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180 P. Arbeille et al.
Fig. 13.3. Instrumentation of the fetus.
Flat Doppler sensors are sewn on the fetal
skin in front of the artery to be investigated
(on the abdomen for the umbilical
arteries, on the neck for the internal
carotid artery, on the maternal abdomen
for the uterine artery). A catheter is inserted
into the fetal femoral artery, for
blood pressure measurements and blood
sampling (blood gases)
der investigation; therefore, any increase of these indices
above the upper limit of the normal range corresponds
to an increase of the vascular resistances
(Fig. 13.4). In contrast, the D/S index decreases as
the resistance to flow increases. The vascular resistance
increase may be due to vascular disease (placental
infarction or fibrosis) or to distal arteriolar vasoconstriction
(brain response to increased pO 2 or to
vasoactive drugs). Conversely, abnormally decreased
resistance to flow values are displayed below the lower
limit of the normal range of the index for S-D/M,
S-D/S, S/D (Fig. 13.4) and above the upper limit for
the D/S index. The decreased resistance to flow may
be due to the existence of arterio-venous shunts, or
to an arteriolar vasodilation (brain adaptation to
hypoxia or to drugs).
Cerebral±Umbilical Ratio
for Evaluating pO 2 Changes
The cerebral±umbilical (C/U) ratio (also known as
the cerebral/placental ratio (CPR)) changes are indicators
of peripheral fetal flow distribution. These parameters,
based on the comparison of the brain (cerebral
to umbilical index, CRI) and the placental (umbilical
to cerebral index, URI) resistances, are expressed
as either CRI/URI [22, 23] or URI/CRI [2],
the cerebral vascular resistances being measured from
one of the intracerebral arteries (anterior or middle)
or from the intracranial part of the carotid artery. By
measuring the flow redistribution between the placenta
and brain, these C/U ratios (in cases of pathologic
pregnancies) take into account the placental disturbances
due to vascular disease at this level, and
the cerebral response (vasodilation) to the hypoxia
induced by placental dysfunction.
In normal pregnancies the diastolic component in
the cerebral arteries is lower than in the umbilical arteries
at any gestational age (Fig. 13.2); therefore, the
cerebral vascular resistances remain higher than the
placental resistances and the C/U ratio (C/U=CRI/
URI) is >1.1 (Figs. 13.4, 13.5). The C/U ratio (CRI/
URI) becomes < 1.1 if any flow redistribution in favor
of the brain occurs (Figs. 13.6±13.8). In such case the
cerebral diastolic flow amplitude is higher than normal,
and the umbilical flow amplitude is lower. Animal
and human studies have demonstrated that the
C/U changes in proportion to fetal pO 2 [35, 36]. The
URI/CRI ratio used by other authors to detect fetal
flow redistribution varies in the opposite direction.
Effect of Fetal Heart Rate
on Doppler Vascular Resistance Indices
It is well known that an elevation of the fetal heart
rate increases the end-diastolic velocity and therefore
decreases the resistance index (Fig. 13.9). On the
other hand, diminution of the heart rate increases the
index value. This effect of the heart rate is eliminated
by the use of the C/U ratios because both indices,
CRI and URI, are measured on the same fetus with
the same heart rate. The two C/U ratios mentioned
above are not heart rate dependent, because the cerebral
as well as the umbilical index are equally affected
by the heart rate changes. Figure 13.5 shows the fluctuations
of both the placental and the umbilical in-