o_1977r8vv9vk1ts2ms0kd8pksa.pdf

a Chapter 30 Doppler Interrogation of the Umbilical Venous Flow 447
Fig. 30.4. Longitudinal assessment of umbilical venous
flow per head circumference (ml/min mm ±1 ). Growth-restricted
fetuses with abnormal umbilical arterial pulsatility
index (full circles). Growth-restricted fetuses with normal
umbilical arterial pulsatility index (empty circles). The 2nd,
5th, 50th, 95th, and 98th percentiles of normal reference
values. HC head circumference
Fig. 30.5. Umbilical venous mean velocity per unit head
circumference (ml/min mm ±1 ). Growth-restricted fetuses
with abnormal umbilical arterial pulsatility index (full circles).
Growth-restricted fetuses with normal umbilical arterial
pulsatility index (empty circles). The 2nd, 5th, 50th, 95th,
and 98th percentiles of normal reference values. HC head
circumference
normalized by the head circumference (ml/min mm ±1 ).
The advantage of this simplified normalization stems
also from the fact that fetal head is an area of preferential
distribution of flow and growth in the deprived
fetus [28]; therefore, the use of this part of the fetal
body as an index of fetal body mass improves the
sensitivity of flow assessment in asymmetrically
growth-restricted fetuses.
The increase in flow volume throughout gestation in
normal fetuses is accounted for mainly by growth of
the umbilical fetal vessels. The diameter of the vein increases
from 4.1 to 8.3 mm, which would lead to a fourfold
increase in cross-sectional area, whereas the velocity
increased only 20% from 0.08 m/s at 20 weeks to
0.10 m/s at 38 weeks [12]. When blood velocity and diameter
is normalized for fetal mass, an interesting result
that can be observed in growth-restricted fetuses
is that the diameter in small fetuses is not narrower
than that of normal fetuses of comparable mass,
whereas the main variable to decrease is blood velocity
(Fig. 30.5). According to these data velocity itself could
be used as a simple diagnostic test in growth-restricted
fetuses. According to fetal sheep experiments [29] pressures
and flow velocities are inversely related in the venous
in-flow tract from the umbilical vein to the ductus
venosus and inferior vena cava, this finding brings in
both a diagnostic potential and a pathophysiological
variable of interest.
In the same paper by Boito and co-workers, elsewhere
quoted [25], a complex set of ultrasound volumetric
measurements was performed on the head and
on the abdomen of restricted and normal fetuses and
a significant (p