Tricuspid Atresia

Parasternal long axis view with posterior sweep indicating tricuspid atresia

Case of tricuspid atresia, normally related great vessels, VSD and pulmonary stenosis

3D volume acquisition in a patient with tricuspid atresia. The LV takes a more globular shape.

Great review presentation on Tricuspid Atresia by Laila Wazneh and Marco Zeid, NNP at the Montreal Children's Hospital

More information on fetal tricuspid atresia here.

More information here (outside link) about tricuspid atresia.

Tricuspid atresia (TA) is a form of cyanotic congenital heart disease. It is characterized by the complete absence of a direct opening or communication between the right atrium (RA) and the right ventricle (RV). Instead of a functional tricuspid valve, there is typically an imperforate membrane or a thick muscular wall. The RV is almost always hypoplastic (underdeveloped).

This condition occurs in approximately 1 in 15,000 live births and ranks as the third most common cyanotic congenital heart defect. The root cause involves a disruption in the normal development of the atrioventricular valves from the endocardial cushion during embryogenesis. There is no significant familial recurrence risk associated with TA, and confirmed genetic predisposition is uncommon, although associations with conditions like trisomies, VACTERL syndrome, and 22q11 deletion have been noted.

Pathophysiology

In tricuspid atresia, systemic venous blood returning to the RA cannot flow forward into the RV. For survival, this blood must be shunted from the high-pressure RA to the left atrium (LA) through an obligatory atrial septal defect (ASD) or a patent foramen ovale (PFO). This results in the mixing of systemic and pulmonary venous blood in the LA. All blood entering the heart from both the body and the lungs mixes in the LA and flows into the single functional ventricle, which is typically the left ventricle (LV).

In tricuspid atresia with normally related great arteries, since the connection between the RA and RV is absent, blood must reach the pulmonary arteries to be oxygenated. This flow is dependent on a communication between the systemic and pulmonary circulations, which typically occurs via a ventricular septal defect (VSD), allowing blood to flow from the LV to the hypoplastic RV and then to the pulmonary artery. Less commonly, a patent ductus arteriosus (PDA) may be the source of pulmonary blood flow. The degree of oxygen saturation in the systemic circulation depends on the balance between pulmonary blood flow and systemic blood flow, and the oxygen saturation of the venous returns.

Classification

Tricuspid atresia is classified based on the relationship of the great arteries and the presence or absence of obstruction to pulmonary blood flow. The main types are:

Type I: Normally Related Great Arteries (Aorta arises from the LV, Pulmonary Artery arises from the RV). This accounts for 70-80% of cases. In this type, the LV is the systemic ventricle, ejecting into the aorta. Blood reaches the pulmonary artery via a VSD and the hypoplastic RV.
- Type IA: With Pulmonary Atresia. There is no (or trivial) VSD, or only a small remnant of RV below the pulmonary valve. Pulmonary blood flow is entirely dependent on a PDA. This is the most rare subtype. Because of the absence of a VSD (or a very small one), the RV does not grow in fetal life and there is pulmonary atresia. These patients are cyanotic and will require an alternate source of blood flow.
- Type IB: With Pulmonary Stenosis (or restrictive VSD). There is a VSD, but either the VSD is small and restrictive or there is obstruction in the pulmonary outflow tract (PS). This limits pulmonary blood flow. This is the most common subtype, accounting for about half of all patients with tricuspid atresia. The VSD in this type has a tendency to close spontaneously, which further restricts PBF. These patients may have a balanced physiology and get operated for first-stage palliation later in life (4 to 6 months of age).
- Type IC: With No Pulmonary Stenosis (large VSD). There is a large, unrestrictive VSD allowing free flow from the LV to the RV and pulmonary artery. This results in increased pulmonary blood flow. These patients may have excessive pulmonary blood flow relative to systemic flow and may need diuretics or pulmonary artery banding as a first stage.
Type II: With Complete Transposition of the Great Arteries (TGA) (Aorta arises from the RV, Pulmonary Artery arises from the LV). This accounts for about 20% of cases. The LV ejects into the pulmonary artery, and blood reaches the aorta (arising from the hypoplastic RV) via a VSD.
- Type IIA: With Pulmonary Atresia. Rare.
- Type IIB: With Pulmonary Stenosis (or restrictive VSD). Limits PBF.
- Type IIC: With No Pulmonary Stenosis (large VSD). Large VSD allowing high PBF. This subtype is described as the second most common type overall after Type IB. Severe pulmonary hypertension is often present. Associated findings like coarctation of the aorta can occur.

Obstruction of the aorta can occur in Type II tricuspid atresia. This can manifest as:

Subaortic stenosis: Obstruction below the aortic valve. This can occur in Type IIC TA if the VSD becomes restrictive. Also this possibility when the aorta arises from a small RV, which is the case in Type II TA.
Aortic arch obstruction: Including narrowing of the aortic arch (isthmus narrowing) and coarctation of the aorta (CoA) or interrupted aortic arch (IAA).
- Aortic arch obstruction is a common associated anomaly in Type II TA, occurring in approximately 30-50% of cases.
- CoA is specifically noted as an association, particularly with Type IIC TA.
- Clinically, these aortic arch obstructions can lead to weak or absent femoral pulses.
- If the aortic obstruction is severe, blood flow to the lower body may be dependent on the PDA remaining open postnatally. This can lead to severe hypoperfusion of the lower body if the PDA closes.
- Furthermore, if a significant aortic obstruction is present, and surgical repair of the obstruction and PDA closure is planned, the VSD needs to be large enough to accommodate the increased systemic flow that must now pass through it from the LV (pumping into the PA) to the rudimentary RV (pumping into the aorta) and then to the body. If the VSD is too small, a pressure gradient will develop, and systemic output may fall.

A VSD is very common with tricuspid atresia, present in about 90% of infants. Other associated anomalies can include persistent left superior vena cava (in about 10-15% of patients), mitral regurgitation (due to volume overload of the LV), aortic arch obstruction (especially with Type II TA), and heterotaxy syndromes (specifically left atrial isomerism).

Clinical Presentation

The clinical presentation of tricuspid atresia in neonates varies significantly depending on the degree of pulmonary blood flow, which is determined by the specific type and the size of the VSD or patency of the PDA.

With Decreased Pulmonary Blood Flow (e.g., Type IA, IB, IIA, IIB, or Type IB where VSD closes): These infants are typically cyanotic shortly after birth, especially as the PDA begins to close. The cyanosis may be severe and progressive. On physical examination, the heart may appear normal or slightly small on chest X-ray. There is often a notable decreased impulse palpated at the lower left sternal border due to the hypoplastic or absent RV. The cardiac impulse is predominantly apical. The first heart sound is accentuated. The second heart sound is typically single and loud, associated with aortic valve closure. A heart murmur may be absent if pulmonary flow is very low, or a soft, continuous murmur of a PDA may be heard in the upper left sternal border/under the clavicle in early infancy. If a VSD is present but restrictive, a systolic murmur may be heard at the lower left sternal border, the character of which changes if the VSD becomes smaller.
With Increased Pulmonary Blood Flow (e.g., Type IC, IIC): These infants may have only mild cyanosis or even appear acyanotic. They often present with signs of congestive heart failure within weeks to months after birth, including tachypnea (rapid breathing), poor feeding, and poor weight gain. Physical examination often reveals prominent pulses and an active, enlarged heart with a prominent apical impulse. A loud pansystolic murmur due to the VSD is usually audible at the lower left sternal border. A low-frequency, mid-diastolic rumble may be heard at the apex due to increased blood flow across the mitral valve. Hepatomegaly (enlarged liver) is common due to increased right-sided pressures or heart failure.

An inadequate atrial communication (ASD/PFO) can restrict flow from the RA to the LA, leading to elevated RA pressure and symptoms like hepatomegaly, regardless of the pulmonary blood flow state. These patients may need a ballood atrial septostomy.

Diagnosis

Suspicion for tricuspid atresia is often raised by the presence of newborn cyanosis combined with specific findings on electrocardiogram (ECG).

Electrocardiogram (ECG): Characteristic findings include left axis deviation in the frontal plane, left ventricular hypertrophy (LVH), and often signs of right atrial hypertrophy (RAH), reflected by tall, peaked P waves. A superior QRS axis is typical, particularly in Type I TA. RV voltages are usually diminished.
Chest X-ray: May show enlargement of the RA. The pulmonary artery segment often appears concave. With reduced pulmonary blood flow, pulmonary vascular markings are decreased, and heart size may be normal or small. With increased pulmonary blood flow, cardiomegaly and increased pulmonary vascular markings are present. A "box-shaped" heart contour is described as a typical radiological finding.
Echocardiography: This is typically the diagnostic imaging modality. An apical four-chamber view is crucial and shows the absence of the tricuspid valve and a thick band of echoes separating the RA and RV. Echocardiography demonstrates the disproportionate ventricular sizes, with the LV being larger than the hypoplastic RV. Color Doppler shows the lack of blood flow across the tricuspid annulus. The study allows evaluation of the VSD size and location, the size of the atrial communication, the relationship of the great arteries, the presence and size of the PDA, and assessment for associated arch anomalies. Measuring the size of the tricuspid valve annulus (often using Z-scores) correlates with RV size and is important for surgical planning.
Cardiac Catheterization: While less often used for initial diagnosis now that echocardiography is widely available, it may be necessary for procedures like balloon atrial septostomy if the atrial communication is restrictive. It is also used later to assess pulmonary artery pressures and resistance, detect kinking, and evaluate LV end-diastolic pressure and mitral regurgitation, particularly before Fontan palliation.

In summary, tricuspid atresia is a complex cyanotic heart defect with varying presentations depending on the specific anatomical subtypes and flow dynamics. Recognizing the characteristic clinical signs (especially the absent RV impulse at the LLSB) and ECG findings (LAD, LVH, RAH) is key to prompting echocardiography, which definitively establishes the diagnosis and guides subsequent management.