FIG 40.5 Left, Dysplastic leaflets of the common truncal valve closed on the crest of the ventricular septum during ventricular diastole; the crest itself is thickened by fibrous tissue However, an interventricular communication remains during ventricular systole Note the persistently patent arterial duct This heart has a balanced arrangement of the intrapericardial components of the aorta and the pulmonary trunk Right, In contrast, this trunk arises exclusively from the right ventricle, and the ventricular septum is intact This heart has aortic dominance but with a confluent pulmonary arterial segment The truncal valve has three leaflets in approximately two-thirds of patients In most of the remaining patients, either two or four leaflets are seen guarding the common arterial orifice The leaflets are almost always in fibrous continuity with the anterosuperior leaflet of the mitral valve (see Fig 40.3, left), but there can be a completely muscular subtruncal infundibulum, particularly when the common trunk arises exclusively from the right ventricle (see Figs 40.3, right, and 40.5, right) Insufficiency of the truncal valve is not uncommon and can be caused by thickened and dysplastic leaflets or by prolapse of unsupported leaflets as a result of dilation of the ventriculoarterial junction Truncal valvar stenosis is relatively uncommon When present, it is usually because the valvar leaflets are dysplastic The greatest anatomic variability is found in the pattern of the branching of the common trunk The presence of a right-sided aortic arch, with mirror-imaged branching of the brachiocephalic arteries, is associated more often with common trunk, occurring in up to one-third of patients, than with any other congenital cardiac malformation Hypoplasia of the aortic arch, with or without coarctation, is a particularly important associated finding but is less frequent than complete interruption of the arch Such interruption is one of the major subgroups recognized in the alpha-numeric system that was suggested by Van Praagh and Van Praagh for classification.8 However, it is better to follow the suggestion made initially by the Van Praaghs, namely to categorize the defects in terms of aortic or pulmonary dominance of the common trunk When the arch is interrupted, the persistently patent arterial duct feeds the descending thoracic aorta and part of the brachiocephalic circulation, the precise proportion depending on the site of interruption As with other forms of interruption of the aortic arch (see Chapter 45), retroesophageal origin of the right subclavian artery is frequently seen Apart from those hearts with severe coarctation or interruption, or in which the pulmonary arteries are discontinuous, and one is fed through a patent duct, it is rare to find ductal patency coexisting with common arterial trunk, although it does exist (see Fig 40.5, left).9 The state of the aortic arch is possibly the most significant clinical associated malformation found with common arterial trunk It was the arrangement of origin of the pulmonary arteries, following the system proposed by Collett and Edwards, which was, in the past, most frequently used for numeric classification.5 This system largely accounts for aortic dominance, with the pulmonary arteries typically arising from the left posterolateral aspect of the aortic dominant common trunk, taking origin a short distance above the truncal valve On occasion, they can take their origin directly from a truncal arterial valvar sinus However, the arrangement in which the right and left arteries can take separate origin from the posterior aspect of the trunk, producing the socalled type III variant is the pattern found with pulmonary dominance (Fig 40.6, right) Describing aortic dominance and providing a precise description for the origin of the pulmonary arteries negate the need to resort to describing “type one and one-half.” In this regard, it is possible to find examples in which only one pulmonary artery arises from the common trunk, the other being supplied initially through a duct that became ligamentous In the clinical setting, this can present as “unilateral absence” of one pulmonary artery, but almost always the “absent” artery is identified within the hilum of the lung When this arrangement is found with common trunk, the discontinuous pulmonary artery initially fed by the duct is most frequently on the same side as the aortic arch This is in contrast to the finding in patients with tetralogy of Fallot when one pulmonary artery is absent because in this setting the discontinuous artery is more frequently on the side opposite the aortic arch In some circumstances, the pulmonary artery feeding the right lung is to the left at its origin from the common trunk relative to the artery running to the left lung The two arteries spiral as they extend to the pulmonary hilums This entity is called “crossed pulmonary arteries.” It is also possible for one pulmonary artery to arise directly from the ascending aorta, whereas the other takes its origin from the right ventricle Some call this malformation “hemitruncus.” This is incorrect because, of necessity, the hearts have separate ventriculoarterial junctions, guarded by separate aortic and pulmonary arterial valves They cannot therefore be examples of common arterial trunk FIG 40.6 According to the pattern of branching of the common trunk, cases can usually be described in terms of aortic (left) or pulmonary (right) dominance Note the interruption of the aortic arch (star) in the heart shown at right, with retroesophageal origin the right subclavian artery In rare circumstances, there can be a balanced pattern of branching (see Fig 40.5, left) VSD, Ventricular septal defect Anomalies of the origin and distribution of the coronary arteries are frequent.10 Unlike the situation when there are separate aortic and pulmonary