Doppler blood flow velocity waveforms
The integration of real-time imaging with pulsed wave Doppler allows for the vessel of interest to be identified and as the diameter of the vessel and the angle of interrogation can be measured, it allows for an estimation of velocity and volume of flow. This, nevertheless is not done in Obstetric scans because of the difficulty and inconsistency encountered in non-linear blood vessels such as the umbilical artery.
As the fetus is completely dependent on the supply of oxygen and nutrients from the placenta, examination of the blood flow through the umbilical circulation would appear to have great potential for the assessment of fetal health.
In the umbilical artery, there is relatively high forward velocities during diastole, consistent with blood flow into a low-impedance vascular bed, the placenta. With advancing gestation, there is an increase in end-diastolic flow velocity relative to peak systolic velocity. This is attributed to decreased resistance in the placental circulation with advancing gestation. This change in the pulse velocity waveform can be quantified by the systolic-to-diastolic ratio (A/B ratio). In pregnancies in which the A/B ratio is elevated, there is an increase in intrauterine growth compromise due to a placental circulation that has diminished in volume owing to placental vascular occlusion.
The A/B ratio in this growth-retarded 24 weeks fetus is 4.6
Click here to see how the ratio progressed eventually.
Umbilical Artery Flow Systolic/Diastolic Ratio*
weeks --------------------- Mean ---------------- Upper Limit
24 ------------------------ 3.5 ------------------------- 4.25
25 ------------------------ 3.4 ------------------------- 4.10
26 ------------------------ 3.3 ------------------------- 3.90
27 ------------------------ 3.2 ------------------------- 3.75
28 ------------------------ 3.1 ------------------------- 3.70
29 ------------------------ 3.0 ------------------------- 3.60
30 ------------------------ 2.9 ------------------------- 3.50
31 ------------------------ 2.8 ------------------------- 3.45
32 ------------------------ 2.8 ------------------------- 3.40
33 ------------------------ 2.7 ------------------------- 3.30
34 ------------------------ 2.6 ------------------------- 3.15
35 ------------------------ 2.5 ------------------------ 3.10
36 ------------------------ 2.4 ------------------------- 3.00
37 ------------------------ 2.4 ------------------------- 2.90
38 ------------------------ 2.3 ------------------------- 2.80
39 ------------------------ 2.3 ------------------------- 2.65
40 ------------------------ 2.2 ------------------------- 2.50
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*Table represents a composite of graphs found in a number of references.(Adapted from Hertzbert BS: Intrauterine growth retardation. In Rifkin MD, Charboneau JW, Laing FC [eds]: Syllabus Special Course: Ultrasound 1991.
The Radiological Society of North America, December 1991: 147-158.)