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.