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a Chapter 37 Evaluation of Pulmonary and Ductal Vasculature 549
Umbilical Cord
Optimal recordings in the umbilical cord have both
arterial and venous tracings. The umbilical venous
tracing is a low-velocity (20±30 cm/s) signal with
high intensity. If there are cardiac pulsations in the
umbilical venous trace, one should suspect increased
venous pressures and cardiac failure.
Uterine Artery
Sampling the maternal circulation at the uterine artery
provides a waveform that appears to correlate
with uterine impedance. Early studies in these vessels
suggested that detection of growth restriction is possible
during pregnancy.
Descending Aorta
The pulsed sample volume should completely insonate
the lumen of the descending aorta. The descending
aorta has a high systolic velocity (50±100 cm/s)
and a low diastolic velocity in the same direction as
the systolic velocity. This waveform has been shown
to change in the presence of growth restriction, probably
indicative of increased total systemic resistance.
In the extreme example of a fetus with advanced
growth disturbance, there is an absence of end-diastolic
flow.
Middle Cerebral Artery
The middle cerebral artery (MCA) is used as an indicator
of brain vascular impedance. The normal pattern
is similar to that in the umbilical cord at a lower
velocity. The diastolic velocity can be elevated with
severe intrauterine growth restriction (IUGR), presumably
indicative of compensatory cerebral vasodilation,
the brain-sparing effect.
Renal Artery
The renal artery waveform is obtained in transverse
sections of the abdomen during visualization of the
kidneys. Color Doppler sonography greatly improves
the percentage of patients from whom high-quality
data can be obtained. The waveform has a high systolic
velocity and a low diastolic velocity, similar to that
in the descending aorta. The normally pulsatile renal
vein is often noted while interrogating the proximal
renal artery.
Ductal and Aortic Arches
The aortic arch and ductal arch may appear similar, but
systolic velocity is always higher in the ductal arch.
Ductal constriction is characterized by an increase in
both systolic and diastolic velocities (see below).
Pulmonary and Aortic Valves
Pulsed-wave Doppler sonography in the semilunar
valves shows a pattern of systolic ejection with a rapid
upstroke in the pulmonary value and a slower upstroke
in the aortic valve. The peak velocities of ejection may
be useful for assessing cardiac output indirectly. Integration
of the waveform gives the time-velocity integral
(TVI) and is proportional to the stroke volume. Correcting
for heart rate, any change in valve flow can
be studied by comparing the TVI-heart rate product
before and after the intervention [9]. If valve stenosis
is present, there will be evidence of turbulence and
the peak velocity will be slightly increased. If valve regurgitation
is present, there will be an abnormal turbulent
jet back into the ventricle during diastole.
Atrioventricular Valves
The mitral and tricuspid valves are assessed using
pulsed-wave Doppler sonography from an apical view.
The 2-mm sample volume is placed at the valve annulus
to obtain a biphasic pattern during diastole consisting
of an E wave (corresponding to the rapid filling
of the ventricle) and an A wave (corresponding to
atrial contraction). Systolic velocity back into the atria
suggests valvar regurgitation and is detected on
the pulsed-wave evaluation; however, the use of continuous-wave
Doppler sonography is helpful for interrogating
the atrioventricular (A-V) valve area. Grading
valvar regurgitation is qualitative, with a nonholosystolic
jet being classed as trivial and a holosystolic
jet as significant. Most fetal valve regurgitation is
trivial or mild in the absence of other signs of fetal
congestive heart failure, such as an abnormal venous
Doppler result. Because the peak velocity of A-V valve
regurgitation is so typical and is higher than any
other velocity in the fetal circulation, the range ambiguity
introduced by the use of continuous-wave Doppler
sonography does not cause any confusion. On
the contrary, continuous-wave Doppler sonography
with high filter settings can detect a tiny jet of A-V
valve regurgitation and characterize the timing of it
and the nature of the early upstroke velocity. Such a
jet is turbulent and creates an abnormal finding on
color Doppler interrogation owing to the high peak
velocity. The most common cause of tricuspid valve
regurgitation in our practice is the effect of indomethacin
on the fetal ductus arteriosus.
Inferior Vena Cava
The waveform in the inferior vena cava (IVC) is normally
biphasic during flow to the heart with an additional
brief period of reversal during atrial systole.
This waveform is useful for detecting abnormal forward
flow states in the fetal circulation. The abnormal