Abstract
Introduction
The Doppler phenomenon or Doppler effect was first described by Christian Johanan Doppler (1803- 1853). His field of interest was Astronomy. He described the apparent changes in colour of light (intensity) emitted by stars when they come near or far from earth. The colour or brightness becomes more apparent if the star is coming towards earth, or, it fades away if the star is going away from earth.
This phenomenon was properly explained as change of frequency when there is a change of velocity, later known as the Doppler effect or the Doppler shift. Many examples were later described : the change in sound of the ambulance when it is coming near OR away from the listener. The same applies to a train when it is coming to the station and when it is leaving it.
In the human body each vessel has its own specific waveform.
This depends on few points: vessel length, caliber, wall elasticity and distance from the cardiac pump. Each wave basically has systolic and diastolic components.
Umbilical artery Doppler waveforms provide an estimate of downstream placental vascular resistance and placental blood flow. There is a strong association between reduced end-diastolic umbilical artery blood flow velocity and increased vascular resistance in the umbilical placental microcirculation.
Also, abnormal umbilical artery Doppler waveforms have been associated with an increased risk of fetal acidosis, as measured during cordocentesis, and may improve the performance of the biophysical profile score in predicting fetal acidemia and hypercarbia.
Why does this matter?
Placental insufficiency is the primary cause of intrauterine growth restriction in normally formed fetuses and can be identified using umbilical artery Doppler velocimetry. The use of Doppler during antenatal fetal surveillance has involved assessment of (1) the umbilical arterial and venous flow velocity waveforms, (2) the fetal cerebral circulation, and (3) the fetal venous circulation, in particular the ductus venosus.
Ultrasound machines with the appropriate probe and capability simultaneously display; two images: a B-scan image to locate the vessel needs to be examined and the other displays the Doppler beam to interrogate the site needed to study blood flow. Blood flow can be coloured either red or blue according to its direction to the transducer : coming to OR going away from it.
Colour Doppler studies, in simple words mean : detection and demonstration of blood flow. Indicated Doppler studies may be carried out at earlier trimesters of pregnancies. This write-up will however, focus on studies after 28 weeks.
Umbilical artery waveform
Doppler ultrasound waveforms reflect blood velocity; therefore, they potentially can provide information on various aspects of blood flow in a circulation, including the presence and direction of flow, velocity profile, volume of flow, and impedance to flow. In perinatal practice, Doppler of the umbilical artery has been used extensively for assessing downstream circulatory impedance (i.e., resistance to pulsatile flow).
Choice of indices
Despite its limitation, the S/D ratio remains the most widely used Doppler index for evaluating the fetal circulation, especially for umbilical arterial haemodynamics.
The RI [Resistance Index] values have defined limits of 0-1.0. The limitation of the RI is due to its inability to reflect impedance increases with the reversal of end-diastolic blood flow. Theoretically, the PI provides more haemodynamic information than the RI and S/D ratio, as it includes data on the whole cardiac cycle in the form of its denominator. Furthermore it expresses haemodynamic alterations associated with absent or reversed end-diastolic flow.
In practice, however, computation of the time-averaged value is not as precise as determination of the peak systolic or end-diastolic frequency shifts.
Doppler waveform analysis is usually based upon the following characteristics of the maximum frequency shift envelope:
- Peak systolic frequency shift value (S)
- End-diastolic frequency shift value (D)
- Average frequency shift value over the cardiac cycle (A)
These three parameters are used to develop indices that reflect the pulsatility of the Doppler waveform. A Doppler index (DI) is calculated as a ratio and is, therefore, virtually independent of the angle of insonation, thus obviating the need for measuring the angle of insonation.
The most commonly used obstetrical applications are the peak systolic frequency shift to end-diastolic frequency shift ratio, (S/D) and the resistance index (RI), which represents the difference between the peak systolic and end-diastolic shift divided by the peak systolic shift. The pulsatility index (PI = S-D/A) is also used.
Flow velocity profiles:
S/D ratio = Systole/diastole
RI (Resistance Index)=Systole-diastole/ systole
PI (Pulsatility Index)=Systole-diastole/mean velocity
Analysis of Doppler waveforms from an arterial source yields information about downstream impedance to flow: the Doppler index (DI) worsens with increasing pathology of fetoplacental angiomorphology.
Early studies reported an elevated S/D ratio was associated with obliteration of small muscular arteries in the tertiary stem villi. Subsequent studies, however, demonstrated sparse, elongated, uncoiled, and less ramified terminal capillary loops, rather than loss of small arteries, as the major lesions in pregnancies complicated by growth restriction and abnormal umbilical arterial Doppler. These observations suggest that fetoplacental vascular maldevelopment results in an increase in impedance which is reflected in the abnormal Doppler waveforms and indices.
An increase in S/D becomes apparent when approximately 30