**5.3 Chamber dominance**

Depending on the overall flow from the atrioventricular orifices to respective ventricles, the chambers are usually the same size which is termed as 'balanced AVSD'. When a common AV valve opens more into the right ventricle or to the left ventricle, it would cause decreased growth of the contralateral ventricle and its great artery, leading to the term 'unbalanced AVSD'. In right ventricle dominant atrioventricular septal defect, left ventricle and aorta are hypoplastic depending on the amount of blood flow, but usually, the atrial and ventricular septal alignment is maintained. In left ventricular dominance, there will be hypoplasia of the right ventricle and pulmonary artery, typically with septal malalignment. This chamber dominance when it involves the atrium would give rise to double outlet atrium (**Figure 7**).

#### **Figure 6.**

*A. Diagram in a normal heart showing aortic valve (AoV) wedged between tricuspid valve (TV) and mitral valve (MV). B. In AVSD, aorta is not wedged between these valves, termed "sprung aortic valve". PV, pulmonary valve; RAVV/LAVV, right and left atrioventricular valve. A. by Dr. Johannes Sobotta - Sobotta's atlas and text-book of human anatomy 1906, public domain, https://commons.wikimedia.org/w/index.php?curid=29901804.*

#### **Figure 7.**

*2D echocardiogram showing right ventricle (RV) dominant AVSD with severely hypoplastic left AV valve and ventricle (LV). Moderately dilated right atrium (RA) and RV with large primum ASD. LA, left atrium.*

### **5.4 Associated malformations**

In partial atrioventricular septal defects, the most common associated malformation includes secundum ASD, patent ductus arteriosus and persistent left superior vena cava to coronary sinus [12].

Tetralogy of Fallot with pulmonary stenosis is found in one-tenth of the patients with common atrioventricular septal defect and in these patients, Rastelli type C is common. Among others, common atrium, double outlet right atrium, double inlet ventricle with discordant ventriculoarterial connections can be seen.

#### **5.5 Atrioventricular conduction tissues**

In normal hearts, AV node is located in the triangle of Koch which is formed by Tendon of Todaro, coronary sinus ostium and septal leaflet of the tricuspid valve [13–15]. In patients with AVSDs, because of deficient AV septum, the atria will meet the ventricle at the crux of the heart, shifting the AV node more posteriorly and inferiorly.
