Tricuspid Atresia: Anatomy, Great Artery Relationships, and Hemodynamics

1. Definition and Anatomic Framework

Tricuspid atresia is a congenital absence of a patent right atrioventricular connection. The right atrium (RA) cannot empty directly into the morphologic right ventricle (RV), which is usually hypoplastic. An interatrial communication is consequently essential: systemic venous blood crosses from the RA to the left atrium (LA), mixes with pulmonary venous return, and enters the dominant left ventricle (LV). In the common muscular form, an imperforate fibromuscular floor separates the RA from the underlying ventricular mass. Morphologic assessment must distinguish true absence of the right atrioventricular connection from other causes of a diminutive RV, including pulmonary atresia with intact ventricular septum. [1,2]

The common circulatory pathway is systemic veins → RA → ASD/PFO → LA → LV. Unlike a normal circulation, a single ventricular chamber supports both systemic and pulmonary blood flow in parallel. Arterial oxygen saturation reflects the degree of pulmonary venous admixture and the pulmonary-to-systemic flow ratio (Qp:Qs), but saturation alone does not establish whether systemic perfusion is adequate.

Three anatomical variables determine the clinical phenotype: great artery relationship, ventricular septal defect (VSD)/bulboventricular foramen (BVF), and pulmonary outflow obstruction. The same diagnosis can produce profound duct-dependent cyanosis, pulmonary overcirculation, or critical systemic outflow obstruction. [1,3]

2. Great Artery Relationships and Their Physiologic Consequences

2.1 Type I: Normally related great arteries

In Type I tricuspid atresia, the aorta arises from the dominant LV, while the pulmonary artery (PA) arises from the hypoplastic RV. The systemic pathway is therefore LV → aorta. Pulmonary flow ordinarily follows LV → VSD → RV → PA or, when antegrade pulmonary flow is inadequate, reaches the pulmonary arteries through a patent ductus arteriosus (PDA) or surgically established source.

Because the VSD feeds the pulmonary outlet, its effective size and the degree of pulmonary valvar/subvalvar obstruction govern pulmonary blood flow. In this morphology, a restrictive VSD primarily threatens pulmonary, rather than systemic, output. The relationship between VSD anatomy and cyanosis must be interpreted together with ductal patency and pulmonary vascular resistance (PVR). [1,3]

2.2 Type II: Transposed great arteries

In Type II tricuspid atresia, the ventriculoarterial connections are discordant: the PA arises from the dominant LV, and the aorta arises from the hypoplastic RV. Pulmonary flow follows LV → PA, whereas effective systemic output generally follows LV → VSD/BVF → hypoplastic RV → aorta.

The BVF therefore has the opposite functional significance. Narrowing of this communication limits systemic blood flow and creates functional subaortic obstruction, despite preserved LV systolic contractility. Additional restriction may occur in the RV outlet, aortic valve, ascending aorta, or aortic arch. This pattern resembles other dominant-LV single-ventricle arrangements with ventriculoarterial discordance, including double-inlet LV with transposition. [4,5]

The operative priority is not merely to balance Qp and Qs; it is to establish durable, unobstructed systemic outflow before or during staged cavopulmonary palliation.

3. Morphologic Subtypes

The conventional subtypes a, b, and c refer to the severity of pulmonary outflow obstruction. They are useful descriptive categories, although the precise VSD, ventricular chamber, and outflow geometry remain more important for individual decisions. [1]

3.1 Type Ia: Normally related great arteries with pulmonary atresia

With pulmonary atresia and no effective ventricular-to-pulmonary pathway, pulmonary perfusion is typically duct dependent. Closure of the PDA may produce severe hypoxemia and cardiovascular collapse. Prostaglandin E1 maintains ductal patency until durable pulmonary flow is secured by catheter-based ductal stenting, a systemic-to-pulmonary arterial shunt, or another individualized approach. Pulmonary artery confluence and the possibility of branch distortion must be assessed.

3.2 Type Ib: Normally related great arteries with pulmonary stenosis

A VSD communicates with the hypoplastic RV, but pulmonary flow is restricted by the VSD, RV infundibulum, or pulmonary valve. The degree of cyanosis varies. Some infants achieve relatively balanced circulation without neonatal surgery; others develop marked hypoxemia or duct-dependent flow. A VSD that becomes increasingly restrictive may precipitate worsening cyanosis over time.

3.3 Type Ic: Normally related great arteries without significant pulmonary stenosis

With a large nonrestrictive VSD and unobstructed pulmonary outlet, falling neonatal PVR permits excessive pulmonary flow. The LV experiences combined systemic and pulmonary volume loading, leading to tachypnea, feeding difficulty, poor growth, and congestive heart failure. Pulmonary artery banding may be appropriate when pulmonary overcirculation cannot be controlled adequately and the anatomy supports band placement, but the initial strategy depends on anticipated timing of cavopulmonary connection and coexisting lesions.

3.4 Type IIa: Transposed great arteries with pulmonary atresia

This anatomically unusual combination requires careful verification of the atretic segment and the actual source of flow to each great artery. If the pulmonary outlet is truly atretic and there is no LV-to-RV communication, neither a direct pulmonary outlet nor the usual LV–BVF–RV–aorta systemic pathway is available; an additional communication or alternate vascular supply must exist for survival. Thus the conventional subtype label cannot substitute for segmental imaging of both outflow pathways. In some diagrams labeled “IIa,” pulmonary or subpulmonary atresia and the presence or absence of a VSD are simplified, and these labels should not be used to infer ductal dependency without defining the exact anatomy.

3.5 Type IIb: Transposed great arteries with pulmonary stenosis

A VSD/BVF supplies the systemic outlet, and pulmonary outflow is restricted. Pulmonary stenosis may protect against pulmonary overcirculation, but it does not protect the systemic circulation from a restrictive BVF. In this morphology, treatment must assess both competing flow pathways independently.

3.6 Type IIc: Transposed great arteries without pulmonary stenosis

Unobstructed LV-to-PA flow predisposes to pulmonary overcirculation and reduced effective systemic perfusion as PVR falls. When the BVF is small, systemic flow must pass through a narrowed LV-to-RV communication. The association of high Qp, a potentially restrictive BVF, and sometimes a small aorta makes this subtype particularly challenging. A pulmonary artery band may reduce Qp, yet banding alone does not remove subaortic obstruction and can complicate subsequent systemic outflow reconstruction. [4–6]

4. Hemodynamics and Evolution After Birth

In all types, obligatory atrial-level right-to-left flow is necessary for systemic venous return to reach the LV. A restrictive atrial septal communication increases RA and systemic venous pressure, impairs effective LV preload, and may cause low output despite an anatomically satisfactory ventricular outlet. Urgent balloon atrial septostomy or surgical atrial septectomy may be indicated when atrial decompression is inadequate. [3]

PVR falls over the first days and weeks of life. In lesions without pulmonary stenosis, this tends to increase Qp and LV volume load; in severe pulmonary obstruction, blood flow may instead be limited by the ductus or surgically created pulmonary source. A high arterial saturation may reflect pulmonary overcirculation rather than favorable hemodynamics. Clinical perfusion, lactate, systemic arterial pressure, ventricular function, and pulmonary venous return must be assessed alongside oxygen saturation.

In Type II, systemic perfusion is especially vulnerable to a restrictive BVF. Doppler gradients may underestimate clinically important obstruction when systemic output is low, and low measured gradient cannot exclude inadequate flow. Serial evaluation is important because the foramen may become relatively or absolutely restrictive as ventricular geometry changes. Volume unloading after a bidirectional Glenn can unmask or worsen systemic outflow obstruction, an issue documented in single-ventricle anatomy with ventriculoarterial discordance. [5–7]

5. Diagnostic Evaluation

Transthoracic echocardiography is the primary diagnostic modality. A segmental, rather than label-based, examination should document atrial situs, systemic and pulmonary venous connections, the absent right atrioventricular connection, ventricular morphology, and the origin of both great arteries. The imaging report should specifically identify:

• The size and adequacy of the atrial communication and the direction/restriction of flow.
• VSD/BVF location, dimensions in more than one imaging plane, relation to outlet septum, and serial change.
• Morphology and dimensions of the hypoplastic RV and its outlet chamber.
• Pulmonary valve, subpulmonary pathway, main and branch pulmonary arteries, and ductal connections.
• Aortic valve, ascending aorta, transverse arch, and potential coarctation.
• LV function, mitral valve function, pulmonary venous return, and associated anomalies.

Color flow and spectral Doppler establish flow pathways, but anatomic dimensions and hemodynamic context are needed to interpret obstruction. For suspected Type II systemic outflow compromise, assess the entire route from the LV through the BVF, RV cavity, aortic outlet, and arch. Cross-sectional CT or cardiovascular magnetic resonance may clarify great-vessel geometry, arch anatomy, and extracardiac vessels when echocardiography is insufficient. Catheterization is reserved for specific diagnostic/interventional questions and for hemodynamic assessment before later staged palliation as appropriate. [2,3]

6. Initial Management and Surgical Strategy

6.1 Pulmonary undercirculation

For severe cyanosis or duct-dependent pulmonary flow, stabilize with prostaglandin E1, monitor systemic oxygen delivery, and establish reliable pulmonary blood flow. Ductal stenting and modified systemic-to-pulmonary shunting have distinct advantages and limitations related to ductal anatomy, branch PA growth, procedural risk, and reintervention. Neither approach is universally preferable. The overall morphology and future Glenn pathway guide choice. [8]

6.2 Pulmonary overcirculation

For unobstructed pulmonary flow, treatment is directed at systemic perfusion, ventricular volume load, and pulmonary vascular protection. Diuresis and nutritional support may be useful, but clinically important overcirculation can require pulmonary artery banding. Band tightness should be assessed by physiologic response rather than a fixed diameter alone. Excessive restriction risks profound hypoxemia and branch PA distortion; insufficient restriction leaves persistent volume overload.

6.3 Type II systemic outflow obstruction

Aortic reconstruction or Damus–Kaye–Stansel (DKS) connection may be needed when systemic outflow traverses a restrictive BVF, particularly if pulmonary artery banding is contemplated. A DKS provides an alternative systemic outlet from the dominant LV via the proximal PA, bypassing the restrictive BVF. Depending on the arch, valve morphology, and neonatal physiology, the initial operation may incorporate DKS with pulmonary blood flow control, arch reconstruction, or a Norwood-type strategy.

Technical planning must consider native pulmonary and aortic root relationships, semilunar valve competence, coronary origins, space beneath the arch, branch PA takeoff, and the geometry of the systemic anastomosis. Obstruction may develop at the proximal anastomosis, neoaortic pathway, or reconstructed arch. Retrospective surgical series support considering systemic outflow relief before clinically significant BVF restriction emerges, although optimal timing is not established for every morphology. [4–7,9]

7. Staged Cavopulmonary Palliation

Once pulmonary vascular conditions are suitable, the bidirectional Glenn diverts superior vena caval return directly to the pulmonary arteries, reduces ventricular volume loading, and prepares for total cavopulmonary connection. Glenn candidacy depends on low pulmonary vascular resistance, adequate PA anatomy, ventricular function, atrioventricular valve competence, and unobstructed pulmonary venous return.

For Type II anatomy, a pre-Glenn evaluation must explicitly reassess systemic ventricular outflow. A borderline BVF may become clinically important following volume unloading; selected patients benefit from concomitant DKS rather than waiting for overt obstruction. [5,6]

Fontan completion incorporates inferior vena caval flow into the pulmonary arteries using a lateral tunnel or extracardiac conduit, sometimes with fenestration. Pulmonary flow is then driven without a subpulmonary ventricle, making low PVR, low downstream atrial pressure, and unobstructed venous pathways essential. Fontan completion is physiologic palliation, not an anatomic cure. Long-term surveillance addresses systemic ventricular function, arrhythmia, pathway and PA obstruction, thromboembolism, exercise limitation, hepatic disease, protein-losing enteropathy, and plastic bronchitis. Contemporary cohorts show substantial improvement in survival but continued late morbidity. [10]

8. Key Clinical Principles

• Atrial communication is essential: all systemic venous return must cross the atrial septum to reach the dominant LV.
• Great artery relationship changes the meaning of the VSD: Type I VSD restriction threatens pulmonary flow; Type II BVF restriction threatens systemic flow.
• Assess Qp and Qs independently: relatively high oxygen saturation can coexist with poor systemic perfusion.
• Type II requires complete systemic outlet assessment: BVF, rudimentary RV outlet, aortic valve, ascending aorta, and arch.
• Pulmonary banding is not a remedy for BVF restriction: consider early systemic outflow reconstruction when appropriate.
• Reassess outflow before Glenn: ventricular volume unloading may reveal or aggravate BVF-related obstruction.
• Fontan physiology requires lifelong surveillance even after anatomically successful staged palliation.

References

1. Rigby ML, Anderson RH, Gibson D, et al. Two dimensional echocardiographic categorisation of the univentricular heart. Ventricular morphology, type, and mode of atrioventricular connection. Br Heart J. 1981. DOI: 10.1136/hrt.46.6.603. PMID: 7317227.
2. Orie J, Anderson C, Ettedgui J, et al. Echocardiographic-morphologic correlations in tricuspid atresia. J Am Coll Cardiol. 1995. DOI: 10.1016/0735-1097(95)00250-8. PMID: 7642870.
3. Rao PS. Echocardiography in the Diagnosis and Management of Tricuspid Atresia. Applied Sciences. 2021. DOI: 10.3390/app11209472.
4. Lan YT, Chang RK, Laks H, et al. Outcome of patients with double-inlet left ventricle or tricuspid atresia with transposed great arteries. J Am Coll Cardiol. 2004. DOI: 10.1016/j.jacc.2003.07.035. PMID: 14715192.
5. van Son JA, Haas GS. Acute aortic outflow obstruction by diminution of bulboventricular foramen size after volume unloading in univentricular physiology. Eur J Cardiothorac Surg. 1996. DOI: 10.1016/S1010-7940(96)80346-0. PMID: 8905287.
6. Alsoufi B, Alwadai A, Khan M, et al. Outcomes of Damus-Kaye-Stansel anastomosis at time of cavopulmonary connection in single ventricle patients at risk of developing systemic ventricular outflow tract obstruction. Eur J Cardiothorac Surg. 2014. DOI: 10.1093/ejcts/ezt251. PMID: 23660552.
7. Miura T, Kishimoto H, Kawata H, et al. Management of univentricular heart with systemic ventricular outflow obstruction by pulmonary artery banding and Damus-Kaye-Stansel operation. Ann Thorac Surg. 2004. DOI: 10.1016/S0003-4975(03)01248-7. PMID: 14726028.
8. Karamlou T, Ashburn DA, Caldarone CA, et al. Matching procedure to morphology improves outcomes in neonates with tricuspid atresia. J Thorac Cardiovasc Surg. 2005. DOI: 10.1016/j.jtcvs.2005.07.024. PMID: 16307990.
9. Kido T, Steringer MT, Vodiskar J, et al. Improved long-term outcome of Damus-Kaye-Stansel procedure without previous pulmonary artery banding. Ann Thorac Surg. 2021. DOI: 10.1016/j.athoracsur.2021.05.022. PMID: 34087235.
10. Lee SY, Kim SJ, Lee CH, et al. The Long-term Outcomes and Risk Factors of Complications After Fontan Surgery: From the Korean Fontan Registry. Korean Circ J. 2024. DOI: 10.4070/kcj.2023.0211. PMID: 39175341.