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a Chapter 22 Doppler Velocimetry in Maternal Alloimmunization 341
cause of fetal anemia. Intermittent, low-frequency 3-
to 5-Hz sinusoidal variations of the fetal heart rate
are more likely a result of stressed cardiac neuroregulation
than actual changes in fetal blood flow [21,
22].
The inability of the medullary centers to control
the fetal heart rate results in a sinusoidal heart rate
pattern. Observations of fetal heart rate in severe anemia,
fetal brain maldevelopment, cytomegalovirus infection
and during alfaprodine or pancuronium bromide
administration as reported by various authors
appear to confirm this mechanism [23, 24].
In the absence of significant hypoxia a mechanism
of sinusoidal heart rate pattern is not well understood.
It is likely that more than one compensatory
mechanism is involved in such a severely affected fetus.
Decreased fetal blood viscosity, increased blood
volume, increased preload, baroreceptor, and volume
receptor stimulation all affect fetal heart rate. Hecher
et al. [15] reported increased fetal blood viscosity
after fetal intravascular transfusion that occurred only
in whole blood but not in serum, suggesting that it is
increased erythrocyte mass that leads to increased
blood viscosity. In anemic fetuses the cumulative effects
of increased blood volume and decreased blood
viscosity may lead to increased venous blood return
and increased preload as reflected by the preload index
in inferior vena cava velocimetry.
No systematic studies of Doppler velocimetry during
sinusoidal rhythm have been reported. In our one
case fetal Doppler velocimetry in umbilical artery,
descending aorta, MCA, and inferior vena cava were
recorded. The elevation of ªaº wave in the inferior
vena cava was noted, and fetal anemia was confirmed
by cordocentesis. Subsequently, sinusoidal rhythm
was relieved by fetal intravascular transfusion with
the return of the inferior vena cava Doppler waveform
to normal values shortly after transfusion. Association
of abnormal Doppler velocimetry with a sinusoidal
heart rate pattern has not been definite and
needs more observations; however, the presence of
fetal sinusoidal rhythm has been reported consistently
as an ominous sign and an emergency requiring
prompt diagnosis and treatment [21].
Changes in Fetal Systemic Arterial
Blood Flow
Fig. 22.1. Sinusoidal fetal heart rate pattern
Doppler Flow Velocity Measurements
Fetal blood flow velocity is the parameter that can be
adequately measured, provided that the angle of insonation
is known and laminar blood flow is taken into
consideration [25]. Accuracy of these measurements
depends on the angle of insonation, < 608 being acceptable
[26]. Volumetric flow measurements are also
possible but require accurate data on the size of the
vessel that are not easy to obtain. With current technology,
they are not of practical use in fetal Rh disease.
According to the law of physics, low viscosity
ªthinnedº fetal blood in anemia flows faster in fetal
blood vessels [16]. With unchanged peripheral resistance
that translates into increased systolic and diastolic
velocities. A similar increase in both systolic
and diastolic blood flow velocities may be an explanation
for generally unchanged S/D ratios and other
indices in fetal arterial circulation. An increase in
blood flow velocity does occur and to measure this
change in flow the maximum velocity (or ªpeak
velocityº) may be a more appropriate measurement
[17]. This single measurement of absolute velocity at
its maximum peak is angle dependent and as such requires
consistency in the technique of measurements.
In extreme conditions, in a terminally ill fetus, additional
factors (hypoxia, neural regulation) may be
affecting blood flow distribution and blood flow
velocity in specific vessels, but it is not observed in
mild to moderate anemia.
Fetal Aortic Arch
Increased fetal blood flow velocity in the aortic arch
can be expected in anemic fetuses consistent with increased
blood flow velocity in the fetal descending
aorta but specific studies are lacking. The standard
measurements have not been established for this
complex blood flow velocity pattern that varies according
to changing physiologic conditions [26].
Fetal Descending Aorta
An example of blood flow velocity measured in the
fetal aorta in an anemic fetus is presented in Fig.
22.2.