. Small elevations in P _det or decreases in flow rates, which might be considered insignificant in a male, may be consistent with BOO in a female. Thus, clinicians should always have a high index of suspicion for BOO in women with a prior history of an antiincontinence procedure, chronic cystitis, pelvic organ prolapse, or incomplete bladder emptying.

Numerous investigators have attempted to establish cutoff values for pressure and flow to diagnose outlet obstruction in women ( ; ). These studies, however, have some limitations in that obstruction was predefined clinically, and only patients with anatomic obstruction were included. A study by used normal asymptomatic women as the control group and found the highest sensitivity and specificity for predicting obstruction was obtained with Q _max less than 12 mL/s and P _det Q _max greater than 25 cm H ₂ O. created a nomogram defining voiding obstruction in women using Q _max from a noninstrumented uroflow and a maximum P _det from the pressure-flow study. More recently, an analysis of the urodynamic data from the Trial of Mid-Urethral Slings (TOMUS) study noted that the difference between these two values increases as voided volumes increase ( ), concluding that values from a pressure-flow study and noninstrumented uroflow cannot be used interchangeably, as has been suggested by the Blaivas–Groutz nomogram for obstruction in women.

The main clinical use of pressure-flow studies is to document the mechanism of abnormal voiding. If a patient has symptoms and signs of abnormal voiding, has low flow rates, and voids with a high P _det , she is probably voiding against an obstruction. On the other hand, if a patient has low flow rates and voids with minimal or no rise in P _det , then her voiding dysfunction is probably secondary to an acontractile or underactive detrusor. As previously mentioned, the limiting factor is that there is no clear cutoff between normal and abnormally high P _det during voiding.

The clinical setting in which pressure-flow studies are most useful in women is in the patient who has undergone pelvic surgery and has developed postoperative voiding dysfunction. The dysfunction may be secondary to denervation, resulting in an underactive or acontractile detrusor, but more commonly the dysfunction is secondary to increased outlet resistance produced from the surgery. Voiding dysfunction or retention occurs in 3% to 20% of patients after various operations to correct stress incontinence. It is always a clinical dilemma whether urethrolysis or a takedown of the repair will restore normal voiding. Filling cystometry and pressure-flow studies may be helpful in this setting. If the patient is able to void around a catheter and is noted to have a high P _det with a low flow rate ( Fig. 11.21 ), then she is obstructed, and relieving the obstruction should improve voiding ( ).

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