Juxtaposition of the Atrial Appendages Juxtaposition of the atrial appendages is a potentially confusing feature of a congenitally malformed heart In the normally constructed heart, the appendages lie one to each side of the arterial pedicle It is the morphologic nature of each appendage that determines the arrangement of the atrial chambers (see Chapter 1) Juxtaposition of the appendages is seen when both the appendages are to the same side of the arterial pedicle Such juxtaposition does not interfere with recognition of the structure of each appendage, since this feature is assessed on the basis of the extent of the pectinate muscles Therefore juxtaposition can occur in the person with the usual atrial arrangement (Fig 49.9), in the individual with a mirror-imaged arrangement, or in the presence of isomeric appendages (Fig 49.10) In the presence of the usual atrial arrangement, juxtaposition occurs most frequently when the right appendage is deviated leftward through the transverse sinus so that it lies above the left appendage (Figs 49.11 and 49.12, see also Fig 49.9) This is conventionally called left juxtaposition It tends to be associated with relatively complex anatomic lesions Noteworthy associated malformations are tricuspid atresia, hypoplasia of the right ventricle, and abnormal ventriculoarterial connections,23 such as doubleoutlet right ventricle FIG 49.9 Reconstruction made from a computed tomographic dataset prepared from a patient with concordant atrioventricular connections and double-outlet right ventricle shows the arrangement in which both atrial appendages are to the left of the arterial pedicle This is left-sided juxtaposition, albeit that the appendages are still recognized as being morphologically right (RAA) and morphologically left (LAA) Ao, Aorta; LV, left ventricle; PT, pulmonary trunk; RV, right ventricle FIG 49.10 In this computed tomographic dataset, prepared from a patient with a topsy-turvy heart, both the appendages are to the right of the arterial pedicle This is right-sided juxtaposition, but the appendages can be recognized as being morphologically right (RAA) and morphologically left (LAA) Ao, Aorta; ICV, inferior caval vein; LA, left atrium; LV, left ventricle; PT, pulmonary trunk; RV, right ventricle