Classification Most ventricular septal defects are classified by location: Perimembranous (also called conoventricular) Trabecular muscular Subpulmonary outlet (supracristal, conoseptal, doubly committed subarterial) Inlet (atrioventricular septal type, atrioventricular canal type) Perimembranous defects (70 to 80%) are defects in the membranous septum adjacent to the tricuspid valve and they extend into a variable amount of surrounding muscular tissue; the most common type of this defect occurs immediately below the aortic valve.