Balanced vs Unbalanced AVSD — From Anatomy to Strategy

Balanced versus unbalanced atrioventricular septal defect should be understood not as a purely descriptive label, but as a question of whether the common atrioventricular junction can be partitioned into a durable two-ventricle circulation. The classic morphometric echocardiographic work established that “unbalance” exists on a spectrum, because the degree of AV-valve commitment and ventricular adequacy varies continuously rather than dichotomously from patient to patient [1].

The most influential quantitative advance was the atrioventricular valve index (AVVI), which provided a practical way to translate that spectrum into surgical decision-making. In the major multi-institutional study, AVVI became the most validated single metric for classifying right-dominant and left-dominant unbalance; in general, values of ≤0.4 suggest right-dominant unbalance, values of ≥0.6 suggest left-dominant unbalance, and very low values were strongly associated with univentricular management [2].

Subsequent conceptual reviews refined the idea further: an AVSD should be considered surgically unbalanced when ventricular size, AV-valve commitment, inflow geometry, or associated obstruction make an anatomically feasible and physiologically sustainable biventricular repair unlikely over time. In other words, the operative goal is not to force a two-ventricle repair, but to choose the circulation that best matches the patient’s anatomy, valve function, growth potential, and long-term durability [3,4].

This distinction is especially important in right-dominant unbalanced AVSD, where the left ventricle may be small but still recruitable. Earlier surgical experience showed that a small right or left ventricular component is not, by itself, an absolute contraindication to biventricular repair; rather, repair becomes reasonable when inlet size, ventricular geometry, and valve commitment indicate that the smaller ventricle can support meaningful growth and flow after septation [5].

Quantitative Assessment of Ventricular Balance

A modern textbook description should emphasize that no single measurement defines the whole lesion. Instead, each index captures a different part of the same geometric problem.

1. Atrioventricular valve index

AVVI remains the most useful starting point because it directly quantifies how much of the common AV valve is committed to the left side:

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

Its strength lies in its simplicity and its external validation, but it should never be interpreted in isolation. AVVI must be read together with AV-valve regurgitation, leaflet morphology, papillary support, ventricular function, and outflow adequacy [2-4].

2. Indexed VSD

In borderline right-dominant lesions, AV-valve commitment alone may not fully capture whether septation is realistic. Lugones and colleagues proposed the indexed ventricular septal defect, which scales VSD size to overall AV-valve diameter and therefore incorporates the ventricular septal component into the balance assessment. Their work suggested that an indexed VSD <0.2 supports biventricular feasibility, whereas intermediate values require careful integration with the rest of the anatomy rather than stand-alone decision-making [6].

3. RV/LV inflow angle

The RV/LV inflow angle adds a more spatial view of the lesion by asking how the common AV inflow is actually directed into the two ventricles. In right-dominant unbalanced AVSD, a smaller systolic inflow angle reflects more unfavorable alignment for a two-ventricle pathway. In the Stanford series, an RV/LV inflow angle ≤114° in systole showed very high sensitivity for selection of a single-ventricle pathway in the validation cohort, making it a particularly useful adjunct in difficult borderline cases [7].

An important later insight is that these measurements are complementary rather than interchangeable. In the Congenital Heart Surgeons’ Society complete AVSD cohort, AVVI, LV inflow index, and RV/LV inflow angle correlated only weakly to moderately with one another and with common AV-valve or ventricular dimensions. That finding is clinically important: two patients may have similar AVVI values but very different inflow geometry, ventricular capacity, and repairability [8].

How the Surgical Pathway Should Be Chosen

The surgical strategy is best organized into three pathways.

1. Primary biventricular repair is favored when both ventricles are adequate, AV-valve commitment is reasonably balanced, and septation can be achieved without creating inflow obstruction, outflow obstruction, or major residual valve dysfunction [2-5].
2. Primary single-ventricle palliation is appropriate when one ventricle is too small or too poorly recruited, when inflow is severely maldistributed, or when AV-valve and septal reconstruction would predictably create an unstable or nondurable two-ventricle circulation [3,4,7].
3. Staged or borderline strategies are most relevant when the anatomy sits between these extremes. In this group, the surgeon’s task is not to declare the patient permanently “biventricular” or “single ventricle” at first glance, but to decide whether time, palliation, and growth may shift the balance toward a more durable circulation [4,5].

This staged mindset is supported by outcome studies showing that biventricular conversion after initial single-ventricle palliation is a legitimate strategy in selected patients with unbalanced complete AVSD. Mid-term outcome data established that such conversion belongs within the treatment algorithm rather than outside it, especially when initial neonatal anatomy does not safely permit primary biventricular repair [9].

More recent staged ventricular recruitment data strengthened that concept by showing that even severe forms of unbalanced atrioventricular canal can be converted after prior single-ventricle palliation, with acceptable mortality, although the price is often a higher burden of reintervention. In other words, staged recruitment is not an easier substitute for primary repair; it is a deliberate strategy to create later biventricular feasibility when that feasibility is not present at the outset [10].

Borderline Anatomy and Contemporary Refinement

For borderline cases, the most accurate contemporary statement is this: AVVI still has the strongest evidence base as the first-line classifier, but the final decision should be multimodal. Valve competence, leaflet tissue quality, papillary muscle architecture, ventricular volume, inlet alignment, VSD geometry, outflow tract adequacy, and associated lesions all matter. The literature now supports using AVVI as the anchor metric and then refining judgment with complementary measurements such as indexed VSD and RV/LV inflow angle, rather than searching for a single “perfect” cutoff [2,6-8].

A particularly important recent advance is the addition of cardiac magnetic resonance imaging in selected borderline patients. In a 2024 study, MRI-derived end-diastolic volume index and LV-RV angle in diastole improved prediction of successful primary biventricular repair, and the best multimodality model combined MRI volume data with echocardiographic AVVI. This does not replace echocardiography, but it does suggest that volumetric imaging can sharpen decision-making when echocardiographic geometry is ambiguous [11].

Accordingly, the most defensible modern formulation is that balanced AVSD is the substrate in which two ventricles can be expected to support a durable circulation after repair, whereas unbalanced AVSD is the substrate in which one ventricle, one AV-valve component, or the inflow geometry is insufficient for that expectation. Borderline anatomy should therefore be approached as a problem of integrated physiology and surgical design, not of threshold values alone [3,4,8-11].

Practical Teaching Summary

1. Balanced AVSD means that both ventricles and both inflow streams are adequate for a durable biventricular repair, not merely that both ventricles are present [1-4].
2. Unbalanced AVSD is defined by clinically meaningful asymmetry in ventricular adequacy and/or AV-valve commitment, usually with right-dominant or left-dominant physiology [2-4].
3. AVVI is the most validated single metric, but it should be interpreted together with valve competence, ventricular size, inflow geometry, and associated lesions [2-4,8].
4. Indexed VSD and RV/LV inflow angle are valuable adjuncts, especially in right-dominant and borderline lesions where AVVI alone may underdescribe the true difficulty of septation [6,7].
5. The best pathway is the one that is anatomically achievable and physiologically sustainable over time—whether that is primary biventricular repair, primary single-ventricle palliation, or staged recruitment with later biventricular conversion [4,9-11].

References

[1] Cohen MS, Jacobs ML, Weinberg PM, Rychik J. Morphometric analysis of unbalanced common atrioventricular canal using two-dimensional echocardiography. J Am Coll Cardiol. 1996;28(4):1017-1023.

[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] 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.

[4] Overman DM, Dummer KB, Moga FX, Gremmels DB. Unbalanced atrioventricular septal defect: defining the limits of biventricular repair. Semin Thorac Cardiovasc Surg Pediatr Card Surg Annu. 2013;16(1):32-36.

[5] De Oliveira NC, Sittiwangkul R, McCrindle BW, Dipchand A, Yun TJ, Coles JG, Caldarone CA, Williams WG, Van Arsdell GS. 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.

[6] 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.

[7] Arunamata A, Balasubramanian S, Mainwaring R, 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.

[8] 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.

[9] Nathan M, Emani S, IJsselhof R, Liu H, Gauvreau K, del Nido PJ. 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.

[10] Oh NA, Doulamis IP, Guariento A, Piekarski B, Marx GR, del Nido PJ, Emani SM. Staged ventricular recruitment and biventricular conversion following single-ventricle palliation in unbalanced atrioventricular canal defects. JTCVS Open. 2023;13:278-291.

[11] Jones AL, White BR, Ghosh RM, Mondal A, Ampah S, Ho DY, Whitehead K, Harris MA, Biko DM, Partington S, Fuller S, Cohen MS, Fogel MA. Cardiac magnetic resonance predictors for successful primary biventricular repair of unbalanced complete common atrioventricular canal. Cardiol Young. 2024;34(2):387-394.