o_1977r8vv9vk1ts2ms0kd8pksa.pdf

184 P. Arbeille et al.
Fig. 13.10. a Mean variations of
the ratio (C/U =cerebral resistance/umbilical
resistance) during
the malaria crises in group
1 (1994 short and long crisis)
and in group 2 (1996 crisis).
The C/U change is expressed in
percentage from cut-off limit
1.1. In group 1 C/U decreased
significantly more during long
crisis than during short ones
(p 150% ?
PPV= 80%, NPV=85%
In normal pregnancies the HI remains equal to
zero as only C/U values lower than 1.1 which indicate
a flow redistribution are taken into account. A flow
redistribution characterized by a C/U ratio equal to
0.77 (±20% below 1.1) means that it could correspond
to a 20% pO 2 reduction. If this situation remains
stable for 10 days, the HI will be equal to 20% ´10
= 200%.
Cerebral Circulation
in Normal Pregnancy
Cerebral Flow Changes
with Gestational Age
The diastolic flow in the cerebral vessels is reduced at
the beginning of the second half of pregnancy (20±
25 weeks) but continues developing progressively.
The increase in the diastolic component with the gestational
age is interpreted as a decrease in the cerebral
resistance due to brain development. The cerebral
vascular resistance changes are measured using
the same Doppler indices as for the placenta (Figs.
13.2, 13.4, 13.5).
The increase in the diastolic component begins
later in the cerebral arteries (at approximately
25 weeks) than in the umbilical arteries (at approximately
15 weeks). The amplitude of the diastolic flow
in the cerebral arteries is always lower than in the
umbilical arteries, meaning that in normal pregnancy
the cerebral vascular resistances are higher than the
umbilical resistances. In other words, the umbilical
flow volume is higher than the cerebral flow volume.
This led to the definition of the C/U ratio (cerebral
resistance/umbilical resistances), which is inversely
proportional to cardiac output distribution between
brain and placenta, and which is higher than 1.1 in
normal pregnancies.