Doppler ultrasound can be used to assess blood flow in a variety of fetal vessels with minimal risk, with the sound waves being reflected by the moving erythrocytes. Arteries which can be studied by this technique include:
(1) umbilical arteries
(2) aorta
(3) carotid arteries
(4) renal arteries
(5) middle cerebral arteries.
The tracing of the Doppler ultrasound varies over the cardiac cycle generating a waveform, with a maximum during systole and a minimum during diastole.
S/D ratio =
= (systolic peak height in kHz) / (diastolic peak height in kHz)
resistance (Pourcelot) index =
= ((systolic peak height in kHz) - (diastolic peak height in kHz)) / (systolic peak height in kHz)
pulsatility index =
= ((systolic peak height in kHz) - (diastolic peak height in kHz)) / (mean peak height in kHz)
Interpretation:
• The indices decrease as gestation progresses, due to a relative increase in the diastolic velocity. As the diameter of blood vessels increases, resistance decreases and blood flow increases.
• Reduction in diastolic flow, its absence or the reversal of direction can be seen with increased vascular impedance downstream, and may be associated with increased perinatal mortality. Absent end diastolic velocity (AEDV) in the umbilical artery is associated with fetal hypoxia, cardiac abnormalities and death.
• Increases in the indices (usually associated with the diastolic velocity not increasing normally) can be seen in growth retardation.
• Doppler studies of multiple gestations can help detect unequal fetal growth.
• Differences in the S/D ratio between the left and right uterine arteries is seen with a divergent uterine blood supply and may be associated with pregnancy induced hypertension and/or intrauterine growth retardation.
Limitations:
• The S/D ratio can be calculated for umbilical and uterine arteries, but it may not be reliable for fetal vessels due to the low diastolic velocities.
• The resistance index can be calculated for umbilical and uterine arteries, but it may not be reliable for fetal vessels due to the low diastolic velocities.
• Doppler velocimetry has proven not to be useful as a screening test for fetal growth retardation.
Evaluation of Umbilical Artery Doppler Ultrasonography Data
Data on the S/D ratio for the umbilical artery in pregnancies with normal outcomes (Schulman 1990, page 400) was analyzed in JMP.
(1) An equation approximating the low limit is:
S/D ratio lower limit = ((0.0001413 * ((weeks gestation)^3)) - (0.011805 * ((weeks gestation)^2)) + (0.2449985 * (weeks gestation)) + 2.0796722)
(2) An equation approximating the upper limit is:
S/D ratio upper limit = ((0.0001471 * ((weeks gestation)^3)) - (0.008642 * ((weeks gestation)^2)) - (0.024583 * (weeks gestation)) + 8.1188817)
Data on the pulsatility index for the umbilical artery in pregnancies with normal outcomes (Reuwer 1984, second reference page 202) was analyzed in JMP.
pulsatility index
(1) An equation approximating the low limit is:
pulsatility index lower limit = ((-0.000174 * ((weeks gestation)^3)) + (0.0187502 * ((weeks gestation)^2)) - (0.688275 * (weeks gestation)) + 9.2148117)
(2) An equation approximating the upper limit is:
pulsatility index upper limit = ((-0.000054 * ((weeks gestation)^4)) + (0.0062881 * ((weeks gestation)^3)) - (0.260467 * ((weeks gestation)^2)) + (4.3952825*(weeks gestation)) - 22.49588)
