o_1977r8vv9vk1ts2ms0kd8pksa.pdf

430 A.A. Baschat
coronary vessels and the ventricular cavity (ventriculo-coronary
fistulae) [16]. The plasticity of the coronary
circulation is responsible for the variation in
myocardial vascular territories and blood flow found
in various fetal conditions and illustrates the critical
importance of myocardial oxygenation for proper cardiac
function.
Myocardial metabolism is almost exclusively aerobic
and in the presence of adequate oxygen various
substrates, including carbohydrates, glucose, lactate,
and lipids, can be metabolized [17±20]. In fetal life,
myocardial glycogen stores and lactate oxidation constitute
the major sources of energy while fatty acid
oxidation rapidly becomes the primary energy source
after birth. To maintain metabolism myocardial oxygen
extraction is as high as 70%±80% in the resting
state. Consequently, a coronary atrioventricular O 2
difference of 14 ml/dl exceeds that of most other vascular
beds and allows little further extraction of O 2
unless blood flow is significantly augmented; therefore,
coronary blood flow is closely regulated to
match myocardial oxygen demands.
The regulation of myocardial perfusion operates at
several levels and time frames. The unique parallel
arrangement of the fetal circulation allows for delivery
of well-oxygenated blood through the ductus venosus
to the left ventricle and thus the ascending aorta.
In the fetal lambthe coronary circulation receives
approximately 8% of the left ventricular output at rest
in this manner [21]. This proportion may be higher
in the human fetus and may be further altered by
modulating the degree of shunting through the ductus
venosus [22]. Once blood enters the coronary vessels
from the ascending aorta the pressure difference
to the right atrium becomes the primary driving
force for coronary blood flow. This perfusion pressure
is further subjected to changes in vascular tone and
extravascular pressure. Autonomic innervation of coronary
resistance vessels regulates overall vascular
tone [23, 24], but ventricular contractions are the
main contributor to extravascular resistance with significant
impact on the flow velocity waveform [25±
27]. Myocardial perfusion predominantly occurs during
diastole when the ventricles relax and pose little
extravascular resistance. This diastolic timing of predominant
perfusion is unique to the coronary circulation
and distinguishes it from other vascular beds in
the human body. In the adult, increases above a resting
heart rate of 70 beats per minute result in a disproportional
shortening of diastole. Fetal heart rates
of 120±160 beats per minute place special demands
on dynamic vascular mechanisms to regulate myocardial
blood flow volume.
Efficiency of myocardial oxygen delivery is further
enhanced by active autoregulatory control mechanisms
ensuring optimal myocardial blood flow despite
fluctuations in arterial perfusion pressure [28,
29]. This is achieved through caliber adjustment of
precapillary resistance vessels allowing channeling of
blood flow to areas of greatest oxygen demand [30,
31]. With maximal dilatation of these sphincters myocardial
blood flow may be elevated four times above
basal flow. The increase in blood flow volume that
can be achieved under these circumstances is the
myocardial blood flow reserve. If myocardial oxygenation
cannot be sustained, long-term adaptation
with formation of new blood vessels may be invoked
thus increasing the myocardial blood flow reserve
[32±34]. Such elevated myocardial blood flow reserve
allows marked augmentation of blood flow during
periods of acutely worsening hypoxemia or increased
cardiac work and increases as high as 12 times the
basal flow have been reported [15, 35, 36].
Ultrasound Examination Technique
Ultrasound Setup
The setup of the ultrasound system is of major importance
for successful examination of the fetal coronary
arteries. Gray-scale ultrasound, color-, and
pulsed-wave Doppler are used in a complementary
manner and machine settings need to be optimized
to provide the best spatial and temporal resolution.
Although visualization of coronary vessels can be
achieved using 4-MHz transducers, higher frequencies
are likely to improve resolution and therefore detection.
The dynamic range of the gray-scale image
should be set to an intermediate level that is generally
used in cardiac setups. Zoom magnification of the
area of interest limits the computing power that
needs to be allocated to the generation of the grayscale
image. These two maneuvers will improve the
frame repetition rate and should therefore be applied
before adding color Doppler imaging. When adding
color Doppler imaging the filter should be set to a
high degree of motion discrimination and the color
box and gate are kept as small as possible to optimize
spatial and temporal resolution of this Doppler modality.
The lateral dimension of the color box has the
greatest impact on computing power and therefore
frame rate. The color amplification gain is set to
eliminate background noise on the screen. The persistence
is set to a low level to minimize frame averaging.
The color velocity scale is adjusted to a range
that allows visualization of intra- and extracardiac
flows without aliasing and suppression of wall-motion
artifacts. A useful velocity range for coronary arteries
is between 0.3 and 0.7 m/s for coronary arteries and
between 0.1 and 0.3 m/s for the coronary sinus. Since
initial detection of the coronary arteries relies on col-