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a Chapter 13 Cerebral and Umbilical Doppler in the Prediction of Fetal Outcome 179
Fig. 13.2. Umbilical and cerebral
velocity waveform in normal
pregnancies at 22, 30, and
37 weeks of gestation. Note
that the cerebral end-diastolic
flow velocity is always lower
than the umbilical flow velocity;
thus, the cerebral to umbilical
index is always higher
than the umbilical to cerebral
index.
Animal Investigations
(Implanted Doppler Sensors)
Although fetal Doppler examinations in the human
pregnancy provide useful information to the obstetrician,
it is not possible to collect all the biological and
hemodynamic data required to understand the physiopathological
mechanisms involved in the development
of the intrauterine growth restriction (IUGR)
and the hypoxia.
With the animal model it is possible to measure
blood pressure, blood velocity and blood volume, to
collect blood samples, to perform pharmacological
tests, and to simulate some human pathologies.
Several studies have been carried out on lamb fetuses
using electromagnetic flow meters placed
around the umbilical cord and catheters with pressure
sensors inserted into the fetal aorta. Mostly, only the
umbilical flow was assessed on the fetal side. Flat
Doppler probes were developed to be implanted in
the fetus and the mother, making it possible to assess
atraumatically the fetal (cerebral/umbilical) and maternal
flows in real time over a period of approximately
20 days' gestation. The 4-MHz continuous
wave (CW) or pulsed wave (PW) Doppler probe consists
of two rectangular piezoelectric transducers,
pre-oriented at 458 from the surface of the probe, and
placed in a plastic case (6 mm high, 2 cm 2 area) with
small holes on the edges to sew the probe to the fetal
skin. The sensors are affixed to the fetal skin, facing
the umbilical cord, the fetal cerebral arteries and in
front of the uterine arteries. The output wires and
the fetal catheter connectors are stowed in a pocket
affixed to the back of the ewe, and at each measurement
session the fetus is simply connected for 1 or
2 h (without any anesthesia) to the Doppler and pressure
system (Fig. 13.3).
On the Doppler waveforms the blood flow volume
and the vascular resistance changes in the area supplied
by the vessel (placenta, brain, uterus) and the
fetal heart rate are calculated. This system has been
tested on normal gestations, during simulated fetal
hypoxia, and during pharmacological treatments
(Fig. 13.3) [7, 27, 28].
Cerebral Hemodynamics
and Doppler Indices
Cerebral Resistance to Flow
Accessibility to the main fetal cerebral arteries by
Doppler has led to the development of various hemodynamic
indices. The amplitude of the end-diastolic
flow in the fetal vessels is directly related to the vascular
resistances in the area supplied by these vessels
[29±31]. In order to quantify the vascular resistances,
various indices, which measure the proportion of systolic
flow within the total forward flow (M) during
one cardiac cycle, or the relative amplitude of systolic
(S) to diastolic (D) flow, have been proposed: PI=(S-
D)/M [32]; R = D/S [30]; RI=(S-D)/S [33]; R = S/D
[34]. Most of these parameters change according to
the resistance to flow into the vascular territory un-