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a Chapter 22 Doppler Velocimetry in Maternal Alloimmunization 345
ciation with low fetal hematocrit was weak with 66%
sensitivity, 75% specificity, 50% positive predictive
value, and 86% negative predictive value.
Diagnostic use of the preload index is further limited
by the difficulty in obtaining consistent measurements
from a specific site [44].
Ductus Venosus
The ductus venosus in anemic fetuses demonstrates
higher flow, reflecting increased venous return, and
cardiac preload. Oepkes found the ductus venosus
elevated in anemic fetuses with marked improvement
following fetal transfusion [37]. A study by Hecher
did not, however, show any significant association
with fetal anemia sufficient for use as a diagnostic
tool [15].
Maternal Uterine Artery
The pulsatility index of the uterine arteries and thoracic
aorta peak velocity were used in a multiple regression
model to predict fetal hematocrit following
fetal transfusion on the assumption that resolving
placental edema after transfusion improves uteroplacental
circulation. The uterine artery pulsatility index
alone has not been found to change in fetal anemia.
Fetal Morphologic Changes
and Doppler Velocimetry
in Alloimmunization
Fetal cardiovascular changes reflected in Doppler velocimetry
precede development of fetal hydrops. During
development of anemia in alloimmunization fetal
hydrops develops gradually and relatively late in the
process. Fetal signs of hydrops appear infrequently if
the fetal hemoglobin level is within 5 g of the median
value for gestational age. With more severe anemia
fetal hydrops appears, often gradually, over a period
of days. The following conditions are observed:
1. Increased amniotic fluid volume
2. Increase in placental thickness
3. Fetal liver enlargement
4. Fetal pericardial effusions
5. Ascites
6. Free loops of bowel in ascites
7. Double-walled bladder
8. Scalp edema (ªhaloº)
9. Facial swellings
10. Pleural effusions
11. Extremities edema
12. Umbilical cord pulsation
Moderate increase in amniotic fluid volume enhances
the resolution of ultrasound imaging and often facilitates
detection of even small amounts of fluid in fetal
body cavities. Findings of fluid in any fetal compartment
in isoimmunized pregnancy should prompt a
complete fetal morphology review for signs of fetal
hydrops. Polyhydramnios in alloimmunization may
reach a significant degree that is detrimental to the
clarity of the imaging. Preterm labor, that often follows,
can make fetal examination even more difficult.
Polyhydramnios will frequently regress with correction
of fetal anemia.
Placental thickness also increases as part of the
process of isoimmunization; however, it is not a reliable
indicator of fetal anemia [45].
Liver enlargement is often seen during fetal anemia
in alloimmunization as a result of increased fetal
extramedullary erythropoiesis [46]. This increased
liver size is associated with increased venous Doppler
blood flow velocity in fetal intrahepatic venous flow
as reported by Oepkes et al. [37]. Although it was
postulated by Vintzileos to be a good indicator of fetal
compromise [47], it was not as accurate as evaluation
with MCA peak velocity measurements [39].
A certain amount of fluid is not unusual in the
pericardium. When measured at the valvular level,
2 mm of fluid is often found and is not abnormal.
This small fluid collection is not associated with abnormal
Doppler velocimetry; however, large pericardial
effusions are one of the signs of fetal hydrops
and indicates fetal anemia.
Ascites are defined as sonolucent areas in the fetal
abdomen with loops of bowel visible, floating in the
ascites and the fetal bladder visible with fluid inside
and outside the bladder wall (Fig. 22.7). The presence
of ascites is commonly associated with fetal anemia
and abnormal Doppler velocimetry of the MCA and
other vessels. Pleural effusions when present in hydrops
fetalis suggest advanced disease.
Fig. 22.7. Fetal ascites