THE INFLUENCE OF OPERATOR TRANSDUCER PRESSURE ON ULTRASONOGRAPHIC MEASUREMENTS OF AMNIOTIC-FLUID VOLUME

OBJECTIVE: Our purpose was to study the effect of operator transducer pressure on amniotic fluid index and single deepest pool measurements and their intraobserver and interobserver variability. STUDY DESIGN: F orty subjects in the third trimester with intact membranes were studie d by two observers at three predetermined operator pressures. Pressure was measured by interfacing a flexible water-filled reservoir connect ed to a manometer between the maternal abdomen and the ultrasonography transducer. The amniotic fluid index and single deepest pool were mea sured on screen by another operator, and data were analyzed by calcula ting components of variance. RESULTS: Compared with medium pressure, l ow pressure resulted in a 13% increase in amniotic fluid index (p < 0. 001), and high pressure resulted in a 21% fall in amniotic fluid index (p < 0.001). The single deepest pool was less sensitive to pressure, with a 11% increase at low pressure (p < 0.001) and a 16% fall at high pressure (p < 0.001). Intraobserver error was 17% for amniotic fluid index and 18% for single deepest pool when operator pressure was contr olled at medium pressure; this increased to 28% and 24%, respectively, when pressure was not controlled (p < 0.01). Interobserver error was consistently very low. CONCLUSION: Both techniques are highly sensitiv e to the pressure applied to the maternal abdomen during scanning. The reproducibility of amniotic fluid index and single deepest pool is co mparable. The use of the same observer for serial measurements of amni otic fluid index would appear to be less important than careful attent ion to transducer pressure.