Straddling and Displaced Atrioventricular Orifices and Valves with Primitive Ventricles

Fourteen cases of straddling or displaced atrioventricular (A-V) orifices and valves are presented. There are three types: (1) straddling tricuspid orifice entering both primitive ventricles with the mitral valve entering the left ventricle (nine cases), (2) displaced tricuspid orifice into the primitive left ventricle, with no A-V orifice in the primitive right ventricle (three cases), and (3) displaced tricuspid orifice with straddling mitral valve, in which the mitral valve has connections in both primitive ventricles, with the tricuspid entering the primitive left ventricle (two cases). Pathologically, these hearts are not single ventricle since there is a primitive right ventricle containing a portion of sinus as well as conus. Embryologically, however, these hearts are related to single ventricle, all being due to insufficient or no passage of the atrial canal region to the right during the process of absorption of the bulbus. Selective angiocardiography may differentiate straddling A-V orifices from single ventricle by showing immediate filling of both ventricles by injection into the right or left atrium.