Umbilical Artery (UA) Impedance Indices are calculated by using
ultrasound to measure the blood flow waveforms from the uterine arteries through a
free-floating portion of the umbilical cord .
The figure at the
right shows the normal saw-tooth appearance of arterial blood flow through
the umbilical cord.
S = Systolic peak (max velocity); The maximum velocity
during contraction of the fetal heart.
D = End-diastolic flow;
Continuing forward flow in the umbilical
artery during the relaxation
phase of the heartbeat.
Vm = Mean velocity
The ratios (indices) of the resistance to blood flow (impedance) are calculated
using the following formulas:
S/D ratio = (systolic / diastolic ratio)
Resistance index (RI) = (systolic velocity - diastolic velocity / systolic velocity)
Pulsatility index (PI) = (systolic velocity - diastolic velocity / mean velocity)
In the normal fetus, the resistance to flow
(impedance) decreases in the umbilical artery due to increased numbers of tertiary stem villi
as the placenta matures.
The calculator below will estimate the 50th and
95th percentile values for some commonly used umbilical artery impedance indices.
The umbilical artery (UA) impedance indices increase when there is decreased
end-diastolic flow due to reduced placental perfusion and "utero-placental
insufficiency" as is seen in intrauterine growth restriction (IUGR). Absent or reversed UA
end-diastolic flow are particularly ominous findings.
1. Gosling RG, et al. The quantitative analysis of occlusive peripheral arterial disease by non-intrusive ultrasound technique.
Angiology 1971;22:52-5 PMID:5101050
2. Acharya G et al. Reference ranges for serial measurements of umbilical artery Doppler indices in the second half of pregnancy.Am J Obstet Gynecol.2005;192:937-44.
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