Doppler examinations have been used to study major fetal circulatory systems which include the umbilical artery, aorta, middle cerebral artery, heart, and umbilical vein. Doppler assessment helps investigate the human fetal hemodynamics which can be used for fetal surveillance.
What is Umbilical Artery Doppler?
The Umbilical Artery Doppler examination is used in fetal surveillance in the third trimester of a pregnancy. It’s the most widely performed Doppler examination in obstetrics. This examination has greatly reduced perinatal mortality and also morbidity in high-risk types of obstetrics situations. In simple terms, Umbilical Artery Doppler is a convenient test for improving morbidity and perinatal mortality.
When to Perform Umbilical Artery Doppler?
Umbilical artery Doppler is performed as from 24 weeks gestation period to term. The application of the Umbilical Artery Doppler velocimetry among high-risk pregnancies with presumed impaired fetal growth of hypertension is related to the current trend of fewer perinatal deaths and also fewer inductions of admissions and labour in hospitals.
How To Do Umbilical Artery Doppler?
In the examination, a color flow image of the fetus cord is scanned. The sample used should be extracted from the cord close to the placenta, at the fetal end, or the free floating cord. Ideally, the indices measurements should be taken in the free cord. On the other hand, for consistency purposes, a fixed side is more appropriate, for instance, the fetal end or the placenta end.
A pencil shaped probe which is used as the transducer is placed on the mother’s abdomen overlying the fetus. It is then systematically manipulated to indicate certain characteristic waveforms from the umbilical artery. Using a pulsed wave doppler system, a scan is taken.
Great caution should be applied to the use of the Umbilical Artery Doppler in Pregnancy. This is due to certain concerns related to thermal/heating effects from the use of high intensities of the doppler ultrasound.
The Most Common Types of the Parameter Used Include
- The Umbilical Arterial s/d ratio: Diastolic Velocity/ Systolic Velocity
- The Pulsatility Index(PI)- (Peaksystolic Velocity – End Diastolic Velocity)/Time Averaged Velocity
- The Resistive Index(RI)- (Peaksystolic Velocity -End Diastolic Velocity)/Peaksystolic
Umbilical Artery Doppler Normal Values
The umbilical arterial waveform flows in the forward direction. The waveform has a saw tooth-like pattern that is used for measuring the umbilical artery Doppler flow. The Doppler indices measured shows changes at the fetal end first.
Usually, an abnormal umbilical artery Doppler is an indication of uteroplacental insufficiency and suspected pre-eclampsia or (IUGR) intrauterine growth restriction. It indicates reversed or absent diastolic flow. However, before the 15th week, the absent diastolic flow can be just a normal finding.
| Doppler Indices | Period of time | Normal Value |
|---|---|---|
| PI | 2nd Trimester | 2 – 1.5 |
| PI | 3rd Trimester | 1.5 – 1 |
| S/D Ratio | Before 28 Weeks | < 5 |
| S/D Ratio | 28 – 34 Weeks | < 4 |
| S/D Ratio | 34 Weeks – Term | < 3 – 3.5 |
The Doppler Indices Decline Gradually with the Gestational age. For Instance,
- (RI)-Resistive Index- Its mean value decreases from 0.756 to about 0.0609
- (PI)-Pulsatility Index- Its mean value decreases from 1.270 to 0.967
- The S/D ratio mean value also decreases from 3.560 to 2.511
The Umbilical Artery Doppler in the perinatal application has been extensively used for examining the downstream circulatory impedance, that is, the resistance to pulsatile flow.
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