Ebstein's anomaly

Section on Circular Shunt Physiology

Background

Ebstein anomaly is a rare congenital heart disease (CHD). It involves a malformation of the tricuspid valve (TV) and the right ventricle (RV). The anomaly occurs in approximately 1 per 200,000 live births and accounts for less than 1% of all congenital heart defects. Its spectrum of anatomical variations and severity is wide, ranging from severe neonatal forms to less symptomatic cases diagnosed in adulthood.

Ebstein anomaly is characterized by apical displacement of the tricuspid valve leaflets (primarily the septal and posterior leaflets) into the right ventricle, resulting in a malformed tricuspid valve, an atrialized portion of the right ventricle, and a small functional right ventricle. This anomaly often includes tricuspid regurgitation (TR), right atrial enlargement, and, in severe cases, associated defects like atrial septal defect (ASD) or patent foramen ovale (PFO). In neonates, severe forms can present with cyanosis/hypoxia, or hemodynamic instability, particularly when complicated by circular shunt physiology.

Circular shunting is a unique and critical phenomenon in some neonates with Ebstein anomaly, where blood flows in a cyclical, non-productive loop between the right heart, lungs, and left heart without effective systemic or pulmonary circulation. This occurs primarily in the presence of severe tricuspid regurgitation, a right-to-left inter-atrial shunt, right left to right patent ductus arteriosus, elevated pulmonary vascular resistance (PVR) favouring blood from from the PDA to reach the main pulmonary artery, and significant pulmonary insufficiency leading to MPA-RV flow.

Ebstein's anomaly has been associated with maternal lithium use during pregnancy, although most cases are sporadic. It is characterized by various degree of displacement of the tricuspid valve towards the apex of the RV, with the posterior and septal leaflets being displaced. This leads to "atrialization" of the RV segment included in the right atrium, with subsequent enlargement of the RA and decreased RV functional cavity. Ebstein's anomaly is associated with atrial septal defect, Wolff-Parkinson-White (and supra-ventricular tachycardia) and risk of strokes. Often, these patients present with an enlarged cardiac silhouette on chest radiography (“Wall to Wall" heart).

Severe Ebstein's can be associated with the "Circular shunt physiology" (or "Circle of death"), a flow physiology by which blood flow goes from Right to Left atrium (via the ASD/inter-atrial shunt), to the Left ventricle, to the Aorta, to the Pulmonary artery (by the PDA) to the right ventricle (by pulmonary insufficiency), to the right atrium (via the tricuspid regurgitation), to the left atrium by the ASD.

Circular shunt physiology in Ebstein:

Aorta → Ductus → Pulmonary artery → Pulmonary Insufficiency → Right Ventricle → Tricuspid insufficiency → Right atrium → Foramen Ovale → Left atrium → Mitral valve → Left ventricle → Aortic valve → Aorta → Ductus arteriosus (Learn more here, and here). It refers to a recirculating loop of blood that bypasses systemic perfusion, often involving the right atrium, right ventricle, pulmonary arteries, ductus arteriosus, aorta, and back into the right heart—creating a futile circulatory loop. In Ebstein anomaly, the tricuspid valve is displaced apically, leading to a massive right atrium and an often severely dysfunctional or hypoplastic right ventricle. This causes severe tricuspid regurgitation, elevated right atrial pressure, and ineffective forward flow into the pulmonary artery. The functional pulmonary atresia seen in severe cases occurs because the RV cannot generate enough pressure to open the pulmonary valve. Some have described the use of maternal NSAIDs as a way to constrict the fetal duct in an attempt to address this fatal physiology for the fetus / newborn. Article here on "Surgical Management of Neonatal Ebstein’s Anomaly Associated With Circular Shunt".

Effective pulmonary blood flow is significantly reduced due to the following mechanisms:

Low right ventricular (RV) output, resulting from decreased RV capacitance caused by atrialization of the RV.
Severe pulmonary valve insufficiency, which leads to regurgitation and further reduces forward flow.

As a result, the pulmonary artery (PA) is underfilled, and the ductus arteriosus (PDA) provides flow to the PA via a left-to-right shunt. However, this flow comes at the expense of systemic perfusion, effectively “stealing” blood from the systemic circulation. The PDA jet is directed toward the incompetent pulmonary valve, allowing blood to flow retrogradely from the PDA into the PA and back into the RV. Pulmonary vascular resistance (PVR) is typically elevated due to low pulmonary blood flow, which triggers reflex vasoconstriction in an attempt to preserve perfusion pressure. Additionally, hypoxemia and acidosis, which are often present, further raise PVR. This hemodynamic environment favors the abnormal circuit of flow: from the PDA to the PA, through the insufficient pulmonary valve, and into the RV. Some of the treatment strategies may involve: pulmonary vascular relaxation (iNO, oxygen exposure, titrated mechanical ventilation), restriction of the duct to break the circular shunt (NSAIDs or ligation), supporting the cardiac function in the context of acidosis, hypoxemia and likely low coronary perfusion pressure (high RA pressure in which the coronary sinus drains), while avoiding too much increase in PVR or SVR. As such, many of those infants require some low dose epinephrine, or milrinone (if the BP is not low).

More on Ebstein's anomaly by clicking here (outside article).

More on fetal Ebstein and circular shunt physiology here.