I just completed a 27 day water only fast. I am alive and did not go into heart failure.
That being said my mother who is a nurse was VERY concerned about my heart. She kept saying that if I kept fasting that I would die of a heart attack. I don't know how true that possibility was, but in doing my own research I found some interesting facts about muscle loss.
Basically without looking up the information again to give you specifics, the most muscle that you lose is in the first few days. Then it really isn't that much after that.
I found the response I gave someone else who asked about that: