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a Chapter 36 Doppler Echocardiographic Studies of Deteriorating Growth-Restricted Fetuses 539
fect the morphology of the velocity waveforms from
different areas, among them preload [28, 29], afterload
[29, 30], myocardial contractility [31], ventricular
compliance [32], and fetal heart rate [33]. It is not
possible to obtain simultaneous recordings of pressure
and volume, so we cannot fully differentiate between
these factors in the human fetus. However, because
each parameter and recording site can be specifically
affected by any one of these factors it is possible
to indirectly elucidate the underlying pathophysiology
by performing measurements at various cardiac
levels.
Venous Circulation
Blood flow velocity waveforms may be recorded from
the superior and inferior vena cava, ductus venosus,
umbilical vein, and pulmonary veins. The vascular
areas most intensively studied are the IVC, ductus venosus
(DV), and umbilical vein.
The IVC velocity waveforms, recorded from the
segment of the vessel just distal to the entrance of the
ductus venosus [34], are characterized by a triphasic
profile with a first forward wave concomitant with
ventricular systole, a second forward wave of smaller
dimensions seen during early diastole, and a third
wave with reverse flow during atrial contraction [23].
Several indices have been suggested for analyzing IVC
waveforms, but the most frequently used is the percent
reverse flow, which is quantified as the percent
of TVI during atrial contraction (reverse flow) with
respect to total forward TVI (first and second wave)
[23]. This index is considered to be related to the
pressure gradient between the right atrium and the
right ventricle during end-diastole, which is a function
of both ventricular compliance and ventricular
end-diastolic pressure [34].
Ductus venosus velocity may be recorded in a
transverse section of the upper fetal abdomen, at the
level of its origin from the umbilical vein [35, 36].
The ductus venosus velocity waveforms exhibit a biphasic
pattern: a first peak concomitant with systole
and a second peak during diastole, with a nadir during
atrial contraction. The ratio between the maximum
systolic (S) peak velocity and the atrial nadir
(A) (S/A ratio) and the preload index (S±A)/S are the
most commonly employed indices to assess DV hemodynamics
[37±39].
Umbilical venous blood flow is usually continuous.
However, in the presence of a relevant amount of reverse
flow, during atrial contraction in the IVC pulsations
occur with the heart rate in the umbilical venous
flow. During normal pregnancies these pulsations
occur only before the 12th week of gestation
and are secondary to the stiffness of the ventricles at
this gestational age, causing a high percentage of reverse
flow in the IVC [40]. The presence of umbilical
vein pulsations later in the gestation is considered a
sign of impaired cardiac function. Only the qualitative
venous Doppler waveform analysis seems to improve
prediction of critical perinatal outcomes in preterm
IUGR fetuses and therefore should be incorporated
into the surveillance of these fetuses [40].
Atrioventricular Valves
Flow velocity waveforms at the level of mitral and tricuspid
valves are recorded from the apical four-chamber
view of the fetal heart. They are characterized by
two diastolic peaks that correspond to early ventricular
filling (E wave) and active ventricular filling during atrial
contraction (A wave). The ratio between the E and
A waves (E/A) is a widely accepted index of ventricular
diastolic function and is an expression of both cardiac
compliance and preload conditions [28, 42].
Outflow Tracts
Flow velocity waveforms from the aorta and pulmonary
artery are recorded, respectively, from the fivechamber
and short-axis views of the fetal heart. PV
and TPV are the most commonly used indices. The
former is influenced by several factors, including
valve size, myocardial contractility, and afterload [29,
30], whereas the latter is believed secondary to the
mean arterial pressure [43].
At the level of the outflow tracts the PV values linearly
increase, and higher values are present in the
aorta than in the pulmonary artery [33]. TPV values
remain almost constant throughout gestation [44].
TPV values at the level of the pulmonary valve are
lower than at the aortic level, suggesting slightly
higher blood pressure in the pulmonary artery than
in the ascending aorta [45]. Quantitative measurements
have shown that the right cardiac output
(RCO) is higher than the left cardiac output (LCO),
and that from 20 weeks onward the RCO/LCO ratio
remains constant at a mean of 1.3 [46, 47]. This value
is lower than that reported in the fetal sheep (RCO/
LCO =1.8), a difference that may be explained by the
higher brain weight in humans, which increases left
cardiac output [48].
Longitudinal Hemodynamic
Modifications in IUGR Fetuses
The timing of the delivery of IUGR fetuses is usually
based on the results of biophysical tests (e.g., fetal
heart rate monitoring or biophysical profile) or because
there is an uncontrollable coexisting maternal
disease (e.g., preeclampsia). The time interval be-