ANTEROLATERAL MUSCLE BUNDLE OF THE LEFT-VENTRICLE IN ATRIOVENTRICULAR SEPTAL-DEFECT - LEFT-VENTRICULAR OUTFLOW TRACT AND SUBAORTIC STENOSIS

The anatomy of the left ventricular outflow tract (LVOT) in 77 hearts with atrioventricular septal defect (AVSD), 36 with a separate AV orifice and 41 with a common AV-orifice, were investigated. In all specimens, an anterolateral muscle bundle of the left ventricle was identified between the superior bridging leaflet and the left coronary aortic cusp. It displaced the attachment of the superior bridging leaflet, resulting in its clockwise rotation. The muscle bundle frequently bulged into the LVOT, but was never prominent enough to have caused significant subaortic stenosis. Measurement of the LVOT aortic ratio was possible in 54 hearts and ranged from 36-100%. In 23 cases (43%), there was mild to moderate subaortic narrowing with a ratio ranging from 53-88%. In six cases (11%), unequivocal subaortic stenosis was present, mainly in AVSD with separate AV orifices (five or six) and iatrogenic in one case with surgically corrected complete defect. A decreased ratio was mainly due to decreased anteroposterior width of the septum in the subaortic area, with anterior displacement of the superior bridging leaflet in cases with dense septal attachment of the superior bridging leaflet (i.e., in AVSD with separate AV orifices, type A complete defect with small ventricular septal defect, or surgically corrected complete defect). Significant subaortic stenosis was caused by hypertrophy of the ventricular septum in the subaortic area with anteroseptal twist in four cases, by anomalous chordal insertion of the superior bridging leaflet in one case, and iatrogenic in one case after surgical correction with left AV valve replacement in a type C complete defect. Other additional obstructive forces were an anomalous papillary muscle, a small left ventricle, and an aneurysm of the membranous septum.