Introduction

Urine residual volume (URV) is defined as the volume of urine remaining in the urinary bladder immediately after completion of voluntary micturition.¹ Excessive URV can be a result of incomplete voiding due to neurologic disease, functional and structural obstructive diseases, and secondary to drugs, such as opioids and epidural administration.^1–5 Subsequently, detrusor atony, urinary bladder rupture, cystitis, vesicoureteral reflux, pyelonephritis, and/or uremia can occur as a complication, highlighting the importance of defining normal URV to guide decision making regarding clinical intervention.¹

An established normal URV from direct measurement is lacking in veterinary medicine. Normal URV in dogs has previously been evaluated using two-dimensional (2D) ultrasonographic measurements and found a median URV of 0.2 mL/kg; however, there was a wide range of variation (0.1–3.4 mL/kg).¹ Two-dimensional ultrasound has been used to estimate urinary bladder volume in dogs.^1,2,6–11 Its use for estimating URV was originally validated in a study measuring volumes from 5–590 mL, with a median volume of 45 mL.⁶ The formulas derived from human medicine that were used in the original studies have inconsistencies associated with bladder shape and size.^2,6–8,12 More recently, a study found an average underestimation of 21.68 mL using 2D ultrasound methods to estimate URV, resulting in a clinically unacceptable variation for predicting actual bladder volume.¹⁰ Similarly, errors in variation due to bladder shape and difficulty of applying geometric formulas to partially filled bladders and small volumes in human medicine have furthermore been shown to have a larger degree of error.^13–15

Numerous different formulas have been studied for estimating URVs in both veterinary and human medicine with varying accuracy. Formulas for both 2D and three-dimensional ultrasound images have been and are currently being investigated, but none have been regarded as the gold standard.^{6,8–11,16–21}

The primary objective of this study was to determine normal URV in a population of healthy male and female dogs based on direct measurement. A second objective was to determine if these results are comparable with corresponding 2D ultrasonographic estimations for minimally distended urinary bladders in dogs. We hypothesized that the URV measured directly will be higher compared to ultrasonographic estimated volume.

Materials and Methods

Procedure

The study protocol was approved and followed the hospital’s Animal Care and Use Committee guidelines. Information collected from the owner included signalment (age, sex, and breed), body weight, and history, including current urination habits (typical time to first urination attempt, single versus multiple attempts, and location taken to urinate), previous abnormal urination behavior (inability to urinate, increased frequency or volume compared to the individual dog’s normal), and general wellness. A physical examination including a neurologic evaluation was performed by a veterinarian. All dogs were walked outside for exactly 5 min followed by a transabdominal ultrasound of the urinary bladder while in dorsal recumbency performed by a veterinarian with a 12 MHz convex transducer if voluntary urination was observed during the walk. Images were obtained in longitudinal and transverse planes, and measurements of the length, width, and height of the bladder lumen were made.

Immediately following transabdominal ultrasonography, a urinary catheter was placed, URV was collected, and the volume recorded. Male dogs were placed in dorsal recumbency and catheterized with a red rubber catheter of appropriate size using sterile technique. A 3 cc, 6 cc, or 12 cc syringe was attached to the catheter, and all URV was removed and measured. Sedation with 5–10 μg/kg dexmedetomidine IV was used if needed during the urine collection. All female dogs were sedated after the abdominal ultrasound with 5–10 μg/kg dexmedetomidine IV ± 0.2 mg/kg butorphanol IV for urine collection. They were placed in dorsal recumbency with the hindlimbs pulled cranially. A red rubber catheter of appropriate size was used for speculum-assisted catheterization using sterile technique. A 3 cc, 6 cc, or 12 cc syringe was attached to the catheter, and all residual urine was removed and measured. A repeat transabdominal ultrasound of the urinary bladder was performed to confirm an empty bladder was achieved before removal of the catheter. All dogs who required dexmedetomidine sedation were reversed with atipamezole intramuscularly immediately following the procedure.

The protocol for ultrasonographic imaging and measurement of urinary bladder volume was performed according to Atalan et al. (Figures 1 and 2).^2,6 The maximal length (L) is defined as the measurement from the intraluminal wall of the apex to the bladder neck, and the maximal depth (DL) in the longitudinal section is measured from dorsal to ventral intraluminal wall. The maximal width (W) is defined as the measurement from the right and left intraluminal wall on transverse plane, and the depth (DT) from dorsal to ventral intraluminal wall on transverse plane. The urine volume was estimated using the previous described formula: Urine volume estimation = Length × Width × × 0.625.^2,6

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The urine volume was measured and recorded. A urine dipstick was completed following labeled instructions, and specific gravity was measured by the author within 15 min of urine collection. Abnormal urine dipstick values were defined as glucose ≥ trace, ketone ≥ 1+, presence of blood, pH protein ≥ 1+, urobilinogen ≥ 1+, nitrite ≥ 1, presence of leukocyte esterase. Abnormal urine concentration was defined by a specific gravity of <1.030. The volume of urine per kilogram of body weight was calculated for both the volumes of urine determined from catheterization and the ultrasonographic measurement for each dog.

Results

Direct Measurement

No statistical significance was found between male and female URV (P = 0.2) (Table 1). There were two outliers identified from visual inspection, and they were confirmed outliers with the Tukey test and were excluded (Figure 3). URV ranged from 0–0.47 mL/kg (confidence interval = 90%), with a mean volume of 0.21 mL/kg and a median value of 0.175 mL/kg.

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TABLE 1 Residual Urine Volumes from Direct Measurement for Male and Female Dog Data After Exclusion of Outliers

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Ultrasonographic Measurement

Of the 22 dogs included in the study, 18 had ultrasound data available for review. The remaining four dogs had transabdominal ultrasounds performed, but the images were unable to be saved as a result of technical complications and, therefore, were unavailable for measurement. There was one visual outlier excluded (Figure 4). There was no significant difference found between URV when it was measured directly with a urinary catheter and estimated using the ultrasonographic method (P = 0.1066).

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Discussion

The present study found a mean normal URV in dogs to be 0.21 mL/kg with a range of 0–0.47 mL/kg based on direct measurements. Our hypothesis was rejected because no difference in URV was detected using 2D ultrasonographic estimation.

The median value of 0.175 mL/kg is comparable to previous reported values, and the individual variation is a consistent finding in previous studies.¹ However, the upper end of the range in this study was substantially lower than the previous reported range of <3.4 mL/kg.¹ Interestingly, the two values found to be outliers in this study, 1.25 mL/kg and 1.3 mL/kg, were also much less. A larger sample size may uncover a larger individual variation to provide a similar range to the historic study. Other factors may have contributed to these differences, including body weight of the dogs, male to female ratio of the sample population, the neuter status of the sample population, and bladder fullness, none of which were controlled for between the two studies.

The current study included dogs ranging from 5.7–49 kg, with an average of 17 kg, which is comparable to the original study including dogs from 3–38 kg with an average of 16.5 kg. The effect of body weight on URV in veterinary medicine is not well established. Previous studies have concluded that dogs of higher body weight had a higher URV,²² whereas another study found conflicting results with heavier dogs having a smaller URV.²³ Further investigation with a larger group of dogs with equal distribution of small, median, and large breed dogs is needed to determine the influence of weight and size on URV in dogs.

Micturition behavior differences between male and female dogs have been documented.^1,22 Male dogs have increased urination attempts compared with female dogs. Micturition behavioral patterns were not specifically evaluated in this study, but no difference between male and female dogs URV was found. This is consistent with previous findings, questioning the impact of this behavior on URV.²² However, it is plausible that the time walked, the location, and the comfort level of the dog could affect the number of urination attempts and therefore the overall volume voided, making it difficult to compare any influence, if present, between different study populations.

Similarly, neuter status can affect urinary behavior habits. The historic study included all intact dogs, and the present study was predominately neutered dogs. Male dogs after neutering have significantly decreases marking behavior. This is thought to be unaffected by age of neuter; however, there are conflicting studies regarding the influence it has.^24,25 In contrast, urinary behavior is not generally affected by neutering female dogs.^25,26 Whether marking behavior affects URV is unknown.

Linear measurements for URV estimation are more accurate compared to cross-sectional area when using 2D ultrasonography; however, there is no consensus on which formula is most accurate in veterinary medicine. The objective of this study was to specifically compare the methods used in the original study that established the URV in dogs on smaller bladder volumes; therefore, the same formula was used. The range of urinary volume measured within the study was from 0–39 mL. It has been shown that estimation of human urinary bladders with ultrasound is less accurate with smaller urinary bladders. There were large degrees of error for bladder volumes <100 mL.^14,15 Underestimation of URV with 2D ultrasonography in dogs has also been documented.¹⁰ In contrast, we found ultrasonographic estimation to be accurate in minimally distended urinary bladders in dogs.

The use of ultrasonography for urine volume estimation offers a minimally invasive alternative method to direct catheterization, eliminating the risk of urinary tract infections associated with catheterization. ^27–30 Furthermore, animals with urinary retention are at an increased risk of developing urinary tract infection, which highlights the appeal for ultrasonography for clinical use to determine urine volume.

A major limitation of the study was the small sample size. A larger sample size would be recommended to develop a reference interval and evaluate the influence of weight/size, sex, and neuter status on URV in normal dogs. Furthermore, it has been shown that dogs hospitalized with IV fluids have larger URV.²³ Investigation of URV for animals with disease and determination of the threshold for increased complications from urinary retention are imperative for clinical application and use to guide medical intervention in animals suspected to have urinary retention.

Two-dimensional ultrasonography has several limitations. It requires trained personnel to perform the ultrasound and acquire appropriate images. The concept relies on an assumption that the urinary bladder is uniform. The formulas do not account for recumbency of a dog providing varying accuracy. It has been found that 2D measurements are most accurate when the animal is in dorsal recumbency, which may not be safely achievable for clinical patients.^1,12,23

Conclusion

In conclusion, this case series suggests URVs in normal dogs has a range of 0–0.47 mL/kg. The median URV of 0.17 mL/kg is consistent with the previous established value. Further investigation with a larger sample size is recommended to evaluate for the influence of size, sex, and neuter status on URV and ultimately develop a reference interval to guide clinical decision making.