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Balanced vs. Unbalanced Anatomy

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In a balanced CAVSD, both lower heart chambers are large enough to pump blood, allowing for a standard two-ventricle repair. If the defect is shifted and one chamber is too small, it is unbalanced and usually requires a staged single-ventricle surgical pathway over the first few years of life.

Key Takeaways

  • • A balanced CAVSD means both lower heart chambers are large enough to support a biventricular surgical repair.
  • • An unbalanced CAVSD occurs when the hole is shifted, leaving one ventricle too small (hypoplastic) to pump blood effectively.
  • • Cardiologists use specific echocardiogram measurements like AVVI, VCR, and MACI to determine if a heart is balanced or unbalanced.
  • • Unbalanced defects typically require the single-ventricle (Fontan) pathway, which involves three staged surgeries over several years.
  • • In borderline cases, surgeons may attempt ventricular recruitment to encourage an undersized ventricle to grow.

While every Complete Atrioventricular Septal Defect (CAVSD) involves a hole in the center of the heart, one of the most important factors for your child’s future is whether the defect is balanced or unbalanced [1]. This distinction determines whether your child’s heart can be repaired to function with two pumping chambers or if it will need a different surgical path using a single pumping chamber [1][2].

Balanced CAVSD: The Two-Ventricle Path

In a balanced defect, the two lower chambers of the heart—the left ventricle and right ventricle—are both large enough and strong enough to do their respective jobs [3]. The right ventricle pumps blood to the lungs, and the left ventricle pumps blood to the rest of the body.

When a heart is balanced, surgeons can perform a biventricular repair [3]. This usually involves:

  • Using patches to close the holes [4].
  • Dividing the single large valve into two separate valves [4].
  • Outcome: After recovery, the heart functions much like a typical heart, with two separate pumping systems [5].

Unbalanced CAVSD: One Chamber is Too Small

In an unbalanced defect, the hole in the center of the heart is “shifted” to one side. Because of this shift, one of the ventricles does not develop fully and remains too small (hypoplastic) to pump blood effectively on its own [1][2].

  • Right-Dominant: The right ventricle is large, but the left ventricle is too small [6].
  • Left-Dominant: The left ventricle is large, but the right ventricle is too small [1].

How Doctors Decide

Cardiologists use specialized measurements from an echocardiogram (heart ultrasound) to determine if a ventricle is “too small” for a two-ventricle repair. They look at:

  • Atrioventricular Valve Index (AVVI): A ratio that compares how much of the common valve opens into the left side versus the right side [6][7].
  • Ventricular Cavity Ratio (VCR): A comparison of the actual size/volume of the two chambers [1][7].
  • MACI (Modified Atrioventricular Canal Index): A complex score that combines several measurements to help the surgical team predict which path is safest for the baby [8][7].

The Single-Ventricle (Fontan) Pathway

If a ventricle is significantly undersized, attempting a two-ventricle repair can be dangerous because the small chamber may fail under pressure [9][10]. In these cases, doctors use the single-ventricle pathway (often called the Fontan pathway) [11].

This is not a single “fix” but a series of three staged surgeries over several years [12]:

  1. Stage 1 (Newborn period): Ensures stable blood flow to both the lungs and the body.
  2. Stage 2 (The Glenn, 4–6 months): Connects major veins directly to the lung arteries.
  3. Stage 3 (The Fontan, 2–4 years): Completes the rerouting so the one strong ventricle only has to pump oxygen-rich blood to the body, while blood flows passively to the lungs [11].

Is there a “middle ground”?

In some “borderline” cases, surgeons may try a strategy called ventricular recruitment [13]. They perform minor procedures to encourage the small ventricle to grow, hoping that it might eventually become large enough for a two-ventricle repair later in childhood [13][12].

While the single-ventricle path is more complex and carries different long-term risks than a balanced repair, many children on this pathway grow up to attend school, participate in activities, and lead meaningful lives [14][12]. Your medical team will choose the path that offers your child the most stable and reliable heart function [15].

Frequently Asked Questions

What is the difference between balanced and unbalanced CAVSD?
In a balanced CAVSD, both lower heart chambers are large and strong enough to pump blood. In an unbalanced defect, the hole in the center of the heart is shifted, causing one chamber to be too small or underdeveloped to work effectively on its own.
How do cardiologists determine if my baby's heart is balanced or unbalanced?
Cardiologists use an echocardiogram to measure the heart chambers and valves. They calculate specific ratios, like the Atrioventricular Valve Index (AVVI) and Ventricular Cavity Ratio (VCR), to decide if the heart can safely support a two-ventricle repair.
What happens if my child has an unbalanced CAVSD?
If one ventricle is too small, doctors typically use the single-ventricle or Fontan pathway. This involves a series of three staged surgeries over several years to safely route blood flow through the single strong pumping chamber.
What do right-dominant and left-dominant mean?
These terms describe which side of the heart is larger in an unbalanced CAVSD. Right-dominant means the right ventricle is large and the left is too small, while left-dominant means the left ventricle is large and the right is too small.
Can a small ventricle grow large enough for a two-ventricle repair?
In some borderline cases, surgeons may try a strategy called ventricular recruitment. This involves minor procedures to encourage the smaller ventricle to grow, with the hope that a two-ventricle repair might be possible later in childhood.

Questions for Your Doctor

  • • Is my baby's unbalance 'right-dominant' or 'left-dominant'? Which ventricle is the smaller one?
  • • What are the specific index scores (like AVVI or MACI) from the echocardiogram that led to this diagnosis?
  • • Does our surgical team believe the smaller ventricle has any potential to grow, or is the single-ventricle pathway the most certain choice for my child?
  • • If we follow the single-ventricle path, what are the names and typical timing of the three surgeries?
  • • Are there any other heart issues, like a narrow aorta, that are contributing to the ventricle being small?

Questions for You

  • • How do I feel about the difference between a one-surgery repair versus a multi-stage surgical plan?
  • • What additional support might I need at home if my child requires a more complex, staged surgical journey?
  • • Have I written down the names of the different parts of my baby's heart that the doctor mentioned were smaller than usual?

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References

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    The Annals of thoracic surgery 2021; (112(2)):610-618 doi:10.1016/j.athoracsur.2020.06.030.

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    Long-Term Outcome Up To 40 Years after Single Patch Repair of Complete Atrioventricular Septal Defect in Infancy or Childhood.

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    Right-Dominant Unbalanced Atrioventricular Septal Defect: Echocardiography in Surgical Decision Making.

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    Can Left Atrioventricular Valve Reduction Index (LAVRI) Predict the Surgical Strategy for Repair of Atrioventricular Septal Defect?

    Schleiger A, Kramer P, Schafstedde M, et al.

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    PMID: 33580286
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    Feasibility of Biventricular Repair in Right Dominant Unbalanced Atrioventricular Septal Defect: A New Echocardiographic Metric to Refine Surgical Decision-Making.

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  9. 9

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    PMID: 28526502
  10. 10

    Left heart growth and biventricular repair after hybrid palliation.

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  11. 11

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  12. 12

    Biventricular repair as an alternative to single-ventricle palliation in a child with hypoplastic left heart structures: What the anesthesiologist should know.

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  13. 13

    Atrioventricular Inflow and Subsequent Ventricular Growth After Staged Ventricular Recruitment for Unbalanced Atrioventricular Canal Defect.

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  15. 15

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This page explains CAVSD anatomy for educational purposes only. Your pediatric cardiologist and surgical team are the best sources for evaluating your child's specific heart anatomy and surgical path.

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