A fetus in trouble with IUGR will respond by lowering cerebral vascular resistance to maintain blood flow to its brain, decreasing the middle cerebral artery S/D ratio in what’s known as a "brain-sparing" wave form. Although there’s nothing that can be done for the fetus at this point, she said, it can be helpful to know that brain-sparing in growth-restricted fetuses was associated with increased risk for abnormal neurobehavioral outcomes in a controlled study of 126 preterm infants (Ultrasound Obstet. Gynecol. 2011;38:288-94).
"It’s a sign of distress," she said. Until recently, clinicians would follow that fetus carefully with various kinds of testing and watch for reverse diastolic flow in the umbilical artery, which is a sign of significant compromise, hypoxemia, and possible death. Today, a finding of brain-sparing next leads Dr. Feldstein to interrogate the ductus venosus in the liver, which is "the hardest to do, but it can be done," she said.
The ductus venosus flow should be phasic but continuous, in a pattern called the "a wave" reflecting forward, continuous flow even during right atrial contractions. If the flow reverses backward into the ductus venosus during right atrial contractions, that’s a sign of cardiac compromise, severe hypoxia, and right ventricular dysfunction, associated with high risk of morbidity and mortality.
Locating the ductus venosus for sampling can be tricky, in part because it is so close to hepatic veins, but if the ductus venosus flow is abnormal enough, there’s a shortcut that is easier to do: Sample the umbilical vein. Abnormal phasicity in umbilical vein pulsation may reflect ductus venosus flow reversal.
A previous study reported that the risk for perinatal mortality increased to nearly 6% with an elevated umbilical artery S/D ratio, to more than 11% with absent or reversed diastolic flow in the uterine artery, and to 39% with an abnormal ductus venosus wave form (Ultrasound Obstet. Gynecol. 2003;22:240-5). In a 2010 Cochrane Review of 18 studies that included more than 10,000 women with high-risk pregnancies, Doppler ultrasound was associated with a 29% reduction in the rate of perinatal deaths (1.2% with Doppler and 1.7% without); analysis showed that using Doppler on 203 high-risk pregnancies would avoid 1 perinatal death (Cochrane Database Syst. Rev. 2010 Jan. 20). "So, there’s a significant impact and not that much excess work," Dr. Feldstein said.
She reported having no financial disclosures.
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