Diagnosis and pathophysiology.
Various tools are available for diagnosis of ARCPA. Conventionally,
coronary angiography is the gold standard for its ability to visualize
the retrograde flow through the RCA, extensive collateralization between
RCA and LCA, dilation and increased flow into the epicardial coronary
system and direct flow between RCA and MPA . Recently, echocardiography
and computed tomography/magnetic resonance (CT/MR) can be fundamental
for diagnosis of ARCPA, thanks to the improvements of these
technologies. Echocardiography plays a crucial role as a screening,
low-cost and diffusely used tool. It can provide first and immediate
suspicion, and definitive diagnosis, in about 25% of cases of
ARCAPA/ALCAPA , thus improving survival of asymptomatic patients.
However, echocardiographic diagnosis of coronary anomaly is not simple.
In our case and in other reports , it is very easy to confuse the origin
of the RCA. As our figure 1 shows, also in the presence of ARCAPA the
sonographer can often visualize an “apparently normal” RCA-origin for
the right aortic sinus of Valsalva: this ultrasound artifact may be due
to the contact of the aortic wall with the proximal part of the RCA
usually coursing strictly nearby and leading to erroneous evidence of
the vessel originating from the aorta. If not highly suspected, for
example during a pediatric routine screening ultrasound, the operator
would never search for a coronary origin from the MPA, thus leading to
misdiagnosis. The evidence of retrograde (commonly “blue”, away from
the probe) systo-diastolic flow into the RCA, is the key diagnostic sign
and should mandatorily induce the sonographer to search for ARCAPA. To
be noted that, as opposed to the normal pulsatile coronary blood flow
which is visible at color-doppler during diastole , the blood flow in
the abnormal vessel is present both during systole and diastole because
the pressure into the coronary system (derived from the left coronary
artery and from the wall stress into the myocardium) is higher than the
pulmonary pressure. Sometimes a proper jet of blood flow draining into
the MPA (fig 2b - the amount of blood shunting from the left to the
right system and increasing the Qp/Qs) can be identified at the
color-doppler mapping (commonly “blue”, away from the probe) and
should “ring-the-bell” about the presence of ARCAPA to the
sonographer. Multiple small tortuous vessels, resembling coronary
fistulae (i.e. collateralizations), with low-velocity and retrograde
flow in the area of the proximal RCA course are also key signs of ARCPA.
However, as in this case, collateralizations do not develop soon after
birth, as sometimes they may only slowly develop during infancy.
Moreover, retrograde inverted (left-to-right) flow in RCA appears only
after PAP declines, but this may take days, weeks or months, especially
in critically ill patients, ECMO patients or newborn with persistently
high PAP for several reasons. Our case is a peculiar example since
several obstacles to the prompt diagnosis were faced: the neonate was on
HFOV for several days, thus preventing a good acoustic window, and he
was put on ECMO at day 2 and this might have delayed the PAP decrease.
As showed , ECMO circuit alters the coronary flow which can become, in
some cases, continuous instead of normally pulsatile . This may
complicate the color-doppler analysis of the coronary flow pattern.
All the previous discussions highlight the importance of an
echocardiographic revision in patients with critical acuteness, poor
acoustic windows, and transient hemodynamic conditions for diagnosis of
anomalies that might have been missed at first CUS. In our center our
clinical practice includes a second complete screening CUS for
critically ill newborns after resolution of the acute event and before
discharge.
The pathophysiology of myocardial ischemia in ARCAPA is dependent from
the PAP and the collateralizations. High PAP after birth drives
antegrade flow of deoxygenated blood from MPA to RCA . As PAP decreases,
inverted systo-diastolic retrograde flow runs from the myocardium to the
MPA and causes a “steal phenomenon”, the reason for progressive
chronic myocardial ischemia in children. If collateralization between
RCA and LCA is not sufficient symptoms and signs of myocardial ischemia
occur earlier in infancy, while, if collateralization is wide enough and
PAP remain low, oxygenated blood from LCA would perfuse the myocardium
and the patient would reach adulthood asymptomatic (about 48% of cases
). Vessel diameter, pulmonary resistances and coronary dominance also
influence the amount of coronary steal and the subsequent ischemia. It
has been estimated to be up to 1/1.4 L/min (20-25% of the cardiac
output, increasing the Qp/Qs).