ULTRASONOGRAPHIC EVALUATION OF HUMAN FETAL URINARY FLOW-RATE - ACCURACY LIMITS OF BLADDER VOLUME ESTIMATIONS

OBJECTIVES: This study was conducted to examine the reliability of fet al bladder volume predictions on the commonly used ovoid volume formul a and to develop a simpler but equally accurate method requiring fewer measurements. STUDY DESIGN: Nine hundred twenty seven measurements we re obtained from 11 dead fetuses in a water bath. Known incremental vo lumes of saline solution were infused and the largest linear bladder d imensions length, depth, and width, were measured ultrasonographically . Bladder Volumes were calculated with the ovoid volume formula, 4/3 . pi . (Length . Depth . Width)/8, and compared with true volumes. The areas of the ultrasonographic planes observed during the measurement p rocess were calculated and plotted against the true volume for regress ion analysis. The sagittal and coronal areas were converted into volum es with the regression equations 0.46323 + 1.39394 . Sagittal area and 1.20640 + 1.07603 . Coronal area, respectively. RESULTS: The fetal bl adder volumes from the ovoid formula had an average error of 6.4 ml, w ith a 95% confidence interval of +/-14.1 ml. The fetal bladder volumes derived from sagittal and coronal area formulas had mean absolute err ors of approximately 0 ml, with 95% confidence intervals of +/-4,0 ml and +/-4.4 ml, respectively. The best predictions were obtained with t he sagittal area measurements. CONCLUSIONS: The ovoid volume formula o verpredicts fetal bladder volume and has a wide 95% confidence interva l. This inaccuracy probably affects measurements of fetal urinary flow rates. Use of the sagittal or coronal area affords improved accuracy and is easier and more convenient.