Biophysical Profile with Amniotic Fluid Volume Assessments
Abstract & Commentary
Synopsis: The AFI offers no advantage in detecting adverse outcomes compared with the single deepest pocket when performed with the BPP. The AFI may cause more interventions by labeling twice as many at-risk pregnancies as having oligohydramnios than with the single deepest pocket technique.
Source: Magann EF, et al. Obstet Gynecol. 2004;104: 5-10.
Assessment of amniotic fluid is a part of all obstetrical ultrasound examinations after the first trimester and has also become a staple in the management of high-risk pregnancies as a component of the biophysical profile (BPP). Basically, the methods to estimate the amount of amniotic fluid vary from a rough clinical assessment by, hopefully, an experienced examiner (the gestalt approach), determination of the deepest pocket of fluid, and the full four-quadrant amniotic fluid index (AFI).
Magann et al recently compared AFI with the single deepest pocket technique in a randomized trial involving 537 patients with a variety of high-risk problems. Oligohydramnios was diagnosed when the AFI was 5 cm or less or the deepest pocket was less than 2 ´ 1 cm. One hundred thirty-two patients (132) of the 264 in the AFI group were diagnosed to have oligohydramnios (38%) vs 46 of the 273 women having the deepest pocket technique (16%). Although the numbers enrolled in the study precluded the statistical power to evaluate many of the morbidity variables, 2 statistically significant differences between groups are worthy of mention. The inductions were doubled in the AFI group (30% vs 15%) and there was a higher rate of cesarean section for fetal distress in the AFI group (13% vs 7%). Yet there were no differences between groups in neonatal complications, Apgar scores, or cord pH.
Comment by John C. Hobbins, MD
Using standard definitions of oligohydramnios, this study suggests that the AFI labels twice as many patients as having it and, by inference, predisposes twice as many patients to induction of labor.
The major problem with this study is that it was spiked with so many patients with ruptured membranes (22%). Monitoring amniotic fluid volume is a way to indirectly assess fetal condition. In ruptured membranes there is an obvious mechanical reason for the oligohydramnios, initially having little to do with fetal condition. Including these patients simply confuses the issue by mixing apples and oranges.
Amniotic fluid volume can be affected by many factors including maternal hydration, fetal glucose levels, placental perfusion, and fetal surface area. It has been shown that between 100 and 250 cc of amniotic fluid is lost to the placenta at term through an osmotically mediated process that can be affected by changes in villous pressures in conditions such as intrauterine growth restriction (IUGR). Nevertheless, the major common denominator involves fetal urine production, which is diminished when the fetus is deprived. As indicated in previous Alerts, fetuses with IUGR will spare their brains in the early stages of compromise (as evidenced by increasing end diastolic flow in the middle cerebral arteries), and will send less blood to the renal arteries. The oligohydramnios that results simply confirms this shift in priority by the fetus, which also happens in seemingly appropriately grown fetuses towards term and postterm. However, to use this rather gross index of fetal condition as a reason alone to interrupt pregnancy, when other more specific and sensitive Doppler and fetal heart rate information is available, is backward, especially if the pregnancy is preterm. Interestingly, even patients without ruptured membranes in the above study were given corticosteroids if less than 34 weeks and delivered if their oligohydramnios "persisted" during the steroid treatment.
If clinicians are still using oligohydramnios as a reason to deliver, then we better get it right when we tag a pregnancy with oligohydramnios, and the single deepest pocket concept seems to buy us fewer unnecessary inductions and cesarean sections without affecting outcome.
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John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor of OB/GYN Clinical Alert.