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a Chapter 23 Doppler Velocimetry in Prolonged Pregnancy 359
ity in technique and defined outcomes once again
limits our ability to arrive at definitive conclusions.
There does, however, appear to be some observable
changes in aortic Doppler indices in compromised
prolonged gestations with reduced velocity, possibly
indicating a degree of compromise.
Aorta and Pulmonary Artery
Doppler
Hemodynamic changes in cardiac function have been
described in growth-restricted fetuses. Weiner et al.
[24] suggested that abnormal cardiac Doppler precedes
the occurrence of abnormal fetal heart rate
(FHR) pattern in these fetuses. These authors [19]
also investigated the aorta and pulmonary outflow
tracts by Doppler ultrasound in uncomplicated prolonged
pregnancies to determine if changes of cardiac
function can cause the development of oligohydramnios
and the occurrence of an abnormal FHR pattern.
They found that both the aortic peak velocity and the
estimated aortic blood flow calculated using vessel diameter
measurements correlated significantly with
both the amniotic fluid index and the FHR pattern.
On the other hand, the pulmonary peak velocity and
estimated outflow volume correlated with the FHR
pattern only; thus, left cardiac output was reduced in
post-term pregnancies with oligohydramnios and
both left and right output were reduced in the presence
of abnormal fetal heart rate. It is well known
that the left cardiac output is directed mainly to the
brain. It is possible that reduced renal perfusion occurred
in association with the lower left cardiac output
directed mainly to the brain, leading to oliguria
and oligohydramnios.
Renal Artery Doppler
Doppler velocimetry of the renal artery, while of limited
practical value in the day-to-day management of
prolonged pregnancies, has the potential to shed light
on the mechanism of fetal deterioration. A limited
number of studies have been published in this area.
Arduini et al. [25] compared the Doppler changes in
the fetal renal artery in IUGR and post-term fetuses
to determine whether the mechanisms of fetal compromise
were the same. Changes in the renal PI in
114 IUGR and 97 post-term fetuses > 42 weeks were
compared. In each population the relationship between
renal PI and fluid volume categorized as adequate
reduced or oligohydramnios was evaluated. The
renal PI in IUGR fetuses was significantly increased
above normal. This increase was most marked in the
oligohydramnios group. This contrasted with findings
in the post-term fetuses where there were no significant
differences in renal artery PI compared with
normative data. Furthermore, there was no correlation
between renal Doppler and amniotic fluid volume.
The authors concluded that while IUGR results
have classic utero-placental insufficiency with redistribution
of fetal cardiac output, a different mechanism
occurred in prolonged gestation. Specifically, it
was noted that animal studies demonstrated increased
sensitivity of the fetal kidney to vasopressin with advancing
gestation. Vasopressin promotes reabsorption
of water by the kidney resulting in reduced urine volume
[26]. Arduini et al. [25] suggested that this
might be the mechanism of oligohydramnios in prolonged
human pregnancies. Veille et al. [27] compared
the renal artery S/D in 33 patients with normal
amniotic fluid volume vs 17 with oligohydramnios
defined as AFI £5 cm. The mean gestational age in
the normal group and the group with low amniotic
fluid volume was 41.3 weeks; however, the number of
cases that were post-term was not given. Despite the
fact that there was no difference in the umbilical
artery S/D (normal fluid group 2.36 Ô 0.05 vs oligohydramnios
group 2.24 Ô 0.05; p =NS), the renal S/D
was significantly higher in the oligohydramnios
group (renal S/D: 6.5 Ô 0.3 vs 7.8 Ô 0.4; p 41 weeks. Fifteen
patients had oligohydramnios (AFI < 5), while
the other 42 had normal fluid volume and served as
controls. There were no differences in either the umbilical
(0.51 Ô0.1 vs 0.052 Ô 0.06) or renal (0.71Ô0.08
vs 0.73 Ô 0.05) artery RI values between oligohydramnios
and control groups, respectively. Power analysis
done by the authors reportedly confirmed the adequacy
of the patient numbers. They therefore did not
find evidence or renal blood flow redistribution in
post-term pregnancies with oligohydramnios.
Studies in laboratory animals [28] indicate that angle-independent
Doppler indices, such as S/D, PI, and
RI, correlate poorly with actual blood flow measured
by more direct methods. Direct velocity measurements,
such as peak systolic (PSV) and end-diastolic
velocities (EDV), were found to correlate more closely