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Blood flow in the fetal descending aorta supplies the
placenta and lower body of the fetus, including kidneys,
splanchnic and pelvic organs, and lower extremities.
The major portion of blood in fetal descending
aorta is directed to the placenta. Thus an increase
in the resistance to flow in the placental vascular
bed can profoundly influence the flow not only in
the umbilical artery but also in the descending aorta.
It would be reflected by typical changes of the velocity
waveform, (i.e., decreased or no diastolic velocity)
(Fig. 11.11). Possibly, vasoconstriction in the lower
fetal body, which is known to be one of the mechanisms
involved in the centralization of flow during
hypoxia, potentiates the effect on aortic velocity
waveforms.
Doppler indication of increased resistance to flow
in the fetoplacental circulation was found to be rea
Chapter 11 Fetal Descending Aorta 151
%
100
50
0
Distribution of Fetal
Aortic Blood Flow
Viscera
Placenta
Lower extr.
28 32 36 40
Weeks
first derivative of the mean velocity and diameter waveforms
is used to synchronize heart cycles.
Intrinsic Influences on Fetal
Aortic Flow and Flow Indices
%
100
Fig. 11.10. Distribution of the fetal aortic blood flow during
the third trimester of normal pregnancy. Blood flow in
the thoracic descending aorta corresponds to 100%. (Reprinted
from [26] with permission)
50
0
Similar to other fetal vessels (e.g., the carotid artery
[51]), a negative correlation has been found between
the aortic PI and fetal heart rate, with correlation
coefficients between ±0.43 and ±0.73 [22, 36, 43, 52].
This inverse relation was pronounced mainly during
periods of behavioral state 2F in term fetuses [36].
In a series of studies, the Rotterdam research
group reported a clear dependence of fetal aortic velocity
waveform indices on fetal behavioral states in
term pregnancies [36, 43] and on the fetal activity
state during the early third trimester [52]. The values
for the fetal aortic PI were higher during fetal quiet
sleep than during active sleep, with decreased impedance
and increased flow in fetal musculature.
All noncompromised fetuses perform periodic
breathing movements, with contraction of the diaphragm,
expansion of the abdominal wall, and retraction
of the thorax during ªinspirationº and a return
to the rest position during ªexpirationº [53]. Fetal
breathing movements have a pronounced effect on
blood circulation in the umbilical cord and the intrafetal
vessels [29]. The aortic blood flow velocity waveforms
exhibit modulation of their shape, with rhythmic
oscillations in the amplitude of their peak velocities
and diastolic velocities. The end-diastolic velocities
sometimes eventually disappear during fetal ªinspiration.º
The time-averaged mean velocity usually
increases ± up to 40% ± during periods of fetal
breathing movements [29]. Probably, the cardiac output
also increases as a consequence of increased
venous blood return to the fetal heart [54].
The above observation is important to consider
when recording fetal aortic blood velocities: To obtain
reproducible results, only Doppler traces recorded during
periods without fetal breathing should be accepted
for analysis. Fetal breathing movements can usually be
recognized in the two-dimensional realtime image of
the fetus, the aortic Doppler shift signals, or the Doppler
traces of umbilical venous velocities.
Fetal Aortic Blood Flow
During Labor
The fetal aortic volume blood flow has been shown to
increase with progression of labor in a study in
which the measurements were performed between
contractions [55]. The increased fetal flow might be a
phenomenon similar to the reactive hyperemia found
in the uteroplacental circulation of experimental animals
between contractions [56]. In the study by Lindblad
et al. [55] there was no change in the aortic PI
with advancing labor, and there was no difference between
patients with and those without ruptured
membranes. Fendel et al. [57] measured the mean fetal
aortic velocity during labor and found a decrease
in the velocity during contractions. The aortic PI and
RI remained unchanged.
Pathophysiologic Changes
of Fetal Aortic Flow During
Intrauterine Hypoxia