Tricuspid Atresia #3: Type I Tricuspid Atresia with Normally Related Great Arteries

1. Definition

Type I tricuspid atresia is defined by normally related great arteries, in which:

• The aorta arises from the left ventricle
• The pulmonary artery arises from the right ventricle
• The left ventricle functions as the systemic ventricle
• Pulmonary blood flow is supplied either through a ventricular septal defect / bulboventricular foramen into the hypoplastic right ventricle and pulmonary artery, or through the ductus arteriosus when antegrade pulmonary flow is absent

Because the tricuspid valve is absent, there is no direct right atrioventricular connection. Systemic venous return must cross an interatrial communication into the left atrium, where it mixes with pulmonary venous return before entering the left ventricle.

The dominant surgical question in Type I tricuspid atresia is therefore not systemic outflow, but how much pulmonary blood flow is present and how it should be controlled.

2. Core Circulatory Anatomy

In Type I tricuspid atresia, the LV has two potential outflow routes.

2.1 Systemic outflow

The LV ejects directly into the aorta:

• LV → aorta

This pathway is usually unobstructed because the great arteries are normally related.

2.2 Pulmonary outflow

Pulmonary blood flow depends on the ventricular-level communication and right ventricular outflow tract:

• LV → VSD / bulboventricular foramen → hypoplastic RV → RVOT → pulmonary artery

The hypoplastic RV functions as an outlet chamber rather than a true inflow ventricle. The size of the ventricular communication and the degree of RVOT or pulmonary valve obstruction determine whether pulmonary blood flow is inadequate, balanced, or excessive.

The bulboventricular foramen (BVF) is a particularly important anatomic determinant. In infants with tricuspid atresia or double-inlet left ventricle with normally related great arteries, an indexed BVF area below approximately 1.8 cm²/m² is associated with increased likelihood of early intervention to augment pulmonary blood flow; very small BVF size, especially below 1.0 cm²/m², indicates high risk for inadequate pulmonary blood flow [1].

3. Physiologic Spectrum

Type I tricuspid atresia is best understood as a spectrum of pulmonary blood flow.

Low pulmonary blood flow

Low pulmonary blood flow occurs when there is:

• Pulmonary atresia
• Severe pulmonary stenosis
• Restrictive VSD or BVF
• Progressive RVOT obstruction
• Ductal-dependent pulmonary circulation

The clinical phenotype is cyanosis, often presenting in the neonatal period.

Balanced pulmonary blood flow

Balanced pulmonary blood flow may occur when pulmonary stenosis provides enough restriction to prevent overcirculation but still permits adequate oxygenation.

This is the most favorable early physiology because it may allow the patient to grow without neonatal shunt placement or pulmonary artery banding.

Excessive pulmonary blood flow

Excessive pulmonary blood flow occurs when there is:

• Large VSD or BVF
• No significant pulmonary stenosis
• Unrestricted LV-to-RV-to-PA flow

The clinical phenotype is pulmonary overcirculation, congestive heart failure, and LV volume overload.

4. Subtypes of Type I Tricuspid Atresia

Type Ia: Pulmonary Atresia, No Effective VSD

Type Ia is characterized by:

• Normally related great arteries
• Pulmonary atresia
• No effective ventricular-level communication to the pulmonary artery
• Ductal-dependent pulmonary blood flow

There is no meaningful antegrade flow from the LV to the pulmonary artery. Pulmonary perfusion depends on the ductus arteriosus.

Physiologic consequence

The dominant problem is severe cyanosis due to critically reduced pulmonary blood flow.

Management implications

Initial stabilization usually requires:

• Prostaglandin E1 to maintain ductal patency
• Careful assessment of ductal and pulmonary artery anatomy
• PDA stenting in selected patients with favorable ductal anatomy
• Surgical systemic-to-pulmonary artery shunt when ductal stenting is unsuitable or insufficient
• Later staged single-ventricle palliation

The goal is to establish stable pulmonary blood flow while avoiding excessive pulmonary blood flow and preserving pulmonary artery growth.

Type Ib: Pulmonary Stenosis with VSD / BVF

Type Ib is characterized by:

• Normally related great arteries
• VSD or BVF
• Hypoplastic RV
• Pulmonary stenosis or RVOT obstruction

Pulmonary blood flow passes through the LV-to-RV communication and then across a stenotic RVOT or pulmonary valve.

Physiologic consequence

The degree of obstruction determines presentation.

• Severe stenosis produces cyanosis and possible ductal dependence.
• Moderate stenosis may provide balanced pulmonary blood flow.
• Progressive restriction of the BVF or RVOT can worsen cyanosis over time.

Management implications

Management depends on the adequacy of pulmonary blood flow.

Patients with inadequate pulmonary blood flow may require:

• Prostaglandin E1 in the neonatal period
• PDA stenting in selected anatomy
• Modified Blalock–Taussig–Thomas shunt or another systemic-to-pulmonary artery shunt

Patients with balanced pulmonary blood flow may be observed and optimized for bidirectional Glenn without neonatal shunt placement.

Type Ic: No Pulmonary Stenosis with Large VSD / BVF

Type Ic is characterized by:

• Normally related great arteries
• Large VSD or BVF
• No significant pulmonary stenosis
• Unrestricted or excessive pulmonary blood flow

In this subtype, the LV ejects both into the systemic circulation and through the ventricular communication into the RV and pulmonary artery.

Physiologic consequence

The dominant problem is pulmonary overcirculation.

This may produce:

• High Qp:Qs
• Pulmonary edema
• Congestive heart failure
• LV volume overload
• Failure to thrive
• Risk of pulmonary vascular remodeling if untreated

Management implications

Medical therapy may provide temporary stabilization, but significant overcirculation often requires pulmonary artery banding to control pulmonary blood flow and protect the pulmonary vascular bed.

5. Surgical Physiology

The essential feature of Type I tricuspid atresia is that the LV is the systemic ventricle.

Therefore:

• Systemic output is usually preserved through the native LV-to-aorta pathway.
• Pulmonary blood flow is the primary determinant of clinical presentation.
• BVF size is a key anatomic variable when antegrade pulmonary blood flow is present.
• Pulmonary stenosis may be protective when it limits overcirculation.
• Severe pulmonary stenosis or pulmonary atresia produces cyanosis and ductal dependence.
• Absence of pulmonary stenosis with a large BVF produces pulmonary overcirculation and LV volume overload.

This physiology differs fundamentally from tricuspid atresia with transposed great arteries, where the ventricular communication may be required for systemic output.

6. Clinical Assessment

Preoperative evaluation should define:

• Great artery relationship
• Size and morphology of the VSD or BVF
• Indexed BVF area when pulmonary flow depends on ventricular-level communication
• RV size and RVOT morphology
• Pulmonary valve anatomy
• Main and branch pulmonary artery size
• Presence, size, and morphology of the PDA
• Degree of pulmonary stenosis or pulmonary atresia
• Adequacy of the atrial communication
• LV size and systolic function
• Mitral valve morphology and regurgitation
• Evidence of pulmonary overcirculation or ductal-dependent cyanosis

Echocardiography is usually the primary diagnostic tool. CT, MRI, or catheterization may be useful when pulmonary artery anatomy, ductal morphology, or interventional planning requires more detailed definition.

7. Management Strategy

Management should match the initial procedure to the patient’s morphology and pulmonary blood flow pattern. Morphology-guided palliation improves the likelihood of successful transition to cavopulmonary connection and later Fontan completion [2].

7.1 Low Pulmonary Blood Flow

Low pulmonary blood flow is most common in Type Ia and severe Type Ib.

Potential strategies include:

• Prostaglandin E1 stabilization
• PDA stenting in selected ductal anatomy
• Surgical systemic-to-pulmonary artery shunt
• Early transition to bidirectional Glenn when age, pulmonary artery size, and pulmonary vascular resistance are suitable

Systemic-to-pulmonary shunts remain important for neonates with inadequate pulmonary blood flow, but they introduce risks of shunt thrombosis, distortion of pulmonary arteries, excessive pulmonary blood flow, and ventricular volume loading. In a multicenter analysis of Type I tricuspid atresia, risk-adjusted 6-year survival was lower after systemic-to-pulmonary shunt compared with pulmonary artery banding or superior cavopulmonary connection, emphasizing the importance of careful patient selection and flow control [3].

7.2 Balanced Pulmonary Blood Flow

Balanced pulmonary blood flow is most often seen in selected Type Ib patients with moderate pulmonary stenosis.

Potential strategy:

• Observation and medical optimization
• Avoidance of unnecessary neonatal shunt
• Serial monitoring of oxygen saturation, BVF size, RVOT obstruction, and pulmonary artery growth
• Planned bidirectional Glenn when the patient reaches appropriate age and hemodynamic suitability

Balanced pulmonary blood flow is favorable because it avoids both severe cyanosis and pulmonary overcirculation.

7.3 Excessive Pulmonary Blood Flow

Excessive pulmonary blood flow is most common in Type Ic.

Potential strategies include:

• Medical management of heart failure
• Diuretic therapy and nutritional optimization
• Pulmonary artery banding when overcirculation is clinically significant
• Protection of the pulmonary vascular bed before Glenn and Fontan stages

Pulmonary artery banding should be calibrated to reduce pulmonary overcirculation without producing excessive cyanosis or compromising pulmonary artery growth.

8. Staged Single-Ventricle Palliation

Definitive management follows the single-ventricle pathway.

8.1 Neonatal or early infant palliation

The first-stage strategy depends on pulmonary blood flow:

• Increase pulmonary blood flow when it is inadequate
• Restrict pulmonary blood flow when it is excessive
• Observe when pulmonary blood flow is balanced

8.2 Bidirectional Glenn / Superior Cavopulmonary Connection

The bidirectional Glenn reduces ventricular volume load by directing superior vena caval return directly to the pulmonary arteries.

Suitability depends on:

• Low pulmonary vascular resistance
• Adequate pulmonary artery size
• Acceptable ventricular function
• Controlled AV valve regurgitation
• Absence of major pulmonary venous or systemic venous obstruction

8.3 Fontan Completion

Fontan completion directs systemic venous return to the pulmonary arteries without a subpulmonary ventricle.

Favorable Fontan candidacy requires:

• Well-functioning systemic LV
• Competent mitral valve
• Low pulmonary vascular resistance
• Well-developed pulmonary arteries
• Controlled pulmonary blood flow before Fontan
• Absence of significant arch obstruction or pulmonary venous obstruction

Historical infant cohorts show that survival and Fontan suitability are strongly influenced by morphology, pulmonary blood flow pattern, and associated lesions. Pulmonary atresia, arch obstruction, and subaortic obstruction are adverse features, whereas balanced pulmonary blood flow and appropriate pulmonary stenosis are favorable features [4].

9. Key Teaching Point

In Type I tricuspid atresia, the great arteries are normally related, so the LV supplies the systemic circulation directly.

The clinical problem is determined by pulmonary blood flow:

• Pulmonary atresia or severe pulmonary stenosis → cyanosis and ductal dependence
• Restrictive BVF / VSD → inadequate pulmonary blood flow and need for intervention
• Moderate pulmonary stenosis → potentially balanced circulation
• No pulmonary stenosis with large VSD / BVF → pulmonary overcirculation and LV volume overload

The surgical strategy is therefore based on whether pulmonary blood flow must be increased, preserved, or restricted.

References

[1] Skaff AM, Parra DA, 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. J Am Soc Echocardiogr. 2022. doi:10.1016/j.echo.2022.11.009.

[2] Karamlou T, Ashburn DA, Caldarone CA, Blackstone EH, Jonas RA, Jacobs ML, Williams WG, Ungerleider RM, McCrindle BW. Matching procedure to morphology improves outcomes in neonates with tricuspid atresia. J Thorac Cardiovasc Surg. 2005. doi:10.1016/J.JTCVS.2005.07.024.

[3] Wilder TJ, Ziemer G, Hickey EJ, Gruber PJ, Karamlou T, Kirshbom PM, Blackstone EH, DeCampli WM, Williams WG, McCrindle BW. Surgical management of competing pulmonary blood flow affects survival before Fontan/Kreutzer completion in patients with tricuspid atresia type I. J Thorac Cardiovasc Surg. 2015. doi:10.1016/j.jtcvs.2015.05.067.

[4] Franklin RCG, Spiegelhalter DJ, Sullivan ID, Anderson RH, Thoele DG, Shinebourne EA, Deanfield JE. Tricuspid atresia presenting in infancy: survival and suitability for the Fontan operation. Circulation. 1993;87(2):427-439.

[5] Williams WG, Rubis L, Fowler RS, Rao MK, Trusler GA, Mustard WT. Tricuspid atresia: results of treatment in 160 children. Am J Cardiol. 1976. doi:10.1016/0002-9149(76)90156-9.