Tricuspid Atresia #1: Core Anatomy and Physiology

1. Definition

Tricuspid atresia is a functional single-ventricle malformation defined by absence of a direct right atrium–right ventricle connection. The tricuspid valve is absent, imperforate, or replaced by an atretic fibromuscular plate, preventing systemic venous return from entering the right ventricle through a normal atrioventricular valve [1].

The essential anatomic consequence is that systemic venous return must leave the right atrium through an interatrial communication, usually a patent foramen ovale or atrial septal defect. This atrial-level communication is mandatory for survival.

2. Fundamental Circulatory Pathway

Because there is no RA–RV connection, tricuspid atresia produces obligatory right-to-left atrial shunting.

The basic pathway is:

Systemic venous return → RA → ASD/PFO → LA → LV

The left ventricle therefore receives both pulmonary venous return and systemic venous return after atrial-level mixing. In most forms of tricuspid atresia, the left ventricle is the dominant functional single ventricle.

The right ventricle is usually hypoplastic. When present, it often functions as an outlet chamber rather than a true inflow-pumping ventricle. Its size is closely related to the size of the ventricular septal defect and the amount of flow passing through it [1].

3. Role of the Interatrial Communication

An adequate atrial communication is physiologically essential.

If the ASD/PFO is restrictive, systemic venous blood cannot decompress from the right atrium into the left atrium. This may cause:

• Elevated right atrial pressure
• Systemic venous congestion
• Reduced ventricular preload
• Low cardiac output
• Severe cyanosis
• Hemodynamic instability

In neonates with tricuspid atresia, the atrial septum must be assessed early. A restrictive atrial communication may require balloon atrial septostomy or surgical atrial septectomy, depending on clinical status and institutional strategy.

4. Functional Single Left Ventricle

In classic tricuspid atresia, the LV becomes the dominant systemic ventricle. It supports systemic output and, depending on the great artery relationship and VSD anatomy, may also provide pulmonary blood flow.

This has several important implications:

• The LV is usually well developed.
• The RV is typically small or rudimentary.
• Pulmonary blood flow depends on the pathway from the LV to the pulmonary arteries.
• Systemic arterial saturation reflects the balance between pulmonary and systemic blood flow after obligatory mixing.
• Long-term palliation is generally directed toward staged single-ventricle circulation with eventual Fontan completion.

The central physiologic problem is therefore not only cyanosis, but the need to maintain balanced pulmonary and systemic flow through a circulation with complete atrial-level mixing.

5. Ventricular Septal Defect and Pulmonary Blood Flow

The VSD is one of the most important determinants of physiology in tricuspid atresia.

In tricuspid atresia with normally related great arteries, the aorta arises from the LV and the pulmonary artery arises from the RV. In this anatomy, pulmonary blood flow usually follows:

LV → VSD → hypoplastic RV → pulmonary artery

Therefore, the VSD often becomes the effective site of pulmonary blood flow limitation. Morphologic studies have shown that VSD size influences RV size and pulmonary artery development in tricuspid atresia with normally related great arteries [1].

Small or Restrictive VSD

A small VSD limits flow from the LV into the RV and pulmonary artery. This results in:

• Reduced pulmonary blood flow
• Cyanosis
• Small RV cavity
• Smaller pulmonary arteries
• Possible ductal dependence when pulmonary blood flow is critically reduced

Restrictive VSD physiology is clinically important because it may worsen early survival, particularly when associated with additional outflow obstruction [2].

Large VSD

A large VSD allows more flow into the RV and pulmonary artery. If pulmonary stenosis is absent or mild, this may produce:

• Pulmonary overcirculation
• Congestive heart failure
• Tachypnea and feeding difficulty
• Failure to thrive
• Risk of progressive pulmonary vascular disease if untreated

Thus, oxygen saturation alone may be misleading. Higher saturation may reflect excessive pulmonary blood flow rather than physiologic stability.

Progressive VSD Restriction

The VSD may become more restrictive over time due to muscular growth or anatomic narrowing. A neonate who initially appears balanced may later develop increasing cyanosis as pulmonary blood flow decreases.

Serial echocardiographic assessment of VSD size, Doppler gradient, pulmonary blood flow, and branch pulmonary artery growth is therefore essential.

6. Outflow Obstruction

Outflow obstruction modifies the clinical phenotype of tricuspid atresia.

In normally related great arteries, obstruction to pulmonary blood flow may occur at several levels:

• VSD
• Hypoplastic RV outlet
• Pulmonary valve
• Subpulmonary region
• Branch pulmonary arteries
• Ductus-dependent pulmonary pathway

Mild or moderate pulmonary stenosis may be physiologically favorable because it limits pulmonary overcirculation. In a large infant cohort, pulmonary stenosis was associated with improved survival, likely because it helped balance pulmonary and systemic blood flow [3].

Severe pulmonary obstruction, however, causes profound cyanosis and may make pulmonary blood flow ductal-dependent.

7. Great Artery Relationship

Tricuspid atresia is not a single uniform anatomy. The relationship of the great arteries fundamentally changes the physiology and surgical priorities.

7.1 Normally Related Great Arteries

In tricuspid atresia with normally related great arteries:

• Aorta arises from the LV
• Pulmonary artery arises from the RV
• Pulmonary blood flow depends on VSD size, RV outlet patency, pulmonary stenosis, and ductal contribution

This is the classic form of tricuspid atresia. The main clinical question is whether pulmonary blood flow is too little, balanced, or excessive.

Possible physiologic patterns include:

• Reduced pulmonary blood flow: cyanosis, ductal dependence, need for prostaglandin or systemic-to-pulmonary shunt
• Balanced pulmonary blood flow: relative neonatal stability
• Excessive pulmonary blood flow: heart failure, need for pulmonary artery banding

Historical surgical management reflected this physiology: patients with inadequate pulmonary blood flow underwent systemic-to-pulmonary shunting, whereas those with excessive pulmonary blood flow underwent pulmonary artery banding [4].

7.2 Transposed Great Arteries

In tricuspid atresia with transposed great arteries:

• Aorta usually arises from the hypoplastic RV or outlet chamber
• Pulmonary artery arises from the LV
• Systemic output may depend on LV-to-RV flow across the VSD

This changes the meaning of VSD restriction. In normally related great arteries, a restrictive VSD primarily limits pulmonary blood flow. In transposed great arteries, a restrictive VSD may instead produce systemic outflow obstruction.

This anatomy is associated with worse outcomes than normally related great arteries. In infant series, discordant ventriculoarterial connection was a major adverse survival factor [3]. In contemporary single-ventricle cohorts, tricuspid atresia with transposed great arteries has also shown higher cumulative mortality than other single-ventricle lesions, particularly when systemic outflow tract obstruction is present at birth [5].

8. Ductal Flow

The ductus arteriosus may be critical depending on whether pulmonary or systemic blood flow is threatened.

Ductal-Dependent Pulmonary Blood Flow

In normally related great arteries with severe pulmonary obstruction, the PDA may supply the pulmonary arteries. Ductal closure can cause abrupt hypoxemia.

In this setting, prostaglandin E1 should be started when ductal-dependent pulmonary blood flow is suspected.

Ductal-Dependent Systemic Blood Flow

In tricuspid atresia with transposed great arteries, systemic output may depend on flow across the VSD into the outlet chamber and aorta. If the VSD is restrictive or if arch obstruction is present, ductal patency may support systemic perfusion.

In this setting, ductal closure may cause shock rather than isolated cyanosis. Arch obstruction and smaller systemic outflow dimensions are important predictors of the need for systemic outflow intervention in patients with tricuspid atresia or double-inlet left ventricle with transposed great arteries [6].

9. Mixing Physiology and Oxygen Saturation

All systemic venous return must cross the atrial septum and mix with pulmonary venous return before entering the LV. Therefore, systemic oxygen saturation depends on the balance between pulmonary blood flow and systemic blood flow.

The major determinants are:

• Size and adequacy of the atrial communication
• VSD size
• Pulmonary stenosis or atresia
• RV outflow tract anatomy
• Ductal patency
• Pulmonary vascular resistance
• Systemic vascular resistance
• Great artery relationship
• Presence or absence of systemic outflow obstruction
• Presence or absence of arch obstruction

The goal in neonatal management is not maximal oxygen saturation. The goal is balanced Qp:Qs, adequate systemic oxygen delivery, acceptable ventricular loading, and preservation of the pulmonary vascular bed for staged palliation.

10. Clinical and Surgical Significance

Management depends on identifying the dominant level of physiologic obstruction or overcirculation.

The key questions are:

1. Is the atrial communication adequate?
2. Is pulmonary blood flow reduced, balanced, or excessive?
3. Is the VSD restrictive?
4. Are the great arteries normally related or transposed?
5. Is systemic outflow obstructed?
6. Is there arch obstruction or coarctation?
7. Is pulmonary blood flow ductal-dependent?
8. Is systemic blood flow ductal-dependent?
9. Are the branch pulmonary arteries suitable for staged single-ventricle palliation?

Initial management may include:

• Prostaglandin E1 infusion
• Balloon atrial septostomy
• Surgical atrial septectomy
• Systemic-to-pulmonary shunt
• Ductal stenting
• Pulmonary artery banding
• Arch repair
• Norwood-type reconstruction or Damus–Kaye–Stansel strategy in selected systemic outflow obstruction anatomy

Most patients ultimately proceed through staged single-ventricle palliation:

1. Neonatal stabilization or first-stage palliation
2. Bidirectional Glenn
3. Fontan completion

In selected patients with tricuspid atresia, normally related great arteries, and significant left-sided/systemic outflow obstruction, Norwood-type reconstruction with arch relief has been used as part of a staged strategy [7].

11. Key Concept

Tricuspid atresia is defined by absence of the RA–RV connection. Survival depends on an adequate atrial communication, while pulmonary and systemic blood flow are determined by VSD size, outflow obstruction, ductal flow, and the relationship of the great arteries.

In normally related great arteries, the VSD primarily determines pulmonary blood flow. In transposed great arteries, the VSD may determine systemic output. This distinction is central to diagnosis, neonatal stabilization, and surgical planning.

References

[1] Guller B, Titus JL. Morphological studies in tricuspid atresia. Circulation. 1968;38(5):977-988.

[2] Kahramanoglu O, Eyisoy O, Demirci O. Prenatal predictors and early postnatal outcomes in fetuses diagnosed with tricuspid atresia. Diagnostics. 2024;14(24):2855.

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

[4] Patel R, Fox K, Taylor JF, Graham GR. Tricuspid atresia: clinical course in 62 cases (1967-1974). Br Heart J. 1978;40(12):1408-1414.

[5] Franken LC, Admiraal M, Verrall CE, Zannino D, Ayer J, Iyengar AJ, Cole AD, Sholler GF, d’Udekem Y, Winlaw DS. Improved long-term outcomes in double-inlet left ventricle and tricuspid atresia with transposed great arteries: systemic outflow tract obstruction present at birth defines long-term outcome. Eur J Cardiothorac Surg. 2017;51(6):1054-1060.

[6] Park WK, Baek JS, Kwon BS, Im YM, Lee JH, Choi ES, Park CS, Yun TJ. Revisitation of double-inlet left ventricle or tricuspid atresia with transposed great arteries. Ann Thorac Surg. 2019;108(5):1470-1477.

[7] Palacio AM, Williams WG, Barron DJ, Argo MB, Jegatheeswaran A, Jacobs ML, Bondarenko I, Welke K, Kirklin JK, Karamlou T, Alsoufi B, McCrindle BW. Management of tricuspid atresia with normally related great arteries and left-sided obstruction. World J Pediatr Congenit Heart Surg. 2024;15(6):632-642.