FIG 31.29 Subcostal image from a patient with a free-floating superior bridging leaflet (Rastelli type C defect) and tetralogy of Fallot The aorta is overriding the ventricular septal defect, which is limited on one side by the superior bridging leaflet and on the other side by the aortic valve Ao, Aorta; lv, left ventricle; rv, right ventricle; sbl, superior bridging leaflet The other important diagnostic feature is again the trifoliate arrangement of the left component of the common atrioventricular valve, as demonstrated in short-axis sections of the left ventricular inlet The elongated, or gooseneck, shape of the left ventricular outflow tract is less obvious in those with the Rastelli C malformation than in those with separate right and left atrioventricular valvar orifices or with the Rastelli A malformation, since the superior bridging leaflet is unattached to the septal crest (Fig 31.30) FIG 31.30 Angulated subcostal long-axis section taken from a patient with a Rastelli type A defect (A) The gooseneck malformation of the left ventricular outflow tract is readily appreciated The gooseneck is far less apparent in (B), taken from a patient with a Rastelli type C defect Ao, Aorta; lv, left ventricle; lvot, left ventricular outflow tract; rv, right ventricle Possible variations in the morphology of the left part of the atrioventricular valve, albeit less common than in defects with separate atrioventricular orifices, should always be excluded If found, they may have major implications for surgical repair Less common variants include virtual absence of the potential for interatrial shunting when the atrial septum meets the bridging leaflets of the common valve and absence of any ventricular component when the bridging leaflets are firmly attached to the ventricular septum This last variant is difficult to distinguish echocardiographically and is clinically indistinguishable from the patients having separate atrioventricular valves and shunting confined at atrial level Variants Found With Either Separate Valves or a Common Valvar Orifice An extremely important variation, usually readily recognized in four-chamber sections, is ventricular imbalance (Figs 31.31A and 31.32) The subcostal modified left anterior oblique view can be used to assess the relationship of the common atrioventricular valve to the underlying ventricles to more accurately define the degree of ventricular imbalance (see Fig 31.31B), with a left ventricle to right ventricle area overlying the common valve of less than approximately two-thirds, suggesting single ventricle palliation as the appropriate surgical management in patients with adequate ventricular level shunting.53 Potential left ventricular volumes as well as the extent of the left ventricular apex should be considered in those with a flattened or bowing interventricular septum.54 FIG 31.31 Apical four-chamber view (A) and modified subcostal echocardiographic image (B) demonstrating a complete atrioventricular septal defect with left ventricular (lv) hypoplasia The modified subcostal view demonstrates that the common atrioventricular valve predominately opens to the right ventricle (rv) la, Left atrium; ra, right atrium FIG 31.32 Apical four-chamber echocardiographic image demonstrating a complete atrioventricular septal defect with right ventricular hypoplasia la, Left atrium; lv, left ventricle; ra, right atrium; rv, right ventricle Left ventricular hypoplasia is commonly associated with obstruction of the left ventricular outflow tract, whatever its cause, and is easily identified on fourchamber sections angled superiorly and anteriorly (Fig 31.33) as well as in longaxis sections Right ventricular hypoplasia, by comparison, often requires delineation from multiple imaging planes (see Fig 31.31) and is usually associated with atrioventricular septal malalignment, often described as doubleoutlet atrium This has important implications for disposition of the conduction tissues (see Fig 31.12) The association of tetralogy of Fallot or double-outlet right ventricle does not fundamentally change the features of the common atrioventricular junction FIG 31.33 (A) Subcostal long-axis section from a patient with a common atrioventricular valve orifice Shown is an obstructed left ventricular outflow