Fetal Pulmonary Atresia with Intact Ventricular Septum

Pulmonary atresia with an intact ventricular septum is a complex congenital heart defect characterized by the complete occlusion of the pulmonary valve in the absence of a ventricular septal opening. During fetal echocardiography, the condition typically presents as a spectrum of right ventricular morphologies ranging from a diminutive, unipartite cavity with severely hypertrophied walls to a normal-sized or even massively dilated chamber (if there is significant tricuspid regurgitation). The tricuspid valve is frequently abnormal; in the hypoplastic variant, the valve annulus is typically small and rudimentary, whereas in the dilated variant, the valve is often incompetent and may exhibit an Ebstein-like deformity. The right atrium may also appear dilated, particularly when significant tricuspid regurgitation is present. The fetal circulation is altered significantly because the right ventricle has no antegrade egress for blood flow, forcing all systemic and umbilical venous return to pass right-to-left across the foramen ovale into the left side of the heart. Consequently, the left ventricle must support the entire combined ventricular output, resulting in an enlarged left ventricular chamber and a dilated ascending aorta. 

Pulmonary blood flow in utero is maintained entirely through retrograde flow across the ductus arteriosus, which carries blood from the aorta back into the pulmonary arteries. Because fetal pulmonary vascular resistance is high relative to systemic vascular resistance—which is markedly reduced by the low-resistance placental circulation—pulmonary blood flow in fetuses with pulmonary atresia with intact ventricular septum (PA/IVS) is entirely ductal and directed from the aorta to the pulmonary arteries. This altered flow pattern influences ductal remodeling, leading to the development of a long, tortuous, and often vertically oriented “pulmonary atresia–type” ductus arteriosus rather than the short, gently curved ductus seen in normal fetal circulation. The ductus is left to right, contrary to the right to left expected shunt in normal fetal physiology.

High pressures within the right ventricular cavity can lead to the development of ventriculocoronary connections, such as sinusoids or fistulas, which may render the coronary circulation dependent on right ventricular pressure for myocardial perfusion. Suspecting a right-ventricular dependent coronary circulation by fetal echocardiography can be challenging and relies on several key views and Doppler interrogation. The term right ventricular sinusoids specifically refers to the endothelial-lined channels found within the hypertrophied myocardium that represent persistent embryonic structures. These sinusoids typically appear in hearts with severely hypertensive and hypoplastic right ventricles where no other antegrade egress for blood exists. A coronary-cameral fistula or ventriculocoronary fistula is the broader term used when these intramyocardial sinusoids create a direct communication between the ventricular cavity and an epicardial coronary artery. The terms ventriculocoronary connections or RV-coronary connections are often used as overarching nomenclature for these abnormalities. While the anatomy involves both sinusoids and fistulas, the most critical clinical designation for management is right ventricle dependent coronary circulation, or RVDCC. This condition is diagnosed when these fistulous connections are associated with proximal coronary artery stenosis or atresia of the coronary ostia at the aortic root. In patients with RVDCC, the myocardium receives its blood supply retrogradely from the hypertensive right ventricle through these connections. It is vital for the imager to distinguish between simple sinusoids and true RVDCC because decompressing the right ventricle in the latter can lead to myocardial infarction and death. This occurs because the drop in right ventricular pressure results in coronary steal or inadequate perfusion of the coronary bed. To help visualize this, you can think of sinusoids as a series of auxiliary pipes built into the wall of a building. The fistula is the accidental junction where one of these wall-pipes joins the main water lines of the city. Because of this phenomenon, a cardiac catheterization is performed in the post-natal setting to better visualized the coronary system before any manipulation of the right ventricular outflow tract.

During fetal life, in the four-chamber view, the imager evaluates right ventricular size and tricuspid valve function, noting that a small tricuspid annulus z-score often correlates with severe ventricular hypoplasia. Long-axis and short-axis views of the right ventricular outflow tract typically reveal a thickened, immobile pulmonary valve membrane with no detectable antegrade flow on color Doppler mapping. Interrogation of the ductus arteriosus in a sagittal or three-vessel view confirms the reverse flow direction from the aorta toward the lungs. Additionally, pulsed Doppler of the hepatic veins and ductus venosus is important for detecting signs of heart failure or increased central venous pressure, which may manifest as prolonged periods of no flow or exaggerated flow reversal during atrial contraction. This can lead to fetal hydrops. Antenatal detection of this condition is vital for planning specialized delivery and immediate postnatal care to ensure ductal patency with prostaglandin infusion. Sequential prenatal examinations are recommended to monitor the growth of right heart structures and the progression of tricuspid regurgitation. The presence of severe right ventricular hypoplasia and right ventricular-dependent coronary circulation remains a significant prognostic indicator that may necessitate a single-ventricle palliation strategy, or a 1.5 pathway.

Example of Fetal Echocardiography