Balanced vs Unbalanced AVSD #4: Surgical Strategy

Management of atrioventricular septal defect (AVSD) is not determined simply by the presence of a common atrioventricular junction. The central surgical question is whether the patient’s anatomy can support a durable biventricular circulation, whether the patient should enter a single-ventricle pathway, or whether a staged strategy is required for borderline ventricular and valvar anatomy. In unbalanced AVSD, this decision is especially complex because ventricular adequacy is not defined by chamber size alone; it reflects the interaction among AV valve commitment, ventricular inflow geometry, VSD size, ventricular morphology, AV valve competence, pulmonary vascular physiology, and growth potential [1].

1. Core Surgical Decision

The management of balanced versus unbalanced AVSD is generally organized into three pathways:

1. Primary biventricular repair
2. Single-ventricle palliation
3. Staged or recruitment-based strategies for borderline anatomy

The goal is not simply to close the atrial and ventricular septal defects. The goal is to establish the circulation that best matches the patient’s ventricular capacity, AV valve anatomy, pulmonary vascular physiology, and long-term hemodynamic potential.

In balanced AVSD, both ventricles are usually capable of supporting their respective circulations. Standard biventricular repair can be performed by closing the ventricular and atrial septal components, partitioning the common AV valve, and achieving competent left and right AV valve function.

In severely unbalanced AVSD, one ventricle and its corresponding AV valve component may be too small or too poorly committed to support an independent circulation. In that setting, forced biventricular repair may result in ventricular failure, AV valve stenosis, residual or progressive AV valve regurgitation, pulmonary venous hypertension, or low cardiac output. These patients are often better served by single-ventricle palliation.

Borderline anatomy is the most difficult category. These patients may require staged palliation, ventricular recruitment, pulmonary artery banding, bidirectional Glenn as an intermediate strategy, or delayed decision-making after somatic growth and physiologic reassessment.

2. Defining Balance and Unbalance

“Balance” in AVSD refers primarily to the distribution of the common AV junction and common AV valve tissue between the right and left ventricles. However, the term also implies whether each ventricle can receive adequate inflow and generate appropriate output after septation.

A balanced AVSD has relatively symmetric AV valve commitment, adequate right and left ventricular volumes, and favorable inflow geometry. An unbalanced AVSD has preferential commitment of the common AV valve to one ventricle, producing relative hypoplasia or functional inadequacy of the contralateral ventricle.

The degree of unbalance exists on a continuum. This is why modern decision-making relies on multiple echocardiographic indices rather than a binary visual judgment alone. The atrioventricular valve index, RV/LV inflow angle, and indexed VSD each describe different aspects of the same anatomic problem, and they are not interchangeable [2–6].

3. Quantitative Indices Used in Decision-Making

Several quantitative measurements help frame the decision between biventricular repair and single-ventricle palliation. These indices should not be used in isolation. They are best interpreted as part of an integrated anatomical and physiologic assessment.

3.1 Atrioventricular Valve Index

The atrioventricular valve index (AVVI) estimates the relative commitment of the common AV valve to the left and right ventricles.

A commonly used concept is:

AVVI = left AV valve area / total AV valve area

This measurement helps classify AVSD as right-dominant, balanced, or left-dominant.

General interpretation:

• Low AVVI: right-dominant AVSD with relatively small left AV valve commitment
• Intermediate AVVI: more balanced or borderline AV valve commitment
• High AVVI: left-dominant AVSD with relatively small right AV valve commitment

In a multi-institutional Congenital Heart Surgeons’ Society study, AVVI effectively characterized the transition between balanced and unbalanced AVSD. An AVVI below 0.19 was strongly associated with single-ventricle repair, whereas values in the intermediate range were associated with heterogeneous strategies and higher risk, reflecting the clinical uncertainty of borderline anatomy [2].

The practical implication is important: AVVI provides a reproducible structural estimate of whether the smaller ventricle has enough inflow commitment to support biventricular physiology. However, AVVI alone cannot determine operability because it does not fully capture inflow direction, ventricular geometry, AV valve morphology, or the size of the ventricular septal component.

3.2 RV/LV Inflow Angle

The RV/LV inflow angle evaluates the spatial relationship between right and left ventricular inflow streams. In balanced AVSD, the inflow axes are generally more symmetric, allowing both ventricles to receive effective filling. In unbalanced AVSD, the common AV valve may be committed preferentially to one ventricle, and the inflow angle becomes more asymmetric.

A markedly abnormal RV/LV inflow angle suggests that one ventricle may receive insufficient inflow despite appearing borderline in size. This is clinically important because ventricular growth and function depend not only on chamber volume but also on the direction and adequacy of inflow.

In right-dominant unbalanced AVSD, RV/LV inflow angle has been shown to be associated with surgical pathway selection. A threshold around 114° has been proposed as a useful discriminator, with smaller angles favoring single-ventricle palliation in that cohort [4]. This does not mean that the angle alone should dictate management. Rather, it provides a geometric measure that complements AVVI and ventricular size.

3.3 Indexed VSD

The indexed VSD compares the size of the ventricular septal defect with the total AV valve area.

Conceptually:

Indexed VSD = VSD area / total AV valve area

This measurement links the ventricular septal component to the common AV valve rather than treating the VSD as an isolated defect. In right-dominant unbalanced AVSD, indexed VSD has been proposed as a method to refine decision-making in patients with mild or moderate unbalance. An indexed VSD below 0.2 has been suggested to favor biventricular repair, whereas larger values may indicate anatomy less suitable for biventricular septation, particularly when combined with low AVVI and unfavorable inflow geometry [5].

The indexed VSD is particularly useful because the size and alignment of the ventricular septal component influence how the VSD patch will partition the ventricular cavities. A patch that anatomically closes the VSD may still create physiologic obstruction if it redirects inflow poorly or leaves one ventricle functionally underfilled.

4. Why Multiple Indices Are Required

No single measurement reliably defines the correct pathway for every patient. AVVI, RV/LV inflow angle, ventricular volumes, and indexed VSD describe related but distinct components of AVSD morphology. Importantly, these indices correlate only weakly to moderately with one another, meaning that each captures a different dimension of unbalance [6].

This has direct surgical relevance. A patient may have a borderline AVVI but favorable inflow geometry and a recruitable ventricle. Another patient may have an apparently acceptable ventricular volume but severe AV valve malcommitment, dysplastic leaflet tissue, or inflow geometry that makes biventricular repair high-risk.

Therefore, pathway selection should integrate:

• AVVI
• RV/LV inflow angle
• Indexed VSD
• Right and left ventricular volumes
• Ventricular morphology and trabecular development
• AV valve leaflet quality
• Chordal attachments and papillary muscle arrangement
• Degree of common AV valve regurgitation
• Predicted left and right AV valve orifice size after partitioning
• Outflow tract obstruction
• Pulmonary venous anatomy
• Pulmonary vascular resistance
• Associated lesions, including heterotaxy and arch obstruction
• Patient age, weight, and growth potential

The decision is not simply whether the AVSD looks balanced or unbalanced. The decision is whether the reconstructed circulation will be durable, unobstructed, competent, and physiologically stable.

5. Primary Biventricular Repair

5.1 Indication

Primary biventricular repair is appropriate when both ventricles are sufficiently developed and the common AV valve can be divided into two functional valves without creating significant stenosis or regurgitation.

Important features supporting biventricular repair include:

• Adequate left ventricular volume
• Adequate right ventricular volume
• Reasonably symmetric AV valve commitment
• Acceptable left and right AV valve orifice size after partitioning
• Favorable ventricular inflow geometry
• Absence of severe ventricular hypoplasia
• Repairable AV valve leaflet morphology
• Acceptable chordal and papillary muscle arrangement
• No prohibitive pulmonary vascular disease

The operative concept is to convert a common AV junction into two competent AV valves while routing systemic venous return to the right ventricle and pulmonary venous return to the left ventricle.

5.2 Biventricular Repair in Small-Ventricle Anatomy

Selected patients with AVSD and a relatively small ventricle may still undergo successful biventricular repair when the hypoplastic chamber has adequate inflow, favorable valve morphology, and sufficient capacity for growth. Reports of biventricular repair in children with AVSD and a small right ventricle emphasize that anatomy must be assessed in detail rather than excluding biventricular repair solely because one ventricle appears small [7].

The critical issue is whether the smaller ventricle can function effectively after septation. This depends on the size of the AV valve component committed to that ventricle, the direction of inflow, the ability to avoid inflow obstruction after patch placement, and the quality of AV valve reconstruction.

5.3 Surgical Objectives

The major objectives of biventricular AVSD repair are:

1. Close the ventricular septal component

The VSD patch must be positioned to preserve adequate ventricular inflow and avoid injury to the conduction axis.

2. Close the atrial septal component

The primum ASD is closed while respecting the AV node region near the inferior margin of the atrial septum.

3. Partition the common AV valve

The superior and inferior bridging leaflets are divided and re-suspended to create separate left and right AV valves.

4. Create a competent left AV valve

The left AV valve cleft is usually closed partially or completely, depending on leaflet morphology, valve orifice size, and the risk of stenosis.

5. Avoid AV valve stenosis

Overzealous cleft closure or excessive leaflet approximation may reduce the effective left AV valve orifice, particularly in borderline left-sided anatomy.

6. Preserve the conduction system

Sutures near the inferior rim of the VSD and primum ASD must respect the location of the AV node and penetrating bundle.

The technical success of biventricular repair depends on more than septal closure. It depends on whether the surgeon can create two competent AV valves, maintain unobstructed inflow to both ventricles, preserve the conduction system, and avoid residual shunts or ventricular outflow obstruction.

6. Single-Ventricle Pathway

6.1 Indication

Single-ventricle palliation is selected when one ventricle is too small, poorly committed, or anatomically unsuitable for safe biventricular repair.

This is most commonly considered when there is:

• Severe ventricular hypoplasia
• Markedly unequal AV valve commitment
• Severe right- or left-dominant AVSD
• Inadequate left AV valve orifice for systemic output
• Inadequate right AV valve orifice for pulmonary output
• Severe malalignment of the ventricular septum
• Unfavorable inflow geometry
• Severe AV valve dysplasia or regurgitation not amenable to durable two-valve reconstruction
• Complex associated lesions making two-ventricle repair high-risk

The single-ventricle pathway is not a failure of repair strategy. It is a deliberate physiologic choice when biventricular repair would produce a circulation that is anatomically possible but functionally unstable.

6.2 Surgical Pathway

The pathway typically includes staged palliation:

1. Neonatal or infant-stage palliation

Depending on pulmonary blood flow and systemic outflow anatomy, this may include pulmonary artery banding, systemic-to-pulmonary shunt, ductal stenting, atrial septectomy, or Norwood-type reconstruction in cases with systemic outflow obstruction.

2. Bidirectional Glenn procedure

Superior cavopulmonary connection reduces ventricular volume load and prepares the pulmonary vascular bed for Fontan circulation.

3. Fontan completion

Systemic venous return is routed directly to the pulmonary arteries, leaving the dominant ventricle to support the systemic circulation.

The quality of the AV valve is critical in single-ventricle physiology. Significant common AV valve regurgitation imposes a major volume burden on the dominant ventricle and is associated with worse outcomes. In children with unbalanced AVSD undergoing single-ventricle palliation, successful AV valve repair has been associated with improved long-term survival and reduced late morbidity compared with persistent or recurrent AV valve dysfunction [9].

7. Borderline AVSD and Staged Strategies

Borderline AVSD is the most nuanced group. These patients do not clearly fit into either primary biventricular repair or definitive single-ventricle palliation at initial evaluation.

A staged strategy may be considered when the underdeveloped ventricle has potential for growth or when physiologic loading conditions may clarify its capacity. Potential strategies include:

• Pulmonary artery banding to control pulmonary overcirculation and promote ventricular remodeling
• Initial single-ventricle palliation with later reassessment
• Bidirectional Glenn as an intermediate stage
• Staged biventricular conversion after ventricular recruitment
• Delayed complete repair after somatic growth
• Hybrid approaches in complex neonatal physiology
• Conversion from single-ventricle pathway to biventricular or 1.5-ventricle repair in selected cases

The essential principle is to avoid making an irreversible decision too early when ventricular adequacy remains uncertain.

Mid-term data suggest that biventricular conversion or ventricular recruitment from a single-ventricle pathway can be achieved in selected patients, with transplant-free survival comparable to primary biventricular repair in specialized programs [8]. However, these strategies are resource-intensive and frequently require multiple reinterventions, particularly for AV valve dysfunction, residual septal defects, pulmonary artery obstruction, venous pathway obstruction, or outflow tract obstruction.

Thus, staged recruitment is not simply a way to avoid Fontan physiology. It is a complex pathway that must be justified by realistic potential for durable biventricular hemodynamics.

8. Surgical Risk of Choosing the Wrong Pathway

8.1 Risks of Forced Biventricular Repair

Attempting biventricular repair in anatomy that cannot support it may result in:

• Low cardiac output syndrome
• Left or right AV valve stenosis
• Residual or progressive AV valve regurgitation
• Pulmonary venous hypertension
• Ventricular failure
• Residual VSD
• Inflow obstruction
• Outflow tract obstruction
• Need for early reoperation
• Late conversion to single-ventricle palliation

The most problematic scenario is an anatomically completed repair that produces physiologic failure. In this situation, the patient may face the cumulative risk of failed biventricular repair, reintervention, and eventual single-ventricle palliation.

8.2 Risks of Unnecessary Single-Ventricle Palliation

Conversely, assigning a potentially recruitable ventricle to single-ventricle palliation may expose the patient to avoidable long-term Fontan physiology.

Potential long-term consequences include:

• Chronic systemic venous hypertension
• Fontan-associated liver disease
• Protein-losing enteropathy
• Plastic bronchitis
• Arrhythmia
• Exercise limitation
• Thromboembolic risk
• Ventricular dysfunction
• Need for transplantation in selected patients

Therefore, when biventricular repair is reasonably achievable, it may offer a more physiologic long-term circulation. The challenge is identifying which borderline patients can truly achieve durable biventricular repair without excessive early or late reintervention.

9. Practical Surgical Framework

A practical decision-making sequence is:

1. Define ventricular dominance

Determine whether the AVSD is balanced, right-dominant, or left-dominant.

2. Assess AV valve commitment

Use AVVI and direct anatomical assessment of leaflet distribution.

3. Assess ventricular adequacy

Evaluate ventricular volumes, morphology, trabecular development, and growth potential.

4. Assess inflow geometry

Use RV/LV inflow angle and detailed echocardiographic or three-dimensional imaging assessment.

5. Assess VSD size and alignment

Consider indexed VSD and the relationship between the septal defect, AV valve, and ventricular cavities.

6. Evaluate AV valve quality

Determine whether the common AV valve can be divided into two competent valves without stenosis or severe residual regurgitation.

7. Evaluate associated lesions

Include outflow obstruction, arch obstruction, pulmonary venous anomalies, heterotaxy, ventricular imbalance, and pulmonary vascular status.

8. Estimate pathway-specific risk

Compare the expected risk of primary biventricular repair, single-ventricle palliation, and staged recruitment.

9. Select the pathway

Choose primary biventricular repair, single-ventricle palliation, or staged strategy.

10. Plan for durability

The best operation is the one that provides stable physiology with the lowest cumulative burden of reintervention.

10. Key Surgical Principle

The optimal strategy for balanced versus unbalanced AVSD is not determined by one number. Quantitative indices such as AVVI, RV/LV inflow angle, and indexed VSD help structure the analysis, but they must be interpreted within the complete anatomical and physiologic context.

The final decision should answer one question:

Which pathway gives this patient the best chance of durable hemodynamics, acceptable AV valve function, ventricular growth, and the lowest cumulative burden of reintervention?

For clearly balanced anatomy, this usually means primary biventricular repair.

For clearly severe unbalance, this usually means single-ventricle palliation.

For borderline anatomy, the safest strategy may be staged decision-making, allowing anatomy, physiology, and growth potential to declare themselves over time.

References

[1] Overman DM, Baffa JM, Cohen MS, Mertens L, Gremmels DB, Jegatheeswaran A, McCrindle BW, Blackstone EH, Morell VO, Caldarone C, Williams WG, Pizarro C. Unbalanced atrioventricular septal defect: definition and decision making. World J Pediatr Congenit Heart Surg. 2010;1(1):91-96.

[2] Jegatheeswaran A, Pizarro C, Caldarone CA, Cohen MS, Baffa JM, Gremmels DB, Mertens L, Morell VO, Williams WG, Blackstone EH, McCrindle BW, Overman DM. Echocardiographic definition and surgical decision-making in unbalanced atrioventricular septal defect: a Congenital Heart Surgeons’ Society multiinstitutional study. Circulation. 2010;122(11 Suppl):S209-S215.

[3] Cohen MS, Jegatheeswaran A, Baffa JM, Gremmels DB, Overman DM, Caldarone CA, McCrindle BW, Mertens L. Echocardiographic features defining right dominant unbalanced atrioventricular septal defect: a multi-institutional Congenital Heart Surgeons’ Society study. Circ Cardiovasc Imaging. 2013;6(4):508-513.

[4] Arunamata A, Balasubramanian S, Mainwaring R, Maeda K, Tierney ESS. Right-dominant unbalanced atrioventricular septal defect: echocardiography in surgical decision making. J Am Soc Echocardiogr. 2017;30(3):216-226.

[5] Lugones I, Biancolini MF, Biancolini JC, de Dios AMS, Lugones G. Feasibility of biventricular repair in right dominant unbalanced atrioventricular septal defect: a new echocardiographic metric to refine surgical decision-making. World J Pediatr Congenit Heart Surg. 2017;8(4):460-467.

[6] Meza JM, Devlin PJ, Overman DM, Gremmels D, Baffa G, Cohen MS, Quartermain MD, Caldarone CA, Pourmoghadam K, DeCampli WM, Fackoury CT, Mertens L. The Congenital Heart Surgeon’s Society Complete Atrioventricular Septal Defect Cohort: baseline, preintervention echocardiographic characteristics. Semin Thorac Cardiovasc Surg. 2019;31(1):80-86.

[7] De Oliveira NC, Sittiwangkul R, McCrindle BW, Dipchand AI, Yun TJ, Coles JG, Smallhorn JF, Williams WG. Biventricular repair in children with atrioventricular septal defects and a small right ventricle: anatomic and surgical considerations. J Thorac Cardiovasc Surg. 2005;130(2):250-257.

[8] Nathan M, Emani S, IJsselhof R, Liu H, Gauvreau K, del Nido P. Mid-term outcomes in unbalanced complete atrioventricular septal defect: role of biventricular conversion from single-ventricle palliation. Eur J Cardiothorac Surg. 2017;52(3):565-572.

[9] Buratto E, Ye XT, Brizard CP, Brink J, d’Udekem Y, Konstantinov IE. Successful atrioventricular valve repair improves long-term outcomes in children with unbalanced atrioventricular septal defect. J Thorac Cardiovasc Surg. 2017;154(6):2019-2027.