Therefore a common arterial trunk is one form of single outlet from the heart As a malformation involving the ventriculoarterial junctions, it must be anticipated to coexist with all possible segmental combinations In almost all instances, nonetheless, there will be a usual atrial arrangement, with concordant atrioventricular connections Examples can be found in combination with discordant atrioventricular connections or with absence of the right atrioventricular connection.6 Although the atrioventricular junctions themselves are usually separate, and guarded by mitral and tricuspid valves, a common trunk can rarely be found in association with an atrioventricular septal defect and a common atrioventricular valve In the presence of the common trunk, the truncal valve is almost always connected across the ventriculoarterial junctions with both ventricles, the valvar orifice overriding the ventricular septal crest, and typically with its leaflets in fibrous continuity with the mitral valve in the left ventricle (Fig 40.3) Such a biventricular connection necessitates the presence of a juxtaarterial interventricular communication The defect is generally large Its floor is the crest of the ventricular septum, reinforced on the right ventricular aspect by the limbs of the septomarginal trabeculation, or septal band, and its roof is the leaflets of the truncal valve The cone of space subtended by the truncal valve has right and left ventricular margins, but it is usually the right ventricular margin that is considered to represent the ventricular septal defect, and it is this space that is closed by the surgeon during repair In the majority of cases, fusion of the inferior limb of the septomarginal trabeculation with the ventriculoinfundibular fold along this right ventricular margin produces muscular discontinuity between the leaflets of the tricuspid and the truncal valves (Fig 40.4, left) In the absence of such fusion, there is continuity between the leaflets of the tricuspid and truncal valves, making the ventricular septal defect perimembranous (see Fig 40.4, right) When present, this muscular bar in the posteroinferior margin protects the specialized axis responsible for atrioventricular conduction In most instances, there is a large distance between the coapting arterial valvar leaflets and the crest of the septum during ventricular diastole when the leaflets are closed However, this space may sometimes be reduced or the leaflets may close directly on the septal crest (Fig 40.5, left) Some have described this latter arrangement as representing an “intact ventricular septum.”7 This is somewhat misleading because, even in this arrangement, a septal deficiency is seen at the ventricular level when the truncal valve opens during ventricular systole (see Fig 40.5, left) Furthermore, hearts are found when the ventricular septum is truly intact, the common trunk arising in most instances exclusively from the right ventricle (see Fig 40.5, right) The interventricular communication can also be restrictive when the common trunk takes an exclusive origin from one or the other ventricle Such a restrictive ventricular septal defect is more likely to produce problems when the trunk arises exclusively from the right ventricle (see Fig 40.3, right) FIG 40.3 Left, Heart sectioned to replicate the parasternal long axis echocardiographic section It shows the truncal valve overriding the crest of the muscular ventricular septum, with the valvar leaflets supported in both ventricles Note the fibrous continuity between the leaflets of the tricuspid and mitral valves Right, In contrast, this heart has the trunk exclusively supported above the morphologically right ventricle, with a completely muscular subtruncal infundibulum FIG 40.4 The presence or absence of a muscular rim along the posteroinferior margin of the interventricular communication determines whether the defect is considered to be perimembranous Left, The ventriculoinfundibular fold fuses with the posteroinferior limb of the septomarginal trabeculation (yellow bars) The muscular bar thus formed (star) protects the atrioventricular conduction axis during surgical correction Right, Defect with fibrous continuity between the leaflets of the truncal and tricuspid valves, putting conduction axis (dotted line) at potential risk during surgical correction