perhaps this would be better suited for the cardiology or internal medicine board, but i think the most straight forward answer is that a CABG (coronary artery bypass graft) is in order when there's a substantial blockage of the artery.
Oftentimes, patients may receive a angioplasty (ballooning of the vessels) or a stent (spring-loaded meshwork that pushes built up plaque against the walls of the vessel) before having a CABG. An initial step in this entire process is often an angiogram, where the cardiologist will shoot dye through the blood stream and observe if blockages do occur, and if they do, to what extent.
I'm pretty sure there's some specific criteria for stress tests and ejection fractions, but I dont know them off the top of my head.