Balanced vs Unbalanced AVSD #2: AV Valve Area and Ratio (AVVI)

In complete atrioventricular septal defect (AVSD), the common atrioventricular junction is shared by both ventricles, but the common AV valve is not always equally committed to the right and left ventricles. The degree of AV-valve commitment is one of the central determinants of whether the anatomy can support a durable biventricular circulation.

The atrioventricular valve index (AVVI) is a practical echocardiographic measure that quantifies this relationship. It is defined as:

AVVI = left AV-valve area / total AV-valve area

Because AVVI reflects how much of the common AV-valve orifice is committed to the left ventricle, it provides a useful framework for classifying complete AVSD as balanced, right-dominant, or left-dominant. In the Congenital Heart Surgeons’ Society multiinstitutional study, AVVI was validated as a discriminator between balanced and unbalanced complete AVSD and was associated with surgical strategy and outcome. In that framework, an AVVI of 0.4–0.6 generally represents balanced AVSD, ≤0.4 suggests right-dominant unbalanced AVSD, and ≥0.6 suggests left-dominant unbalanced AVSD [1].

However, AVVI should not be interpreted as an isolated surgical rule. It must be integrated with ventricular size, ventricular geometry, inflow angle, AV-valve morphology, AV-valve regurgitation, ventricular function, outflow tract adequacy, and associated lesions.

1. Why AV-Valve Commitment Matters

The common AV valve is the principal inflow structure in complete AVSD. Therefore, its distribution across the ventricular septum strongly influences ventricular filling.

In a balanced AVSD, the common AV valve is reasonably committed to both ventricles. Both ventricles receive adequate inflow, develop sufficient cavity volume, and can usually support biventricular repair. The surgical objective is to close the atrial and ventricular septal components while dividing the common AV valve into competent left and right AV valves.

In an unbalanced AVSD, the common AV valve is preferentially committed to one ventricle. The dominant ventricle receives most of the inflow, whereas the contralateral ventricle may be small, underfilled, or geometrically unfavorable. The clinical question then shifts from “Can the septal defects be closed?” to “Can the smaller ventricle support its assigned circulation after septation?”

This distinction is essential. A ventricle may appear present anatomically but remain functionally inadequate if its inflow is restricted, its compliance is poor, its outflow is obstructed, or its associated AV-valve component is too small to support physiologic flow.

2. Interpretation of AVVI

AVVI provides a quantitative estimate of left-sided AV-valve commitment:

• AVVI around 0.5: relatively symmetric AV-valve commitment
• AVVI ≤0.4: right-dominant AVSD, with relatively reduced left AV-valve commitment
• AVVI ≥0.6: left-dominant AVSD, with relatively reduced right AV-valve commitment

In the landmark multiinstitutional cohort, AVVI correlated with the operative pathway. Most patients underwent biventricular repair, but those with extremely low AVVI were consistently managed along a univentricular pathway; specifically, patients with AVVI <0.19 uniformly underwent univentricular repair in that dataset [1]. This illustrates that AVVI is most decisive at the extremes of imbalance.

The gray zone is more difficult. Patients near the 0.4 or 0.6 thresholds require detailed anatomic assessment rather than automatic assignment to biventricular repair or single-ventricle palliation. In this borderline group, surgical decision-making depends on whether the smaller ventricle has adequate inflow, volume, compliance, outflow, and growth potential.

3. Right-Dominant AVSD

In right-dominant unbalanced AVSD, the common AV valve is predominantly committed to the right ventricle. The left AV-valve component is relatively small, and the left ventricle may be underfilled or hypoplastic.

Important anatomic concerns include:

• Small left AV-valve component
• Reduced left ventricular inflow
• Small or borderline left ventricular cavity
• Potential left ventricular outflow tract obstruction
• Coarctation or arch hypoplasia
• Risk of postoperative left AV-valve stenosis after biventricular repair

The key surgical question is whether the left ventricle can accept pulmonary venous return and generate systemic output after septation. If the left ventricle is inadequate, biventricular repair may lead to pulmonary venous hypertension, elevated left atrial pressure, low cardiac output, residual left AV-valve stenosis, or early reintervention.

Additional echocardiographic indices help refine this decision. In a multiinstitutional analysis of right-dominant unbalanced AVSD, the right ventricle/left ventricle inflow angle emerged as an important marker of inflow geometry. A more acute or unfavorable inflow relationship reflects malalignment of the AV junction and may identify patients less suitable for biventricular repair [2]. A later study focusing on surgical decision-making in right-dominant AVSD found that AVVI and RV/LV inflow angle in systole were reproducible measures associated with operative strategy. In that selected right-dominant cohort, AVVI ≤0.70 and RV/LV inflow angle ≤114° in systole were sensitive markers for single-ventricle palliation [3]. These values should be interpreted only within the context of right-dominant AVSD and should not replace the general 0.4/0.6 AVVI classification.

4. Left-Dominant AVSD

In left-dominant unbalanced AVSD, the common AV valve is preferentially committed to the left ventricle. The right AV-valve component is relatively small, and the right ventricle may be underfilled or hypoplastic.

This pattern is less common than right-dominant imbalance but remains surgically important. The central question is whether the right ventricle can support the pulmonary circulation after septation.

Potential problems include:

• Small right AV-valve component
• Restricted right ventricular inflow
• Small right ventricular cavity
• Abnormal right ventricular geometry
• Risk of right AV-valve stenosis
• Inadequate pulmonary blood flow after biventricular repair

In left-dominant AVSD, the surgical threshold for biventricular repair depends not only on right ventricular size but also on the feasibility of creating an unobstructed right-sided inflow pathway. If the right AV-valve component is too small or the subvalvar apparatus is unfavorable, repair may create functional right AV-valve stenosis despite technically successful septation.

5. AVVI Is Useful but Incomplete

AVVI is valuable because it is simple, intuitive, and directly related to AV-valve commitment. Nevertheless, it captures only one morphologic dimension of unbalance.

A large core-lab echocardiographic analysis of complete AVSD demonstrated that commonly used indices of unbalance correlate only weakly to moderately with one another. This finding is important because AVVI, ventricular dimensions, valve annular measurements, and inflow geometry are not interchangeable; each describes a different component of the same complex anatomy [4].

Therefore, AVVI should be used as part of a structured multiparametric assessment:

1. AV-valve commitment

AVVI estimates the relative distribution of the common AV-valve orifice.

2. Ventricular dimensions and volume

The non-dominant ventricle must have adequate cavity size and growth potential.

3. Inflow geometry

Favorable inflow alignment may support biventricular repair even when valve commitment is borderline.

4. AV-valve morphology

Leaflet dysplasia, bridging leaflet configuration, chordal attachments, and papillary muscle arrangement influence repairability.

5. AV-valve regurgitation

Significant regurgitation may distort functional assessment and worsen postoperative risk.

6. Outflow tract adequacy

The smaller ventricle must have a suitable outflow pathway for the intended circulation.

7. Associated lesions

Coarctation, arch hypoplasia, pulmonary stenosis, double-outlet right ventricle, heterotaxy, and pulmonary venous anomalies may substantially alter strategy.

6. Surgical Implications

The operative pathway in complete AVSD is generally organized into three broad strategies:

1. Primary biventricular repair
2. Single-ventricle palliation
3. Staged recruitment or delayed biventricular conversion in borderline anatomy

In clearly balanced AVSD, primary biventricular repair is usually appropriate. The surgeon closes the VSD and primum ASD components and reconstructs the common AV valve into separate left and right AV valves.

In severely unbalanced AVSD, single-ventricle palliation may be the safest pathway, especially when the non-dominant ventricle is markedly hypoplastic or the corresponding AV-valve component is severely deficient.

The borderline group is the most difficult. Surgical decision-making must balance the long-term advantages of biventricular circulation against the risks of forcing an inadequate ventricle into a circulation it cannot support. Reports of biventricular repair in selected patients with small ventricles show that repair can be feasible when anatomy is favorable. In children with AVSD and a small right ventricle, biventricular repair achieved acceptable long-term survival in selected patients, although low AVVI values required caution and careful anatomic review [5].

The fundamental surgical objective is not merely to divide the common AV valve. The goal is to create two unobstructed inflow pathways, two competent AV valves, adequate ventricular outputs, and acceptable long-term freedom from reoperation.

7. Practical Surgical Interpretation

A practical approach to AVVI is:

• AVVI 0.4–0.6

Usually supports balanced AVSD physiology, but repairability still depends on AV-valve morphology, ventricular size, and associated lesions.

• AVVI ≤0.4

Suggests right-dominant AVSD. The surgeon must determine whether the left AV valve, left ventricle, LVOT, aortic valve, and aortic arch can support systemic circulation.

• AVVI ≥0.6

Suggests left-dominant AVSD. The surgeon must determine whether the right AV valve, right ventricle, RVOT, and pulmonary circulation can support a biventricular repair.

• Extreme AVVI values

Strongly suggest single-ventricle palliation unless there is compelling evidence of recruitable ventricular capacity.

• Borderline AVVI values

Require integrated decision-making using ventricular volume, inflow angle, valve morphology, function, and institutional experience.

A concise operative principle is:

AVVI tells us where the inflow is committed. It does not, by itself, prove that the smaller ventricle can support the circulation.

This is why unbalanced AVSD remains one of the most nuanced lesions in congenital cardiac surgery. Severe imbalance is usually straightforward; mild or borderline imbalance is where surgical judgment becomes critical. The transition between biventricular feasibility and single-ventricle suitability is not defined by one number alone [6].

Summary

AVVI is a central quantitative tool in the evaluation of balanced versus unbalanced complete AVSD. It is calculated as the left AV-valve area divided by the total AV-valve area. Values between 0.4 and 0.6 generally indicate balanced AV-valve commitment, whereas values ≤0.4 suggest right-dominant AVSD and values ≥0.6 suggest left-dominant AVSD.

The strength of AVVI is that it directly describes AV-valve commitment, one of the core determinants of ventricular inflow. Its limitation is that it does not fully describe ventricular adequacy, inflow geometry, valve morphology, valve competence, outflow anatomy, or associated lesions.

Therefore, AVVI should be interpreted as a decision-support metric within a broader surgical framework. The final decision between biventricular repair, single-ventricle palliation, and staged recruitment must integrate valve anatomy, ventricular size and function, inflow geometry, outflow suitability, associated lesions, and expected long-term durability.

References

[1] 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.

[2] Cohen MS, Jegatheeswaran A, Baffa JM, Gremmels DB, Overman DM, Caldarone CA, McCrindle BW, Blackstone EH, Williams WG, Morell VO, Mertens L, Pizarro C. 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.

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

[4] 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.

[5] De Oliveira NC, Sittiwangkul R, McCrindle BW, Dipchand A, Yun TJ, Coles JG, Smallhorn JF, Van Arsdell GS, 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(1):250-257.

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