(A) Diagram of the cardiac conduction system. (B) The right atrium is opened to show the triangle of Koch delimited by the hinge line of the tricuspid valve anteriorly (broken line), the tendon of Todaro (dotted line) posteriorly, and the coronary sinus (CS) inferiorly. The sinus node lies in the terminal crest at its antero-lateral junction with the superior caval vein (SCV). (C) These four panels depict the normal components of the atrioventricular conduction system and the variants of interruption that are the anatomic substrates of congenital heart block. AV, atrioventricular; BB, branching bundle; LBB, left bundle branch; ER, Eustachian ridge; EV, Eustachian valve; ICV, inferior caval vein; LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle; Trans., transitional.