o_1977r8vv9vk1ts2ms0kd8pksa.pdf

60 T. Kiserud
Fig. 5.7. The pressure variation of the left atrium is reflected
in the velocity recording of the pulmonary veins
(upper panel). With the loss of connection the pressure
variation is not transmitted into the vein and the velocity
pattern reflects instead the general pressure variation in
the chest (typical for anomalous pulmonary venous drainage;
lower panel). A atrial contraction wave, D diastolic
peak, S systolic peak. (From [27])
details of the cardiac events, but rather the general
pressure variation of the fetal chest (Fig. 5.7). The Doppler
recording thus supports the diagnosis.
Another important transmission line is formed by
the inferior vena cava (IVC), ductus venosus and umbilical
vein (Fig. 5.8a) [30, 31]. Agenesis of the ductus
venosus has been shown to interrupt the transmission
of the cardiac wave to the umbilical vein
[29]. The wave propagating along this line reflects the
changes in both the left and the right atrium since
the IVC is connected to the left atrium through the
foramen ovale in addition to the connection to the
right atrium. Conversely, the pulmonary veins reflect
predominantly the left atrium, and to some extent
the right atrium, depending on the size of the foramen
ovale [27].
Fig. 5.6. Effect of cardiac compliance on the venous waveform
in the ductus venosus. Although a stiff myocardium
due to acidosis or hypoxia is the most common cause, in
this case it was the fetal pleural effusion (Pl) at 30 weeks
of gestation (upper panel) that had a constrictive effect on
the heart. The reduced compliance was reflected in the
rapid downstroke of the peak velocity during ventricular
systole (S) causing the dissociation between S and the diastolic
peak (D; middle panel). Doppler recording 2 min after
the pleural effusion has been drained off (lower panel)
showed an instantaneous improvement in myocardial compliance
(less pointed S and reduced dissociation between S
and D), and the end-diastolic pressure was less, signified
by the less pronounced atrial contraction wave (a)
Wave Reflections
The pulse wave travelling along the transmission line
is modified according to the local physical conditions
[6, 15, 16, 27, 30, 32±35]. Pulsation at the ductus venosus
outlet is more pronounced than at the inlet
[36]. The stiffness of the vessel wall is different at the
ductus venosus outlet, ductus venosus inlet and intraabdominal
umbilical vein, and so are cross-section
and compliance [37]. The single most important
mechanism for changing the propagating pulse in the
veins is reflections. In much the same fashion as light
is reflected or transmitted when the beam encounters
a medium with a different density, the pulse wave in
the veins is reflected and transmitted when it hits a
change in impedance (Fig. 5.8 b) [30, 31, 38]. Vascular
junctions often represent a significant change in cross
section (and thus impedance). The junction between
the ductus venosus inlet and the umbilical vein is of
great diagnostic interest and has been particularly
well examined. During the second half of pregnancy