Transitional AV Canal (aka intermediate AV canal)

The term transitional AV canal is variably defined; however, it is considered a variant of a partial AV canal.  A transitional AVSD has two separate AV valve annuli.  In addition to a primum ASD and a cleft mitral valve, there is often a small inlet VSD that is often restricted or obliterated by dense chordal attachments of the AV valves to the crest of the ventricular septum.  Therefore, the effective ventricular level shunt is typically trivial to small.  As a result of the effectively small ventricular defect, physiologically, a transitional atrioventricular canal behaves more like a partial atrioventricular septal defect.