You’re called to counsel the 36-weeker with abnormal dopplers and go to review the obstetric scans. Unfortunately, all you see are TLAs* and numbers. You try to make sense of it, but you can’t work out what’s going on—time to get a handle on umbilical dopplers in neonatology.
*TLAs – three-letter acronyms
In order to understand what the dopplers mean, we need to understand the basics of fetal circulation. The umbilical vein (UV) takes oxygenated blood from the placenta. It bypasses the liver through the ductus venosus (DV) into the inferior vena cava (IVC). From the IVC it enters the right atrium (RA). There, it shoots through the foramen ovale into the left atrium. It enters the left ventricle, and goes through the aorta, diverting to the upper half of the developing fetus (especially the brain). The blood returns to the RA and enters the right ventricle (RV). It bypasses the lungs through the ductus arteriosus between the pulmonary artery and aorta. This diverts the less oxygenated blood to the lower half of the fetus, and leaves via the umbilical arteries (UA) back to the placenta.
Relevant Fetal Doppler Measurements
Resistive Index (RI)
Indicates the resistance in pulsatile vessels. The narrower a vessel, the higher the resistance and the higher the RI.
Pulsatility Index (PI)
Assesses the pulsatility against the resistance of the artery. Throughout the pregnancy, there is a gradual fall in PI as the umbilical vascular bed opens up.
End-Diastolic Flow (EDF)
Diastole is when the heart relaxes after contraction and blood passively fills the heart. EDF is the speed of blood flow at the end of diastole. A normal UA EDF is often denoted as UA EDF positive.
Fetal Doppler Abnormalities
Low MCA PI
Normally raised but slightly decreases throughout gestation. Low PI reflects redistribution of cardiac output to the brain to preserve blood flow to the brain in pathological states.
Raised UA PI
UA PI is indicative of uteroplacental perfusion. It is initially raised and then falls throughout gestation. A raised PI suggests impaired placental perfusion.
Absent UA EDF
This occurs when there is no blood flow at the end of diastole because of increased placental resistance.
Rather than the flow moving forward, it flows in reverse at the end of diastole due to increased placental resistance (following an absent EDF).
These doppler abnormalities are markers of placental insufficiency. This typically leads to intrauterine growth restriction (IUGR). In addition, there may be reduced gastrointestinal blood flow, requiring the careful introduction of feeds postnatally to prevent necrotising enterocolitis.