Balanced vs Unbalanced AVSD #3: AV Valve Ratio and the VSD Component

1) Why the VSD component matters (beyond AV-valve partitioning)

In complete AVSD, AV-valve commitment/partitioning is the primary “inflow territory” signal and remains the anchor for judging biventricular (BiV) feasibility. The Congenital Heart Surgeons’ Society (CHSS) multi-institutional experience helped operationalize this concept by linking AVVI to surgical strategy across a large cohort, while highlighting a “gray zone” where outcomes are heterogeneous and decision-making is most difficult [1].

However, VSD-related geometry adds orthogonal information because it affects how inflow is distributed in 3D, not only how much inflow is available. Practically, it informs:

• Inflow–cavity alignment: whether diastolic inflow vectors align with each ventricular long axis.
• Functional “access” to a ventricle: whether streaming preferentially reaches one ventricle even when nominal valve area seems adequate.
• Septation mechanics: whether VSD/bridging leaflet geometry predicts an unfavorable baffle, inflow restriction, or postoperative distortion.

Key concept:

AVVI estimates partitioned inflow capacity; the VSD component metrics estimate inflow directionality + septation geometry—especially relevant in borderline anatomy [1–3].

2) AVVI remains the anchor metric (where this “#3” topic plugs in)

2.1 Definition (typical clinical use)

• AVVI (classic concept): left AV-valve area / total AV-valve area
• Many centers use a modified AVVI (mAVVI) depending on measurement conventions; be explicit in your protocol [1,4].

2.2 Strategy signal (CHSS experience)

• Extremely low AVVI strongly tracks univentricular repair (e.g., very small left-sided commitment) [1].
• Intermediate AVVI values represent the decision “stress test” zone, where additional geometric markers can refine interpretation [1,3,5].

Why you care: this is exactly where RV/LV inflow angle and indexed VSD become most valuable—when AVVI alone is not decisive.

3) RV/LV inflow angle (a geometry-based discriminator)

3.1 What it is

The RV/LV inflow angle is a reproducible echocardiographic surrogate for inflow alignment across the common AV valve and its relationship to the interventricular communication. Conceptually:

• More “open/symmetric” alignment → greater likelihood of functional balance.
• More acute/misaligned configuration → tends to behave as dominant-ventricle physiology.

In a multi-institutional analysis of right-dominant unbalanced AVSD, geometric clustering suggested that non-area variables can meaningfully stratify the “unbalanced spectrum,” not merely label it [4].

3.2 Threshold commonly taught

• RV/LV inflow angle ≤ 114° (systole) has been reported to correlate with clinical selection of a single-ventricle pathway, with high sensitivity in a small validation cohort [2].
• The inflow angle also performed as a strong discriminator among candidate metrics in comparative modeling [3].

3.3 How to measure it (to keep it teachable + reproducible)

Standardize three items:

1. Imaging plane: a consistent 4-chamber equivalent (apical/subcostal) that clearly shows AV valve hinge points and ventricular long axes.
2. Vector definition: define the two inflow axes consistently (e.g., AV valve plane → ventricular apex/long axis).
3. Cardiac phase: match phase across studies (the published threshold above references systolic measurement in that series) [2].

Interpretive cautions

• Severe AV-valve regurgitation, loading conditions, and acoustic limitations can shift apparent vectors. Treat the inflow angle as supportive evidence, not a standalone rule [2–4].

4) Indexed VSD (relative VSD dominance vs total AVV size)

4.1 What it is (be explicit about your definition)

Two related definitions appear in the literature and in teaching figures—choose one for your textbook and list the alternative:

• Diameter-based indexed VSD:

Indexed VSD = VSD diameter / total AVV diameter [5]

• Area-based indexed VSD (conceptually similar):

Indexed VSD = VSD area / total AVV area (not numerically interchangeable with the diameter ratio)

4.2 Suggested thresholds (for the decision-support role)

A proposed framework links indexed VSD to surgical feasibility, particularly in mild–moderate unbalance:

• < 0.2: tends to support BiV repair
• 0.2–0.35: “conditional BiV” (depends on global anatomy/physiology)
• 0.35–0.5: consider alternative strategies
• > 0.5–0.55: univentricular palliation may be reasonable [5]

4.3 Physiologic intuition (why the ratio helps)

• A smaller indexed VSD implies the ventricular septal component is less dominant relative to total inflow orifice size—often compatible with more controlled septation geometry.
• A larger indexed VSD may reflect a configuration where interventricular communication and streaming are “too dominant,” increasing the likelihood that septation will create inflow obstruction or unfavorable baffle geometry unless other features are strongly favorable [5].

5) How to integrate AVVI + VSD metrics into a surgical decision framework

A practical, “textbook-usable” approach is three-layer integration:

Layer 1 — Inflow territory (anchor)

• AVVI / mAVVI: defines how much AV inflow is committed to each side [1,4].

Layer 2 — VSD geometry (this slide’s focus)

Use both:

• RV/LV inflow angle (alignment/streaming surrogate) [2–4]
• Indexed VSD (relative VSD dominance) [5]

Clinical use-case:

When AVVI is borderline, these metrics help interpret whether the heart will behave balanced or unbalanced after septation.

Layer 3 — Global feasibility + durability checks

Even if Layers 1–2 look “BiV-friendly,” confirm durability drivers:

1. Ventricular adequacy (volume/length, apex formation, filling pressures when available) [2,4]
2. Outflow tract suitability (LVOT/RVOT constraints, arch issues, rerouting complexity)
3. AV-valve reparability (leaflet/chordal morphology; risk of residual regurgitation)
4. Patient-specific modifiers (heterotaxy, pulmonary vascular status, timing)

Principle: A metric supporting BiV feasibility matters only if the expected BiV circulation is durable—low risk of late inflow/outflow obstruction or progressive AV-valve dysfunction.

6) What “latest practice” adds: staged strategies and evolving endpoints

Contemporary management increasingly recognizes that strategy is not always binary at birth:

• BiV conversion / staged ventricular recruitment after initial single-ventricle palliation can achieve transplant-free survival comparable to primary BiV repair in selected cohorts, albeit with a higher reintervention burden in some groups [6].
• This underscores a modern reframing: initial pathway choice should consider the possibility of future conversion when anatomy/physiology is likely to evolve favorably.

7) Summary (high-yield takeaways)

1. AVVI is the anchor for inflow partitioning, but the “gray zone” requires additional geometry [1].
2. RV/LV inflow angle operationalizes inflow alignment; ≤114° has been associated with SV selection in published cohorts [2–3].
3. Indexed VSD contextualizes the VSD component relative to AVV size and may refine decision-making in mild–moderate unbalance [5].
4. Final decisions should be integrative, prioritizing durability, AV-valve reparability, and the feasibility of staged options when appropriate [4–6].

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):S209-15.

[2] 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.

[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] 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] 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] 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.

[7] Schleiger A, Gitter R, Gorenflo M, Loukanov T, Abdul-Khaliq H. Can Left Atrioventricular Valve Reduction Index (LAVRI) Predict the Surgical Strategy for Repair of Atrioventricular Septal Defect? Pediatr Cardiol. 2021;42(7):1541-1552.

[8] Overman DM. Decision-Making in Unbalanced Atrioventricular Septal Defect: Examining Another Piece of the Puzzle. World J Pediatr Congenit Heart Surg. 2017;8(4):468-469.