ventricular septal defect muscular ventricular septal defect mVSD ventricular septal defect, muscular 2010-09-27T04:13:52Z VSD, Type 4 abnormal communications between the two lower chambers of the heart, involving the muscular septum and often occurring as multiple communications, and includes central muscular or midmuscular, apical, or marginal communications when the defect is along the RV-septal junction; any single defect observed from the LV aspect may have several openings on the RV aspect csmith MPheno.ontology Swiss-cheese septum trabecular ventricular septal defect MP:0010420 Fyler:1320 abnormal interventricular septum muscular part morphology