Three‐dimensional bladder ultrasound to measure daily urinary bladder volume in hospitalized dogs

Abstract Background Urinary bladder volume (UBV) and urine residual volume (URV) provide important information for hospitalized dogs and might allow recognition of urine retention. Objective Using 3‐dimensional (3D) ultrasound to monitor daily URV is a safe and effective way to recognize urinary retention. Animals Twenty‐five client‐owned hospitalized dogs. Methods Prospective, observational study. UBV and URV were measured using 3D ultrasound daily at approximately the same time. UBV was measured, the dog was taken for a 5‐minute controlled leash walk, then URV was estimated. Concurrent use of opioids, anesthetics, and fluids administered IV were recorded. Results Daily URVs were >0.4 mL/kg in 22 of 25 dogs on at least 1 day of hospitalization. Seventeen of 25 dogs had an abnormal URV at the time of discharge. Of 18 dogs that were anesthetized while hospitalized, 16 had a URV >0.4 mL/kg with a mean of 4.34 mL/kg (range, 0.5‐13.4 mL/kg). No statistical difference in degree of URV was found based on the use of anesthesia, administration of fluids IV, or opioids. Weight was significantly associated with URV; dogs <10 kg had a higher URV per unit mass than dogs >10 kg (P = .001). Conclusions and Clinical Importance Use of a 3D ultrasound device to measure daily UBV and URV in hospitalized dogs provides a safe estimate of bladder volume in real‐time. Monitoring daily URV might help in early identification of patients that are retaining urine, thereby preventing potential adverse effects of urethral catheterization or prolonged urinary retention.


| INTRODUCTION
Urine residual volume (URV) is the volume of urine remaining in the bladder after the completion of voiding and is a clinically important measurement for assessing bladder function. Using 2-dimensional (2D) ultrasound calculations, dogs have a reported normal URV of 0.2 to 0.4 mL/kg, but with a wide range of values of 0.1 to 3.4 mL/kg. 1 Dogs retaining >0.4 mL/kg are suspected to have incomplete voiding.
Furthermore, in human medicine, a persistent elevated URV is suggestive of urine retention and can lead to serious clinical consequences such as detrusor atony and urinary tract infection (UTI). [2][3][4][5] Multiple factors, such as opioids, anesthesia, and surgery are implicated to cause urinary retention in people, which has led to routine in-hospital monitoring of bladder size. 2,6 Monitoring for urinary retention in hospitalized dogs is not routinely performed and the prevalence of urinary retention in dogs is unknown.
Urine residual volume can be measured directly via urethral catheterization or indirectly via 2D ultrasound. [7][8][9][10][11] However, these techniques impose risks such as sedation-related adverse events or catheter-associated UTI, and require appropriate operator skill and equipment, respectively. [12][13][14][15] The incidence of CAUTI in hospitalized dogs with indwelling urinary catheters or intermittent catheterization is 8%-32%. [12][13][14][15] The incidence of catheter-associated UTIs in dogs along with the daily maintenance requirements of a urethral catheter highlights the need and importance for alternative methods to monitor daily URV. [12][13][14][15] While 2D B-mode ultrasound is an accurate method of measuring URV in dogs, it requires trained personnel for adequate image acquisition and dogs positioned in dorsal recumbency, which might not be suitable for critically ill hospitalized dogs. 1,8,16 Application of a 3-dimensional (3D) ultrasound device is used for point-of-care volumetric assessments of the human urinary bladder and has been the method of choice for monitoring urinary bladder volume (UBV) in hospitalized people. [9][10][11]17,18 The 3D ultrasound device is intended to be used by operators with varying levels of expertise, allows for quick "bedside" measurements, and limits examination time because it reports an estimated volume in real time. Clinical application of 3D ultrasound in veterinary medicine is validated in dogs, and accurate in determining UBV and comparable to the gold standard 2D ultrasound method. 19,20 Detection of an increased URV and frequent monitoring in hospitalized dogs could be useful in identifying early signs of urinary retention. Use of 3D ultrasound could allow for easy, point-of-care daily monitoring without the need for urethral catheterization.
The purposes of this study are to (a) investigate for daily urinary retention in hospitalized dogs using a 3D ultrasound device by measuring URV, and (b) evaluate for any associated factors that could be contributing to increases in URV, when present. We hypothesized that the majority of hospitalized dogs would have evidence of urinary retention characterized by an increased URV as previously reported. 1 We further hypothesized that various factors such as general anesthesia, IV fluids, or use of opioids would contribute and that those with higher body weight would have a higher URV based on previous studies. 21 2 | MATERIALS AND METHODS

| Study design and animals
This prospective, observational study was performed at the North Carolina State University Veterinary Hospital (NCSU-VH). Dogs hospitalized in any of the available housing areas in NCSU-VH, including the general hospital ward, intermediate care ward, or the intensive care unit were recruited. If the dog met the inclusion criteria, the protocol was reviewed with the client and signed or verbal consent was obtained.
The study protocol was reviewed, approved, and conducted in accordance with the North Carolina State Animal Care and Use Committee.
Inclusion criteria included hospitalization in any of the above areas for a minimum of 24 hours, ability to walk and urinate without assistance, no behavioral aggression, and no reported lower urinary tract disorders that might affect their ability to naturally void. Enrolled dogs had to weigh ≥5 kg based on increased accuracy of the device to capture bladder tracings in dogs above this weight. 19,20 Dogs were excluded if they had suspected oliguric or anuric renal failure, any history of a neurologic disorder, because of the known impact of these disorders on urine production or bladder function, or if they received an epidural during their hospitalization.

| Data collection
Dogs were enrolled in the study from the sample of dogs that presented to the NCSU-VH. Recorded data for each dog included: age (years), sex, the center of the screen with a surrounding green line, a simple "point and click" technique was used. In less than 5 seconds, V MODE technology automatically captured 12 B-mode slices of the bladder and displayed the calculated volume results in real time. A total of 3 acceptable (green line) measurements were acquired for each dog, then averaged to determine the UBV and the URV. The URV was then calculated based on mL/kg.

| Statistical analysis
All analyses were conducted in R version 3.6.3. Numerical data were assessed for normality by visual inspection. An unpaired t test was used to compare urinary retention rates among dogs that did or did not receive opioid therapy, anesthesia, and IV fluids. The package lme4 was used to fit the models and P values were provided by the lmerTest package. Statistical significance was set at P < .05.
Because of the exploratory nature of this study and the small sample sizes, mostly descriptive rather than inferential statistics were utilized in order to eliminate type II statistical errors. Variables were described as mean ± SD. Median and ranges are also reported due to small sample size.
When assessing daily URV, 3 different cutoff values were used to evaluate including 0.4, 1.0, and 3.0 mL/kg based on previous studies. 1 3 | RESULTS 3.1 | Breed, age, weight, sex, hospitalization length, reason for hospitalization Twenty-five dogs met the inclusion criteria and were enrolled in the study. One dog represented to the hospital at a later time and was re-enrolled for the study and measured at 2 different hospitalizations, however was counted as a single dog for the overall sample size. The demographic information for the study sample is summarized in Table 1. Reasons for hospitalization included a surgical procedure (15), supportive care (8), or radiation therapy (2). Eighteen of the 25 dogs were anesthetized during their hospitalization, occurring either daily (3), or on day 1 (5), day 2 (9), or day 3 (1) of hospitalization. Six dogs had a soft tissue surgical procedure including a total ear canal ablation (2), thoracotomy (1), splenectomy (1), forelimb amputation (1) (6), and anemia and lethargy (1).

| Micturition behavior
The median number of attempts to urinate was 1 (range, 0-4) and the median successful attempts was 1 (range, 0-3  Weight was significantly associated with URV, with dogs <10 kg having a higher post-void retention per unit mass than dogs >10 kg (P = .001).

| Effect of anesthesia on urinary retention
There were 28 anesthetics, 13 were daily general anesthesia for radia-  F I G U R E 2 Line graph depicting daily urine residual volume (URV) in hospitalized dogs. Each line presents 1 dog in hospital increased URV on day 1, suggesting that the effects of urinary retention might have persisted after the initial hospitalization.
Dogs that had anesthesia during their hospitalization did not retain significantly more than those that did not have anesthesia (mean 4.51 and 3.67 mL/kg respectively; P = .07).

| Length of hospitalization
The median length of hospitalization was 3 days (range, 1.5-6).
Regardless of the cutoff value used to define urinary retention (0.4, 1.0, or 3.0 mL/kg), there was no direct effect on length of hospitalization and increasing URV (P = .93).

| DISCUSSION
The use of 3D ultrasound has been shown as a safe, efficient, and reliable tool for measuring canine UBV, thus allowing for daily monitoring of URV as an indicator of urinary retention. 19,20 In this study, we dem- Contrary to previous studies which found that those of higher body weights had higher URV, 21 we found that heavier dogs had smaller URV. These differences could be artifacts of the small sample size used in each study. Another speculative theory is that larger breed dogs could have less in-hospital anxiety than smaller breed dogs. Behavioral traits, though not evaluated in this study, might impact micturition behavior; this potential association would be interesting to study further. Micturition behaviors were subjectively evaluated and observed during hospitalization in this study. We found that male dogs made more urination attempts than did female dogs but there was no difference in URV between male and female dogs. A previous study also found that normal male dogs made more urination attempts than did normal female dogs. 1 The importance of this on URV is not known.
A major limitation of this study was the majority of dogs enrolled received an opioid along with fluids IV and multiple other medications including sedatives. This made it difficult to determine the effect of these factors on URV. This was an explorative study to determine if URV increases in hospitalized dogs and further investigation with a larger group and a control group is needed to further assess the effects of these variables on URV.
In conclusion, this study suggests that hospitalized dogs have URV's >0.4 mL/kg and might experience urinary retention during hospitalization; however, the long-standing effects of this elevated URV are unknown at this time. The use of a safe and efficient "cage-side" 3D ultrasound device to measure daily UBV and URV in hospitalized dogs provides a quick estimate of bladder volume in real-time and decreases the need for urethral catheterization. The device could help in early identification of dogs that are retaining urine and ultimately prevent the possible complications of urinary retention.