**5. Pathophysiology**

The presentation and pathophysiology of Ebstein's anomaly depend on the severity of the morphology and associated congenital heart defects. Symptomatic neonates generally present with cyanosis, cardiomegaly, arrhythmias, and congestive heart failure [18]. At the extreme end in Carpentier type C and D of Ebstein's anomaly, there is severe displacement of the tricuspid valve that results in severe tricuspid regurgitation and an ineffective functional right ventricle. With the physiological elevation in pulmonary vascular resistance typically seen in neonates, the small and ineffective functional right ventricle is unable to generate antegrade pulmonary blood flow, especially when the ductus arteriosus is still patent, which leads to "functional pulmonary atresia." True anatomic pulmonary valve atresia is also associated with Ebstein's anomaly and requires a patent ductus (PDA) to provide pulmonary blood flow.

Another serious condition observed in neonatal Ebstein's patients is a "circular shunt," where there is ineffective systemic output due to recirculation of blood with a poorly functioning right ventricle and severe tricuspid regurgitation in association with an atrial septal defect and a patent ductus arteriosus. Retrograde flow from the ductus arteriosus (**Figure 7** arrow 1) through a regurgitant pulmonary valve

**Figure 7.** *Diagram illustrating the "circular shunt" physiology.*

(**Figure 7** arrow 2) circulates into the right atrium due to severe tricuspid regurgitation (**Figure 7** arrow 3) and then passes into the left heart (**Figure 7** arrow 5, 6) through an atrial septal defect (**Figure 7** arrow 4) for another cycle through this circular shunt via the ductus arteriosus (**Figure 7**). High perinatal mortality is associated with the presence of a "circular shunt"; in utero NSAIDs constrict the ductus arteriosus improving fetal survival and resulting in greater gestational age at delivery [19]. In neonates, a "circular shunt" creates unstable hemodynamics with severe hypoxia and low-cardiacoutput syndrome. Patients may be temporized with mechanical ventilation and inotropic support, but more definitive correction of the physiology with surgical intervention, typically the Starnes procedure may be required [7].

The goal of medical treatment during the neonatal period is to assist with the generation of antegrade pulmonary blood flow by supporting the functional right ventricle. Antegrade pulmonary flow improves as the pulmonary vascular resistance falls and can be augmented by the initiation of pulmonary vasodilators, such as inhaled nitric oxide (iNO). Prostaglandin infusion is crucial to maintain patent ductus in neonates with anatomic pulmonary atresia. A trial of withdrawing prostaglandin may be required to assess for the presence of functional pulmonary atresia. Early prostaglandin withdrawal may also be necessary for neonates with a "circular shunt." With less severe forms of Ebstein's anomaly (Carpentier type A and B), the functional right ventricular can generate antegrade pulmonary blood flow. These neonates may recover out of the neonatal period without any intervention. Such neonates need to be followed into infancy as the tricuspid regurgitation may worsen over time leading to worsening cardiomegaly from worsening right atrial dilatation and thinning out of the atrialized right ventricle [7].

In the neonatal period, cyanosis due to inadequate pulmonary flow and/or right to left shunting, as well as congestive heart failure are the main issues. Neonates with adequate antegrade pulmonary blood flow and a reasonable size functional right ventricle are candidates for a biventricular repair beyond the neonatal period.

Neonates with pulmonary atresia fall into two groups: true anatomic pulmonary valve atresia and "functional pulmonary atresia." In neonates with Ebstein's anomaly and anatomic pulmonary atresia, initial prostaglandin administration followed by either stenting of the ductus arteriosus or placement of a surgical Blalock-Tausig shunt may be the option to get out of the neonatal period in patients with an adequate functional right ventricle. When the functional right ventricle is small, patients may undergo a Starne's repair followed subsequently by a Cone procedure and/or a Fontan procedure.

Neonates with functional pulmonary atresia are often very unstable, as some patients may develop a "circular shunt" with retrograde flow back through the pulmonary valve. Such neonates usually present in extremis and need a Starne's repair. The strategy for stable patients depends again on the size of the functional right ventricle.
