If I am understanding you correctly, you have no fears while going to sleep, but something IS waking you out of sleep which causes you fear. Do I have that right?
I would suggest you speak with your cardiologist about this. It is very possible it's just your subconscious playing on the fears of having had a triple by-pass, but something else might be waking you and your cardiologist is the only one who can assess and reassure you.