Andersons pediatric cardiology 1061

valves, in which virtually without exception the coronary arteries arise from one or other of the aortic valvar sinuses adjacent to the pulmonary trunk, and usually both sinuses, there is no constant pattern of sinusal origin in the presence of a common ventricular outflow tract Instead, the coronary arteries can arise from any of the truncal valvar sinuses, albeit that in most instances there are still two coronary arteries, with the left artery giving rise to anterior interventricular and circumflex branches The arteries often arise close to a zone of apposition between the valvar leaflets, and origin above the sinutubular junction is quite common This can produce potential difficulties during surgical correction should the high origin be adjacent to the origin of the pulmonary arteries Knowledge of location of the atrioventricular conduction axis is also important when planning surgical repair The sinus node and the atrioventricular node are normal in their location and structure Having taken origin from the atrioventricular node, the penetrating atrioventricular bundle pierces through the central fibrous body, and the left bundle branch originates along the left ventricular septal endocardium (see Chapter 3) The right bundle branch travels within the myocardium of the ventricular septal crest, attaining a subendocardial course at the level of the moderator band In those hearts in which a muscular bar interposes between the attachments of the truncal and tricuspid valves in the posteroinferior margin of the septal defect, the membranous septum is intact behind the muscular tissue, and the atrioventricular conduction tissues are somewhat distant from the rim of the defect (see Fig 40.3, left) In contrast, in patients in whom the ventricular septal defect is perimembranous, the conduction tissue passes directly along the left aspect of the fibrous posteroinferior rim of the defect (see Fig 40.3, right) It is then at greater surgical risk We have already mentioned most of the common associated cardiovascular anomalies found in the setting of common arterial trunk, including a right aortic arch, interrupted aortic arch, patency of the arterial duct, discontinuity of one pulmonary artery, coronary arterial anomalies, and incompetent truncal valve A defect within the oval fossa has been noted in up to one-fifth of patients, persistence of the left superior caval vein draining to the coronary sinus in up to one-tenth, and an aberrant subclavian artery in between one-tenth and one-twentieth.8 Partially anomalous pulmonary venous connection has also been reported.11 Morphogenesis The evidence now available shows that many cases of common arterial trunk result from a genetic defect The evidence comes from interpretation of morphology, experiments in animals, studies on the role of cells migrating from the neural crest in the development of the outlet components of the heart and the arterial trunks,12 and the discovery of deletions in chromosome 22q11 in patients with malformations involving the outflow tracts—so-called conotruncal defects.13,14 The morphology of common arterial trunk supports very strongly the notion that, during development, there has been failure of septation of the ventricular outlets and the outflow segment of the heart tube It had been suggested that the entity was produced in consequence of failure of formation of the subpulmonary infundibulum, with the common arterial trunk in essence representing the aorta.8 No evidence has accrued over the past decades to support this latter notion, but much has appeared to contradict it Thus studies of cardiac development, on both normal and abnormal hearts, have always shown that the initially common ventricular outflow tract is separated by the contained endocardial cushions to produce the separate arterial roots and their supporting ventricular outflow tracts.15 Failure of such separation during embryologic development was demonstrated conclusively as producing common arterial trunk in an elegant study using Keeshond dogs published as long ago as 1978.16 The subsequent studies of Kirby and colleagues showed that, when the migration of the cells from the neural crest is perturbed, the cushions do not develop properly, and one of the lesions produced is common arterial trunk.17,18 Common arterial trunk has now been produced is multiple mouse models, such as the mouse with perturbation of the Furin enzyme (Figs 40.7 and 40.8) These studies also show that the variations in patterning of the intrapericardial arterial components depends on the extent of the aortopulmonary septum, formed by the protrusion from the dorsal wall of the aortic sac (see Chapter 3) Some of the perturbed embryos had doubly committed ventricular septal defects, rather than common arterial trunk This is of particular interest because, as we have already discussed, the morphology of the outflow tracts is almost identical in the setting of a common arterial trunk and in doubly committed defects, apart from the finding of separate aortic and pulmonary valvar orifices in the latter malformations FIG 40.7 Ventriculoarterial junctions of two mouse embryos in which the Furin enzyme was perturbed Both have common arterial trunk (see Fig 40.8) Left, Two major outflow cushions have failed to fuse, but with normal formation of the intercalated (IC) cushions The situation will produce a quadrifoliate truncal valve The heart is photographed from above Right, Failure of fusion of the outflow cushions, but with absence of the aortic IC cushion, providing the potential for forming a trifoliate truncal valve This heart is photographed from the apex looking toward the cardiac base FIG 40.8 Hearts from mice in which the Furin enzyme was perturbed Both hearts have common arterial trunk, but there is a balanced arrangement of the aortic and pulmonary components of the common trunk