What is the difference between a balanced atrioventricular (AV) septal defect and a complete atrioventricular (AV) canal defect?

Differences Between Balanced Atrioventricular Septal Defect and Complete Atrioventricular Canal Defect

Complete atrioventricular canal defect (CAVC) and balanced atrioventricular septal defect (AVSD) are actually synonymous terms describing the same congenital heart defect. 1

Anatomical Features of Complete AVSD/CAVC

• Complete AVSD/CAVC is characterized by a common atrioventricular junction, an interatrial communication just above the atrioventricular valve, an interventricular communication just below the atrioventricular valve, and varying degrees of malformation of the left-sided component of the common atrioventricular valve 1
• The defect includes an unrestrictive interventricular communication (no interventricular pressure gradient), and bridging leaflets that usually float to varying extent within the atrioventricular septal defect 1
• The anatomical components include a primum atrial septal defect, an inlet ventricular septal defect, and a common atrioventricular valve 2, 3

The "Balanced" Descriptor

• The term "balanced" refers to the relative sizes of the ventricles and the distribution of the common atrioventricular valve between the ventricles 1, 4
• In a balanced AVSD, the right and left valve components are equally divided between the ventricles 5
• The atrioventricular valve index (AVVI), defined as the left atrioventricular valve area divided by the total atrioventricular valve area, is used to quantify balance 4
• A balanced AVSD typically has an AVVI between 0.4 and 0.6 4

Unbalanced AVSD

• Unbalanced AVSD occurs when there is dominance of either the right or left ventricle 4, 5
• Right dominant unbalanced AVSD: AVVI ≤ 0.4 4
• Left dominant unbalanced AVSD: AVVI ≥ 0.6 (less common) 4
• Extreme unbalanced cases may require single ventricle palliation rather than biventricular repair 4, 5

Classification Systems

• Rastelli classification is commonly used to describe anatomic variations of the superior bridging leaflet of the atrioventricular valve in complete AVSD 1, 2
• Three types according to Rastelli (A, B, and C) are based on the morphology of the superior leaflet of the common atrioventricular valve 2, 3
• The relative sizes of the ventricles (balanced or unbalanced) guide the type of repair (biventricular vs. single ventricle) 1

Clinical Implications

• Balanced complete AVSD typically undergoes biventricular repair 4
• Unbalanced AVSD with significant hypoplasia of one ventricle may require univentricular repair strategies 4, 5
• If not repaired early in life (typically <6 months of age), irreversible pulmonary vascular disease usually develops resulting in Eisenmenger physiology 1
• Long-term follow-up is required to monitor for left atrioventricular valve regurgitation and stenosis, left ventricular outflow tract obstruction, and arrhythmias 1

Diagnostic Approach

• Echocardiography is the primary diagnostic tool, with apical, subcostal, and parasternal views being most useful 3
• The AVVI measurement helps distinguish between balanced and unbalanced forms 4
• Cardiac catheterization is not mandatory in infants (less than 6 months) but is indicated in older patients if irreversible pulmonary hypertension is suspected 2

Treatment Considerations

• Surgical repair is typically performed between 3-6 months of life 2
• Patients with AVVI between 0.19-0.39 may have heterogeneous repair strategies (either univentricular or biventricular) with higher mortality risk 4
• Medical treatment with digitalis, diuretics, and vasodilators serves as a bridge to surgery 2