Anatomy and Morphogenesis As emphasized, it is the basic combinations of concordant atrioventricular and discordant ventriculoarterial connections that produce the entity we call transposition (Fig 37.1).15 The anatomic situation can be complicated by the presence of a ventricular septal defect, an obstruction within the left ventricular outflow tract, both of these malformations, or by other associated malformations Cases with an intact interventricular septum and no obstruction of the left ventricular outflow tract are usually considered simple, even if they are complicated by other lesions such as persistent patency of the arterial duct Basic Segmental Combinations The combination of segmental connections (Fig 37.2) can be found with either usual or mirror-imaged atrial arrangements (Fig 37.1) However, this specific combination cannot exist when there is isomerism of the atrial appendages Hearts with isomeric atrial appendages can, of course, have biventricular atrioventricular connections, right-hand topology, and discordant ventriculoarterial connections Such hearts are closely related to transposition as it is defined for the purposes of this chapter, but the patients with isomeric atrial appendages have grossly abnormal venoatrial connections as their major feature (see Chapter 26) In the arrangement as defined for the purposes of this chapter, the atrial anatomy is basically normal, although most frequently the oval foramen is patent, or there is a deficiency of the floor of the oval fossa Even if the flap valve overlaps the rim of the oval fossa, it is flimsy and can be ruptured easily by balloon septostomy In keeping with this normal atrial anatomy, the sinus and atrioventricular nodes are in their anticipated position FIG 37.2 (A) Morphologically right ventricle in the setting of transposition It is connected by the tricuspid valve to the morphologically right atrium and supports the aorta above a muscular infundibulum The ventricular septum is intact (B) Morphologically left ventricle from the same heart It is connected to the morphologically left atrium through a mitral valve and gives rise to the pulmonary trunk Note the fibrous continuity between the leaflets of the pulmonary and mitral valves Unlike the atrial chambers, ventricular morphology is subtly different from normal The ventricular septum is much straighter than usual, not showing the multiple curves so typical of the normal heart The pulmonary valve is not wedged as deeply between the mitral and tricuspid valves as is the aortic valve in the normal heart This, in turn, means that the area of off-setting of the leaflets of the atrioventricular valves is much less marked, as is the area occupied by the membranous septum Another consequence of this abnormal arrangement is that the ratio of the dimensions of the inlet and outlet components of the ventricular mass are abnormal in favor of the outlet dimension, although not to the extent seen in atrioventricular septal defects (see Chapters 31 and 32) At birth, the walls of the morphologically left ventricular wall are marginally thicker than those of the right ventricle The right ventricular mural thickness then rapidly increases in the first 2 years of life, becoming much thicker than that of the left ventricle The most obvious external abnormality is the relationship of the aorta to the pulmonary trunk In the majority of patients with an intact ventricular septum, the aortic root is to the right of the pulmonary trunk in hearts in the setting of usual atrial arrangement (Fig 37.2) and to the left in the mirror-imaged variant (Fig 37.3, left panel) Patients are found, nonetheless, with usual atrial arrangement and intact ventricular septum when the aorta is to the left (Fig 37.3, right panel) Rarely, the aorta may be right sided and posterior, even when the ventricular septum is intact