Amnioreduction: how much to drain?

OBJECTIVES

To assess the relationship between the volume of amniotic fluid removed and the change in amniotic fluid index (AFI) and calculate an equation describing this association.

MATERIALS AND METHODS

A retrospective analysis of 19 amnioreduction procedures performed in our unit. Multiple regression analysis was used to assess the effect of gestational age and pre-procedure AFI on the change in AFI (DeltaAFI) after adjusting for the volume removed.

RESULTS

As expected, a significant linear relationship was found between the change in AFI and the volume removed (r = 0.82, n = 19, p < 0.0001). DeltaAFI was not dependent on the gestational age or the pre-procedure AFI. The equation describing the association between the volume removed and DeltaAFI was: volume = (DeltaAFI - 2. 26)/0.008, which is close to 1 cm DeltaAFI for every 100 ml removed.

DISCUSSION

Using the described equation, it is possible to predict the required volume to be removed in order to achieve a particular DeltaAFI, which may reduce the need to interrupt the procedure to measure the AFI. However, the limitation of AFI as a semiquantitative assessment of the liquor volume, together with its inter- and intra-observer variations mean this equation should be used only as a guide.