o_1977r8vv9vk1ts2ms0kd8pksa.pdf

a Chapter 5 Venous Hemodynamics 63
Fig. 5.11. Mathematical model showing how umbilical venous
pulsation changes with wall stiffness (b UV ), a determinant
for compliance. Index of pulsation for the pressure
wave (IP =pulse amplitude divided by time-averaged pressure)
increases with stiffness. The model also demonstrates
the effect of increased diameter ratio between the umbilical
vein and ductus venosus (D UV /D DV =g) on pressure
transmission to the umbilical vein. Increased ratio is associated
with less pulsation in the umbilical vein due to increased
degree of reflections at the junction. (From [34])
of the fetal portal vein [47, 48]. Compared with the
umbilical vein, the left portal branch has a smaller diameter
(i.e. compliance), which increases the likelihood
for visible pulsation [49].
Direction of Pulse and Blood Velocity
Fig. 5.12. Upper panel: The physiological constriction of
the umbilical vein at the abdominal inlet (b) represents a
reduction in compliance compared with the section in the
cord (a) or intra-abdominal portion (c; from [43]). Lower
panel: High velocity is recorded at the constriction (b) compared
with outside (a) or inside (c) the abdominal wall. The
pulsation from the neighboring umbilical artery induces
velocity pulsation in the vein at the constriction area but
not in the neighboring sections where the compliance is
higher (a and c). (From [45])
A short velocity deflection of the umbilical venous
flow is commonly recognized as the atrial contraction
wave; however, pulsations may appear differently and
have various causes. Recent research has addressed
this part of physiology. One important determinant is
the direction of the pulse wave compared with the direction
of the blood flow. The concept of wave intensity
was introduced to explain the wave in arteries
[50], but the concept is equally valid for veins [27,
28, 38]. When the pressure wave travels in the opposite
direction of the blood velocity (Fig. 5.13), the
pressure wave causes a deflection in the velocity (e.g.
atrial contraction wave in the hepatic veins, ductus
venosus and umbilical vein); however, if the pressure
wave and blood velocity travel in the same direction,
the pressure wave will impose a velocity increase (e.g.
umbilical artery waveform), an effect also seen in the
venous system (e.g. left portal vein; Fig. 5.13).
A particularly instructive example is found at the
junction between the ductus venosus and the umbilical
vein/portal system (Fig. 5.14) [49]. The pressure
wave travels down the ductus venosus in the opposite
direction of the blood flow (atrial contraction wave is
negative). When the pressure wave enters the umbilical
vein it propagates in two directions: down the
umbilical vein, or up the left portal vein. In the former
case the velocity wave is negative, in the latter it
is positive (Figs. 5.13, 5.15). In the compromised fetal
circulation the left portal vein also acts as a watershed
area between the left and right part of the
liver [49]; thus, blood in this section may pendulate
or reverse. Depending on the direction of flow in this
section, the waveform will turn the same way or be
inverted when compared with the umbilical vein
pulse.