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a Chapter 15 Pulsed Doppler Ultrasonography of the Human Fetal Renal Artery 217
ratio and is a better predictor of IUGR than SGA
[27].
In another study, the renal volume in fetuses with
IUGR fetuses was 31% (95% CI, 20%±40%), which
was less than the renal volume obtained in the group
of non-IUGR fetuses after adjusting for gestational
age. The ratio of renal volume to estimated fetal
weight was 15% (95% CI, 1%±26%), which was less
than the same ratio in the non-IUGR fetuses. No differences
were seen in the renal artery Doppler measurements.
These authors concluded that IUGR appears
to be associated with a decrease in fetal renal
volume. Because renal volume is a likely proxy for
nephron number, this study supports the hypothesis
that IUGR may be linked to congenital oligonephropathy
and potentially to hypertension in later life and
other related vascular diseases [28].
Some authors have determined the fetal blood flow
redistribution and the amount of amniotic fluid in
appropriate-for-gestational-age (AGA) fetuses and
growth-restricted fetuses. In one study, Yoshimura
determined the blood flow velocity waveforms of the
umbilical artery, descending aorta, middle cerebral
artery, renal artery, and uterine artery using pulsed
Doppler ultrasonography in 100 AGA fetuses and 39
growth-restricted fetuses. The PI values and the
amount of amniotic fluid were compared between the
two groups. The PI values of the umbilical artery and
renal artery were significantly higher in AGA fetuses
with oligohydramnios than in fetuses with an adequate
amount of amniotic fluid. The PI values of the
umbilical artery and renal artery were significantly
higher and the PI of the middle cerebral artery was
significantly lower in growth-restricted fetuses with
oligohydramnios than in fetuses with an adequate
amount of amniotic fluid. Furthermore, there was a
significant negative correlation between the PI value
of the renal artery and the vertical diameter of amniotic
fluid, and between the PI value of the renal artery
and the amniotic fluid index. The PI value of the
renal artery was related to the amount of amniotic
fluid in growth-restricted fetuses, and the same relationship
was demonstrated in AGA fetuses [29].
Arduini and Rizzo reported on the renal blood
flow velocity waveforms of 114 IUGR fetuses and 97
postterm fetuses [16]. They found that the IUGR fetuses
had a higher PI than a group of normally
grown fetuses especially if there was oligohydramnios.
Interestingly, postterm fetuses had PIs similar to
those of normal term fetuses. In the postterm fetuses
there was no correlation between the amount of amniotic
fluid and the fetal renal PI values. To explain
these apparent discrepancies, the authors suggested
that the etiology of the oligohydramnios could have
different mechanisms in these two subsets of fetuses.
They speculated that the oligohydramnios in the
IUGR fetuses was related to changes in intrarenal vascular
resistance [16], and in postterm fetuses it was
related to changes in tubular reabsorption.
Mari et al. [17] found that among four human fetuses
affected by asymmetric IUGR who had oligohydramnios
and abnormal fetal renal artery velocimetry
the perinatal mortality was high (three of four), confirming
the findings of Veille and Kanaan [13].
Thus most of the published studies on fetal renal
artery waveforms support the concept that IUGR fetuses
with oligohydramnios have a PI above the established
values for the 95th percentile (Fig. 15.7).
The combination of IUGR, oligohydramnios, and elevated
PI of the fetal renal artery seems to be associated
with an increase in perinatal morbidity and
mortality. These Doppler studies support an intrarenal
increase in impedance, which in turn affects renal
perfusion and urine production.
Akita et al. evaluated renal blood flow in 102 normal
human fetuses between weeks 20 and 40 of gestation
and compared these normative results to those
of 11 IUGR fetuses with normal amniotic fluid, 15 fetuses
with oligohydramnios, and 10 IUGR fetuses
with oligohydramnios [30]. Color duplex PW Doppler
ultrasound was used to evaluate the fetal renal artery.
The ascending aorta and pulmonary arteries were
evaluated at the same time. Akita et al. concluded
that the kidneys of IUGR fetuses with oligohydramnios
were poorly perfused because of a decrease in
stroke volume, which was found to be associated with
these fetuses [30].
Although a prerenal etiology for oligohydramnios
is always possible, human and animal data strongly
suggest that the human kidney is capable of modifying
intrarenal resistances according to alterations in
the in utero environment. Evidence obtained from fetal
renal arteries of guinea pigs, for example, suggests
that the fetal circulation exhibits heterogeneity in
Fig. 15.7. Confidence intervals for a group of normal fetuses.
This graph and the one in Fig. 15.6 are comparable
and point to an elevated pulsatility index (PI) of the fetal
renal artery. (Reprinted from [16] with permission)