7.3 Right Heart Lesions (Pulmonary Atresia, Critical PS, Ebstein Anomaly)

Right Ventricular Outflow Tract Obstructions

Anomalies affecting the right heart frequently involve varying degrees of obstruction to pulmonary blood flow, ranging from moderate stenosis to complete atresia. The integrity of the ventricular septum plays a defining role in the morphological development of the right ventricle in these conditions.

Pulmonary Atresia with Intact Ventricular Septum (PA-IVS)

Pulmonary Atresia with Intact Ventricular Septum (PA-IVS) is a severe congenital heart defect characterized by complete membranous or muscular atresia of the pulmonary valve with no ventricular septal defect (VSD). Because the ventricular septum is intact, the right ventricle (RV) cannot decompress forward into the pulmonary artery or across a VSD.

This leads to profound morphological changes:

• Right Ventricular Hypoplasia: The right ventricle becomes severely hypoplastic, thick-walled, and hypertrophied. The cavity size can range from mildly reduced to virtually obliterated.
• Tricuspid Valve Changes: The tricuspid valve is typically hypoplastic and may exhibit severe regurgitation. The annulus size heavily influences the ultimate viability of a biventricular repair postnatally.
• Right Ventricle-to-Coronary Artery Fistulae: In severe cases, the suprasystemic pressure in the closed RV cavity forces blood into the myocardial sinusoids, creating fistulous connections with the coronary arterial circulation. Identifying these fistulae prenatally is crucial, as their presence indicates a "RV-dependent coronary circulation," complicating postnatal surgical options.
• Color Doppler: Interrogation of the main pulmonary artery reveals retrograde flow from the ductus arteriosus supplying the pulmonary branches. There is absolutely no antegrade flow across the pulmonary valve.

Critical Pulmonary Stenosis (PS)

In Critical Pulmonary Stenosis, the pulmonary valve is severely stenosed but not completely atretic. While some antegrade flow exists, it is insufficient to support adequate pulmonary circulation. The right ventricle may be hypertrophied but is generally less hypoplastic than in PA-IVS.

Prenatal echocardiography demonstrates a thickened, doming pulmonary valve with high-velocity, aliasing antegrade flow on color and spectral Doppler. Crucially, the ductus arteriosus may exhibit bidirectional or purely retrograde flow, indicating that the lesion is ductal-dependent for adequate pulmonary blood flow. In severe cases, the massive right ventricular pressure can lead to severe tricuspid regurgitation and secondary right atrial dilation.

Ebstein Anomaly of the Tricuspid Valve

Ebstein Anomaly is a rare, complex malformation of the tricuspid valve characterized by the failure of the septal and posterior leaflets to delaminate properly from the right ventricular myocardium during embryogenesis.

Diagnostic Criteria and Echocardiographic Appearance

The hallmark of Ebstein Anomaly is the apical displacement of the functional tricuspid valve annulus. The anterior leaflet is typically elongated and "sail-like," while the septal and posterior leaflets are tethered deep within the right ventricle.

Diagnostic parameters include:

• Apical Displacement: Displacement of the septal leaflet hinge point by > 8 mm/m2 (indexed to body surface area, though absolute distance is often used in fetal echo) relative to the anterior mitral valve leaflet insertion at the crux of the heart.
• The "Atrialized" Right Ventricle: The downward displacement of the valve divides the right ventricle into two functional components: an inlet portion that is integrated into the right atrium (the "atrialized" RV) and a smaller, trabecular/outlet portion that functions as the true pumping RV. The atrialized RV has thin, atrial-like walls but ventricular electrical properties.
• Tricuspid Regurgitation (TR): The malformed valve is invariably incompetent, often resulting in severe, central tricuspid regurgitation.

Severe Presentations and Prognosis

Severe forms of Ebstein Anomaly can be devastating in utero. Massive tricuspid regurgitation leads to profound right atrial enlargement and an overall massive increase in the cardiothoracic ratio (cardiomegaly). The heart can become so enlarged that it fills the entire fetal thorax.

This massive cardiomegaly has critical consequences:

• Pulmonary Hypoplasia: The grossly enlarged heart compresses the developing fetal lungs, leading to lethal pulmonary hypoplasia. This is the primary cause of neonatal mortality in severe cases.
• Hydrops Fetalis: Severe right heart volume overload and subsequent failure can lead to elevated central venous pressures, manifesting as fetal hydrops (ascites, pleural effusions, skin edema).
• Arrhythmias: The abnormal geometry and electrical properties of the atrialized RV predispose the fetus to supraventricular tachycardias (SVT) and Wolff-Parkinson-White (WPW) syndrome.

Prenatal counseling must emphasize the spectrum of the disease. While mild forms may be asymptomatic until adulthood, the severe fetal presentation characterized by massive cardiomegaly and hydrops carries an exceedingly poor prognosis. Evaluation of the lung-to-head ratio (LHR) or total fetal lung volume via MRI may be indicated to assess the severity of pulmonary compression.