GE LOGIQ 500 Advanced Reference Manual page 199

Methodology
LOGIQ 500 Advanced Reference Manual
2276614–100 Rev. 0
Doppler studies on the umbilical arteries are carried out after
manual palpation of the uterus and fetus. In pregnancies before
28 weeks, the fetus cannot be easily felt hence the probe is
placed on the upper third of the uterus at the midline. The probe
is gently rotated until the umbilical signal is heard and clearly
identified on the screen. There should be 3
waveforms of equal amplitude. If there is fetal breathing, the
waveforms will not be equal in intensity or frequency; hence,
measurements must be postponed until the fetus is quiet. If
there is tachycardia or bradycardia, the measurements have no
clear meaning because now the waveform represents a
complex mixture of resistance, cardiac output, and time needed
to empty a vascular region. The image is frozen and the
measurements are taken. We have found it useful to
continuously record the examination on audiotape because the
signal is immediately captured and not lost when the fetus or
cord moves.
This process is repeated at least twice more and the results
from the three studies are averaged. Although the S/D ratio is,
-
in theory, an angle independent measurement, there may be
differences in the ratio when the artery is studied near the
abdomen or near the insertion in the placenta. When three
different angle measurements are taken, we found an average
experimental error of 6% and a maximum error of 16%. The
uterine artery signals may be more difficult to obtain and require
more training to develop expertise. The probe is directed in the
lower quadrants at the paracervical area where the uterine
artery enters the uterus. Three potential signals may be
encountered, hence the examiner should be persistent until the
correct waveform is identified. The mature waveform is
generally present by 20 weeks, but should be achieved no later
than 26 weeks. The pulse wave frequency should coincide with
the maternal pulse and 4 equal and dense images should be
seen on each side of the uterus. The results from both sides are
averaged to give a single number. The S/D ratio appears to
provide a simple and reproducible measurement, but a number
of other calculations have also been proposed. Interobserver
error with this technique is 4% with a maximum error of 10%.
Bioeffects
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4 clear dense
6–15