o_1977r8vv9vk1ts2ms0kd8pksa.pdf

126 J. Itskovitz-Eldor, I. Thaler
Fig. 9.12. Sites of pressure
measurements (arrows) and
calculations of vascular resistance
(R) to umbilical-placental
blood flow. This total resistance
comprises the following
individual resistances: R [Ao-
Uvp] resistance presented by
the umbilical arteries plus placenta.
R[UVp-PS] resistance
across the unbilical veins. R
[PS-IVS] combined resistance of
the ductus venosus and liver.
(Reprinted from [118] with permission)
tus is a function of umbilical blood flow and oxygen
content of umbilical venous blood. In general, maternal
conditions affect uterine blood flow (hypotension,
obstruction of the IVC) or uterine arterial blood oxygen
content (hypoxia, anemia) without affecting umbilical
blood flow significantly. Fetal conditions principally
influence umbilical blood flow (umbilical cord
compression, hemorrhage) but may also affect umbilical
venous oxygen content (anemia, polycythemia)
[119]. Oxygen delivery to the fetus can be reduced
experimentally by administering a low-oxygen gas
mixture to the mother [120] restricting uterine [53,
97] or umbilical [59, 94] blood flows, fetal hemorrhage
[92], or altering fetal hemoglobin concentration
[120, 121] or its affinity for oxygen [122, 123].
Acute Hypoxia
Circulatory adaptation to acute hypoxic stress is accomplished
by changes in the distribution of cardiac
output secondary to the responses of the vasculature
in the various organs. In general, blood flow to the
priority organs (brain, heart) increase with maintenance
of oxygen delivery, whereas blood flow and
oxygen delivery to organs that are less essential for
immediate survival decreases [115, 116, 124].
Umbilical blood flow reduction is associated with
decreased fetal cardiac output, which is closely related
to the decrease in umbilical venous return; when flow
is reduced by 50%, fetal cardiac output is reduced by
about 20% [94]. With maternal hypoxemia or uterine
blood flow reduction, fetal cardiac output may be
maintained during mild hypoxemia but decreases by
about 20% with more severe hypoxia [97, 114]. The
changes in distribution of fetal cardiac output and actual
organ blood flows associated with umbilical
blood flow reduction show several similarities, as well
as a number of significant differences, when compared
with the findings during maternal hypoxemia
or uterine blood flow reduction. In the latter situation,
the proportion of cardiac output and the actual
blood flow increase to the brain and myocardium but
decrease to the lungs. Whereas the percentage of cardiac
output and the actual blood flow to the carcass
(skin, muscle, bone) and to the splanchnic organs decrease
during maternal hypoxemia or uterine blood
flow reduction, they increase during cord compression
(Fig. 9.13). Thus although the total umbilicalplacental
vascular resistance increases during cord
compression, total fetal body vascular resistance does
not change significantly [94]. Maternal hypoxemia
and uterine blood flow reduction are associated with
increased fetal body vascular resistance but with only
a small change in umbilical-placental vascular resistance
[97, 114, 118].
It is possible that the increase in peripheral blood
flow during umbilical blood flow reduction may serve
as a mechanism for augmenting venous return to the