The intracardiac anatomy of right isomerism
Both atrial appendages have similar external and internal configurations. 2,3,7,12,13,30 They have a broad junction, separated by an extensive terminal groove, with the venoatrial components. Presence of pectinate muscles extending to the crux of the heart on both sides is the essential diagnostic feature. Because of the presence of bilateral morphologically right appendages, the pulmonary venous connections will always be anatomically anomalous, even when the veins connect to one or other of the atrial chambers (Figure 2). The connection is usually through a midline confluence that connects to the atrial roof (Figures 3 and 4). The channel often becomes stenotic. The superior caval veins, frequently bilateral, connect directly with the atrial roofs. The coronary sinus is universally absent. Hence, the coronary veins drain directly into the atrial cavities, usually at the level of the vestibules, but on occasion at a distance from the atrioventricular junction.36 It is most unusual to find interruption of the inferior caval vein when there is right isomerism. In some instances, all of the pulmonary veins can drain to either the right-sided or the left-sided atrial chamber, while all the systemic veins connect to the other atrial chamber. Such quasi-usual or quasi-mirror-imaged venous drainages do not detract from the fact that the atrial arrangement remains that of right isomerism.
The atrial chambers typically connect to the ventricles through a common atrioventricular junction, and often the atrial septum is no more than a myocardial strand. Although found most frequently with double inlet right ventricle, each of the atrial chambers can be connected to its own ventricles across the common junction. Half of the heart will then be concordantly connected, while the other half will be discordantly connected. The biventricular atrioventricular connection, therefore, is mixed. It is frequent in this setting to have duplicated atrioventricular nodes, producing the so-called Monckeberg sling.37 It is very unusual to find concordant ventriculo-arterial connections. Most usually the connections are double outlet right ventricle or transposition. Obstruction or atresia of the outflow tract of the morphologically right ventricle is expected. It is very rare to find a common arterial trunk, or tetralogy of Fallot. When pulmonary atresia is present, then the pulmonary arteries are usually fed through an arterial duct, which can be right or left-sided (Figure 5). The aortic arch, along with the cardiac apex, can be right-sided or left-sided.