They can form scar tissue and block, or a clot can form. Same with your
bypass grafts, they can block and veins can close up. I had a triple
bypass and three months later I had an episode while out walking. It lasted around 4 mins, but then seemed to settle down. I had angina from that time again. I told the surgeon who said "there is nothing wrong with your bypass, ECG normal, and so there's nothing I can do".