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a Chapter 29 Doppler Ultrasound Examination of the Fetal Coronary Circulation 431
or Doppler, these aspects of the setup are essential
preliminary steps. Once the coronary vessel is identified
using these techniques the transducer position
should be adjusted to provide an insonation angle
close to 08 prior to obtaining pulsed-wave measurements.
The pulsed-wave Doppler gate should be adjusted
to exclude other cardiac and extracardiac flows
and should be the only active display when measurements
are taken. Concurrent activation of multiple
image modalities (duplex or triplex mode) drastically
increases computing requirements and affects the
spatial and temporal resolution of the spectral Doppler
waveform.
Examination of Coronary Arteries
Using gray-scale ultrasound the coronary ostia are
discernable in late gestation (Fig. 29.1). Before this
time, the size of the main-stem arteries is below
1 mm in diameter and thus frequently below the resolution
threshold of current sonographic equipment
in the majority of cases [37]. For this reason color
and pulsed-wave Doppler ultrasound are necessary to
detect and verify coronary artery blood flow. The
Doppler examination of the fetal coronary vessels has
been adopted from techniques developed for infants
and neonates [38]. The main-stem right and left coronary
arteries are best examined in a long-axis view of
the left ventricular outflow tract and ascending aorta
or a precordial short-axis view of the aorta. The LAD
branch of the LCA is best identified from an apical
short-axis view. In the standard precordial short-axis
view the left coronary artery courses forward towards
the transducer, whereas the right coronary artery
runs more parallel. This view therefore facilitates examination
of the LCA. In the lateral, or long-axis,
view of the left ventricular outflow tract the RCA is
more readily imaged if imaged from the right side of
the fetus. In this view it may also be possible to visualize
both coronary arteries (Fig. 29.2) [39, 40].
The LAD may be identified scanning from the apical
four-chamber view. From this view the transducer is
tilted towards the head until the level of the superior
cardiac surface and interventricular groove is reached
[41]. Cardiac wall motion, high blood flow velocities
in the ventricles and ventricular outflow tracts and
movement of pericardial fluid can all interfere with
the relatively low coronary blood flow velocities on
color Doppler imaging. Back and forward motion of
pericardial fluid outlining the ventricular walls in
particular may be mistaken for a coronary artery
[42]. For these reasons identification of coronary artery
blood flow by color Doppler imaging should always
be followed by verification of the typical waveform
pattern by pulsed-wave Doppler to provide assurance
that the coronary arteries have indeed been
identified.
Spectral Doppler measurement of coronary blood
flow velocities is easiest proximally since vessel diameter
is greatest and motion during the cardiac cycle
is less than distally. After coronary vessels are identified
by color Doppler, the pulsed-wave Doppler gate
is positioned at the origin of the vessel. The gate may
require adjustment to achieve continuous sampling of
the waveform allowing for the movement of the aortic
root in the cardiac cycle. The coronary artery flow
velocity waveform has a biphasic pattern with systolic
and diastolic peaks and antegrade flow throughout
the cardiac cycle (Fig. 29.3). Predominant diastolic
perfusion produces a unique waveform pattern with
higher velocities during diastole than systole. In normal
fetuses coronary blood flow has been visualized
from 29 weeks onwards (median gestational age of
33Ô6 weeks). The median systolic and diastolic peak
blood flow velocities are 0.21 and 0.43 m/s, respectively,
and show little change during the latter part of
gestation (Figs. 28.4, 28.5) [43]. Gestational age at visualization
and coronary artery blood flow velocities
are in part determined by the fetal condition (see below).
Fig. 29.1. The fetal heart is examined
in a short-axis view of
the aorta at 34Ô2 weeks gestation
(a) and in a long-axis view
of the left ventricular outflow
tract at 35Ô5 weeks' gestation
(b). The ostia of the left and
right coronary arteries are discernible
in a (arrows) in the area
of the left posterior and right
anterior aortic sinus. (From [87]).
In b the ostium of the left coronary
artery is discernible (arrow)