Accuracy limits of ultrasonographic estimation of human fetal urinary flow rate.

OBJECTIVE

Our purpose was to define the accuracy of currently available methods of ultrasonographically estimating human fetal urinary flow rate in a controlled setting.

STUDY DESIGN

Eleven fetal cadavers were studied in a water bath. Saline solution was incrementally infused into the bladder to simulate a rate of 1 ml/min. Serial fetal bladder volumes were calculated from ultrasonographic measurements by means of the ovoid volume formula (ovoid volume = 4/3.pi.(D1.D2.D3)/8 and ellipse sagittal-area and coronal-area volume formulas that we previously reported (Sagittal-area volume = 0.46323 + 1.39394. Sagittal area and Coronal-area volume = 1.20640 + 1.07603. Coronal area). Fetal urinary flow rate was determined by (1) subtracting the mean of two fetal bladder volumes at the start and end of a simulated 30-minute bladder filling or (2) linear regression of three, four, five, and six fetal bladder volume observations against time. The means of fetal urinary flow rate estimates and errors derived with each method were compared to the actual rate of 1 ml/min by means of the Student t test.

RESULTS

The volume subtraction technique with ovoid volume yielded a fetal urinary flow rate of 1.68 ml/min (95% confidence interval 0.86 to 2.50 ml/min). Similar overprediction of fetal urinary flow rate occurred with regression with ovoid volume (1.45 ml/min, 95% confidence interval 0.61 to 2.29 ml/min). Estimated fetal urinary flow rates (from sagital-area volume and coronal-area volume (0.99 ml/min, 95% confidence interval 0.64 to 1.34 ml/min) were significantly more accurate than those from ovoid volume (p < 0.0001). Regression with 3 (95% confidence interval +/- 40%) or 4 points (95% confidence interval +/- 37%) was marginally less accurate than with 5 (95% confidence interval +/- 36%) or 6 points (95% confidence interval +/- 35%, p = 0.02).

CONCLUSIONS

Ultrasonographic estimates of fetal urinary flow rate based on the ovoid volume formula overestimate the true rate by 40% to 70%. Fetal urinary flow rate calculated by regression of three to six sagittal or coronal bladder area measurements is a better estimate of true rate with a satisfactory margin of uncertainty. This technique can be used to predict human fetal urinary flow rate with an expected accuracy of +/- 35%.