Are conventional pressure-flow measurements dependent upon filled volume?


Robert Pickard, Freeman Hospital, Newcastle upon Tyne, NE7 7 DN, UK.



To determine, in a prospective study, whether detrusor pressure (pdet.Qmax) and maximum urinary flow rate (Qmax) measurements obtained after filling to maximum cystometric capacity (MCC) differ from those obtained with filling restricted to average voided volume (Vvoid), as standard protocols for pressure flow studies (PFS) mandate bladder filling until the subject has a strong desire to void, which aids standardization but further divorces the test from real-life experience.


After calculating the appropriate sample size, 84 patients attending for PFS with an adequately completed 3-day frequency-volume chart were recruited. Each underwent two consecutive PFS with filling to MCC and average Vvoid in a random order, and measurements of pdet.Qmax and Qmax were compared. For men, the agreement for a diagnosis of obstruction between the tests was also assessed.


Complete data were obtained from 76 (90%) of the patients, with a mean (range) age of 64 (20–94) years. The mean (sd) difference between MCC and average Vvoid was 134 (113) mL (P < 0.01). There were no significant differences between estimates of Qmax, at − 0.1 (3) mL/s (P = 0.75), and of pdet.Qmax, at − 1 (13) cmH2O (P = 0.91), obtained within each patient. For men there was 91% agreement (32 of 35) in the classification of obstruction.


Restriction of filling to the average Vvoid during PFS allows a closer approximation to normal voiding and results in no clinically relevant change to the value of standard pressure-flow measurements or alters individual classification of obstruction.


maximum urinary flow rate


detrusor pressure at Qmax


pressure-flow study


maximum cystometric capacity


detrusor overactivity


fill, voided or filling volume


frequency-volume chart


Abrams-Griffiths (number).


Voiding dysfunction in patients with bothersome LUTS is currently diagnosed by plotting on a nomogram simultaneous readings of maximum urinary flow rate (Qmax) and detrusor pressure at Qmax (pdet.Qmax) obtained from a conventional pressure-flow study (PFS), allowing urodynamic categorization into obstructed, equivocal or unobstructed groups [1]. The reliability of such categorization depends on several factors, which may include the filled volume of the bladder [2]. Based on recommendations of the ICS, standard practice for PFS is to continue bladder filling until the subject experiences a strong desire to void, this being defined as maximum cystometric capacity (MCC) [3]. The main reasons for this practice are the increased likelihood of detecting detrusor overactivity (DO) during an extended filling phase, and to encourage the ability to void in an atypical setting. However, a possible disadvantage of filling beyond the functional capacity is over-stretching the detrusor, leading to reduced force of contraction [4] and hence reduced Qmax[5].

As the role of PFS is to provide diagnostic measurements in the context of symptoms experienced by an individual, it would seem reasonable to recreate real-life conditions as far as possible within the clinical environment of the urodynamic laboratory. As part of this aim, restricting filling to an individually comfortable bladder volume might be advantageous, provided the diagnostic reliability was not compromised. We therefore carried out a prospective study to test the following hypotheses: (i) There is no difference in simultaneous measurements of Qmax and pdet.Qmax made during voids initiated at MCC and average voided volume (Vvoid); (ii) the classification of obstruction is not changed by restricting bladder filling to the average Vvoid.


A preliminary calculation of sample size showed that the recruitment of ≥ 75 subjects was required to detect clinically relevant differences of 15 cmH2O or 2 mL/s in pdet.Qmax and Qmax, respectively, at 5% significance [6]. All adult patients with no neurological disease who attended our urodynamic laboratory for conventional PFS and with an adequately completed 3-day frequency-volume chart (FVC) were invited to participate in the study. After a careful explanation of the aims and methods of the study, and obtaining of informed consent, we recruited 84 patients over a 5-month period.

For each patient the average Vvoid was calculated by summing the volumes of all voids recorded during three consecutive 24-h periods and dividing the total by the number of voids. The patients then had two consecutive PFS with filling to both MCC, indicated by a strong desire to void, and to average Vvoid, the sequence being determined by previous randomization. A standard technique for PFS was used which conformed to ICS good practice [2]. Briefly, after a private void and under aseptic conditions, the bladder was catheterized with a 10 F filling line and a 4 F bladder pressure line, whilst a 6 F catheter covered with a vented finger cot was inserted into the rectum for abdominal pressure recording. The water-filled lines were connected to pressure transducers placed at the level of the pubic symphysis and zeroed to atmospheric pressure. The initial fill was with the patient supine before changing to standing or seated for voiding, whilst the second fill was with the patient standing (men) or seated on a commode (women). Both voids were therefore in the same position for each patient. Non-physiological filling at a rate of 100 mL/min was used throughout. Intravesical pressure (pves), abdominal pressure (pabd), subtracted detrusor pressure (pdet), flow rate (Q) together with filled and Vvoid were continuously recorded at a sampling frequency of 10 Hz. Residual urine was estimated at the end of the second void by abdominal ultrasonography.

Values for filled volume, Qmax and pdet.Qmax for each PFS were recorded and expressed as the mean (sd). Values for residual volume obtained at the end of the second void were noted but not added to the measured filled volumes during PFS. The Abrams-Griffiths (AG) number (pdet.Qmax− 2Qmax) was calculated for each void and men were categorized as being obstructed, equivocally obstructed or unobstructed by plotting the values of Qmax and pdet.Qmax from each void on the provisional ICS nomogram [1]. Cystometry traces recorded for the two fills were examined by two experienced urologists for the presence of DO, defined according to current ICS criteria [3]. Differences in paired values of Qmax and pdet.Qmax within each patient were assessed using Student's t-test and the Bland-Altman analysis to estimate bias and measurement error [7]. The 95% CI for the mean was calculated when appropriate and statistical significance assessed at the 5% level.


In all, 84 patients were recruited (63 men and 21 women, mean age 65 years, range 20–94); of these, 76 (90%; 59 men and 17 women) completed the study, whilst eight were excluded because of expulsion of the bladder pressure line during voiding (two), failure to void into the uroflowmeter (two), or failure to void (four). Two of those who failed to void did so on both the fills, and another two failed after filling to the average Vvoid alone. Of those who completed the study, 38 were randomized to filling to MCC first, whilst 38 were initially filled to average Vvoid.

The mean (sd) infused volume required for MCC, at 327 (135) mL, was 134 (114) mL (70%) higher than that of average Vvoid, at 193 (78) mL, calculated from the FVC (P < 0.001). Overall, measurements of Qmax and pdet.Qmax obtained after filling to MCC and average Vvoid showed no statistical or clinically relevant differences (Table 1, Fig. 1a,b). In all, 30 (39%) patients had a difference in Qmax of > 2 mL/s and 16 (21%) had a > 15 cmH2O difference in pdet.Qmax. There were systematic differences between voiding variables recorded from the first and second fills, irrespective of filled volume, with both Qmax and pdet.Qmax being significantly greater on the first fill (Table 1). The residual urine volume after the second void was > 150 mL for 12 (16%) patients, with a mean (range) of 290 (170–500) mL. Analysis of the data from this group showed no significant differences from those with more complete bladder emptying (Table 1).

Table 1.  Comparison of Qmax, pdet.Qmax and Vfill measured during PFS after filling to MCC and Vvoid.mean, with randomized order of filling, with sequential filling, and as a comparison between the two fills for patients whose residual urine was > 150 mL
Paired dataMean (sd) [95% CI] differenceP
Random (76 patients):
value at MCC − value at average Vvoid for
Qmax, mL−0.1 (3.2) [−0.9 to 0.6]0.75
pdet.Qmax, cmH2O−0.2 (12.6) [−3.0 to 2.7]0.91
A–G number 0.08 (11.8) [−2.6 to 2.8]0.95
Vfill, mL134 (114) [108–160]<0.01
Sequential, value at 1 − value at 2 (76 patients)
Qmax, mL  1.3 (3.0) [0.6–2.0]<0.01
pdet.Qmax, cmH2O  5.3 (11.4) [2.7–7.9]<0.01
AG number  2.7 (11.5) [0.1–5.3]<0.04
Vfill, mL 13 (179) [−27 to 54]0.5
Residual volume > 150 mL (12 patients)
value at MCC − value at average Vvoid for
Qmax, mL−1.4 (2.6) [−3.0 to 0.2]0.08
pdet.Qmax, cmH2O−5.7 (12.7) [−13 to 2.4]0.15
AG no.−2.8 (11.8) [−10 to 4.7]0.42
Vfill, mL188 (153) [91–286]<0.01
at MCC
Vfill− Vvoid, mL135 (220) [−5.0 to 274]0.06
at average Vvoid
Vfill− Vvoid, mL−27 (100) [−90 to 37]0.37
Figure 1.

Bland-Altman analyses of: a, Qmax (MCC) and Qmax (Vvoid.mean); and b, pdet.Qmax (MCC) and pdet.Qmax (Vvoid.mean).

For men the diagnostic category for obstruction was changed for four (5%), with three moving from obstructed to equivocal and one from equivocal to obstructed (Table 2). The reproducibility of the diagnosis of BOO using filling to average Vvoid in the subgroup of 59 men was 91%. All those with no BOO were correctly identified by filling to average Vvoid. There were phasic pressure rises indicative of DO in 24 (32%) patients (Table 3). Statistical analysis of the detection rates for DO according to filled volume and order of filling, assuming all cases of DO were identified, showed significantly more cases during the second fill, irrespective of the filled volume used (P < 0.001, Fisher's exact test), whilst the increased detection rate after filling to MCC compared to average Vvoid was of only marginal significance (P < 0.041, Fisher's exact test).

Table 2.  Comparison of diagnostic categorization of men, using the provisional ICS nomogram, according to Vfill and fill sequence
Fill atDiagnosis
Average Vvoid33206
P 0.85 0.84 
P 0.57 0.55 
Table 3.  Comparison of the rate of diagnosis of phasic DO during filling cystometry according to filled volume and sequence of fill for 24 patients who had DO on at least one fill
Fill atDONo DO
MCC17 7
Average Vvoid 915
P 0.041 
First 717
Second19 5
P 0.001 


The present results suggest that for the purposes of a conventional PFS used to investigate the urodynamic cause of voiding dysfunction, restriction of bladder filling to average Vvoid allows a valid estimation of Qmax and pdetQmax, and does not alter the classification of BOO for > 90% of patients.

The effect of filled volume on measurements obtained during conventional PFS has not been systematically characterized to date. Studies examining the volume-dependency of free urinary flow rate showed an increase with Vvoid, particularly at 0–200 mL [8]. Theoretical modelling and experimental studies suggest that over-stretching can impair detrusor contraction strength and hence reduce Qmax[4,5]. Further information was provided by studies comparing ambulatory natural-fill urodynamics with conventional PFS, which showed that the 40–50% reduction in Vvoid after natural-fill was associated with increases in both Qmax and pdet.Qmax compared to sequential conventional PFS [9,10]. A subsequent comparative study where nonphysiological filling during conventional PFS was restricted to ‘normal desire to void’ found that the increase in Qmax recorded after ambulatory filling was less pronounced and voiding pressure was similar, suggesting a dependency on filled volume rather than fill rate [11]. In an attempt to characterize more precisely the effect of filled volume with a constant fill rate, Groen et al.[12] compared values of Qmax and pdet.Qmax obtained by conventional PFS after filling to both MCC and modal functional capacity in young female volunteers. They found that flow rate was lower and voiding pressure unchanged during voids initiated at functional capacity. These findings were an advance on existing knowledge, but the quality of their data was compromised by a high exclusion rate, use of unpaired data and lack of randomization of voiding sequence. The effect of void sequence is illustrated by findings from most studies investigating the test/re-test reliability of conventional PFS, which have shown sequence bias with values of both Qmax and pdet.Qmax being higher on the first fill [6,13], changing the urodynamic classification of obstruction for ≈ 30% of cases [13,14], as also in the present study. These changes are thought to reflect variation in contraction strength and outlet status rather than measurement error [13]. The design of the present adequately powered study using a constant fill rate, identical voiding conditions and randomization to nullify the effect of void sequence, allows a more robust conclusion that pressure-flow variables are unchanged by restricting filling, despite this being partly contrary to previous work [12].

The relationship between bladder sensation and filled volume during natural bladder filling and nonphysiological filling for the purposes of conventional PFS was elegantly assessed by DeWachter and Wyndaele [15]. They found that although volume at strong desire to void (MCC) was similar for both fill rates, > 90% of natural voids occurred at or before an initial desire to void, with volumes ≈ 60% of MCC. The present results are in keeping with these data and suggest that average Vvoid is a valid, individually standardized variable that provides a fair reflection of normal micturition.

The present data showed no overall bias in measurement of Qmax and pdet.Qmax when filling was restricted to average Vvoid, but there was a degree of random variation. Analysis of this variation using pre-set limits (Qmax < 2 mL/s, pdet.Qmax < 15 cmH2O) showed clinically significant changes for flow rate in 39% of patients and for pressure measurement in 21%, compared to values of 20% and 15%, respectively, in a recent reliability study [13]. In contrast, Bland-Altman analysis, where random variation is expressed by the sd of differences, showed similar findings to previous studies [6,12]. On balance, it therefore appears reasonable to conclude that our methods caused no substantial increase in random variation of standard pressure-flow measurements.

The FVC is now well established as a useful tool for evaluating patients with LUTS but for the purpose of informing the conduct of PFS, a representative standard Vvoid for an individual must be defined [16]. Previous authors suggested the maximum Vvoid (previously defined as functional capacity), modal Vvoid and average Vvoid over 24 h or during daytime alone. A review of relevant reports suggests that the average Vvoid over three or four consecutive 24-h periods is best, a measure that has consistently been shown to be ≈ 60% of MCC [15,17–19]. The precise volume within the bladder just before voiding will be the sum of filled volume, urine production during the test and residual volume. We decided to measure filled volume only, as this represents the major component of bladder capacity during conventional PFS, can be accurately measured and can be related directly to data from the FVC. It is possible that high residual urine volumes may affect the reliability of pressure-flow measurements, but in the few present patients with residuals of > 150 mL the results were unchanged.

Given the lack of bias found in measurements of Qmax and pdet.Qmax it is not surprising that there were no clinically significant changes in classification of obstruction using either the AG number or position on the provisional ICS nomogram. Although such classification has only been validated for men with LUTS, we felt it appropriate to include women in the overall study, to give a wide range of pressure and flow readings and hence widen the applicability of the findings.

The present results, together with previous published work, suggest that pressure-flow measurements are consistent irrespective of whether the void was initiated at functional or maximum capacity, indicating that reduced contraction strength caused by overfilling is not of practical importance during conventional PFS. Part of the rationale for filling to MCC is to detect DO, although filled volume is only one of many factors that influence this diagnosis [20]. The present study was not designed to answer this question, but there were marked variations in detecting DO, with fill sequence appearing the more important factor rather than filled volume. This may have been influenced by performing the second fill standing or sitting, a known provocative factor for DO [20]. Another reason for filling to MCC is to encourage the subject to void in the laboratory environment. This has not been tested before, but in the present study only 2% of patients failed to void only when filling was restricted to functional capacity. The patient experience during cystometry has not been well documented. A recent study suggested that men in particular find the procedure uncomfortable, whilst women experience more shame and embarrassment [21]. Whether this discomfort was lessened by restricting filling to a functional capacity was not assessed in the current study, but it may benefit those who experience urge at higher volumes.

In conclusion, in men with LUTS and no significant urge component who solely require diagnosis of possible BOO, a PFS with filling restricted to average Vvoid calculated from the FVC allows valid pressure-flow measurements. Those patients with mixed symptoms, particularly involving urgency or urge incontinence, require further provocation by extended filling, change to upright position or a second fill, to reliably document DO. It may be possible therefore to better tailor invasive urodynamic evaluation to individual requirements provided the rationale and conduct of the examination is fully documented in the report.


None declared.