Tricuspid Atresia

Definition and Incidence

Tricuspid atresia is defined by the congenital absence of the tricuspid valve, resulting in a lack of direct communication between the right atrium and the right ventricle.
It accounts for 1% to 3% of all congenital cardiac malformations, with an estimated incidence of 5 to 13 cases per 100,000 live births.
The anomaly creates a functionally univentricular heart, universally requiring staged palliative procedures for long-term survival.

Due to the atretic tricuspid valve, an obligate interatrial communication (such as a patent foramen ovale or an atrial septal defect) is absolutely required to permit systemic venous blood to exit the right atrium and allow survival at birth.
Systemic venous blood completely mixes with oxygenated pulmonary venous blood within the left atrium and left ventricle.
The morphologic right ventricle is classically hypoplastic and incomplete, lacking its inlet portion.
The disease is subclassified based on ventriculoarterial connections: approximately 80% of patients have concordant (normally related) great arteries, while the remaining 20% have discordant connections (transposition of the great arteries).
In cases with normally related great arteries, the left ventricle ejects blood into the aorta and also pumps blood through a ventricular septal defect (VSD) into the hypoplastic right ventricle to reach the pulmonary artery.
In cases with transposition of the great arteries, left ventricular blood flows directly into the pulmonary artery (causing massive pulmonary overcirculation), while systemic blood must traverse the VSD and right ventricle to reach the aorta.

The majority of patients (those with normally related great arteries and restricted pulmonary blood flow) present with severe cyanosis shortly after birth.
A unique hallmark of tricuspid atresia among cyanotic congenital heart defects is the presence of a left ventricular type of apical impulse, rather than a right ventricular heave.
Examination of the jugular venous pulse frequently reveals prominent, large 'a' waves due to the right atrium contracting against an atretic valve.
Hepatomegaly with presystolic liver pulsations may be appreciated on abdominal examination.
Patients are at high risk for spontaneous narrowing or spontaneous closure of the VSD, which can rapidly exacerbate cyanosis and cause life-threatening hypoxia.

The second heart sound (S2) is usually single, representing aortic closure.
A holosystolic murmur is frequently audible along the left sternal border, generated by blood flow across the VSD.

The classic triad includes left axis deviation (mean QRS around -45 degrees), left ventricular hypertrophy, and right atrial enlargement.
Left axis deviation distinguishes tricuspid atresia from most other cyanotic congenital heart lesions, which typically present with right axis deviation and right ventricular hypertrophy.
The normally prominent 'R' wave in the right precordial leads (V1, V2) is replaced by an 'rS' complex, while left precordial leads show a 'qR' complex.
P waves are usually biphasic or tall and spiked in lead II, indicating right atrial enlargement.

In patients with normally related great arteries, the lung fields typically demonstrate pulmonary undercirculation (oligemia).
Cardiomegaly may be present, often demonstrating a flattening of the left heart border and an absent mediastinal shadow of the left main pulmonary artery.

Two-dimensional echocardiography is the definitive diagnostic modality, demonstrating an echo-bright fibromuscular membrane or plate in place of the normal tricuspid valve apparatus.
The right ventricle is visibly hypoplastic, and the left ventricle is mildly to moderately enlarged.
Color flow Doppler confirms the complete lack of direct inflow from the right atrium to the right ventricle and demonstrates the right-to-left shunt across the atrial septum.

Hemodynamic measurements reveal normal or slightly elevated right atrial pressure with a prominent 'a' wave.
Right atrial angiography shows a diagnostic filling sequence: immediate opacification of the left atrium from the right atrium, followed by left ventricular and aortic filling.
The absence of direct right ventricular filling creates a characteristic angiographic filling defect (a "window") between the right atrium and the left ventricle.

Moderately or severely cyanotic neonates must be started on an intravenous infusion of Prostaglandin E1 (0.05-0.1 µg/kg/min) to maintain ductal patency and secure a reliable source of pulmonary blood flow.
If the interatrial communication is restrictive (causing profound hypoxemia or hepatic congestion), an urgent Rashkind balloon atrial septostomy is indicated to tear the fossa ovalis and ensure adequate right-to-left mixing.

Because biventricular repair is impossible, patients undergo a staged, functionally univentricular palliation pathway aimed at establishing a Fontan circulation.
Stage 1 (Neonatal Period): In neonates with severe cyanosis, an initial source of pulmonary blood flow is established via a systemic-to-pulmonary artery shunt (e.g., modified Blalock-Taussig shunt) or transcatheter stenting of the arterial duct. In the minority of patients with transposition of the great arteries and unrestrictive pulmonary blood flow, pulmonary artery banding is performed to protect the pulmonary vascular bed.
Stage 2 (2 to 6 months of age): A bidirectional Glenn shunt (superior cavopulmonary anastomosis) is performed, directly connecting the superior vena cava to the pulmonary arteries to provide stable pulmonary blood flow and unload the volume-stressed left ventricle.
Stage 3 (2 to 3 years of age): The completion Fontan procedure (total cavopulmonary connection) routes systemic venous return from the inferior vena cava directly to the pulmonary arteries via an extracardiac conduit or an intracardiac lateral tunnel. This bypasses the right heart entirely, placing the systemic and pulmonary circulations in series and eliminating the cyanosis-causing right-to-left shunt.