The Cutoff Value of Bladder Voiding Efficiency for Predicting Surgical Outcomes After GreenLight HPS ™ Laser Photoselective Vaporization of the Prostate

Abstract

Purpose:

A large postvoid residual (PVR) has been considered to be an indicator for the need to further management. However, the threshold level of PVR has been not defined. We determined the PVR volume by using bladder voiding efficiency (BVE) that would predict surgical outcomes after GreenLight HPS™ laser photoselective vaporization of the prostate (HPS-PVP).

Methods:

A total of 193 patients who underwent HPS-PVP were divided into two groups based on BVE: >75% (group A) and ≤75% (group B). BVE was calculated by dividing voided volume by voided volume plus PVR. Surgical outcomes were assessed from the ratio of the IPSS, the difference in Qmax, and the difference in the quality of life IPSS subscore (QoL) before surgery and at 3, 12, and 36 months postoperatively.

Results:

There were significant improvements in all outcome measures after 3, 12, and 36 months; the improvement in Qmax was greater in group A than group B at each time point. The IPSS ratio was significantly greater in group A than group B at 3 months. There was no difference in QoL between the groups. The multivariate logistic regression analysis showed that BVE correlated significantly with short-term surgical success, but not with prostate volume or age. In the area under curve of ROC analysis, the BVE cutoff value of 75% showed the highest sensitivity and specificity for predicting the short-term success of surgical outcomes.

Conclusions:

Our results suggest that a large PVR may predict relatively slight improvements in short-term surgical outcomes after HPS-PVP. The cutoff value of PVR in predicting surgical success is 25% or more of residual fraction.

Introduction

Urodynamic studies are considered to be the gold standard tools for the diagnosis of bladder outlet obstruction, but are invasive, expensive, and time-consuming.¹ Therefore, there remains a need for a simple, inexpensive, safe, and accurate diagnostic test.

Postvoid residual (PVR) urine volume is frequently measured when evaluating lower urinary tract symptoms (LUTS) with benign prostatic hyperplasia (BPH). A large PVR has traditionally been considered to be an indicator for the need for further investigation and management.² However, because of marked inter- and intraindividual variability,³ the threshold level of PVR has been variously thought to lie within the range of 50–500 mL.^4

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Bladder voiding efficiency (BVE), post/pre ratio of PVR, is the proportion of the total bladder capacity voided, and is a reflection of bladder contractility against urethral resistance.¹⁰ Ku et al. found that the residual fraction was a less variable, more accurate, and reliably reproducible index than PVR alone, as it took into account the premicturitional bladder volume.¹¹ There have been no prior studies that have examined whether BVE can predict the outcome of treatment, nor has a threshold been established that might predict surgical outcomes.

We examined whether PVR can predict the surgical outcome after GreenLight High-Performance system laser photoselective vaporization of the prostate (HPS-PVP), and whether a PVR threshold that would predict good surgical outcome could be determined.

Materials and Methods

Of the 506 consecutive patients who underwent HPS-PVP for LUTS caused by BPH by a single surgeon (H.S.) between January 2008 and June 2012, data of the 362 patients who underwent preoperative urodynamic studies were analyzed. The indications for surgery were age ≥50 years, clinical LUTS (International Prostate Symptom Score, IPSS ≥8), and anatomical BPH (prostate volume ≥25 mL). Patients with a Qmax >15 mL/sec and a voided volume <150 mL on one or more occasions before surgery were excluded.¹² After approval by our institutional review board, we retrospectively reviewed the records of 193 patients.

Preoperative evaluation included complete medical history, physical examination, urinalysis, prostate-specific antigen, IPSS, quality of life index of IPSS (QoL), uroflowmetry with PVR, and transrectal ultrasonography to measure prostate volume. PVR urine volume was measured by transabdominal ultrasonography.

We calculated BVE according to the following equation: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*} & Bladder \ voiding \ efficiency \\ & \quad = \left( \frac { voided \ volume } { voided \ volume + post \ void \ residual } \right) \times 100 \end{align*} \end{document}

We then divided the patients into two groups based on the BVE: group A consisted of patients with BVE >75%, and group B consisted of those with BVE ≤75%. The area under the receiver operating characteristic curve analysis showed that 75% of BVE was the point of most sensitive and specific for predicting therapeutic success. The bladder outlet obstruction index (BOOI) was determined by the formula of detrusor pressure at maximum flow rate (PdetQmax)–2Qmax. The bladder contractility index (BCI) defined as PdetQmax+5Qmax.

As baseline characteristics were different between the groups, surgical outcome was assessed using the criteria suggested by Homma et al: the ratio of IPSS before and after surgery; the difference in Qmax before and after surgery; and the difference in QoL before and after surgery.¹³ Values were recorded 3, 12, and 36 months after surgery. For each variable, success was defined as a fair or greater improvement in outcomes (Table 1); the overall surgical outcome, at each time point, was considered successful if two or more variables were judged positive.¹⁴

Table 1.

Criteria for Efficacy of Each Outcome and Overall Success in HPS-PVP

Variables	Efficacy	Outcomes
Symptom		Post/pre ratio of IPSS
	Excellent	≤0.25
	Good	≤0.5
	Fair	≤0.75
	Poor	>0.75
Function		Post–pre difference of Qmax (mL/sec)
	Excellent	≥10
	Good	≥5
	Fair	≥2.5
	Poor	<2.5
QoL		Post–pre difference of QoL index
	Excellent	≥4
	Good	3
	Fair	2, 1
	Poor	≤0
Overall success	Two or more variables were judged as a fair or greater improvement

HPS-PVP=GreenLight HPS™ laser photoselective vaporization of the prostate; IPSS=International Prostate Symptom Score; Qmax=maximal flow rate in uroflowmetry; QoL=quality of life.

All data are presented as mean±standard deviation. Student's t test or Mann–Whitney U test was used to compare continuous variables and the chi-square test or Fisher exact test for categorical variables. Multiple linear regression was used to evaluate the preoperative variables as predictors of surgical outcomes. A two-sided p-value<0.05 was considered statistically significant. Receiver operating characteristic curves were constructed to examine the predictive value. All statistical analyses were undertaken using the SPSS software package (SPSS 13.0, Inc., Chicago, IL).

Results

The baseline characteristics of the groups are shown in Table 1: in group A, the prostate volume and preoperative prostate-specific antigen were lower than group B. There were no differences in the mean QoL scores, but the mean IPSSs, mean Qmax, and mean PVR differed between the groups.

The extents of postoperative improvements in objective and subjective voiding parameters are shown in Figure 1. Mean Qmax improved from 9.8 mL/sec at baseline to 20.2, 18.3, and 17.6 mL/sec at 3, 12, and 36 months in group A, respectively. In group B, mean Qmax improved from 8.2 to 11.6, 13.9, and 12.4 mL/sec at the same time points, respectively. There was a decrease in the mean IPSS from 20.0 to 9.0, 9.8, and 11.4 in group A, respectively, and from 22.1 to 12.2, 12.5, and 12.9 in group B, respectively, at each follow-up. The mean QoL also decreased from 4.0 to 2.4, 2.4, and 2.4 in group A, and from 4.1 to 2.6, 2.5, and 2.5 in group B. All outcome measures improved significantly after surgery compared with baseline, except the IPSS subscore for voiding at 3 months.

FIG. 1.

Improvements of clinical parameters during follow-up (mean±SD) (N=193 at base, 164 at 3 months, 103 at 12 months, 111 at 36 months). Black lines indicate group A (BVE >75%), gray lines indicate group B (BVE ≤75%). BVE=bladder voiding efficiency; Qmax=maximal flow rate; PVR=postvoid residual; IPSS=International Prostate Symptom Score; QoL=quality of life.

Table 2 shows comparisons of postoperative parameters used to measure the surgical outcome for each group. At 3 months after surgery, there were significant differences in Qmax and the IPSS ratio between the groups. For Qmax, the significant difference was maintained thereafter, but the IPSS ratio did not differ between the groups by 12 months. There was no significant difference in QoL measures between the groups at any time point.

Table 2.

Baseline Characteristics (Mean±SD)

	Group A (BVE ≥75%)	Group B (BVE <75%)	p-Value
N (%)	116 (60.1%)	77 (39.9%)
Age, year	67.2±7.2	69.1±8.6	0.092
Prostate volume, mL	49.5±23.7	62.6±35.6	0.006
PSA, ng/mL	3.1±3.7	4.6±4.4	0.014
IPSS	20.0±6.5	22.1±7.2	0.037
Voiding subscore	12.3±4.4	13.3±4.7	0.134
Storage subscore	7.7±3.2	8.8±3.4	0.024
QoL	4.15±1.05	4.07±1.07	0.603
Qmax, mL/sec	9.8±2.6	8.2±3.2	<0.001
Voided volume, mL	278.6±92.5	234.9±65.2	<0.001
PVR, mL	44.2±33.0	158.7±77.3	<0.001

BVE=bladder voiding efficiency; PSA=prostate specific antigen; PVR=postvoid residual.

The overall success rates of surgery were 79.3%, 75.9%, and 75.4% at 3, 12, and 36 months, respectively. Success rates at 3 and 12 months were significantly different between the two groups, but not at 36 months (Table 3). In patients with hypocontractile bladder function (BCI <100) or with equivocal to unobstructed voiding pattern (BOOI <40), the predictive value of urodynamic study indices became more precise when the BVE was added (Table 4). The likelihood of success after surgery rose from 3.0 to 9.4 at 3 months and from 2.4 to 7.8 at 12 months with considering all three indices: BOOI, BCI, and BVE.

Table 3.

Comparisons of Surgical Outcomes Between Two Groups (Mean±SD)

	3 Months	p-Value	12 Months	p-Value	36 Months	p-Value
Difference of Qmax		<0.001		0.044		0.033
Group A	10.4±8.7		8.2±6.1		8.1±6.9
Group B	3.2±6.4		5.4±8.0		4.0±6.4
Ratio of IPSS		0.020		0.392		0.672
Group A	0.45±0.33		0.48±0.36		0.59±0.39
Group B	0.59±0.38		0.55±0.43		0.63±0.56
Difference of QoL		0.235		0.953		0.420
Group A	1.7±1.4		1.58±1.7		1.5±1.5
Group B	1.4±1.6		1.56±1.5		1.2±1.8

Group A: BVE >75%, group B: BVE ≤75%.

Table 4.

Success Rates After Surgery at Each Time Point According to Indices

	3 Months				12 Months				36 Months
	Suc.	p	OR	95% CI	Suc.	p	OR	95% CI	Suc.	p	OR	95% CI
A	86.7	0.003	3.0	1.4, 6.3	82.6	0.043	2.4	1.03, 5.83	76.9	0.672	1.2	0.5, 2.8
B	68.0				66.0				73.5
BCI <100	79.5				73.6				73.6
+A	88.2	0.001	4.2	1.7, 10.3	85.7	0.057	2.8	0.9, 6.8	76.9	0.388	1.5	0.5, 3.9
+B	63.8				61.3				68.6
BOOI <40	77.0				77.5				69.1
+A	88.6	<0.001	7.7	2.9, 20.4	87.8	0.003	5.9	1.8, 19.7	74.4	0.218	1.9	0.6, 5.5
+B	50.0				54.5				60.0
BCI+BOOI	79.8				77.4				68.9
+A	91.0	<0.001	9.4	3.0, 29.1	88.6	0.002	7.8	2.1, 28.9	74.4	0.220	2.0	0.6, 6.1
+B	51.9				50.0				59.1

CI=confidence interval; OR=odds ratio.

Multivariate analysis with logistic regression revealed that BVE correlated significantly with short-term surgical success after adjusting for age, prostate-specific antigen, prostate volume, and voided volume (Table 5). However, there was no significant association between medium or long-term success and BVE. Receiver operating characteristic curves were constructed to analyze the relationship between BVE and success of surgical outcome. A BVE cutoff value of 75% showed the highest sensitivity and specificity for predicting short-term surgical success. A BVE >75% had a negative predictive value of 71.3%, a positive predictive value of 55.8%, a sensitivity of 70.4%, a specificity of 60.5%, a likelihood ratio of a positive test result of 2.044, and a likelihood ratio of a negative test result of 0.561. The area under the receiver operating characteristic curve of BVE was 0.654 for predicting the short-term success of surgical outcomes (Figure 2).

FIG. 2.

Relationship between bladder voiding efficiency and bladder outlet obstruction index. With increasing bladder voiding efficiency, the bladder outlet obstruction index decreased as well (Spearman rank test: 0.251, p<0.001).

Table 5.

Multivariate Logistic Regression Analysis for Predicting Surgical Success at Each Time Point

	Short			Intermediate			Long
Variables	p-Value	OR	95% CI	p-Value	OR	95% CI	p-Value	OR	95% CI
Qmax	<0.001	1.34	1.16, 1.56	0.57	1.04	0.89, 1.22	0.72	1.02	0.88, 1.19
BVE	<0.001	5.91	2.40, 14.55	0.06	2.43	0.94, 6.27	0.61	1.25	0.51, 3.09
Voided volume	0.597	0.99	0.99, 1.00	0.54	0.99	0.99, 1.00	0.66	0.99	0.99, 1.00
PSA	0.146	1.09	0.96, 1.23	0.89	0.99	0.87, 1.12	0.45	0.95	0.83, 1.08
Prostate volume	0.044	0.98	0.96, 1.00	0.95	1.00	0.98, 1.01	0.68	0.99	0.97, 1.01

Discussion

To our knowledge, this study demonstrates for the first time that BVE was significantly related with surgical outcomes after HPS-PVP, and that a large PVR fraction may predict a relatively slight improvement after surgery. Our findings suggest that a BVE cutoff value of 75% predicted a relatively poor outcome, and that BVE ≤75% was significantly correlated with a BOOI <40.

The degree of bladder emptying is clinically more significant than the absolute value of PVR. The mean PVR in normal men is 141 mL. The cutoff value of PVR that predicts urologic adverse outcomes has variously been found to be 50, 100,⁴ 150,⁵ 180,⁶ 250,⁷ 300,⁸ and 500 mL.⁹ These studies all measured absolute PVR volume rather than the relative PVR fraction. Because PVR has marked inter- and intraindividual variability, the test should be performed several times to improve precision, particularly if the first residual urine volume is substantial.³ Ku et al. suggested that BVE is influenced by the individual premicturitional bladder capacity, which reduces intraindividual variation.¹¹ Therefore, BVE is considered a more accurate and reliably reproducible index of bladder emptying, and may reflect voiding function in patients with LUTS/BPH more accurately than PVR.

Interestingly, although BVE is thought to be a consequence of bladder contractility against urethral resistance, our results showed that BVE correlated with BOOI rather than BCI. The likelihood of a large PVR was a 2.5 times more likely when BOOI was ≥40 than <40, but BCI did not influence PVR, even after taking age, prostate volume, and IPSS into account. In our cohort, it appears that the PVR volume was determined more by bladder outlet obstruction than hypocontractility. In rats, it has recently been found that bladder mechanoreceptors become desensitized in an experimental model of bladder outlet obstruction, causing premature termination of micturition, and thus, a relatively larger PVR volume.¹⁵ However, satisfactory reflex contractions were still evoked during micturition, supporting the argument that bladder contractility changes might not be the main cause of a large PVR in BOO.

The predictive value of urodynamic parameters for surgical outcome in our cohort was contrary to those in other studies.^16,17 We found that patients with an unobstructed or hypocontractile index also experienced significant improvements in all outcome measures after surgery. Abrams suggested that BVE should be taken into account along with BOOI and BCI to describe an individual patient's voiding function more precisely.¹⁰ In this study, BVE alone for success at 3 months showed a sensitivity of 67%, a specificity of 61%, a positive predictive value of 84%, and a negative predictive value of 37%, but in patients with BOOI <40 and BCI <100, the values increased to a sensitivity of 85%, a specificity of 70%, a positive predictive value of 90%, and a negative predictive value of 59%. Although BVE was an accurate predictor of surgical outcome, the predictive value of BVE was superior in patients with equivocal to unobstructed and hypocontractile urodynamic findings, in whom it would have been challenging to make management decisions on the basis of pressure-flow studies alone.

Our study has several limitations, the main one being its retrospective design. Nevertheless, we were careful to maximize the quantity of data available for analysis and minimize bias by using a number of strict inclusion and exclusion criteria. Another limitation was that a large proportion (42%) of patients was lost to follow-up, although the numbers were similar in groups A and B. The main reason for patients being lost to follow-up was that their LUTS had improved sufficiently to warrant discharge; consequently, they tended not to return for surveillance or outpatient appointments. Therefore, their nonparticipation would not have substantially influenced our findings. Other limitations include a relatively small cohort and the lack of a longer term follow-up. There is a need for further prospective studies with a longer follow-up and a larger number of participants.

Conclusions

In conclusion, this study demonstrates that a large PVR may predict a relatively slight improvement in short-term outcome after surgery using HPS-PVP. The cutoff value for PVR to predict short-term surgical success was 25% or more of residual fraction.

Footnotes

Disclosure Statement

No competing financial interests exist.

Abbreviations Used

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