In this particular case, my heart attack was caused by a blood clot. I have a bad history of blood clots, even when on blood thinners, including several pulmonary embolisms, several DVTs, a couple of minor strokes, etc. A heart catheterization took place a couple of years ago when they suspected additional heart issues, and the results were very good - no atherosclerosis. "Squeaky clean" was the official diagnosis! I unfortunately don't know the cause of the blood clots though, even after taking MANY blood tests (Factor V Leiden, Antiphospholipid antibodies, etc) to determine it. I have had a couple of serious car vs bike incidents, so I'm thinking that the trauma from those might have caused a narrowing of blood vessels somewhere, but that's just pure speculation at this point.
I also understand that this is a question for a cardiologist, but they, or at least those that I have been involved with, don't seem to have a lot of knowledge about exercise beyond walking. Most of their clientele isn't aiming to win a time trial! It seems that, since my heart muscle itself is quite damaged, it has enlarged to compensate, but I'm just trying to figure out if the heart itself is my current limiting factor, or if it is a combination of other things. The advice that Carbonfiberboy gave was very good - get a pulse oximeter. I ordered one and am awaiting its arrival. That might answer a lot of questions I've had.
Dave