Tricuspid Atresia #6: Subaortic Stenosis and Left Ventricular Outflow Tract Obstruction

1. Overview

Subaortic stenosis is a major determinant of systemic output in tricuspid atresia with transposed great arteries, particularly Type II tricuspid atresia. In this anatomy, the pulmonary artery arises from the dominant left ventricle, whereas the aorta arises from the hypoplastic right ventricular outlet chamber. Systemic blood must therefore pass from the left ventricle through the ventricular septal defect, or bulboventricular foramen, into the rudimentary right ventricle before reaching the aorta.

The functional systemic outflow pathway is:

Dominant left ventricle → bulboventricular foramen/VSD → rudimentary right ventricle → aorta

A restrictive bulboventricular foramen therefore functions as subaortic stenosis. Associated aortic annular hypoplasia, ascending aortic hypoplasia, coarctation, or aortic arch obstruction may further restrict systemic output. In a series of patients with double-inlet left ventricle or tricuspid atresia and transposed great arteries, 20 of 30 patients ultimately required a systemic outflow relief operation, demonstrating the frequency with which initially acceptable outflow anatomy becomes clinically inadequate [1].

Subaortic stenosis has important prognostic implications. In a historical cohort of 237 infants with tricuspid atresia, subaortic stenosis and aortic arch obstruction were independently associated with poorer survival [2].

2. Anatomic Basis of Systemic Outflow Obstruction

In Type II tricuspid atresia:

1. The tricuspid valve is absent.
2. The right ventricle is markedly hypoplastic and functions primarily as an outlet chamber.
3. The pulmonary artery arises from the dominant left ventricle.
4. The aorta arises from the rudimentary right ventricle.
5. The dominant left ventricle communicates with the rudimentary right ventricle through the VSD or bulboventricular foramen.

Because the aorta does not arise directly from the dominant ventricle, the bulboventricular foramen becomes an obligatory component of the systemic outflow tract.

The term left ventricular outflow tract obstruction is physiologically appropriate even though the obstruction may not lie within the conventional anatomic left ventricular outflow tract. It represents obstruction to blood leaving the dominant left ventricle toward the systemic circulation.

Similar physiology occurs in double-inlet left ventricle with ventriculoarterial discordance.

3. Mechanisms of Subaortic Stenosis

3.1 Restrictive Bulboventricular Foramen

When the bulboventricular foramen is small, systemic blood must accelerate through a restricted interventricular communication. This produces a pressure gradient between the dominant left ventricle and the rudimentary right ventricular outlet chamber.

As restriction progresses:

• Dominant left ventricular systolic pressure increases.
• Ventricular hypertrophy may develop.
• Systemic cardiac output becomes increasingly dependent on a small communication.
• Coronary, cerebral, renal, and mesenteric perfusion may become compromised.
• Ventricular compliance may deteriorate.
• Low-output physiology may occur despite apparently adequate pulmonary blood flow.

The obstruction may be caused by the size of the VSD itself, hypertrophied muscular margins surrounding the defect, or unfavorable geometry between the dominant ventricle, outlet chamber, and aortic root.

3.2 Progressive Restriction

The bulboventricular foramen may be adequate during the neonatal period but become progressively restrictive during staged palliation. Potential mechanisms include:

• Somatic growth without proportional enlargement of the bulboventricular foramen
• Muscular hypertrophy surrounding the defect
• Increased dominant ventricular afterload
• Alteration of ventricular geometry
• Progressive narrowing following pulmonary artery banding
• Coexisting aortic annular or arch hypoplasia

Pulmonary artery banding reduces pulmonary overcirculation but increases dominant ventricular pressure. In susceptible anatomy, the resulting hypertrophic response may aggravate restriction of the bulboventricular foramen.

4. Predictors of Bulboventricular Foramen Restriction

Several morphologic and procedural factors have been associated with subsequent systemic outflow obstruction.

4.1 Aortic Arch Obstruction

Aortic arch obstruction is an important marker of a globally inadequate systemic outflow pathway. In a 30-patient series, arch obstruction was strongly associated with earlier need for a systemic outflow relief operation, with a hazard ratio of 20.6 [1].

Arch obstruction should therefore not be regarded as an isolated extracardiac lesion. Its presence may indicate that the bulboventricular foramen, outlet chamber, aortic annulus, ascending aorta, and arch collectively provide an inadequate systemic pathway.

4.2 Systemic Outflow Tract Area

A smaller indexed systemic outflow tract area at end systole has also been associated with the need for subsequent outflow intervention. For every 10 mm²/m² decrease in systemic outflow tract area index, the risk of earlier systemic outflow relief increased in the Park series [1].

Assessment should therefore include the complete cross-sectional area of the systemic pathway rather than the VSD diameter alone.

4.3 Bulboventricular Foramen-to-Aortic Annulus Ratio

The relationship between bulboventricular foramen size and the systemic arterial outlet may provide more useful information than an isolated BVF measurement.

In a 2026 series of 17 patients with a dominant single left ventricle and ventriculoarterial discordance, bulboventricular foramen restriction developed in 47.1%. A pre-bidirectional cavopulmonary shunt BVF-to-aortic annulus ratio of 0.65 or less predicted restriction with 71% sensitivity and 83% specificity [3].

This ratio should be interpreted as a risk-stratification parameter rather than an absolute operative threshold because the available evidence is derived from a small retrospective cohort.

4.4 Pulmonary Artery Banding With Arch Reconstruction

Initial pulmonary artery banding combined with aortic arch reconstruction was associated with a higher incidence of subsequent BVF restriction than other initial strategies in the same series [3].

This finding probably reflects both:

• The adverse substrate of arch obstruction and a small systemic outflow pathway
• The potential for pressure loading after pulmonary artery banding to promote ventricular and septal hypertrophy

The association should not be interpreted as proof that pulmonary artery banding itself causes obstruction in every patient. Rather, patients selected for arch repair and banding require particularly close surveillance.

5. Hemodynamic Consequences

Subaortic stenosis in Type II tricuspid atresia primarily compromises systemic rather than pulmonary output.

Potential consequences include:

• Reduced systemic cardiac output
• Increased dominant left ventricular pressure
• Ventricular hypertrophy
• Impaired ventricular relaxation and compliance
• Elevated left atrial and pulmonary venous pressures
• Atrioventricular valve regurgitation
• Metabolic acidosis
• Oliguria and end-organ hypoperfusion
• Myocardial ischemia
• Hemodynamic instability during infection, anesthesia, or perioperative stress

The combination of bulboventricular foramen restriction and aortic arch obstruction is particularly hazardous because systemic blood flow is limited at multiple sequential levels.

Pulmonary blood flow may remain excessive despite severe systemic outflow obstruction. Consequently, satisfactory oxygen saturation does not exclude critical systemic hypoperfusion.

A low Doppler gradient also does not necessarily indicate mild obstruction. In a patient with depressed cardiac output, limited flow across a severely restrictive pathway may produce only a modest measured gradient.

6. Diagnostic Assessment

Evaluation must define the entire systemic outflow pathway.

6.1 Echocardiography

Important measurements and observations include:

• Bulboventricular foramen dimensions in multiple planes
• BVF area and indexed BVF area
• BVF-to-aortic annulus ratio
• Morphology and thickness of the muscular BVF margins
• Peak and mean Doppler gradients
• Size of the rudimentary right ventricular outlet chamber
• Aortic annular dimension
• Ascending aortic dimension
• Presence and severity of coarctation or arch hypoplasia
• Dominant ventricular systolic and diastolic function
• Degree of ventricular hypertrophy
• Atrioventricular valve regurgitation
• Pulmonary artery dimensions
• Pulmonary artery band gradient when present
• Semilunar valve morphology and function

Serial imaging is essential because systemic outflow obstruction may develop between the initial palliation and bidirectional Glenn procedure.

6.2 Cross-Sectional Imaging

Computed tomography or cardiac magnetic resonance imaging may be useful for defining:

• Aortic arch anatomy
• Spatial relationships of the great arteries
• Cross-sectional area of the systemic outflow pathway
• Branch pulmonary artery anatomy
• Coronary anatomy before a palliative arterial switch
• Three-dimensional relationships relevant to surgical reconstruction

6.3 Cardiac Catheterization

Catheterization may be required when noninvasive findings are inconclusive or when intervention is planned. Relevant data include:

• Dominant ventricular pressure
• Pressure in the rudimentary outlet chamber
• Systemic outflow gradient
• Aortic and pulmonary artery pressures
• Pulmonary vascular resistance
• Cardiac output
• Angiographic appearance of the BVF, outlet chamber, and aortic arch

7. Principles of Surgical Management

The central objective is to establish a durable and unobstructed systemic outflow pathway while maintaining appropriately controlled pulmonary blood flow.

Pulmonary flow control alone is insufficient when clinically significant subaortic stenosis is present. The surgical strategy must address both:

1. Systemic outflow obstruction
2. Pulmonary blood flow regulation

The major options include:

• Palliative arterial switch operation
• Modified Norwood-type reconstruction
• Arch repair with pulmonary artery banding
• Damus–Kaye–Stansel anastomosis
• Direct bulboventricular foramen enlargement or muscular resection

The optimal operation depends on:

• Severity of BVF restriction
• Size of the aortic annulus and ascending aorta
• Presence of arch obstruction
• Great artery relationship
• Coronary anatomy
• Pulmonary artery anatomy
• Dominant ventricular function
• Atrioventricular valve function
• Previous procedures
• Planned timing of the bidirectional Glenn procedure

Contemporary evidence does not demonstrate universal superiority of one neonatal strategy. In a retrospective series of 71 neonates with single-ventricle anatomy, transposed great arteries, and systemic outflow obstruction, overall survival and Fontan completion were comparable among palliative arterial switch, pulmonary artery banding with or without arch repair, and modified Norwood strategies [4].

These results support anatomy- and physiology-specific selection rather than application of a single operation to all patients.

8. Significant Subaortic Stenosis at Initial Palliation

When significant obstruction is present during the neonatal period, the initial operation should bypass or reconstruct the restricted systemic pathway.

8.1 Palliative Arterial Switch Operation

A palliative arterial switch establishes direct continuity between the dominant left ventricle and the systemic arterial root.

Following the switch:

• The native pulmonary root becomes the systemic outflow root.
• Dominant left ventricular output no longer depends on passage through the restrictive BVF.
• The former aortic root becomes the pulmonary outlet from the rudimentary right ventricle.
• Pulmonary blood flow is controlled with pulmonary artery banding.
• Associated aortic arch obstruction may be repaired simultaneously.

This creates band physiology, in which pulmonary flow remains antegrade but is regulated by a pulmonary artery band.

Potential advantages include:

• Direct dominant ventricular-to-systemic arterial continuity
• Elimination of BVF dependence for systemic output
• Avoidance of systemic-to-pulmonary shunt dependence
• Preservation of antegrade pulmonary flow before the Glenn stage

Important technical considerations include:

• Coronary transfer
• Great artery size discrepancy
• Semilunar valve morphology
• LeCompte maneuver and branch pulmonary artery geometry
• Aortic arch reconstruction
• Pulmonary artery band placement and calibration

In the largest reported palliative arterial switch cohort, 23 patients underwent palliative arterial switch as part of a 71-patient comparison. Survival, Fontan completion, systemic valve competence, ventricular function, and overall reintervention-free survival were comparable with pulmonary artery banding and modified Norwood strategies [4].

8.2 Modified Norwood-Type Reconstruction

A modified Norwood procedure establishes systemic outflow through the pulmonary root and reconstructs the systemic arterial pathway.

The operation generally includes:

• Damus–Kaye–Stansel-type connection or aortopulmonary amalgamation
• Reconstruction of the diminutive ascending aorta and aortic arch
• Creation of a systemic-to-pulmonary shunt or right ventricle-to-pulmonary artery conduit
• Elimination of dependence on the restrictive BVF for systemic output

This creates shunt physiology, in which pulmonary blood flow depends on a surgically created source.

The Norwood-type strategy is particularly applicable when:

• The native aortic annulus is markedly small
• The ascending aorta is hypoplastic
• Significant arch hypoplasia or coarctation is present
• The entire systemic pathway requires reconstruction
• Coronary or great artery anatomy makes palliative arterial switch less favorable

Potential disadvantages include:

• Interstage shunt dependence
• Risk of shunt thrombosis
• Diastolic runoff
• Pulmonary overcirculation
• Imbalance between systemic and pulmonary blood flow
• More extensive neonatal reconstruction

In the contemporary comparative series, modified Norwood palliation achieved survival and Fontan completion comparable with palliative arterial switch and pulmonary artery banding strategies [4].

9. No Significant Subaortic Stenosis at Initial Palliation

Arch Repair With Pulmonary Artery Banding

When the bulboventricular foramen and native aortic pathway are initially adequate, treatment may focus on associated arch obstruction and pulmonary overcirculation.

The initial operation may include:

• Aortic arch reconstruction
• Pulmonary artery banding
• Preservation of the native pathway from the dominant ventricle through the BVF and rudimentary right ventricle to the aorta

This approach avoids neonatal coronary transfer or Norwood-type systemic outflow reconstruction.

However, it leaves systemic output dependent on the bulboventricular foramen. The combination of arch obstruction, a relatively small systemic outflow tract, and pressure loading after pulmonary artery banding identifies a population at increased risk for progressive obstruction [1,3].

Historical data showed worse survival among patients undergoing pulmonary artery banding with arch repair than among those undergoing other palliative procedures [2]. This finding arose from an earlier surgical era and was likely influenced by unfavorable morphology and patient selection. More recent data demonstrate comparable survival and Fontan completion when pulmonary artery banding is applied selectively within an individualized treatment strategy [4].

10. Reassessment Before the Bidirectional Glenn Procedure

The systemic outflow tract must be reassessed before superior cavopulmonary connection.

Evaluation should include:

• BVF dimensions and area
• BVF-to-aortic annulus ratio
• Doppler gradient
• Dominant ventricular pressure and hypertrophy
• Aortic annular and arch dimensions
• Ventricular systolic and diastolic function
• Atrioventricular valve regurgitation
• Clinical evidence of systemic hypoperfusion

10.1 No Significant Obstruction

When the systemic outflow pathway remains adequate, a bidirectional Glenn procedure may be performed without additional systemic outflow intervention.

Continued surveillance remains necessary because obstruction may develop later.

10.2 Significant or Progressive Obstruction

When obstruction is present or strongly anticipated, the Glenn procedure should be combined with systemic outflow relief.

The principal options are:

• Damus–Kaye–Stansel anastomosis
• Direct BVF enlargement
• Muscular resection around the BVF
• Less commonly, conversion to another systemic outflow reconstruction

In patients with a pre-Glenn BVF-to-aortic annulus ratio of 0.65 or less, prophylactic or concomitant systemic outflow relief should be considered in the context of the complete anatomy, although this threshold has not been validated in large multicenter cohorts [3].

11. Damus–Kaye–Stansel Anastomosis

The Damus–Kaye–Stansel procedure connects the pulmonary root to the systemic arterial circulation. Systemic output can then leave the dominant left ventricle through the pulmonary root and enter the aorta without passing through the restrictive BVF.

At the bidirectional Glenn stage, pulmonary blood flow is supplied primarily by the cavopulmonary connection, allowing the native pulmonary root to be incorporated into the systemic outflow.

Potential advantages include:

• Reliable bypass of the restrictive BVF
• Avoidance of resection near the conduction tissue
• Durable relief of progressive or recurrent obstruction
• Applicability when the BVF is deeply positioned or difficult to expose
• Avoidance of ventricular septal incision in selected patients

Potential concerns include:

• Distortion of the pulmonary and aortic roots
• Development or progression of semilunar valve regurgitation
• Coronary compression or unfavorable great artery geometry
• Branch pulmonary artery distortion
• Anastomotic obstruction
• Need for long-term surveillance of the reconstructed systemic outflow

A Damus–Kaye–Stansel anastomosis is particularly attractive when direct enlargement carries a substantial risk of conduction injury, atrioventricular valve damage, or recurrent muscular obstruction.

12. Direct Bulboventricular Foramen Enlargement

Direct relief may involve:

• Resection of hypertrophied muscle surrounding the BVF
• Enlargement of the ventricular septal communication
• Patch augmentation of the opening
• Combined muscular resection and septal enlargement

Potential advantages include:

• Preservation of the native aortic outflow pathway
• Avoidance of proximal great artery reconstruction
• Maintenance of native semilunar valve relationships
• Ability to combine the intervention with the bidirectional Glenn procedure

Important limitations include:

• Difficult exposure
• Risk of complete heart block
• Injury to the conduction axis
• New or worsened atrioventricular valve regurgitation
• Ventricular injury
• Residual obstruction
• Recurrent muscular narrowing
• Potential injury to semilunar valve structures

In a retrospective comparison of 34 patients undergoing either Damus–Kaye–Stansel reconstruction or direct BVF enlargement, both procedures effectively relieved systemic outflow obstruction. However, direct BVF enlargement was associated with higher rates of heart block and new atrioventricular valve regurgitation [5].

Direct enlargement is therefore most appropriate when the obstructive anatomy is discrete, exposure is favorable, and the BVF can be enlarged without unacceptable risk to the conduction tissue or atrioventricular valve apparatus.

13. Selection of Surgical Strategy

Palliative Arterial Switch

Consider when:

• Significant neonatal systemic outflow obstruction is present.
• Coronary transfer is technically feasible.
• The native pulmonary root is suitable for systemic function.
• Great artery geometry permits safe reconstruction.
• Pulmonary flow can be controlled with a band.
• Avoidance of shunt-dependent physiology is desirable.

Modified Norwood Reconstruction

Consider when:

• The aortic annulus and ascending aorta are markedly hypoplastic.
• Significant arch reconstruction is required.
• The native systemic pathway is globally inadequate.
• A comprehensive neonatal systemic outflow reconstruction is necessary.
• Palliative arterial switch anatomy is unfavorable.

Arch Repair With Pulmonary Artery Banding

Consider when:

• Arch obstruction is present.
• The BVF is initially nonrestrictive.
• The aortic annulus and ascending aorta are adequate.
• The native systemic outflow pathway can be preserved.
• Close surveillance for progressive obstruction is feasible.

Damus–Kaye–Stansel Anastomosis

Consider when:

• Significant obstruction is present at the Glenn stage.
• The BVF is small or progressively restrictive.
• Direct enlargement carries substantial conduction or valvar risk.
• Recurrent muscular obstruction is considered likely.
• A durable bypass of the BVF is preferred.

Direct BVF Enlargement

Consider when:

• The obstruction is discrete and surgically accessible.
• The conduction axis can be protected.
• The atrioventricular valve apparatus is remote from the intended incision.
• Adequate and durable enlargement can be achieved.
• Great artery reconstruction is undesirable or anatomically unnecessary.

14. Perioperative Considerations

Patients with systemic outflow obstruction are vulnerable to rapid hemodynamic deterioration.

Management priorities include:

• Maintaining adequate systemic arterial pressure
• Preserving coronary and cerebral perfusion
• Avoiding excessive pulmonary blood flow
• Maintaining sufficient preload
• Avoiding severe tachycardia
• Correcting metabolic acidosis
• Monitoring lactate and urine output
• Monitoring cerebral and somatic near-infrared spectroscopy
• Assessing dominant ventricular function
• Evaluating atrioventricular valve regurgitation
• Confirming unobstructed arch and systemic outflow
• Assessing pulmonary artery band, shunt, or Glenn function
• Evaluating branch pulmonary artery geometry

After palliative arterial switch or pulmonary artery banding, the band must provide sufficient restriction to prevent pulmonary overcirculation without causing excessive hypoxemia.

After Norwood-type reconstruction, systemic-to-pulmonary shunt patency and balance between pulmonary and systemic blood flow are critical.

After Damus–Kaye–Stansel reconstruction, attention should be directed toward:

• Anastomotic patency
• Semilunar valve competence
• Coronary perfusion
• Branch pulmonary artery distortion
• Systemic outflow geometry

Postoperative oxygen saturation must always be interpreted together with markers of systemic perfusion.

15. Long-Term Surveillance

Surveillance should continue throughout staged palliation and after Fontan completion.

Important targets include:

• Recurrent or residual systemic outflow obstruction
• Aortic arch obstruction
• BVF restriction
• Systemic semilunar valve regurgitation
• Atrioventricular valve regurgitation
• Dominant ventricular hypertrophy
• Systolic and diastolic ventricular dysfunction
• Branch pulmonary artery distortion
• Pulmonary artery band-related stenosis
• Damus–Kaye–Stansel anastomotic obstruction
• Fontan pathway hemodynamics

The systemic outflow pathway should be reassessed before every major stage because obstruction may evolve despite previously acceptable anatomy.

16. Key Surgical Concepts

• In Type II tricuspid atresia, the bulboventricular foramen is an obligatory component of systemic outflow.
• A restrictive BVF functions as subaortic stenosis.
• Aortic arch obstruction and a small systemic outflow tract area identify patients at increased risk for later outflow intervention.
• A pre-Glenn BVF-to-aortic annulus ratio of 0.65 or less is associated with subsequent BVF restriction.
• Pulmonary artery banding controls pulmonary blood flow but does not directly relieve systemic outflow obstruction.
• Significant neonatal obstruction may be treated with palliative arterial switch or modified Norwood reconstruction.
• Arch repair with pulmonary artery banding is most appropriate when the native systemic pathway is initially adequate.
• Progressive obstruction at the Glenn stage may be treated with Damus–Kaye–Stansel reconstruction or direct BVF enlargement.
• Direct BVF enlargement carries specific risks of heart block and atrioventricular valve regurgitation.
• No single operation is universally superior; surgical selection should be individualized according to anatomy, physiology, and anticipated Fontan pathway.

References

[1] 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. doi:10.1016/j.athoracsur.2018.11.052.

[2] Franklin RC, 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. doi:10.1161/01.CIR.87.2.427.

[3] Ergün S, Tüzün B, Recep E, Çoban Ş, Sağlam S, Şahin Işılay İ, Kutanis K, Öztürk E, Hatemi A. Predictors of bulboventricular foramen restriction in patients with a dominant single left ventricle and ventriculoarterial discordance. Cardiol Young. 2026. doi:10.1017/S1047951126112244.

[4] 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 Tech. 2023. doi:10.1016/j.xjtc.2023.06.006.

[5] Albahlooli N, Alahdal S, Alfaraidi Y, Alqethamy H, Alsaeedi A, Elwy A, Alnajashi K, Donato R. Damus–Kaye–Stansel operation versus bulboventricular foramen enlargement for the management of univentricular heart with systemic outflow obstruction: 17 years’ experience, a retrospective study. J Saudi Heart Assoc. 2015. doi:10.1016/j.jsha.2015.05.192.