Atrio-ventricular septal defects (AVSDs)

The term “defects of the atrio-ventricular septum” (AVSDs) refers to a variety of cardiac anomalies characterized by the presence of a common atrio-ventricular (AV) junction . This key morphological feature differentiates AVSDs from other cardiac malformations where separate left and right AV junctions are present. Depending on whether there is a fusion of the bridging leaflets of the common AV valve, the latter has either a single common or two separate orifices within the common AV junction.

Other associated features include the presence of a trifoliate left AV valve with a zone of apposition (the term “cleft” is inaccurate) and an unwedged aorta. Abnormal position of the aortic annulus, which instead of being “wedged” between the mitral and tricuspid annuli is displaced anteriorly by the common AV valve. This results in an elongation of the left ventricular outflow tract. The spatial relationship between the atrial and ventricular septae and the bridging leaflets of the common AV valve determines whether there is a shunt at atrial and/or ventricular level. In rare cases of spontaneous AV septal defect closure, there is no shunt at any level and only an isolated trifoliate valve is present.

Despite some inaccuracies and limitations, the classification of AVSDs into a complete, partial, and transitional form ( Figure 1 ) is intentionally used in this book for simplicity. Complete AVSD is characterized by a single orifice of the common AV valve, the presence of an ostium primum atrial septal defect (ASD) and usually an unrestrictive inlet ventricular septal defect (VSD). In partial (or incomplete) AVSD, the common AV valve has two separate orifices due to partial fusion of the bridging leaflets. The left AV valve is trifoliate. The attachment of the fused bridging leaflets to the crest of the interventricular or interatrial septum (or rarely both) results in an isolated shunt at atrial or ventricular level (or rarely no shunt).

In transitional AVSD, the common AV valve has two separate orifices due to partial fusion of the bridging leaflets. The atrial component is usually large. The ventricular component is functionally restricted by aneurysmal AV valve tissue and dense chordal attachments extending from the AV valve to the crest of the interventricular septum. This form is anatomically close to a complete AVSD, but the physiology is more akin to a partial AVSD.

AVSDs account for approximately 5% of congenital heart defects and are very frequent in patients with Down syndrome. They are often associated with other anomalies such as atrial isomerism, double outlet right ventricle and tetralogy of Fallot. Corrective cardiac surgery represents the only treatment in patients with AVSDs.