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Fetal Assessment Comparing Nonstress Test with Umbilical Artery Doppler Velocimetry
Abstract & Commentary
Synopsis: Umbilical artery Doppler as a screening test for fetal well-being in a high-risk population was associated with a decreased incidence of cesarean delivery for fetal distress compared to the nonstress testing, with no increase in neonatal morbidity.
Source: Williams KP, et al. Am J Obstet Gynecol. 2003;188:1366-1371.
For the past 20 years 2 tests have been used almost exclusively in the assessment of fetal condition in high-risk pregnancies, and the nonstress test (NST) by far has been the preferred diagnostic regimen used alone, as part of the full biophysical profile (BPP), or as one of 2 components of the "modified" BPP (NST plus amniotic fluid assessment by ultrasound).
Williams and associates carried out a randomized controlled trial (RCT) pitting the standard NST against umbilical artery Doppler analysis in pregnancies > 32 weeks complicated by hypertension, diabetes, decreased fetal movement, intrauterine growth restriction (IUGR) and post-dates pregnancy (see Table). A total of 1360 patients were randomly allocated to have twice-weekly testing with umbilical artery Doppler (UAD) (699) or NSTs (691). Patients were induced if the NST was abnormal as defined by standard criteria or if there was absent or reversed end diastolic flow in the umbilical arteries. If the UAD S/D ratio was above the 90th percentile or if the NST was equivocal, then the decision to induce was based on the presence or absence of oligohydramnios, defined as an amniotic fluid index (AFI) of less than the fifth percentile.
Although there was only 1 fetal demise in the whole study (in the NST group), there was a decrease in the need for cesarean section for fetal distress in labor in the Doppler group (30, 4.6%) vs the NST group (60, 8.7%) with P £ .006. There were no differences in any of the other variables such as Apgar scores, meconium at birth, admission to NICU, or birth weight.
Although there was a difference in cesarean section rate for fetal distress in every category, the most dramatic difference was in hypertension and IUGR.
Comment by John C. Hobbins, MD
The use of UAD has been the focus of meta-analyses looking at perinatal mortality as an end point. These studies clearly have shown a superiority of Doppler over NST alone in lowering perinatal mortality. However, these studies have addressed its use predominantly in IUGR. Now in a wider scope of high-risk categories, UAD has been shown to decrease the need for cesarean section due to fetal distress. The clear implication here is that if one waits for the NST to become positive to deliver, then the fetus would be in poorer condition to tolerate labor than if one acted on the Doppler information earlier. In fact, the induction rate was indeed significantly higher in the UAD group (4.8%) than in the NST group (1.9%). Yet, it had no effect on the total cesarean section rate.
Recent studies in which IUGR fetuses were followed with a variety of fetal assessment tests have shown that umbilical artery Doppler becomes nonreassuring 7-21 days before the NST or biophysical profile becomes "positive," and a landmark study in 1993 in which growth-restricted fetuses were evaluated with UAD and NSTs (while percutaneous fetal blood sampling was intermittently undertaken) demonstrated that if both UAD and NSTs were reassuring, fetal hypoxia/acidosis was nonexistent. If the UAD was abnormal and the NST was normal, then hypoxia/acidosis was noted in only 5% of fetuses, but if both are abnormal, there was a 60% incidence of hypoxia/acidosis. This strongly suggests that waiting until the NST becomes positive to deliver will result in longer fetal exposure to metabolic acidosis and, with it, neonatal morbidity and neonatal sequelae.
The diagnostic story got a bit more complicated when it recently was shown that late in pregnancy the UAD is not an invariably perfect predictor of fetal distress in patients with abnormal uterine artery waveforms whose fetuses are showing evidence of brain sparing by increased flow in the middle cerebral arteries. Some of these fetuses will show signs of compromise in labor despite normal UADs.
Nevertheless, this isolated study should not in any way taint the compelling message in Williams et al paper that UAD at least can play an extremely important role as an adjunctive predictor of fetal condition in a variety of high-risk situations.
Dr. Hobbins is Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver.
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