The following is an extract from this site (taken in March 1999). The site is well worth a visit!

Doppler Ultrasound

The doppler shift principle has been used for a long time in foetal heart rate detectors. Further developments in doppler ultrasound technology in recent years have enabled a great expansion in it's application in Obstetrics, this time in the area of assessing and monitoring the well being of the foetus.

Blood flow characteristics in the foetal blood vessels can be assessed with Doppler 'flow velocity waveforms'. Diminished flow, particularly in the diastolic phase of a pulse cycle is associated with compromise in the foetus. Various ratios of the systolic to diastolic flow are used as a measure of this compromise.

 

The blood vessels commonly interrogated include the umbilical artery, the aorta, the middle cerebral arteries and the uterine arcuate arteries. The use of colour flow mapping can clearly depict the flow of blood in foetal blood vessels in a real-time scan, the direction of the flow being represented by different colours. 'Colour' doppler is particularly indispensable in the diagnosis and assessment of congenital heart abnormalities. Another recent development is the Power Doppler (Doppler angiography). It uses amplitude information from doppler signals rather than flow velocity information to visualize slow flow in smaller vessels. A colour perfusion-like display of a particular organ such as the placenta overlapping on the 2-D image can be very nicely depicted.

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 foetus 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 foetal health.

In the umbilical artery, there is a relatively high forward velocity 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.