syn. sidedness. The position of the morphologic right atrium determines the sidedness and is independent of the direction of the cardiac apex, or the positions of the ventricles or the great arteries. situs ambiguus Indeterminate sidedness (in the setting of atrial isomerism). situs inversus Mirror-image sidedness, i.e. opposite of normal. Left-sided morphologic right atrium. situs inversus totalis Total mirror-image sidedness. The position of all lateralized organs is inverted. situs solitus Normal sidedness. Right-sided morphologic right atrium.