Tricuspid Atresia #4: Type II Tricuspid Atresia With Transposed Great Arteries

1. Core Anatomic Concept

Type II tricuspid atresia is defined by tricuspid atresia with d-transposed great arteries.

In this anatomy, the systemic and pulmonary outflow pathways are fundamentally rearranged:

• The right atrium has no direct connection to the right ventricle.
• Systemic venous blood crosses the atrial septum into the left atrium.
• The left ventricle ejects primarily toward the pulmonary artery.
• The aorta arises from the hypoplastic right ventricle.
• Systemic blood flow therefore depends on a communication between the dominant left ventricle and the hypoplastic right ventricle, usually through a ventricular septal defect, also described functionally as the bulboventricular foramen.

This is the major distinction from Type I tricuspid atresia with normally related great arteries. In Type I, the VSD/BVF is primarily relevant to pulmonary blood flow. In Type II, the VSD/BVF is critical for systemic blood flow.

2. Why Type II Physiology Is High Risk

The central physiologic issue in Type II tricuspid atresia is not simply cyanosis or pulmonary blood flow.

The key question is:

Can the left ventricle deliver unobstructed blood flow to the aorta through the VSD/BVF, hypoplastic right ventricle, and right ventricular outflow tract?

Because the aorta arises from the hypoplastic RV, systemic output must pass through:

1. Dominant LV
2. VSD / bulboventricular foramen
3. Hypoplastic RV cavity
4. RV outflow tract
5. Aorta

Any narrowing along this pathway creates functional subaortic obstruction.

This obstruction may occur at the level of:

• Restrictive VSD / BVF
• Small or hypertrophied hypoplastic RV chamber
• Narrow RV outflow tract
• Small ascending aorta or aortic valve
• Associated coarctation or arch hypoplasia

Although Type II tricuspid atresia–specific outcome data are limited, broader studies of single-ventricle anatomy with transposed great arteries and systemic outflow obstruction emphasize that the initial palliation must preserve an unobstructed systemic outflow pathway while preparing the patient for staged Fontan circulation [1].

Therefore, Type II tricuspid atresia must be evaluated as a single-ventricle lesion with potential systemic outflow tract obstruction, not merely as tricuspid atresia with transposed great arteries.

3. Role of the VSD / Bulboventricular Foramen

The VSD or BVF is the essential connection between the dominant left ventricle and the hypoplastic right ventricle.

In Type II anatomy:

• A large, nonrestrictive VSD/BVF allows systemic blood to pass from the LV to the RV and then to the aorta.
• A restrictive VSD/BVF limits systemic cardiac output.
• Progressive narrowing of the VSD/BVF may cause worsening systemic obstruction over time.
• A restrictive BVF may produce ventricular hypertrophy, poor systemic perfusion, metabolic acidosis, or ductal dependence.

Evidence specific to Type II tricuspid atresia remains sparse. In related single-ventricle anatomy with normally related great arteries, an indexed BVF area below 1.8 cm²/m² predicted need for early intervention, and all patients with BVF area below 1.0 cm²/m² required intervention [2]. This finding should not be applied mechanically to Type II anatomy because the physiologic consequence differs. In Type I anatomy, BVF restriction often limits pulmonary blood flow; in Type II anatomy, BVF restriction threatens systemic output. Nevertheless, the study supports the broader principle that BVF size is a clinically important determinant of obstruction risk in single-ventricle physiology [2].

This is clinically important because a neonate may initially appear stable if the ductus arteriosus remains open, but systemic perfusion can deteriorate as the PDA constricts.

4. Pulmonary Blood Flow in Type II Tricuspid Atresia

Pulmonary blood flow depends on whether there is pulmonary stenosis, pulmonary atresia, or unobstructed LV-to-PA flow.

Because the LV ejects toward the pulmonary artery, pulmonary blood flow may be excessive when there is no pulmonary stenosis. This can cause:

• Pulmonary overcirculation
• Congestive heart failure
• Poor feeding
• Tachypnea
• Failure to thrive
• Elevated pulmonary artery pressure
• Risk of pulmonary vascular disease if prolonged

However, pulmonary overcirculation is not the only concern. Even when pulmonary blood flow is abundant, systemic blood flow may still be compromised if the VSD/BVF or RV outflow pathway is restrictive.

This creates the characteristic paradox of Type II tricuspid atresia:

The lungs may receive too much blood, while the systemic circulation remains obstructed or ductal-dependent.

5. Subtypes of Type II Tricuspid Atresia

Type IIa: Pulmonary Atresia, No VSD

In Type IIa, there is pulmonary atresia and no effective VSD.

The pulmonary artery is not supplied directly by ventricular ejection. Pulmonary blood flow depends on the ductus arteriosus or other collateral sources.

Physiologic consequences include:

• Severe cyanosis
• Ductal-dependent pulmonary blood flow
• Need for prostaglandin E1 in the neonatal period
• Early intervention to establish a reliable pulmonary blood supply

The surgical strategy depends on pulmonary artery size, ductal anatomy, branch pulmonary artery anatomy, and the overall single-ventricle pathway.

Type IIb: Pulmonary Stenosis With VSD

In Type IIb, pulmonary stenosis is present and a VSD/BVF allows communication between the LV and hypoplastic RV.

This subtype may have relatively balanced physiology if pulmonary stenosis limits overcirculation while the VSD/BVF remains adequate for systemic output.

However, the patient remains at risk for:

• Cyanosis if pulmonary stenosis is severe
• Systemic obstruction if the VSD/BVF becomes restrictive
• Progressive ventricular hypertrophy
• Need for early palliation depending on oxygen saturation and systemic perfusion

The management must assess both pulmonary blood flow and systemic outflow adequacy.

Type IIc: No Pulmonary Stenosis, Restrictive VSD/BVF, Small Aorta

Type IIc is particularly important because pulmonary blood flow may be excessive while systemic output is limited.

The dominant LV ejects toward the pulmonary artery without major pulmonary stenosis, creating pulmonary overcirculation. At the same time, systemic flow must pass through a restrictive VSD/BVF into a hypoplastic RV and then to a small aorta.

This produces a physiology of:

• Pulmonary overcirculation
• Systemic hypoperfusion
• Functional subaortic obstruction
• Possible ductal-dependent systemic circulation
• Risk of acidosis, poor feeding, tachypnea, and shock as the PDA closes

This subtype often requires careful neonatal decision-making because pulmonary artery banding alone may worsen systemic obstruction if the BVF is restrictive or becomes restrictive after band placement.

6. Clinical Assessment

Evaluation should focus on two parallel questions.

Is pulmonary blood flow appropriate?

Assess:

• Oxygen saturation
• Pulmonary stenosis or atresia
• Pulmonary artery size
• Degree of pulmonary overcirculation
• Pulmonary venous return and left atrial pressure
• Respiratory symptoms and feeding tolerance

Is systemic output unobstructed?

Assess:

• VSD/BVF size and Doppler gradient
• RV cavity size
• RV outflow tract to aorta
• Aortic valve size
• Ascending aorta and arch dimensions
• Presence of coarctation or arch obstruction
• PDA size and direction of flow
• Systemic perfusion, lactate, renal function, and acidosis

In Type II tricuspid atresia, systemic obstruction may be underestimated if attention is focused only on pulmonary blood flow.

7. Surgical and Interventional Strategy

The final pathway is usually staged single-ventricle palliation toward Fontan circulation. However, the initial neonatal strategy depends on the balance between pulmonary blood flow and systemic output.

Potential strategies include:

• Prostaglandin E1 to maintain ductal patency when systemic or pulmonary flow is ductal-dependent
• Pulmonary artery banding for uncontrolled pulmonary overcirculation
• Systemic-to-pulmonary shunt or ductal stenting when pulmonary blood flow is inadequate
• Arch repair if coarctation or arch hypoplasia is present
• Damus–Kaye–Stansel connection when systemic outflow obstruction is present or anticipated
• Norwood-type reconstruction in selected neonates with severe systemic outflow obstruction, small aorta, or arch hypoplasia
• Palliative arterial switch in selected single-ventricle transposition anatomy where this strategy creates a more favorable systemic outflow pathway
• Bidirectional Glenn followed by Fontan completion when pulmonary artery pressure, ventricular function, atrioventricular valve function, and pulmonary artery anatomy are suitable

In a contemporary cohort of neonates with single-ventricle anatomy, transposed great arteries, and systemic outflow obstruction, 71 patients underwent initial palliation with pulmonary artery banding, Damus–Kaye–Stansel/Norwood-type palliation, or palliative arterial switch. The cohort included 27 patients with tricuspid atresia. Overall survival was 93%, with mortality occurring only during the interstage period, and Fontan completion rates were comparable across strategies [1].

The implication is not that one operation is universally superior. Rather, the initial operation should be individualized according to:

• Degree and level of systemic outflow obstruction
• BVF size and risk of progressive restriction
• Aortic valve and arch dimensions
• Pulmonary blood flow balance
• Pulmonary artery anatomy
• Ventricular function
• Institutional experience with staged single-ventricle reconstruction

The critical surgical principle is that restriction of the BVF in Type II tricuspid atresia threatens systemic output. Therefore, any intervention that changes loading conditions, especially pulmonary artery banding, must be planned with attention to the risk of worsening subaortic obstruction.

8. Pulmonary Artery Banding Versus Systemic Outflow Reconstruction

Pulmonary artery banding may be appropriate when pulmonary blood flow is excessive and systemic outflow is clearly unobstructed. However, in Type II tricuspid atresia, the surgeon must be cautious because banding increases ventricular afterload and may unmask or accelerate systemic outflow obstruction across a restrictive BVF.

When systemic outflow obstruction is present or anticipated, strategies that directly secure systemic output may be required. These include Damus–Kaye–Stansel or Norwood-type reconstruction. Broader functional single-ventricle data support the concept that early relief of potential systemic ventricular outflow tract obstruction can preserve ventricular performance and support long-term staged palliation [3].

Thus, the decision is not simply “band versus shunt.” The decision is whether the proposed neonatal palliation protects both:

1. Pulmonary vascular bed from overcirculation
2. Systemic circulation from subaortic obstruction

Failure to address either side may compromise candidacy for Glenn and Fontan completion.

9. Practical Surgical Message

Type II tricuspid atresia should be approached as a lesion in which the dominant left ventricle is connected preferentially to the pulmonary artery, while the aorta depends on a hypoplastic RV pathway.

Therefore:

• Pulmonary blood flow may be excessive if there is no pulmonary stenosis.
• Cyanosis may dominate if pulmonary stenosis or pulmonary atresia is severe.
• The most dangerous hidden problem is systemic outflow obstruction through a restrictive VSD/BVF.
• A small aorta, arch obstruction, or ductal-dependent systemic circulation should prompt early consideration of systemic outflow reconstruction.
• The initial palliation must protect both pulmonary vascular resistance and systemic cardiac output.
• The evidence base for Type II tricuspid atresia specifically is limited; contemporary decision-making is often extrapolated from broader single-ventricle transposition cohorts and BVF obstruction studies [1,2].

10. Key Takeaway

In Type II tricuspid atresia, the VSD/BVF is not merely a pathway for pulmonary blood flow.

It is the gateway for systemic output.

A restrictive VSD/BVF creates functional subaortic obstruction because the aorta arises from the hypoplastic RV. Pulmonary flow may be excessive without pulmonary stenosis, but the decisive question is whether systemic blood flow from the LV to the aorta remains unobstructed.

References

[1] Kalustian AB, Spigel Z, Greenleaf CE, Doan TT, Chavez AI, Adachi I, Heinle J, Binsalamah Z. Comparing palliation strategies for single-ventricle anatomy with transposed great arteries and systemic outflow obstruction. JTCVS Techniques. 2023. doi:10.1016/j.xjtc.2023.06.006.

[2] Skaff AM, Parra D, Soslow JH, Shuplock JM. Association of bulboventricular foramen size and need for early intervention in infants with tricuspid atresia or double inlet left ventricle with normally related great arteries. Journal of the American Society of Echocardiography. 2022. doi:10.1016/j.echo.2022.11.009.

[3] Kido T, Steringer MT, Vodiskar J, Burri M, Ewert P, Strbad M, Cleuziou J, Hager A, Hörer J, Ono M. Improved long-term outcome of Damus-Kaye-Stansel procedure without previous pulmonary artery banding. Annals of Thoracic Surgery. 2021. doi:10.1016/j.athoracsur.2021.05.022.