Complete heart block is most commonly caused by myocardial infarction. An inferior wall MI (RCA in 85% of population) can damage the AV node, leading to complete heart block. An anterior wall MI (LAD) can damage the distal conducting system, also preventing atrial-ventricular communication.
Note that the sinus node artery is from the RCA in only 55% of the population (from the LCx in the other 45%).
Review of Coronary Anatomy: Left anterior descending (LAD): courses anteriorly in the interventricular groove and gives rise to diagonal (anterolateral) and septal (IV septum, bundle branches) branches. The LAD itself terminates at the apex of the left ventricle. The diagonal branches of the LAD distribute blood to the anterolateral aspect of the heart. The septal branches of the LAD supply the interventricular septum, the bundle branches, and the Purkinje system. Left circumflex artery: gives rise to obtuse marginal (lateral wall) branches Right coronary artery (RCA): acute marginal (R anterior wall of RV) branches. In 85% of the population, the RCA gives rise to the posterior descending artery (PDA).