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a Chapter 11 Fetal Descending Aorta 149
Fig. 11.9. Doppler shift spectrum recorded from the thoracic
descending aorta of a healthy fetus at 28 weeks' gestation
with a low proportion of diastolic flow (Fig. 11.9); recorded
in the fetal abdominal aorta, the proportion of
diastolic flow increases [22]. It is therefore crucial to
standardize the site of recording; typically, in the
thoracic aorta the sample volume is located just above
the diaphragm.
Fetal Aortic Flow
in Uncomplicated Pregnancies
Blood flow in the fetal descending aorta is characterized
by high blood velocity ± higher than that found
in adult descending aorta [23], and the waveform of
the aortic velocity is influenced by the low vascular
resistance in the placenta. In the uncompromised fetus
during the second half of pregnancy, aortic diastolic
velocity is present throughout the cardiac cycle,
the proportion of diastolic flow being higher in the
abdominal than in the thoracic descending aorta [22].
Consequently, the pulsatility index (PI) [24] and resistance
index (RI) [25] are typically lower in the abdominal
than in the thoracic fetal aorta. The narrow
spectrum of Doppler shift signals recorded from the
thoracic aorta during systole indicates a fairly flat
flow profile. In the abdominal aorta, lower and more
dispersed frequencies can be seen owing to the
change of flow profile toward a more parabolic pattern.
The time-averaged mean velocity in the thoracic
descending aorta does not change significantly during
the third trimester (35.0Ô5.5 cm/s, meanÔSD) [26],
the increase in the volume flow being related to the
growing aortic diameter. The volume flow corrected
for fetal weight was found to be stable during late
pregnancy, the reports in the literature ranging from
206 to 280 ml ´min ±1 ´kg ±1 (Table 11.1). The mean
aortic PI was reported to range from 1.83 to 2.80 and
to be rather stable until 36 weeks (Table 11.2). Thereafter
a slight increase was observed toward term [22].
Concomitantly, a slight decrease in the volume flow
was reported [26].
Estimation of flow at two levels of the fetal descending
aorta and in the abdominal part of the
umbilical vein enabled calculation of the percentual
distribution of the aortic flow (Fig. 11.10). The placental
proportion of blood flow in the descending
thoracic aorta related to the fetal weight diminished
with increasing pregnancy length: At 28 weeks it was
59% and at term 33% [26]. This finding is in good
agreement with the reports of other authors, who
have described the umbilical venous blood flow to be
64% [44], 55% [31] or 54% [28] of the fetal aortic
blood flow from 26 weeks onward. In fetal lambs
65% of the blood flow in the descending aorta was
found to be directed to the placenta [45]. The decrease
in the placental proportion of the flow with
the progression of pregnancy might be due to the
changing ratio of fetal to placental weight [46].
The velocity waveform in the fetal aorta is subject
to several influences, one of them being fetal heart
function. In a study on exteriorized lambfetuses, a
strong correlation was found between the rising slope
of the velocity waveform and myocardial contractility
(measured as dP/dt) in the left heart ventricle [47].
Interestingly, the next best correlation was found for
the aortic PI.
Råsånen et al. [32] found in human fetuses a good
correlation between the growth of the fetal heart, the
lumen of the descending aorta, and aortic volume
flow. The aortic velocity indices were independent of
cardiac size and fractional shortening. The authors
interpreted this finding as myocardial contractility
that remained stable despite the changing peripheral
vascular resistance so long as the changes of resistance
remained within the normal range.
The nonsimultaneous measurements of pulsatile
mean flow velocity and vessel diameter in the fetal descending
aorta can be synchronized by transabdominal
fetal electrocardiography (ECG) [48±50]. A more accurate
estimation of the aortic flow is then possible, and
the aortic stroke volume can be calculated. In the above
studies, the stroke volume has been reported to range
from 2.8 to 5.6 ml/min in the thoracic aorta and from
2.4 to 4.3 ml/min in the abdominal aorta of healthy
third trimester fetuses. The relative stroke volume
was found to be stable during the last trimester of gestation
(1.7±2.1 ml ´ min ±1 ´kg ±1 ) [48, 50]. The method
used for these examinations is laborious, mainly because
of the difficulty obtaining transabdominal fetal
ECG signals of acceptable quality. To circumvent this
problem, Tonge et al. [49] applied their experience
from animal experiments and recommended that the