Atrioventricular Junctions As with hearts in which the atrial appendages are lateralized, chambers with isomeric appendages can each be connected to their own ventricles, or the atriums can be connected to only one ventricle When the atrioventricular connections are biventricular, it is essential also to describe the ventricular topology This is because there are two patterns to be found, irrespective of whether the isomeric appendages are of right or left morphology In the first pattern, the right-sided atrium, whether associated with a morphologically right or left appendage, will be connected to the morphologically right ventricle and the left-sided atrium to the morphologically left ventricle This ventricular topology is right-handed (Fig 26.17, left) In the second arrangement, the rightsided atrial chamber, which again may possess either a morphologically right or left atrial appendage, will be connected to a morphologically left ventricle, and the left-sided atrium will be connected to a morphologically right ventricle In this second pattern, the ventricular topology is left-handed (see Fig 26.17B, right) When we initially defined such atrioventricular connections, we described them as ambiguous We now recognize that it is better to describe them as biventricular and mixed, proceeding always to describe the ventricular topology present and, if necessary, any abnormal and unexpected ventricular relationships Biventricular and mixed atrioventricular connections are much more frequent in the setting of isomeric left atrial appendages, with most of these patients having right-hand pattern ventricular topology FIG 26.17 Concept of biventricular and mixed atrioventricular connections The connections would still be mixed if the appendages were of left morphology bilaterally, rather than the isomeric right variant shown in Fig 26.5 Univentricular atrioventricular connections, typically with double inlet via a common atrioventricular valve, are significantly more frequent in hearts with right isomerism In about one-third of such instances, the univentricular connection will be to a morphologically right ventricle, usually left-sided, but in some it will not be possible to find a second ventricle The majority of hearts will have double inlet to a dominant morphologically left ventricle, but solitary and indeterminate ventricles are more frequent with isomerism than in any other setting Absence of the left-sided or right-sided atrioventricular connection is rare but can be found with any ventricular morphology Thus, when the atrial appendages are isomeric, any type of univentricular atrioventricular connection must be anticipated, along with any possible ventricular morphology Fewer hearts with left isomerism have univentricular atrioventricular connections, but the same variability must be expected Irrespective of the presence of biventricular or univentricular atrioventricular connections, most hearts with right isomerism have a common atrioventricular junction guarded by a common valve (see Fig 26.4) As would be expected, with such a high incidence of atrioventricular septal defects, the ventricular septum in the majority of hearts with biventricular atrioventricular connections is deformed in the anticipated fashion In the very rare cases with right isomerism but without an atrioventricular septal defect, ventricular septal defects are the rule and can be perimembranous or muscular In all hearts with univentricular atrioventricular connection, whether a right or left isomerism, those with incomplete ventricles have interventricular communications as anticipated for the ventricular morphology present Ventriculoarterial Junctions Among the entire group of hearts with isomeric atrial appendages, there is just as much variability in type and mode of ventriculoarterial connection, infundibular morphology, and arterial relationships as encountered in congenital cardiac disease as a whole.28 Certain patterns occur with significantly different frequencies in hearts with isomeric right as opposed to left appendages Pulmonary obstruction or atresia is significantly more common in association with right than with left isomerism When there is pulmonary atresia, the pulmonary supply is almost always duct dependent, although supply through systemic-to-pulmonary collateral arteries can sometimes be found Obstruction of the left ventricular outflow tract, with aortic coarctation or atresia, is much more common with left isomerism Analysis of the ventriculoarterial junctions as a whole shows significant differences between the two isomeric arrangements An anterior right-sided aorta, along with subaortic or bilateral infundibulum, is more common with right isomerism, while concordant ventriculoarterial connections are more frequent in left isomerism Conduction Tissues The morphology of the sinus node reflects the arrangement of the atrial appendages In right isomerism, there are bilateral sinus nodes, each related to the terminal crest and the cavoatrial junction in normal fashion.29 In left isomerism, in contrast, there are no terminal crests and no right atrial appendages The sinus node therefore cannot occupy its normal position Indeed, in most hearts studied, it has not proved possible to identify with certainty the sinus node.22 When a candidate for the sinus node is discovered, it tends to be grossly hypoplastic and located in the atrial wall adjacent to the atrioventricular junctions The disposition of the atrioventricular conduction axis reflects both the atrioventricular connection present and the ventricular topology When there are biventricular atrioventricular connections, the dominant feature is the ventricular topology An atrioventricular node in its regular position, with a posteroinferior penetrating atrioventricular bundle, is found with right-hand topology When there is left-hand ventricular topology, there is usually an anterior atrioventricular node, and the conduction system is found as is typical for congenitally corrected transposition Alternatively, a sling of conduction tissue is present running along the crest of the ventricular septum, producing a connection with two atrioventricular nodes With univentricular atrioventricular connection, ventricular morphology becomes the dominant feature When the left ventricle is dominant, the atrioventricular node is found in anterior position, with the bundle varying its relationship to the outflow tract of the left ventricle according to the position of the incomplete right ventricle (see Chapter 49) With connection to a dominant right ventricle, the conduction system is normally situated when the incomplete left ventricle is left-sided, but an anterior node or sling may be found