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Diagnosis of Postoperative Urinary Retention Using a Simplified Ultrasound Bladder Measurement

Daurat, Aurélien MD*; Choquet, Olivier MD*; Bringuier, Sophie PharmD, PhD; Charbit, Jonathan MD*; Egan, Michael MD*; Capdevila, Xavier MD, PhD*

doi: 10.1213/ANE.0000000000000595
Technology, Computing, and Simulation: Research Report

BACKGROUND: In this study, we sought to determine whether a simplified ultrasound measurement of the largest transverse diameter, using a standard ultrasound machine, could be used to diagnose postoperative urinary retention (POUR). This method may replace expensive bladder volume measuring devices or a more complex ultrasound procedure (involving the measurement of 3 bladder diameters).

METHODS: Patients at risk of POUR if unable to void after orthopedic surgery were evaluated in the postanesthesia care unit before discharge. Bladder diameter was first measured using a portable ultrasound device (Vscan®; GE Healthcare, Wauwatosa, WI). An automated evaluation of bladder volume was then performed (Bladderscan® BVI 3000; Diagnostic Ultrasound, Redmond, WA). Finally, when a bladder catheterization was performed, the actual urinary volume was measured. The main outcome was a bladder volume ≥600 mL as measured using the automated ultrasound scanner (Bladderscan BVI 3000) or by catheterization. Correlations between bladder volumes and diameter were studied and receiver operating characteristic curves were constructed to determine the performance in predicting a bladder volume ≥600 mL. A “gray zone” approach was developed because a single cutoff value may not always be clinically significant.

RESULTS: One hundred patients were included and underwent a Bladderscan measurement. Urinary volume after catheterization was obtained in 49 patients. A significant correlation was found between the largest transverse diameter and urinary volumes assessed by the 2 methods (Bladderscan and catheterization). Pearson correlation coefficients were r = 0.80 (95% confidence interval [CI], 0.72–0.86; P < 0.001) and r = 0.79 (95% CI, 0.65–0.88; P < 0.001), respectively. The area under the receiver operating characteristic curves for the prediction of a bladder volume ≥600 mL were 0.94 (95% CI, 0.88–0.98) and 0.91 (95% CI, 0.79–0.97), respectively, for urinary volumes assessed by Bladderscan and catheterization. The optimal cutoff value was 9.7 cm for both methods. The gray zone was narrow, ranging from 9.7 to 10.7 cm thus limiting inconclusive measurements.

CONCLUSIONS: A simple ultrasound measurement of the largest transverse bladder diameter seemed to be helpful to exclude or confirm POUR.

Published ahead of print January 30, 2015

*Department of Anesthesia and Critical Care Medicine, Lapeyronie University Hospital, Montpellier, France; and Departments of Anesthesia and Critical Care Medicine and Biostatistics, Lapeyronie University Hospital, Montpellier, France.

Accepted for publication October 31, 2014.

Published ahead of print January 30, 2015

Funding: Institutional funding.

The authors declare no conflicts of interest.

This report was previously presented, in part, at the Congres de la SFAR 2012.

Reprints will not be available from the authors.

Address correspondence to Aurélien Daurat, MD, Department of Anesthesia and Critical Care Medicine, Lapeyronie University Hospital, 5 rue Nozeran 34090 Montpellier, France. Address e-mail to a-daurat@chu-montpellier.fr.

Postoperative urinary retention (POUR) is an overdistention of the bladder that occurs frequently in the postoperative period. It can lead to both local and general complications, such as infection, delirium, detrusor muscle damage,1 or even cardiac arrhythmia,2 and also may delay hospital discharge.3 Because this adverse event primarily affects patients with risk factors,4,5 screening of such patients is recommended after surgery.6

Real-time ultrasonography may provide a reliable estimation of bladder volume. However, it is a technically demanding procedure, involving the measurement of 3 diameters (transverse, anteroposterior, and superoinferior) in 2 different planes (transverse and sagittal) and the application of a proportionality constant.7 Specific automated bladder ultrasound devices that measure bladder volumes have been developed to facilitate the diagnosis of POUR. Indeed, several studies have confirmed their high degree of accuracy compared with actual urine volumes obtained after bladder emptying.8 Nonetheless, these devices remain costly and often are not readily available.

During the past 10 years, medical professionals have continued to expand the clinical applications of ultrasound. This is especially true in the field of anesthesia, and ultrasound devices are now readily available in most centers. This raises the question of the need to continue to rely on specific expensive devices.

The aim of this study was to determine whether a simplified ultrasound measurement of the largest transverse diameter using a standard ultrasound device could reliably diagnose POUR.

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METHODS

Design

After ethical review board approval (Comité de Protection des Personnes Sud Méditerranée III, Nîmes, France) and registration in the French Database for Clinical Trials (ID RCB number: 2013-A00757-38), a prospective observational study was performed from July to October 2013 in the orthopedic surgery unit of a tertiary university hospital. Written informed patient consent was obtained preoperatively.

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Patients

All postoperative patients with at least one of the following risk factors for POUR immediately after surgery, i.e., age ≥50 years, male sex, prostate adenoma, surgery ≥60 minutes, IV fluids ≥750 mL intraoperatively, spinal anesthesia, and IV morphine, were evaluated immediately before discharge from the postanesthesia care unit (PACU) if unable to void, as has been recommended.2 Patients younger than 18 years of age, patient refusal, those already included in another study, or those who received only peripheral nerve blockade were excluded.

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Measurements

A small portable ultrasound device (Vscan®; GE Healthcare, Wauwatosa, WI) with a low-frequency transducer (from 1.7 to 3.8 Hz) was used. The largest transverse bladder diameter was measured in the PACU by a nurse not involved in the care of the particular patient. The probe was positioned directly above the pubis (Fig. 1, A), and after visualization of the largest transversal image of the bladder, the screen was frozen and the largest transverse diameter measured (Fig. 1, B). The axis used to obtain the largest transverse diameter was left to the discretion of the operator.

Figure 1

Figure 1

To assess the reproducibility of this measurement, the first author, who was blinded to the nurse’s findings, performed a second measurement on a subset of patients. Volume assessment of the bladder was then performed using an automated bladder ultrasound scanner according to the manufacturer’s instructions (Bladderscan® BVI 3000; Diagnostic Ultrasound, Redmond, WA) by a nurse also blinded to the result of the initial transverse diameter measurement. Finally, when a bladder catheterization was performed, the actual urinary volume was measured.

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Nurse Training

All PACU nurses from the unit were provided with brief training in the use of ultrasonography before the study, which included a 30-minute explanatory review and a session of practical application.

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Catheterization

Catheterization of the urinary bladder was performed in the PACU according to the ward protocol, that is, if the volume measured by the Bladderscan was ≥600 mL or if the anesthesiologist-in-charge of the patient deemed it necessary (e.g., indwelling catheter to monitor urinary output). The main outcome was a bladder volume of ≥600 mL, as measured using the automated ultrasound scanner (Bladderscan BVI 3000) or by catheterization.

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Analysis

A minimal sample size of 84 patients was calculated to obtain an area under the receiver operating characteristic curve (ROC) of 0.75, as recommended by Ray et al.9 The POUR incidence of 25% found in the study by Balderi et al.10 was used for this calculation. Because all the patients would not be catheterized, we finally increased this number to 100 patients.

Distribution of continuous variables was tested for normality using the Shapiro-Wilk test, all P > 0.05 among the data were considered normally distributed, and these data were expressed by mean (SD). Nonparametric data were expressed by median (interquartile range). Comparisons between groups for parametric continuous data were performed using Student t test. The Pearson correlation coefficient (r) was calculated to examine the relation between diameter and volume. Because we could not predict a priori whether the largest bladder diameter was dependent on the urine volume, and because an empty bladder may have a diameter >0, we did not use linear regression with forced intercept of zero for this purpose.

ROCs were constructed to determine the performance of the largest transverse diameter in predicting a bladder volume ≥600 mL as measured by the Bladderscan or catheterization. The area under the curve was calculated and the optimum cutoff assessed using the method reported by Youden.11 Calculation of the 95% confidence intervals (CIs) of the different cutoff and of the area under the curve was performed using the binomial exact method.

Because a single cutoff value may not always be clinically significant, a “gray zone” approach as described by Ray et al.9 was adopted. The same method as Cannesson et al.12 was applied. It was decided a priori that the values associated with neither a sensitivity lower than 90% nor a specificity inferior to 90% be considered inconclusive, thereby defining a gray zone of clinical uncertainty. Two cutoff points of clinical interest were determined for each urinary volume measurement method (Bladderscan and catheterization), thus corresponding to the lower and upper limits of the gray zone.

Interobserver reliability between the measures performed by the principal investigator and the nurses was assessed by calculating the intraclass correlation coefficient (ICC) in a subset of patients. An ICC > 0.75 corresponds to a good agreement. ICC was calculated using a 2-way mixed-effect model with an absolute agreement definition as described by McGraw and Wong13 (for the CI calculation, see the “ICC (A,1)” equation in Table 7 in the study by McGraw and Wong). Statistical analyses were performed using MedCalc for Windows version 12.5.0.0 (MedCalc Software bvba, Mariakerke, Belgium) and the R software (version 3.0.2). Results with P values ≤0.05 were considered statistically significant.

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RESULTS

Of the 1932 patients who underwent an orthopedic surgical procedure in our unit, 100 patients were eventually included and received bladder diameter and Bladderscan measurement. Urinary volume after catheterization was obtained in 49 patients (Fig. 2). Characteristics and intraoperative data are listed in Table 1.

Table 1

Table 1

Figure 2

Figure 2

The mean largest diameter was significantly greater in patients with a Bladderscan ≥600 mL compared with those with a Bladderscan <600 mL: 11.0 (±1.2) vs 8.2 cm (±1.6); P < 0.001. It was also larger in patients with a catheterized urinary volume ≥600 mL compared with those with a urinary volume <600 mL: 11.2 (±1.4) vs 8.6 cm (±1.6); P < 0.001. The median bladder volume measured by Bladderscan BVI was 350 mL (interquartile range, 212–600), and mean urinary volume after catheterization was 572 ± 312 mL.

A significant positive correlation was found between the largest bladder diameter and the urine volume as measured by the Bladderscan and after catheterization, respectively, r = 0.80 (95% CI, 0.72–0.86) and r = 0.79 (95% CI, 0.65–0.88; Fig. 3).

Figure 3

Figure 3

ICC between 20 largest transverse diameter measurements performed by an investigator and the nurses was 0.92 (95% CI, 0.82–0.97).

The area under the ROC curves for the prediction of a bladder volume ≥600 mL measured by the Bladderscan or catheterization was 0.94 (95% CI, 0.88–0.98, P < 0.001) and 0.91 (95% CI, 0.79–0.97; P < 0.001), respectively (Fig. 4). The best cutoff value determined using the Youden method was 9.7 cm for prediction of a urine volume ≥600 mL measured both by Bladderscan and catheterization. According to predefined sensibility and specificity values, the cutoff values determining the gray zone of clinical uncertainty were the following:

  • (1) The lower threshold was 9.7 cm for both volume measurement methods. This threshold provided, respectively, for the prediction of a catheterized urine volume and a Bladderscan volume ≥600 mL, a sensitivity of 0.96 (95% CI, 0.79–0.99) and 0.96 (95% CI, 0.80–0.99) and a negative predictive value of 0.95 (95% CI, 0.74–0.99) and 0.98 (95% CI, 0.92–1.00).
  • (2) The greater threshold was 10.7 cm for the urinary volume after catheterization and 10 cm for the Bladderscan. Respectively, the specificity values associated with these threshold were 0.92 (95% CI, 0.73–0.99) and 0.93 (95% CI, 0.85–0.98) and the positive predictive values were 0.87 (95% CI, 0.62–0.98) and 0.81 (95% CI, 0.62–0.94).
Figure 4

Figure 4

Values of sensitivity, specificity, positive and negative predictive values, and likelihood ratio for the various thresholds are listed in Table 2.

Table 2

Table 2

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DISCUSSION

This observational study of 100 patients suggests that the postoperative measurement of the largest transverse diameter is an efficient tool for the diagnosis of POUR in at-risk patients. Indeed, there was good correlation between this measurement and the urinary volume measured when Bladderscan BVI 3000 or catheterization was used (r = 0.80 and r = 0.79, respectively). The area under the ROC curves were large (0.94 and 0.91, respectively), indicating good performance for the diagnosis of a urinary volume ≥600 mL. Moreover, a largest transverse diameter of ≤9.7 cm excluded a bladder urinary volume ≥600 mL with a high negative predictive value of 98% (95% CI, 0.92–1.00) and 95% (95% CI, 0.74–0.99), whereas measurements of >10 and 10.7 cm indicated a bladder volume ≥600 mL with a positive predictive value of 81% (95% CI, 0.62–0.94) and 87% (95% CI, 0.62–0.98), values referring to Bladderscan or catheterized volume, respectively. Nurses appeared to perform the procedure efficiently after only a brief and standardized hands-on training session with a good interobserver reliability, therefore avoiding the recourse to complex 3-diameter bladder evaluation.14

The performance of largest transverse diameter measurement was compared with both the catheterized volume and the Bladderscan because catheterized urine volume would have not been available for all patients but mainly for those with a large urinary volume according to the local protocol. This could have lead to an overestimation of the performance of the diameter measurement in predicting a large bladder volume. This risk might be limited by combining the 2 areas of uncertainty, so we suggest using a clinical gray zone ranging from 9.7 to 10.7 cm and considering measurements between these thresholds inconclusive.

The findings are interesting from several perspectives. First, the high negative predictive values suggest that patients at risk of POUR with a largest transverse diameter ≤9.7 cm could be discharged without catheterization, similar to the recommendations using an automatedscanner when the volume is <600 mL (e.g., Bladderscan BVI 3000).2 Second, given the positive predictive value of this threshold, it would appear reasonable to catheterize patients with a bladder diameter >10.7 cm, therefore avoiding potential POUR complications.1,3 Finally, the gray zone between these 2 cutoff points appears relatively slight, thus minimizing the number of inconclusive results. Given the relatively small sample size, however, CIs are rather large and the number of inconclusive results may have been underestimated, leading us to advise caution for boundary values. To our knowledge, this is the first study assessing a simplified ultrasound bladder measurement for the detection of POUR. In this study, a pocket-sized ultrasound apparatus equipped with a low-frequency transducer was used because of its portability. Images produced by this device are comparable with those provided by conventional machines.15,16

Nevertheless, this study has several limitations. There is no universally accepted definition of POUR with bladder volumes ranging from 400 to 600 mL. In this study, a bladder volume ≥600 mL was chosen because catheterization is performed at this volume as recommended in our local protocol.2,6 The automated ultrasound device (Bladderscan BVI 3000) was regarded as comparative because it provided a noninvasive evaluation of the bladder volume for all patients, whereas catheterization was only performed in 49 patients. Indeed, good agreement between the Bladderscan estimates of urinary bladder volume and urine volume measured after emptying the bladder has been found in several studies.8,17,18 However, this measurement does not have the accuracy of a “gold standard,” with a mean difference of −21.5 mL and limits of agreement between −147 and +104 mL.8 Obtaining actual bladder volumes by catheterization for all patients would have provided more precise results to evaluate the accuracy of our ultrasound measurements. This would, however, have necessitated catheterizing all patients, which would be ethically questionable when one considers the possible complications, for example, urinary tract infection.19 We also did not include patients who voided before discharge to the PACU. Indeed, even if the voided volume could have been compared with an ultrasound assessment of the urinary bladder, the residual volume would have introduced a bias. Finally, a follow-up procedure on patients after discharge to the PACU was not performed. It may have obviated the need to catheterize some of these patients.

In conclusion, our findings suggest that a simple ultrasound measurement of the largest transverse diameter using a standard ultrasound device provides valuable aid in the management of patients at risk of POUR postoperatively. Those with a largest transverse diameter of ≤9.7 cm may be discharged without voiding. Catheterization could be considered if this measurement is >10.7 cm. The findings of this study directly call into question the usefulness of expensive specific devices for assessing POUR in the postoperative period.

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DISCLOSURES

Name: Aurélien Daurat, MD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Aurélien Daurat has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Name: Olivier Choquet, MD.

Contribution: This author helped design the study and conduct the study.

Attestation: Olivier Choquet approved the final manuscript.

Name: Sophie Bringuier, PharmD, PhD.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Sophie Bringuier has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Name: Jonathan Charbit, MD.

Contribution: This author helped design the study, analyze the data, and write the manuscript.

Attestation: Jonathan Charbit approved the final manuscript.

Name: Michael Egan, MD.

Contribution: This author helped conduct the study and write the manuscript.

Attestation: Michael Egan approved the final manuscript.

Name: Xavier Capdevila, MD, PhD.

Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.

Attestation: Xavier Capdevila has seen the original study data and approved the final manuscript.

This manuscript was handled by: Maxime Cannesson, MD, PhD.

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ACKNOWLEDGMENTS

We thank all recovery room nurses for their participation in this study.

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