I have done some tests and think I have worked it out. Partly at least.
It's due to exercise.
Let me explain -
I do small sessions on a stepper - 500 steps - several sessions in the day for about 5 minutes each, before I get on the treadmill.
I never get angina of any significance on the stepper.
Then, later in the day, I do a longer treadmill workout - that's my main daily workout, when I hit the angina barrier.
What I've done is to double my small stepper sessions from about three or four to eight, or ten. Then, when I do the treadmill I find I can go much longer before the angina becomes a problem (ie chest pain) and I have to stop. So the "pre-conditioning" (I'll call it that) from the stepper boosts my treadmill performance. Importantly, in my case, it seems to boost it a lot and I can go for 3 or 4 times longer on the treadmill and even then I pull up feeling a lot better than I ever did with the shorter "preconditioning".
Another thing I've found is that if I boost my treadmill performance by "preconditioning", and I try the treadmill again the next morning it's still there. Great! That is, the improved resistance to the angina carries over and remains there the next day. So it does not drop back immediately. Great too!
I am still coming to grips with this - working out what the pattern is.
What I plan next is to try and keep it at the high treadmill perfomance level by using the treadmill both in the morning and the afternoon.
That way I reckon I can stop from dropping back. (It's a lot of excerise though.
The question that remains is even if I keep up the exercise and push the angina threshold higher - will it remain there and what do I have to do to keep it up there? If it quickly drops back after say no exercise for 24hrs that may mean I have flexed the arteries out a bit, got some better blood flow, but they quickly go back again. I pretty well know that will happen from my experience now. Maybe I need to keep them "open" for a month or so to get something permanent.
This is all hopeful thinking and just my crude ideas on it. But the facts stand and I am in there fighting!
Thanks for reading the post, and I welcome comments from you - or anyone in fact who might be interested.