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a Chapter 16 Doppler Velocimetry of the Uteroplacental Circulation 229
Fig. 16.3. Fully developed physiologic
changes in the uteoplacental arteries
during normal pregnancy. Hatched portions
of the wall of these vessels indicate
the extent of the physiologic
changes. I. V. S. intrvillous space. (From
[113] with permission)
changes. In all, approximately 100±150 converted
spiral arteries supply the placental bed. It is believed
that these uteroplacental vessels have lost their ability
to respond to vasoactive substances.
The question arises whether this change from
small muscular arteries to dilated tortuous vessels is
entirely responsible for the increase in flow from
100 ml/min to 500±800 ml/min. All large vessels of
the pregnant uterus, regardless of whether they supply
the placental bed, undergo hyperplasia and hypertrophy
[17]. Thus the increase in cross-sectional
area leads to a reduction in resistance and further development
of the uterine circulation. This increase in
luminal diameter of the large vessels probably accounts
for the increase in uteroplacental flow during
the third trimester. In addition, the effects of the hormones
estrogen and progesterone, the increased
blood volume and cardiac output, the diminished
blood viscosity, and the lowered peripheral resistance
influence uterine flow.
Abnormal Development
of the Uteroplacental Circulation
in the Presence of Essential
Hypertension, Preeclampsia,
and Intrauterine Growth Restriction
According to Brosens et al. [15], Robertson et al.
[16], and Khong et al. [18], a lack of endovascular infiltration
by trophoblasts into the myometrial portion
of the placental bed spiral arteries is a consistent
finding in the presence of preeclampsia. The physiologic
changes of the placental bed spiral arteries
extend only to the deciduomyometrial junction
(Fig. 16.4). With preeclampsia the spiral arteries may
remain unconverted throughout their decidual and
myometrial length (Fig. 16.5). Thus there is both an
incompleteness in the degree of endovascular trophoblastic
invasion of the spiral arteries, being confined
to the decidual portion, and a reduction in the number
of uteroplacental arteries formed. The diameter of
the spiral arteries remains at 200±300 lm. Sheppard
and Bonnar [19] reported the picture not to be so
clear-cut, as they observed physiologic changes in the
myometrial spiral arteries in preeclamptic pregnan-
Fig. 16.4. Difference between normal
and preeclamptic pregnancies regarding
the extent of physiologic changes
in the uteroplacental arteries. With
preeclampsia these changes do not
extent beyond the deciduomyometrial
junction. (From [113] with permission)