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).