Validation of 3-dimensional mathematical computation of feline bladder volume using orthogonal ultrasonographic bladder images

Urine output measurement is critical for the management of hospitalized cats and their underlying conditions. Ultrasoundguided estimation of urinary bladder volume (UBV) is a non-invasive surrogate measurement that can provide important clinical information. The purpose of this study was to validate the accuracy of a novel 3D computation method in estimating UBV using 2D point-of- care ultrasonographic images. Bladder volume estimation was performed using coordinates from bladder circumference tracings on paired longitudinal and transverse ultrasonographic images (n = 359) aligned in 3D space for mathematical algorithmic computation. Ultrasonographic images were obtained by 2 different observers at 18 different time points on 10 healthy, purpose-bred male cats under general anesthesia in sternal recumbency. Actual urine volumes were measured via urinary catheterization and compared to UBV estimations using Lin's concordance correlation coefficient and Bland-Altman analysis. Estimation of UBV using the 3D computational bladder circumference tracing method showed moderate strength- of-agreement with actual bladder volume (p(c) = 0.94 to 0.95) with clinically insignificant bias (3D computation-derived minus actual volume) of 21.96 mL (IQR = 23.89 to 20.57 mL, P, 0.001) and 22.42 mL (IQR = 24.64 to 20.66 mL, P, 0.001) for the 2 observers, respectively. Our study demonstrated acceptable accuracy of 3D computation method for UBV estimation in healthy cats. This method may provide a bridging alternative until 3D ultrasound becomes more readily accessible.