**4. Classification**

There are two major types of AV septal defects: complete and partial AVSDs. Two sub-types are described: intermediate and transitional, which are variations of complete and partial AV septal defects, respectively (**Figure 4**). It is preferable to describe the features of these subtypes rather than identifying them as an entity. Different combinations of shunting across atria and ventricles could happen based on the attachments and relationship of the bridging leaflets to septal structures. In general, we would see ostium primum defect and ventricular septal defect (VSD). If the bridging leaflets are attached to the atrial septum, there could be only a ventricular level shunt (**Figure 5**). When the bridging leaflets are attached to the crest of ventricular septum, it results in an atrial level shunt with an ostium primum defect. In rare instances, where the bridging leaflets close the septal defect(s), we will still see features of the

#### **Figure 4.**

*Summary of AVSD. Anatomic and physiologic similarities between the different forms of atrioventricular septal defect (AVSD) are illustrated. Complete AVSDs have one orifice with large interatrial and interventricular communications. Intermediate defects (two orifices) are a subtype of complete AVSD. Complete AVSDs have physiology of VSDs and atrial septal defects (ASDs). In contrast, partial AVSDs have physiology of ASDs. Transitional defects are a form of partial AVSD in which a small inlet VSD is present or the ventricular level shunt has been obliterated by chordal tissue. Partial AVSDs and the intermediate form of complete AVSD share a similar anatomic feature: A tongue of tissue divides the common atrioventricular valve into distinct right and left orifices. LA, left atrium; LPV, left pulmonary vein; LV, left ventricle; RA, right atrium; RPV, right pulmonary vein; RV, right ventricle. "From: Cetta F, Truong D, Minich LL, Maleszewski JJ, O'Leary PW, Dearani JA & Burkhart HM. Chapter 29: Atrioventricular Septal Defects. In: Allen HD, editor. Moss & Adams' Heart Disease in Infants, Children, and Adolescents, Including the Fetus and Young Adult, 9th Edition. Philadelphia: Lippincott Williams & Wilkins, 2016; used with permission of Mayo Foundation for Medical Education and Research, all rights reserved."*

**Figure 5.**

*2D echocardiogram apical four-chamber view: A rare form of AVSD with large inlet ventricular septal defect (\*) without a primum atrial septal defect. Note that AV valves are at same level. RA, right atrium; RV, right ventricle; LA, left atrium; LV, left ventricle.*

common atrioventricular valve [8–10]. Complete AVSDs are classified further into three types based on the morphology of anterior bridging leaflet and is named after Giancarlo Rastelli who made significant contributions in his short career and life span:

