Estimation of fetal urinary bladder volume using the sum-of-cylinders method vs. the ellipsoid formula.

OBJECTIVES: A changing hourly fetal urine production rate (HFUPR) has been demonstrated when fetuses are subjected to hypoxia, which is of clinical interest. Conventionally, calculations of the HFUPR have been based on repeated two-dimensional ultrasound bladder images during the filling phase and the formula for ellipsoids. However, the bladder shape changes from the ellipsoidal to super-ellipsoidal during filling and the volume could be underestimated. In this study the sum-of-cylinders method was used, a method which does not depend on the longitudinal bladder shape being an ellipse. METHODS: The volumes from 102 urinary bladder images from 34 fetuses (three per fetus) were assessed using the sum-of-cylinders method. These results were compared with the corresponding findings when using the ellipsoid formula on the identical material of bladder images in a previous study. RESULTS: When using the sum-of-cylinders method vs. the ellipsoid formula, the mean volume difference was +1.6 mL. Applying the sum-of-cylinders method for bladder volumes of 5-40 mL resulted in standard deviations of 8.8-3.5% vs. 12.9-5.5% when using the ellipsoid formula. No deviation was found when using the sum-of-cylinders method vs. a known volume formula for cylinders, which simulated super-ellipsoids on the computer screen. CONCLUSIONS: Volume calculation by the sum-of-cylinders method is independent of the longitudinal bladder shape when it comes to producing accurate estimations. This method demonstrated a systematically larger volume compared with the ellipsoid formula. The sum-of-cylinders method is therefore preferable for volume estimations of the fetal urinary bladder.