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Traces: making sense of urodynamics testing--Part 10: evaluation of micturition via the voiding pressure-flow study.

Part 10 of the Traces series focuses on urodynamics evaluation of micturition via the voiding pressure-flow study (VPS). This article reviews techniques for generating a high-quality voiding pressure-flow study, and initial interpretation of findings. Part 11 of the Traces series will focus on advanced analysis of the VPS using nomograms that measure the magnitude of urethral resistance (bladder outflow obstruction) and detrusor contraction strength.

Key Words: Urodynamics, voiding pressure-flow study, urethral resistance, bladder outflow obstruction, detrusor contraction strength.

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Our knowledge of the physiology of micturition in the human is surprisingly sparse. In the healthy, continent adult, micturition occurs as the result of a voluntary decision. Functional imaging studies of the brain demonstrate that the prefrontal cortex and anterior cingulate gyrus are activated during voluntary voiding, along with specific neurons in the hypothalamus and periaqueductal gray, an area in the brainstem known to coordinate neural modulatory centers in the upper brain with those of the brainstem (Griffiths, Derbyshire, Stenger, & Resnick, 2005). These events are associated with the activation of the pontine micturition center, which inhibits sympathetic outflow to the lower urinary tract, while also activating parasympathetic outflow to the bladder wall, causing contraction of the detrusor muscle, along with relaxation of smooth muscle in the urethral sphincter mechanism (Yoshimura & Chancellor, 2012). Activation of the pontine micturition center also causes reflex relaxation of the striated muscle of the urethral sphincter mechanism and periurethral striated muscle. The precise mechanisms that determine the maximum amplitude of a given detrusor contraction and sustain the detrusor contraction sufficiently for complete evacuation of the vesicle, as well as the afferent signals that alert the individual to the end of micturition, are not well understood.

Urodynamic evaluation reveals the process of micturition begins with relaxation of the pelvic floor and periurethral striated muscles that are observed on the pelvic floor electromyography (EMG). Between 5 and 15 seconds later, Pdet will rapidly rise, indicating the onset of a voluntary detrusor contraction. Uroflow begins when the detrusor contraction overcomes urethral resistance, and it is sustained until the bladder vesicle is empty or nearly empty of urine. At this point, uroflow stops, followed by a gradual decline in Pdet, indicating relaxation of the detrusor muscle, and a subsequent increase in baseline pelvic floor muscle activity.

Urodynamic Assessment Of Micturition

The voiding pressure-flow study (VPS) is considered the best available study for evaluating micturition. It requires simultaneous measurement of intravesical, abdominal, and detrusor pressures combined with uroflowmetry with or without pelvic floor EMG (Griffiths et al., 1997). Data generated by the VPS are used to determine the uroflow pattern, detrusor contraction strength, urethral resistance, and magnitude of any obstruction. The addition of pelvic floor EMG allows assessment of striated sphincter and pelvic floor muscle response to micturition.

The urodynamic clinician generates a VPS by asking the patient to urinate following a filling cystometrogram. Voiding is requested after the patient perceives an imminent desire to urinate or when terminal detrusor overactivity results in involuntary voiding. The patient should be placed in an upright position, either sitting or standing, so he or she is able to urinate into a funnel positioned over the uroflow transducer. Women are accustomed to voiding in a sitting position and should be allowed to urinate in this position during the VPS. In contrast, adult men tend to void in a standing position, and limited evidence reveals that male patients tend to void more effectively in a standing position (Choudhury et al., 2010). However, this must be carefully weighed against the risk of a vasovagal response and potential fall when voiding during urodynamic testing. Therefore, it is recommended for both men and women to void while seated unless the male patient is only able to effectively urinate while standing (Gray, 2010).

Most VPS studies are generated with a urethral catheter, and the urodynamic clinician should select a smaller catheter for this procedure, ideally 7-French or smaller. Klingler, Madesbacher and Schmidbauer (1996) compared the effects of 10-French versus 5-French urethral catheters in 41 men with prostatic enlargement and obstruction, and 21 men without obstruction. They found that the 10-French catheter significantly diminished maximum flow rate (Qmax) and detrusor pressure at Qmax (Pdet@Qmax) along with urethral resistance measured by Schaefer's nomogram when compared to values generated using a 5-French urethral catheter. Klausner, Galea, and Vapnek (2002) also compared the effect of 10-French versus 5-French urethral catheters in a group of 31 men with prostatic enlargement; they found that the use of the larger catheter led to a false-positive diagnosis of obstruction in 10 (32%) and a false-positive diagnosis of a higher grade of obstruction in 17 (55%).

Reynard, Lim, Swami, and Abrams (1996) compared the effect of 8-French catheters versus no catheters in a group of 59 men with varying degrees of bladder outlet obstruction and found no statistically significant results when the Qmax rates were compared. They reported a significantly greater maximum Pdet during micturition but no statistically significant difference in the urethral resistance algorithm, a calculated value used to determine the magnitude of bladder outlet obstruction.

In contrast to these findings, Zhao and colleagues (2007) compared Qmax and urethral resistance in a group of 39 men with prostatic enlargement and obstruction to uroflowmetry without any catheter. They reported a significantly lower Qmax and higher degree of obstruction among men with moderately severe to severe obstruction grades (III-V). These data must be compared with care to the findings of Reynard, Lim, and colleagues (1996) because they focus on men with more severe obstruction where the introduction of smaller catheters is expected to produce a more significant clinical effect.

Groutz, Blaivas, and Sassone (2000) retrospectively evaluated the effect of a 7-French urethral urodynamic catheter in 6000 women with a variety of lower urinary tract symptoms. They reported differences in the flow pattern in 43% of these women. Specifically, women were more likely to generate an interrupted flow pattern during the VPS as compared to non-instrumented uroflowmetry. Costantini and coinvestigators (2005) evaluated the effects of 9-French catheters in a group of 126 women to 7-French catheters in a group of 126 women. They reported significantly diminished Qmax in women with 7- and 9-French catheters, but also found that catheter size did not act as an independent predictor of diminished Qmax.

Considered collectively, the results of these studies suggest that a smaller urethral catheter should be used whenever possible to generate a VPS. When compared to non-instrumented uroflowmetry, 10-French catheters were consistently found to lower Qmax, and to raise Pdet@Qmax and urethral resistance in both non-obstructed and obstructed patients. Even smaller catheter sizes may produce a more obstructed effect when compared to a non-instrumented uroflowmetry in patients with more severe obstruction. The VPS is nevertheless valuable in these patients because it provides an assessment of moderately severe to severe obstruction along with measurement of detrusor contraction strength. Groutz and colleagues (2000) observed that women are more likely to void with an interrupted flow pattern during VPS as compared to non-instrumented uroflowmetry. Urodynamic clinicians are therefore encouraged to compare uroflow patterns based on free uroflowmetry versus VPS and reproduce the VPS if necessary until the non-instrumented flow pattern has been reproduced.

Generating accurate and reproducible results in the context of the urodynamic laboratory is challenging. Micturition is an intensely private activity. In developed societies, voiding occurs only in highly circumscribed situations, such as a private or public toilet, and urination in public is socially taboo and often illegal (USLegal.com, 2012). These psychosocial and legal restrictions of voiding behavior profoundly influence patients undergoing urodynamic testing when asked to urinate in the urodynamic test setting. Creating an environment that promotes privacy and maximal comfort in this highly unusual setting is essential. Arranging the urodynamic test setting so that the micturition seat does not directly face the door to the room is recommended, combined with strategies to render the environment less clinical and a system for maximizing privacy when asked to urinate. Facing the micturition chair away from the entrance is recommended because clinical experience strongly suggests that opening the door while a patient is attempting to void greatly destroys perceptions of privacy and dramatically reduces the likelihood of a robust and reproducible micturition episode.

Strategies to reduce the "clinical" environment include placement of a non-threatening picture on the wall or even on the ceiling easily viewed by the patient during preparations for urodynamic testing. Soothing music may be played to reduce patient awareness of other activities in the clinical area and to promote pelvic floor muscle relaxation in preparation for micturition. Maximizing privacy includes reducing the number of clinicians in the urodynamic setting to essential persons, establishing expectations that clinicians or staff will not enter the urodynamic test setting during testing unless absolutely necessary, and removing telephones from the room and muting or switching cellular phones to the off setting. One family member is encouraged when evaluating a child, but clinical experience reveals that allowing one or more family members when evaluating an adult may paradoxically inhibit the patient's ability to generate a robust VPS.

Ideally, the infusion pump is turned to the off position and all clinicians leave the room when the patient completes the VPS study. A small bell can be left within the patient's reach and can be used to alert the clinician when voiding is completed. In some cases, it may not be possible for everyone to leave the room (for example, during videourodynamic testing when a radiology technologist generates a voiding urethrogram, or when a younger child may need reassurance and encouragement to initiate voiding). In this case, taking up a position in the room away from the patient's direct line of vision is recommended, along with gentle and persistent encouragement to urinate. Clinical experience suggests that despite maximal privacy and encouragement, a minority of patients (< 10%) who normally empty their bladders via urination will be unable to void in the urodynamic setting. In this case, allowing the patient to complete micturition in a non-instrumented uroflowmetry setting followed by measurement of a postvoid residual via bladder ultrasound or catheterization is recommended.

Generating and Interpreting Results

The VPS generates multiple variables that may be used to analyze micturition (see Table 1 and Figure 1). This article discusses only the most commonly used parameters in clinical practice. Identification of the uroflow pattern is based on the same criteria described in Part 2 of the Traces series (Gray, 2010). Common uroflow variables generated by the VPS include the maximum and average flow rates and the voided volumes. The clinician augments these variables by measuring the post-void residual volume either via catheterization or bladder ultrasound. The uroflow curve is visually inspected to identify the flow pattern. Three patterns are described:

continuous, interrupted (sometimes called intermittent), and prolonged (Kelly & Krane, 2000; Nielsen, 1995). A continuous flow pattern is consistent with normal voiding function, in contrast to the interrupted or prolonged flow patterns that may indicate abnormal micturition associated with poor detrusor contraction strength, bladder outlet obstruction, or a combination of these factors. Uroflowmetry alone does not allow the clinician to identify the cause of an abnormal flow pattern with or without an elevated post-void residual volume (Chancellor, Blaivas, Kaplan, & Axlerod, 1991). However, combining this analysis with the assessment of detrusor contraction strength and urethral resistance enables the urodynamic clinician to identify the cause of abnormal micturition.

[FIGURE 1 OMITTED]

The initial evaluation of detrusor contraction strength is based on the assessment of two parameters: amplitude and duration. The amplitude of a detrusor contraction, referred to as the maximum detrusor pressure by the International Continence Society (ICS), is defined as the maximum detrusor pressure generated during micturition (Griffiths et al., 1997). It is measured using the Pdet tracing and expressed in centimeters of water (cm [H.sub.2]O). The duration of the detrusor contraction is defined as the time that elapses between its onset and return to baseline Pdet following the end of micturition. It can be measured in seconds. While technically feasible, few clinicians routinely measure the duration of the detrusor contraction because it can be difficult to determine the precise point of its onset and the precise point when Pdet returns to baseline, especially in the patient with bladder outlet obstruction and incomplete bladder emptying. Instead, most clinicians qualitatively assess this contractility parameter based on visual inspection of the contraction and associated uroflowmetry data.

The ICS defines three categories of detrusor contractility during micturition: normal, underactive, and acontractile. Normal detrusor contraction strength is sufficient to result in complete bladder evacuation in the absence of bladder outlet obstruction. Nevertheless, the amplitude and duration of the detrusor contraction vary based on urethral resistance. Individuals with higher urethral resistance, such as late middle-aged men, tend to have greater urethral resistance as compared to post-menopausal women. As a result, the late middle-aged male will tend to have a higher contraction amplitude that persists for a longer period of time than an age-matched woman. When confronted by increased urethral resistance and bladder outlet obstruction, the amplitude of the detrusor contraction will be higher and its duration longer regardless of gender.

The urethral resistance relationship is initially assessed by visually comparing detrusor pressure and urethral flow. Several variables are used for the initial assessment of urethral resistance. The urethral opening pressure is the Pdet required to initiate urinary flow; a higher urethral opening pressure indicates greater urethral resistance. Both the detrusor pressure at maximum flow and detrusor pressure at minimum flow are used to identify the magnitude of contraction force needed to sustain urinary flow. The urethral resistance relationship is ideally measured via a flow rate nomogram that compares Pdet to Q via an X-Y plot. Such a plot can be used to identify bladder outlet obstruction and rank its severity when compared to age and gender-matched persons. The generation and interpretation of pressure-flow plots will be discussed in detail in Part 11 of the Traces series. This article focuses on initial assessment of the VPS to differentiate normal voiding function from bladder outlet obstruction and poor detrusor contraction strength.

Initial interpretation of findings from the VPS focuses on differentiation of micturition as normal, abnormal due to impaired detrusor contractility, or abnormal due to bladder outlet obstruction (see Table 2). Normal voiding function is characterized by a continuous flow pattern, a detrusor contraction with sufficient amplitude and duration to result in complete bladder emptying, and little urethral resistance. Since women have a shorter urethral course than men, they tend to void with higher maximum and average flow rates, with lower amplitude and briefer detrusor contractions and lower urethral resistance than men (see Figures 2 and 3). In contrast, bladder outlet obstruction is characterized by a prolonged or interrupted flow pattern, and a high amplitude detrusor contraction that tends to be longer in duration, reflecting increased urethral resistance (see Figure 4). Figure 5 illustrates bladder outlet obstruction caused by vesicosphincter dyssynergia. In this case, the level of obstruction can be identified using the voiding pressure flow study tracings alone. Videourodynamic imaging is required for identifying the level of obstruction when it is not associated with vesicosphincter dyssynergia. Abnormal micturition associated with impaired detrusor contractility is characterized by a prolonged or interrupted flow pattern with a low amplitude and/or poorly sustained detrusor contraction in the absence of elevated urethral resistance (see Figure 6). Observation of intravesical and abdominal pressure is especially helpful in this case because it tends to reveal abdominal straining to augment bladder evacuation.

Quality Control and Reproducibility

Since micturition represents muscular performance of the lower urinary tract, some variability is anticipated when measuring the VPS. This variability is reflected in studies of several variables reflective of micturition efficiency, including voided and post-void residual volumes (Griffiths, Harrison, Moore, & McCracken, 1996; Rule et al., 2005) and uroflow parameters Qmax and Qave (Bower, Kwok, & Yeung, 2004; Reynard, Peters, Lim, & Abrams, 1996).

Limited evidence concerning variability in common VPS parameters reveals intra-individual variability over time that does not tend to reach statistical significance when inter-individual variability is compared. Rahmanou, Chaliha, Kulinskaya, and Khullar (2008) evaluated the test-retest reliability of VPS in 31 women with stress and mixed stress and urge incontinence. Study participants voided in a sitting position; voiding occurred with a 4-French single channel urethral catheter for measurement of intravesical pressure; abdominal pressure was measured via a rectal tube. Baseline evaluation was followed by repeat assessment two weeks later. VPS parameters Qmax, Pdet@Qmax, urethral opening pressure, and detrusor pressure at the termination of uroflow were not significantly different when baseline and two-week measurements were compared. Sorensen (1988) compared VPS parameters Qmax and time to onset of flow; larger within-subject variability was seen than the variability between subjects.

Brostrom, Jennum, and Lose (2002) compared VPS parameters, including Qmax, Qave, Pdet@Qmax, and urethral opening pressure, in a group of 32 continent, otherwise healthy women who underwent sequential assessment in a single day. Variability in each of these parameters was noted, but none reached statistical significance. Gupta, Defreitaas, and Lemack (2004) evaluated the test-retest reliability of VPS parameters Qmax, pdet@Qmax, and voided volume in a group of 20 continent, otherwise healthy women. Baseline evaluation was followed by a second VPS test within 4 to 20 weeks. They reported low repeatability coefficients for voided volume and Qmax but relatively high repeatability for Pdet@Qmax. Comparison of these variables revealed a statistically significant difference in Pdet@Qmax but not voided volume and Qmax.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Digescu, Hutchings, Salvatore, Selvaggi, and Khullar (2003) evaluated the interobserver and intraobserver reliability of VPS in 162 women with lower urinary tract symptoms, including stress, urge and mixed urinary incontinence, and test reliability, in a subgroup of nine women. They reported good intraobserver reliability for VPS Qmax and urethral opening pressure, but poor reliability for Pdet@Qmax. They also reported good interobserver reliability in Qmax, Pdet@Qmax, and urethral opening pressure.

Test-retest variability has also been studied in men. Kraus and colleagues (2010) evaluated VPS in 200 men with moderate to severe obstruction and found that standardization of technique performing the study resulted in smaller than anticipated standard deviations (variability in data reported across multiple patients) when best urodynamic practices advocated by Schafer and associates (2002) were followed. Eri, Wessel, and Berge (2001) assessed short-term test-retest reliability (based on repeat studies during the same day) versus long-term test-retest variability (based on VPS repeated at 24 weeks) in a group of 84 men with obstruction. They reported statistically significant variability in the Abrams-Griffiths number, also known as the urethral resistance algorithm, during shortterm retesting, but not when long-term test-retest variability was evaluated.

Dorkin, Leonard, and Pickard (2003) evaluated Qmax and Pdet@Qmax during VPS with a 10-French urethral catheter in a group of 79 men with prostatic enlargement and bladder outlet obstruction. Qmax and Pdet@Qmax yielded the same magnitude of obstruction in 80% of men when compared to parameters generated during a voluntary void.

Considered collectively, the findings of these studies support the anticipated physiologic variability expected when measuring VPS in patients with and without obstruction. Kranse and van Mastrigt (2003) performed spectral analysis of detrusor contractility in 131 patients who underwent urodynamic testing and concluded variability in Qmax, and Pdet@Qmax reflected true physiologic variability consistent with performance of a neuromuscular system. However, knowledge of this variability must be combined with careful attention to detail when generating and interpreting results from a VPS study.

Conclusion

The VPS study combines uroflowmetry with pressure measurements (Pves, Pabd, and Pdet) to identify uroflow pattern, detrusor contraction strength, and urethral resistance. Because the study measures performance in a neuromuscular structure (the lower urinary tract), test-retest variability for the study is a limitation. This variability must be addressed by careful attention to technical detail and psychosocial environment when generating this important component of multichannel urodynamic testing.

Note: Publication of this article is supported by a grant provided by LABORIE.

References

Bower, W.F., Kwok, B., & Yeung, C.K. (2004). Variability in normative urinary flow rates. Journal of Urology, 171(6, Pt. 2), 2657-2659.

Brostrom, S., Jennum, P., & Lose, G. (2002). Short-term reproducibility of cystometry and pressure-flow micturition studies in healthy women. Neurourology and Urodynamics, 21, 457-460.

Chancellor, M.B., Blaivas, J.G., Kaplan, A.S., & Axlerod, S. (1991). Bladder outlet obstruction versus impaired detrusor contractility: The role of outflow. Journal of Urology, 145(4), 810-812.

Choudhury S., Agarwal M.M., Mandal A.K., Mavuduru R., Mete U.K., Kumar S., & Singh S.K. (2010). Which voiding position is associated with lowest flow rates in healthy adult men? Role of natural voiding position. Neurourology & Urodynamics, 29(3), 413-417.

Costantini, E. Mearini, L., Biscotto S., Ginnantoni, A., Bini, V., & Porena M. (2005). Impact of different sized catheters on pressure-flow studies in women with lower urinary tract symptoms. Neurourology and Urodynamics, 24, 106-110.

Digesu, A.D., Hutchings A., Salvatore, S., Selvaggi, L., & Khullar, V. (2003). Reproducibility and reliability of pressure flow parameters in women. British Journal of Obstetrics and Gynaeco logy, 110, 774-776.

Dorkin, T.J., Leonard, A.S., & Pickard, R.S. (2003). Can bladder outflow obstruction be diagnosed from pressure flow analysis of voiding initiated by involuntary detrusor overactivity? Journal of Urology, 170, 1234-1236.

Eri, L.M., Wessel, N., & Berge, V. (2001). Test-retest reliability of pressure flow parameters in men with bladder outlet obstruction. Journal of Urology, 165, 1188-1192.

Gray M. (2010). Traces: Making sense of urodynamics testing--Part 2: Uroflowmetry. Urologic Nursing, 30(6), 321-326.

Griffiths, D., Derbyshire, S, Stenger, A., & Resnick, N. (2005). Brain control of normal and overactive bladder. Journal of Urology, 174(5), 1862-1867.

Griffiths, D.J., Harrison G., Moore K., & McCracken P. (1996). Variability of post-void residual volume in the elderly. Urologic Research, 24(1), 23-26.

Griffiths D., Hofner, K., van Mastrigt, R., Rollema, H.J., Spanberg A., & Gleason, D. (1997). Standardization of terminology of lower urinary tract function: Pressure-flow studies of voiding, urethral resistance, and urethral obstruction. Neurourology and Urodynamics, 16, 1-18.

Groutz, A., Blaivas, J.G., & Sassone A.M. (2000). Detrusor pressure uroflowinetry studies in women: Effects of a 7FR transurethral catheter. Journal of Urology, 164, 109-114.

Gupta A, Defreitas, G., & Lemack, G.E. (2004). The reproducibility of urodynamic findings in health female volunteers: Results of repeated studies in the same setting and after a short term follow-up. Neurourology and Urodynamics, 23, 311-316.

Kelly C.E., & Krane R.J. (2000). Current concepts and controversies in urodynamics. Current Urology Reports, 1,217-226.

Klausner, A.P., Galea, J., & Vapnek, J.M. (2002). Effects of catheter size on urodynamic assessment of bladder outlet osbruction. Urology, 60, 875-880.

Klingler, H.C., Madesbacher, S., & Schmidbaner, C.P. (1996). Impact of different sized catheters on pressure-flow studies in patients with benign prostatic hyperplasia. Neurourology and Urodynamics, 15, 473-481.

Kranse, R., & van Mastrigt, R. (2003). Causes for variability in repeated pressure-flow measurements. Urology, 61, 930-935.

Kraus, S.R., Dmochowski R., ALbo, M.E., Xiu, L., Klise, S.R., & Roehrborn, C.G. (2010). Urodynamic standardization in a large-scale, multicenter trial examining the effects of daily tadalafil in men with lower urinary tract symptoms with or without benign prostatic obstruction. Neurourology and Urodynamics, 29, 741-747.

Nielsen B. (1995). Evaluation of micturition. Journal of Wound, Ostomy and Continence Nursing, 22(1), 44-50.

Rahmanou, P., Chaliha, C., Kulinskaya, E., & Khullar, V. (2008). Reliability testing of urodynamics, pressure flow studies and cough leak point pressure in women with urodynamic stress incontinence with and without detrusor overactivity. International Urogynecology Journal, 19, 933-938.

Reynard, J.M., Lim, C., Swami, A., & Abrams, P. (1996). The obstructive effect of a urethral catheter. Journal of Urology, 155, 901-903.

Reynard, J.M., Peters, T.J., Lira, C., & Abrams, P. (1996). The value of multiple free flow studies in men with lower urinary tract symptoms. British Journal of Urology, 77(6), 813-818.

Rule, A.D., Jacobson, D.J., McGree, M.E., Girman, C.J., Lieber, M.M., & Jacobsen, S.J. (2005). Longitudinal changes in post-void residual and voided volume among community dwelling men. Journal of Urology, 174, 1317-1321.

Schafer, W., Abrams, P., Liao, L., Mattiasson, A., Pesce, F., Spangberg, A., ... International Continence Society. (2002). Good urodynamic practices: Uroflowmetry, filling cystometry, and pressure-flow studies. Neurourology and Urodynamics, 21(3), 261-274.

Sorensen, S. (1988). Urodynamic investigatiosn and their reproducibility in healthy postmenopausal females. Scandinavian Journal of Urology and Nephrology, 114(Suppl.), 42-47.

USLegal.com. (2012). Public urination law and legal definition. Retrieved January 2, 2012, from http://definitions. uslegal.com/p/public-urination/

Yoshimura, N., & Chancellor, M.B. (2012). Physiology and pharmacology of the bladder and urethra. In A.J. Wein, L.R. Kavoussi, A.C. Novick, A.W. Partin, & C.R. Peters (Eds.), Campbell-Walsh urology (10th ed., pp. 1786-1833). Philadelphia: Elsevier.

Zhao, S.C., Zheng, S.B., Tan, W.L., Mao, X.M., Zhang, P., Huang, Z.M., ... Zuo, Y. (2007). Effect of transurethral catheterization on the uroflow rate in the pressure flow study of patients with benign prostatic hyperplasia. Zhong Hua Ke Xua, 13(8), 710-712.

Mikel Gray, PhD, FNIP, PNP, CUNP, CCCN, FAANP, FAAN, is a Professor and Nurse Practitioner, Department of Urology, University of Virginia, Charlottesville, VA.
Table 1.
Elements of a Voiding Pressure-Flow Study

                     Abbreviation/Unit of
                     Measure                     Description
Uroflow Variables

Maximum              Qmax; measured in           Maximum flow rate
flow rate            ml/second                   sustained for at
                                                 least 1 second during
                                                 micturition.

Average (mean)       Qave; measured in           Average (mean) flow
flow rate            ml/second                   rate calculated by
                                                 dividing voided
                                                 volume by the voided
                                                 volume by flow time.

Flow time            Measured in seconds         The interval during
                                                 which measurable flow
                                                 occurs; excludes
                                                 periods when no
                                                 measurable flow
                                                 occurs.

Voiding time         Measured in seconds         The total interval
                                                 required for
                                                 micturition,
                                                 including periods
                                                 when flow is
                                                 interrupted.

Voided volume        VV; measured in             The voided volume.
                     milliliters

Pressure-Flow Variables

Maximum              Pdet/max; measured          Maximum detrusor
detrusor pressure    in cm [H.sub.2]O            pressure measured
                                                 during micturition.

Detrusor             Pdet/max@Qmax;              Detrusor pressure
pressure at          measured as cm [H.sub.2]O   measured at maximum
maximum flow         at ml/second                flow.

Detrusor             Pdet/max@Qmin;              Detrusor pressure
pressure at          measured as cm [H.sub.2]O   measured at minimum
minimum flow         at ml/second                flow.

Urethral opening     Pdet/open or UOP;           Detrusor pressure
pressure             measured as cm [H.sub.2]O   when uroflow begins;
                                                 indicates detrusor
                                                 pressure required to
                                                 overcome urethral
                                                 resistance to
                                                 outflow.

Sources: Gray, 2010; Griffiths et al., 1997; Nielsen, 1995.

Table 2.
Initial Interpretation of a Voiding Pressure-Flow Study

Voiding Pressure   Flow               Detrusor           Urethral
Flow Pattern       Characteristics    Contractility      Resistance

Normal male        Continuous flow    Pdet/max usually   Low
                   pattern; Qmax      varies between
                   usually 12         30 and 60 cm
                   ml/second or       [H.sub.2]O.
                   greater

                   (Qmax and Cave
                   typically lower
                   than normal
                   study in female
                   patient)

Normal female      Continuous; Qmax   Pdet/max usually   Very low
                   usually 15         varies between 5
                   ml/second or       and 30 cm
                   greater (Qmax      [H.sub.2]O.
                   and Qave
                   typically higher
                   than normal
                   study in male
                   patient)

Impaired           Prolonged or       Low amplitude or   Low
detrusor           interrupted        poorly sustained
contraction                           contraction;
strength                              often augmented
                                      by abdominal
                                      straining.

Bladder outlet     Prolonged or       High amplitude     High
obstruction        interrupted        detrusor
                                      contraction,
                                      often with
                                      longer duration;
                                      Pdet/max often
                                      exceeds 90 cm
                                      [H.sub.2]O with
                                      moderately
                                      severe to severe
                                      obstruction.
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Title Annotation:Special Series on Urodynamics
Author:Gray, Mikel
Publication:Urologic Nursing
Article Type:Report
Geographic Code:1USA
Date:Mar 1, 2012
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