FIG 49.11 Cross-sectional echocardiogram, in the short axis of the heart, showing left juxtaposition of the right atrial appendage (RAA) in the setting of double-outlet right ventricle (RV) Note that the morphologically right appendage has passed through the transverse sinus to lie alongside the morphologically left appendage (LAA) LA, Left atrium; RA, right atrium FIG 49.12 Subcostal four-chamber echocardiogram from the patient shown in Fig 49.11 showing that the right atrial appendage (RAA) is displaced to the right Note that the roof of the right atrium is indented by the arterial trunk (arrows), in this case the pulmonary trunk (PT) LA, Left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; SCV, superior caval vein Sometimes the juxtaposition is only partial, with a small pouch of right appendage continuing to protrude forward on the right side of the arterial trunk The juxtaposition distorts the internal morphology of the atrial septum On opening the relatively small atrial surface, the orifice of the juxtaposed appendage tends to occupy the anticipated site of the oval fossa The oval fossa is more slit-like, and is deviated posteroinferiorly The distortion of the right atrium can also produce potential problems in atrial redirection procedures, albeit that nowadays these are usually performed only as part of the double switch procedure, and this is rarely undertaken in patients with juxtaposition Another consequence of the juxtaposition is to distort the landmarks to the sinus node The terminal groove is orientated in horizontal rather than vertical fashion, and the sinus node itself is deviated toward the atrioventricular junction Much more rarely, the appendages can be juxtaposed to the right of the arterial pedicle when there is the usual atrial arrangement The left appendage occupies the transverse sinus, coming to lie superiorly relative to the right appendage (see Fig 49.10) In some cases, right juxtaposition is found in hearts when the only associated lesion is an atrial septal defect In other instances, it may accompany exceedingly complex lesions In the person with a mirror-imaged arrangement, of course, right-sided juxtaposition is the expected variant It is associated with the same constellation of lesions as is left-sided juxtaposition in the individual with the usual atrial arrangement When there is left juxtaposition, a straight or concave right lower heart border, and a prominent bulging contour of the left mid-heart border, are seen on the chest radiograph This is because the right atrial appendage is not in its usual place but is displaced to the left (Fig 49.13) The diagnosis is now readily made with cross-sectional echocardiography (see Figs 49.10 and 49.11), magnetic resonance imaging, or computed tomography During cardiac catheterization, if the diagnosis has not already been made by echocardiography, the first pointer to the diagnosis may be the sampling of systemic venous blood from the left side of the heart in a low-pressure zone If necessary, selective injection of contrast medium into the appendage will establish the diagnosis