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436 A.A. Baschat
Clinical Applications in Fetuses
with Normal Cardiac Anatomy
In fetuses with normal cardiac anatomy disorders are
frequently only apparent through alterations in cardiovascular
status. Under these circumstances coronary
blood flow dynamics may be altered to accommodate
changes in myocardial oxygen requirements. Since
spectral Doppler of the coronary sinus is rarely
achieved, clinical observations revolve primarily
around color- and pulsed-wave Doppler characteristics
in coronary arterial vessels.
The ªHeart-Sparing Effectº
in Fetal Growth Restriction
Severe fetal growth restriction (IUGR) can progress
to decompensation of cardiovascular status. Such deterioration
can be documented through progressive
deterioration of arterial and venous Doppler studies
[50]. This progression often accompanies the deterioration
of acid±base status from chronic hypoxemia
to acidemia [51±54]. Under these circumstances the
combination of elevated central venous pressure, elevated
afterload, and worsening oxygenation places
unique demands on myocardial oxygen balance. Elevated
afterload increases myocardial oxygen demand
because of an increase in cardiac work. Elevated central
venous pressure and aortic pressures decrease the
pressure difference across the coronary vascular bed
and therefore diminish the driving force for coronary
perfusion. The summation of these factors has detrimental
effects on coronary perfusion at a time when
myocardial oxygen balance and fetal metabolic state
are critical. Consequently, adaptive mechanisms need
to be evoked in order to maintain myocardial oxygen
balance. The necessary augmentation of coronary
blood flow can be achieved in two principal ways.
One way is to increase the proportion of oxygenated
left ventricular output available for myocardial delivery.
The second way is through autoregulationmediated
coronary vasodilatation.
Several mechanisms operate in IUGR fetuses that
increase the potential delivery of oxygenated blood to
the myocardium. Under conditions of elevated placental
resistance the relative proportion of left ventricular
output increases [55±57]. Decreases in oxygen
tension may further increase the proportion of oxygenated
umbilical venous blood that is delivered
through the ductus venosus to the left side of the
heart [58, 59]. Prolonged chronic myocardial hypoxemia
allows for angiogenesis and increases in vascular
cross-sectional area and therefore myocardial flow
reserve. These responses constitute chronic heart
sparing in IUGR. When acute worsening of cardiovascular
status and/or oxygenation is superimposed
the only mechanism to significantly augment myocardial
blood flow is marked coronary vasodilatation
with massive recruitment of coronary vascular reserve.
This vascular response is more acute, often occurring
over the course of 24 h, and is most consistently
associated with severe elevation of precordial
venous Doppler indices [60, 61].
The chronic initial phase of heart sparing can be
implied by demonstrating certain Doppler abnormalities
in the arterial and venous circulations. These include
absent or reversed umbilical artery end-diastolic
velocity and/or end-diastolic blood flow reversal in
the aortic isthmus [62]. In the second trimester the
magnitude of coronary blood flow may still be below
the visualization threshold of ultrasound equipment;
therefore, augmentation of coronary blood flow cannot
be documented by spectral Doppler measurement
of coronary arteries. With acute worsening of fetal
cardiovascular and respiratory status color- and
pulsed-wave Doppler measurement of coronary artery
blood flow is readily achieved as a reflection of maximal
augmentation of coronary blood flow ± now exceeding
the visualization threshold [40]. In IUGR
both diastolic and systolic coronary artery peak
blood flow velocities are significantly higher than in
appropriately grown fetuses providing additional evidence
of blood flow augmentation. There are no associated
changes in the coronary sinus diameter as evidence
of increased coronary venous return [63]. Since
coronary artery blood flow may be visualized in normal
and IUGR fetuses at overlapping gestational ages,
concurrent examination of the arterial and venous
circulations is mandatory to assess fetal status. Clinical
management cannot be based on the evaluation of
coronary vascular dynamics alone. In IUGR fetuses
with abnormal arterial and venous Doppler, heartsparing
prognosis is poor with a high perinatal mortality
and a high risk for acidemia and neonatal circulatory
insufficiency requiring the highest level of
neonatal care.
Fetal Anemia
Severe fetal anemia can result in reduction of oxygencarrying
capacity and subsequently impaired myocardial
oxygenation. Fetal hydrops with tricuspid insufficiency
and abnormal precordial venous flow is associated
with elevated right-heart pressures and a decline
in coronary perfusion pressure. Under these circumstances
short-term augmentation of myocardial
blood flow of four to five times basal flow can be
achieved through autoregulation. Color- and spectral
Doppler measurement of coronary artery blood flow
velocities has been successful in circumstances of
acute fetomaternal hemorrhage, non-immune hy-