Novel Technique for Assessing Amniotic Fluid Volume: Use of a Three-Dimensional Bladder Scanner

Abstract

Objective: Abnormalities of amniotic fluid (AF) volume are associated with significant perinatal morbidity and mortality. Although current ultrasound techniques provide a linear amniotic fluid index (AFI) or 2-D area of AF, these indices have limited correlation with actual AF volume. The bladder volume instrument (BVI) 2500 ultrasound (Diagnostic Ultrasound Corp., Redmond, WA) utilizes a rotating 2 MHz transducer, a computer-defined fluid interface, and computer integration of 12 cross-sectional images to calculate 3-D fluid volume. In term pregnancies with normal AF volume, we previously demonstrated a correlation between the AFI and the 3-D volume as determined by BVI. In the present study, we sought to establish normative gestational values for BVI-determined 3-D volume and the relation to simultaneous AFI determinations.

Methods: Following written informed consent, 73 gravidas (17-41 weeks) with uncomplicated pregnancies and normal AF volume were prospectively studied with BVI and an ATL Ultramark 6 sector scan. The largest vertical fluid pocket in each quadrant of the abdomen was identified with the sector scan and vertical measurements for AFI recorded. Simultaneous AF 3-D volume measurements of each pocket were performed three times with BVI and maximum values were utilized. Gestational changes in the 3-D volume were correlated with measured AFI as well as known changes in AF volume.

Results: The mean ± SD 3-D volume and AFI for all patients was 369 ± 172 ml and 12.4 ± 4-5 cm, respectively. One AFI centimeter was equivalent to a volume of approximately 30 ml and did not change significantly by gestational age. The 3-D AF volume demonstrated a parabolic pattern throughout gestation and significantly correlated with known changes in AF volume.

Conclusions: The BVI scanner provides a useful measure of AF volume as the BVI values are correlated with known changes in AF volume. 3-D volume assessment with BVI may aid in diagnosing oligo- and/or polyhydramnios.

Key Words:

• fetal urine output
• amniotic fluid volume
• ultrasound