Introduction

Non-invasive urodynamics (UDS) consists of tests [voiding diaries, flowmetry, post-void residual (PVR) estimation and pad tests], which do not require any patient manipulation. In contrast, invasive UDS warrants the insertion of catheters, transducers, and/or needle sets into patients. While non-invasive tests are useful tools for screening (i.e., flowmetry) or diagnosis (i.e., voiding diaries), invasive tests are necessary to confirm the diagnosis and refine the findings. The invasiveness of these tests raises the problem of their non-physiological recording. Acknowledging their limitations, such tests must always be interpreted by an experienced urologist with knowledge of recording conditions and patients’ complaints and symptoms.

Method

Data were obtained from various sources, including MEDLINE search via PubMed, for articles published between January 1956 and February 2011, International Continence Society (ICS) meeting abstracts and Standardization Committee reports as well as the bibliographies of retrieved articles and book chapters. The key terms searched were UDS, cystometry, urethral pressure profilometry, leak point pressure, PVR urine, video UDS, normal volunteer, pressure flow studies, and electromyography (EMG). Based on a literature review, we report normal UDS values and ranges in women.

Limits

Such a review, gathering together results from normal volunteers as well as patients, and trying to define normality have limits. The heterogeneity of the population, the absence of standardized technical methods for some testings, and the frequent absence of gender and age consideration in the interpretation of data, are many limiting factors for such articles. Taking this into consideration, we hope that future research in urodynamics will address this heterogeneity by studying a large cohort of normal volunteers to conform or modify our findings.

Cystometry

The principal aim of cystometry is to reproduce patient symptoms and relate them to any synchronous urodynamic events [1]. During the filling phase, abdominal and bladder pressures are recorded via rectal and urethral catheters, respectively, whereas detrusor pressure (P det) is calculated by subtracting abdominal pressure from bladder pressure. Initial resting abdominal and bladder pressures are 5–20 cm H2O in the supine position, 15–40 cm H2O in the sitting position, and 30–50 cm H2O in the standing position [2]. P det in an empty bladder varies between 0 and 10 cm H2O in 90% of cases [3]. Normal P det during bladder filling at a rate of 50–60 ml/s should be <20 cm H2O [4].

The ICS report 2002 divides the filling rate of the bladder during filling cystometry into: (a) physiological filling rate, which is defined as filling rate less than the predicted maximum − predicted maximum body weight in kilogram divided by 4 expressed as milliliter/minute and (b) non-physiological filling rate, which is defined as filling rate greater than the predicted maximum − predicted maximum body weight in kilogram divided by 4 expressed as milliliter/minute [5].

Several other parameters are recorded during the filling phase: bladder sensations, bladder compliance, detrusor overactivity (DO), and maximum cystometric capacity (Fig. 1 and Table 1).

Fig. 1
figure 1

The normal cystometrogram curve has two phases: a filling phase, including all normal parameters during the storage phase (first sensation, detrusor function, bladder compliance, and capacity) and a voiding phase on pressure flow study

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Table 1 Normal reported cystometric parameters during filling in females
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1. Bladder sensations

Wyndaele et al. studied bladder sensations in 50 normal volunteers (32 female and 18 male) by cystometry. They described three normal bladder sensation patterns: (a) the first sensation of bladder filling, which is felt when the volunteers first become aware of bladder filling (it is vague sensation, felt in the lower pelvis, which waxes and wanes, and could be easily ignored for few minutes); (b) first desire to void, a familial constant sensation that would lead the patient to void in the next convenient moment, but still voiding can be delayed (it is felt in the lower abdomen and gradually increases with bladder filling); and (c) strong desire to void, a persistent desire to void without fear of leakage, and felt in the perineum or urethra. Furthermore, Wyndaele and De Wachter [6] reported that volumes in all three types of sensation were lower in women: The first sensation manifested at 175 ml, the first desire to void was felt at 272 ml, and the strong desire to void presented at 429 ml.

2. Bladder compliance

Bladder compliance is the relationship between changes in volume and pressure. It is calculated by dividing change in volume by change in P det and is expressed in milliliter/centimeter H2O [5]. Normal values of bladder compliance have not been well defined. In patients with neurogenic bladder, values of 13–40 ml/cm H2O have been associated with a high risk of upper urinary tract complications [7]. Accordingly, normal bladder compliance values vary between 30 and 100 ml/cm H2O. They are higher in women than in men [5, 8–10]. Bladder compliance is considered to be compromised if it is below 30 ml/cm H2O [11] (Table 2). Harris et al. [12] studied 270 neurologically intact women and reported that normal bladder compliance was >40 ml/cm H2O. Wyndaele [10] conducted a urodynamic study of 30 volunteers (20 male and 10 female volunteers), and found that compliance was higher in female than in male volunteers, with normality exceeding 100 ml/cm H2O. The ICS [5] recommends two standard points for measuring bladder compliance: detrusor pressure at empty bladder and at maximum bladder capacity or immediately before the start of any detrusor contraction that causes significant leakage. Both points are measured excluding any detrusor contraction. Wahl et al. [13, 14] developed another method for measuring bladder compliance that they claimed was more accurate and practical, especially in children. They standardized bladder compliance according to a complex mathematical formula.

Table 2 Normal bladder compliance (reproduced with permission from Corcos and Schick [50])
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3. Detrusor stability during bladder filling

The absence of involuntary detrusor contractions is considered to be normal and is defined as “stable detrusor activity” [10, 15]. Uninhibited detrusor contractions occur in 10–18% of asymptomatic volunteers and do not need any further evaluation in asymptomatic patients [10, 16]. Advance in ambulatory UDS has complicated discussion of DO, which has been shown to arise in 60% of asymptomatic women undergoing ambulatory UDS [17]. The significance of these DO in normal volunteers is not very well understood as shown by the study looking for correlation with other variables in this population. The mere absence of documented overactivity on cystometrograms (CMG) does not rule out its existence. Up to 40% of patients with urge incontinence do not show DO on CMG [18].

The 1988 ICS report [8] differentiated types of detrusor storage function, as determined by filling cystometry, into: (a) normal detrusor function, which allows filling with little or no change in pressure and no involuntary phasic contractions despite provocation, and (b) overactive detrusor function, which is urodynamic observation characterized by involuntary detrusor contractions during the filling phase and may be spontaneous or provoked [19].

DO has a variety of patterns on urodynamic tracing. The 2002 ICS report [5] describes two types: (a) phasic DO, defined by its characteristic waveform, which may or may not lead to urinary incontinence, and (b) terminal DO, classified as a single involuntary detrusor contraction occurring at cystometric capacity, which cannot be suppressed and causes incontinence often resulting in complete bladder emptying [17].

The first ICS report [20, 21] stated that, in order to diagnose “detrusor instability,” the contraction should be at least 15 cm H2O. However, it was subsequently realized that involuntary detrusor contractions of <15 cm H2O could cause significant symptoms. The ICS [5, 8] stated that there is no standardized minimum value. In practice, it may be difficult to be certain whether an involuntary detrusor contraction has occurred if the phasic wave is <5 cm H2O [17].

4. Maximum cystometric bladder capacity

Maximum cystometric capacity is the bladder volume at the end of filling CMG when patients have a strong desire to void, feel they can no longer delay micturition, and are given permission to void [5]. This volume includes both the amount voided and residual urine left after the void (PVR) [17]. Normal cystometric capacity varies widely, but is normally between 300 and 500 ml, with higher values in men than in women [10].

Wyndaele [10] conducted a urodynamic study of 30 volunteers (20 male and 10 female), with a mean age of 24 years, and reported wide variability of normal ranges. Bladder capacity was found to be larger in men than in women, ranging from 300 to 550 ml.

Summary

The bladder should have constantly low pressure that usually does not reach more than 6–10 cm H2O above baseline at the end of filling (end-filling pressure), and there should be no involuntary contractions, with normal first sensation ranging between 100 and 250 ml, bladder compliance between 30 and 100 ml/cm H2O, and maximum bladder capacity between 450 and 550 ml.

Pressure flow (P/Q)

P/Q study simultaneously measures P det and flow rate during voiding. P/Q assessment is considered to be the gold standard for quantifying and grading bladder outlet obstruction (BOO) and differentiating between BOO and detrusor underactivity [22–28]. P/Q data can be plotted on pressure flow nomograms to classify patients as being either obstructed or not obstructed and, at the same time, grade the severity of obstruction. Different types of nomograms have been developed. The most common in clinical practice are ICS nomogram [22], Abrams–Griffiths nomogram, Schafer nomogram, bladder contractility nomogram, urethral resistance factor, and the composite nomogram [17], but these nomograms are used in men only.

During P/Q assessment, several parameters are recorded, including detrusor opening pressure, maximum detrusor pressure (P det.max), P det at maximum flow (\( {P_{{\det .{Q_{{\max }}}}}} \)), minimum detrusor pressure during voiding, maximum flow rate (Q max), voided volume (VV), and PVR. The ICS has defined the following terms in the interpretation of P/Q [3, 5]. Pre-micturition pressure is intravesical pressure just before the onset of isovolumetric detrusor contraction. Detrusor opening pressure is P det recorded at the onset of measured flow, which tends to be elevated in patients with infravesical obstruction. Opening time is the time that elapses from the initial rise in P det to the onset of flow through the urethra.

However, because flow rate is measured at a downstream location (i.e., flowmeter outside the urethra), flow rate measurement is slightly delayed from bladder pressure measurement. This flow delay, generally between 0.5 and 1 s, should be factored into the analysis [29]. \( {P_{{\det .{Q_{{\max }}}}}} \) is the magnitude of detrusor contraction when flow rate is at its maximum [5]. P det.max is the maximal pressure recorded regardless of flow. This pressure can exceed pressure at maximal flow if the bladder is contracting isometrically against a closed outlet. Isometric detrusor pressure is obtained by mechanical obstruction of the urethra or by active contraction of the distal sphincter mechanism during voiding.

Post-micturition contraction (after contraction) is a reiteration of detrusor contraction after flow has ceased, and its magnitude is typically greater than that of micturition pressure at maximal flow. After contractions are not well-understood, but they seem to be more common in patients with unstable or hypersensitive bladders [30]. PVR is the volume of urine remaining in the bladder immediately after voiding. Although the test situation often leads to inefficient voiding and falsely elevated residual urine, the absence of residual urine does not exclude infravesical obstruction or bladder dysfunction [17].

Wyndaele [10] attempted to define what can be considered as normal parameters by urodynamics in 38 healthy adult volunteers (28 men and 10 women) with a mean age of 24 years. Free flow rate, water cystometry, and P/Q assessment were performed in all of them. Micturition bladder pressure was higher in men than in women, reflecting higher outflow resistance in men, but P det was not statistically different between the sexes. Flow time was significantly longer, and Q max was significantly lower during P/Q evaluation than during free flow rate measurement in both sexes. There was no residual urine at all in the majority of volunteers estimated by urethral catheter, but six men and three women had <50 ml residual urine.

Pressure flow (P/Q) studies (Table 3)

Blaivas and Groutz studied 50 women with BOO (mean age, 65 years) and 20 normal controls (mean age, 67 years) by videourodynamics (VUDS) assessment [31]. The P/Q parameters recorded in the control group were: free Q max of 24 ± 9 ml/s, Q max of 13 ± 6 ml/s, \( {P_{{\det .{Q_{{\max }}}}}} \) of 18 ± 8 cm H2O, P det.max of 22 ± 9 cm H2O, VV of 312 ± 131 ml, and PVR of 103 ± 100 ml. These authors described a nomogram to diagnose BOO in women, known as Blaivas nomogram (Fig. 2). Two parameters are needed to construct this nomogram: free Q max and P det.max. Free Q max is preferred to Q max during P/Q because \( {P_{{\det .{Q_{{\max }}}}}} \) and Q max cannot be evaluated if the patient does not void during the test. Blaivas nomogram consists of four zones, which classify patients into four categories: zone 0 (normal or no obstruction), zone 1 (mild obstruction), zone 2 (moderate obstruction), and zone 3 (severe obstruction) [31].

Fig. 2
figure 2

Bladder outlet obstruction nomogram (Blavias nomogram) for women indicating that normal women should be in an area of no obstruction (grade 0)

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Table 3 Normal reported P/Q parameters in women
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Defreitas et al. [32] investigated 169 women with BOO and 20 asymptomatic volunteers by P/Q assessment. They reported normal Q max and \( {P_{{\det .{Q_{{\max }}}}}} \) as 16 ml/s and 24 cm H2O, respectively, in the asymptomatic group. The cut-off values to detect BOO with \( {P_{{\det .{Q_{{\max }}}}}} \) and Q max were 25 cm H2O and 12 ml/s, respectively, with sensitivity, specificity, and accuracy of 68%. Chassagne et al. [33] mentioned that the combined cut-off values for diagnosing BOO in women are Q max <15 ml/s and \( {P_{{\det .{Q_{{\max }}}}}} \) >20 cm H2O, yielding sensitivity of 74.3% and specificity of 91.1%.

Brostrom et al. [34] performed a P/Q study in 30 normal female volunteers with a mean age of 52 years. Two sets of measurements were recorded. They found that, between two repeated measurements, there was a statistically significant increase in first desire to void (171 and 205 ml) and a normal desire to void (284 and 351 ml) with a decrease in bladder opening pressure, whereas no change was noted in maximum cystometric capacity (572 and 570 ml). Kuo [35] investigated 441 women with BOO and stress urinary incontinence (SUI) as well as 30 asymptomatic volunteers. He reported that \( {P_{{\det .{Q_{{\max }}}}}} \) ≥30 cm H2O combined with Q max ≤15 ml/s indicated BOO with specificity of 93.9% and sensitivity of 81.6%.

Summary

In asymptomatic females, Q max ranges from 13 to 25 ml/s, \( {P_{{\det .{Q_{{\max }}}}}} \) ranges from 18 to 30 cm H2O, P det.max ranges from 22 to 46 cm H2O and VV ranges between 250 and 650 ml.

Leak point pressures

1. Detrusor leak point pressure

Detrusor leak point pressure (DLPP) is defined by the ICS as the lowest P det at which urine leakage occurs in the absence of either detrusor contraction or increased abdominal pressure [5]. The rise in bladder pressure is secondary to low bladder compliance. This value reflects resistance that the urethra offers to the bladder, mainly by the action of the striated sphincter [17, 36].

In patients with neurogenic bladder, a high DLPP can jeopardize upper urinary tract function. McGuire et al. [37] followed the urodynamic evolution of 42 myelodysplastic children and observed that those with DLPP ≥40 cm H2O developed upper tract damage if not treated.

2. Abdominal leak point pressure or Valsalva leak point pressure

Abdominal leak point pressure (ALPP) or valsalva leak point pressure (VLPP) is intravesical pressure at which urine leakage occurs because of increased abdominal pressure in the absence of detrusor contraction [5]. It measures the ability of the urethra to resist an increase in abdominal pressure. ALPP should be tested during cystometry after the bladder has been filled to at least 150–200 ml. The patient is then asked to do a valsalva maneuver until he or she leaks. VLPP is the lowest pressure at which incontinence occurs [17]. This test assesses the severity of SUI and may be useful in detecting intrinsic sphincter deficiency (ISD). In normal individuals, no incontinence should be recorded, whatever the increase in abdominal pressure. Therefore, there is no “normal ALPP.” However, studies have attempted to determine a cut-off between patients with or without ISD [17].

McGuire et al. [38] employed VUDS and demonstrated that 80% of women with VLPP below 60 cm H2O had type III SUI. They also showed that VLPP values above 90 cm H2O could rule out ISD. In fact, in women suffering from SUI without genital prolapse, high ALPP of 100 cm H2O or more is usually associated with urethral hypermobility. Those with values between 60 and 100 cm H2O have features of both ISD and hypermobility [39].

Summary

There should be no DLPP in normal individuals. In neurogenic patients, DLPP higher than or equal to 40 cm H2O is considered dangerous for the upper tract.

In normal individuals, no abdominal pressure increase should cause incontinence. Therefore, there is no “limit of normal ALPP.”

In women with SUI, VLPP below 60 cm H2O is highly suggestive of ISD. VLPP of 100 cm H2O or more is usually associated with urethral hypermobility. Values between 60 and 100 cm H2O are suggestive of both ISD and hypermobility.

Urethral pressure profilometry

Urethral pressure is defined by the ICS as the fluid pressure needed to just open a closed urethra [5], and the urethral pressure profilometry (UPP) is a graph indicating changes in intraluminal pressure along the length of the urethra.

UPP quantifies the occlusive pressure generated by active and passive structures of the urethra and allows the evaluation of urethral competence. Two variations of this measurement are commonly reported: static UPP, with its variants stress UPP and pressure transmission ratios, and micturitional UPP [17].

Maximum urethral pressure (MUP) is categorized as maximum pressure of the measured profile, while maximum urethral closure pressure (MUCP) is defined as the maximum difference between urethral pressure and intravesical pressure.

UPP rises in “normal” healthy individuals with increasing bladder volume. It is the so-called “guarding reflex.” However, continuous recording of MUP shows variations and oscillations between 10 and 25 cm H2O [40]. MUCP below 20 cm H2O is considered hypotonic, raising the possibility of ISD. MUCP values above 75 cm H2O in women and 90 cm H2O in men are considered hypertonic [17]. Sorensen et al. [41] analyzed urethral pressure variations in 10 healthy, fertile female volunteers (mean age, 32 years) and 12 healthy post-menopausal volunteers (mean age, 58.7 years). In the fertile group, they observed that mean maximum urethral pressure (mMUP) and mean maximum urethral closure pressure (mMUCP) had median values of 66.5 and 60 cm H2O, respectively. Postmenopausal women had significantly lower mMUP and mMUCP: 55.5 and 43.5 cm H2O, respectively. Van Geelen et al. [42] studied 27 nulliparous healthy women between the ages of 19 and 35 years and recorded mMUP of 98 ± 17 cm H2O in the supine position, with mean MUCP of 84 ± 18 cm H2O. Pfisterer et al. [43] examined bladder function parameters in pre-, peri-, and postmenopausal, continent women, discerning mMUCP of 94, 74, and 42 cm H2O, with functional urethral lengths of 3.3, 3.3, and 3.5 cm, respectively.

It is desirable that the urethral catheter is perfused at a constant rate. This necessitates the use of a motorized syringe pump or a very accurate peristaltic pump. A perfusion rate of between 2 and 10 ml/min gives an accurate measurement of closure pressure. Perfusion rates of <2 ml/min usually fail to record the true urethral pressure unless the withdrawal rate is extremely slow [1].

Stress UPP measures the rise in intra-abdominal pressure transmitted to the proximal urethra. In normal women without urethral hypermobility, increases in intravesical pressure and proximal urethral pressure should be similar. The pressure transmission ratio is a different parameter, recording the increment of urethral pressure with stress as a percentage of intravesical pressure elevation. In normal women, this value should exceed 100 cm H2O [44].

Micturitional UPP serves to identify the presence and location of BOO [45]. It is performed in a manner similar to static UPP except that the patient voids as the catheter is being withdrawn. This allows bladder pressure to be compared with urethral pressure at points along the urethra. If a significant drop is encountered on catheter withdrawal, it corresponds to the site of obstruction.

Summary

MUCP below 20 cm H2O raises the possibility of ISD. MUCP values above 75 cm H2O for women are considered hypertonic [17].

Electromyography of the pelvic floor and external urethral sphincter

Clinical neurophysiological studies, which include sphincter EMG, record bioelectric potentials generated during muscle depolarization. They enable clinicians to completely evaluate the striated sphincter complex and pelvic floor activity during bladder filling, storage, and voiding. Clinically, the most important information obtained from sphincter EMG is coordination or discoordination between the external urethral sphincter (EUS) and the bladder [17].

EMG is undertaken with electrodes. The needle is placed lateral to the urethral meatus and is advanced parallel to the urethra to a distance of about 1–2 cm. For representative EMG studies of the perineal floor, needle and wire electrodes may be placed in the superficial anal sphincter in women [17].

Kinesiologic investigations may be performed with a variety of electrodes and display methods. Needle/wire electrodes are preferable because they are positioned in the muscle of interest, allowing the detection of activity in individual motor units [17].

Normally, EMG activity from the EUS is low at rest. It intensifies as fluid volume in the bladder grows, during bladder filling, due to EUS contraction. It is known as the guarding reflex. During voiding, EMG activity disappears completely for a few seconds before detrusor contraction starts. Once the bladder is empty, EMG activity resumes [46, 47].

Failure of the sphincter to relax or stay completely relaxed during micturition is abnormal [5]. When it occurs in patients with neurologic disease, it is termed detrusor–sphincter dyssynergia; it is typical in patients with suprasacral spinal cord injury. The term detrusor–sphincter dyssynergia cannot be used in the absence of neurologic disease. Instead, the applicable term is pelvic floor hyperactivity or dysfunctional voiding [17].

Summary

EMG activity increases progressively during bladder filling. The rise in EMG activity during heightened abdominal pressure (cough, straining, etc.) is proportional to the level of stress. EMG activity of the external sphincter and pelvic muscles should be silent during voiding.

Reproducibility of urodynamics in healthy women

Reliability and reproducibility of different urodynamic procedures are questionable. Short-term reproducibility (same session or duplicate) was studied in several reports [34, 48]. An increase in first and normal desires on second fill was noted, yet maximum cystometric capacity reamined unchanged.

Conclusions

Urodynamic tests are useful tools to evaluate LUT dysfunction. They are gold standards for the diagnosis of BOO and urinary incontinence. Urodynamic evaluation is a good predictor of outcomes after therapeutic intervention. Urodynamic normality in healthy populations is not well known and illustrates a wide variety of data and patterns. Several important parameters, such as age, sex, and body mass index, affect urodynamic values, rendering it more challenging to precisely define normality from tests performed on patients.

Mathematical models and simulation may help in the future to generate more data on normality, but additional studies on healthy volunteers must be encouraged to gather more information.