Your quickened heartbeat could be caused by a number of things--excess toxins in the system, for example. But please be aware of any further changes such as this. And quit the fast if your body tells you to.
According to your post, you're actually "juice fasting" not water fasting. Physiologically speaking, when you feed your body any form of carb during a fast, regardless of the amount, the true state of water-fasting never kicks in. You never reach ketosis or the protein-sparing phase.
Perhaps, you want to do a juice fast, instead? That can be just as beneficial as a water fast, in a slightly different way.
If so, I would recommend drinking more raw, freshly squeezed juice. You might even consider linking in with the juice-fasting forums in Curezone. Maybe even the Master Cleanse Forum
Juice fasting is the more commonly accepted form of fasting these days, although water fasting, due to Joel Fuhrman, is experiencing a resurgence.
I'm presently on day seven of a water-only fast. Let me know if I can be of further assistance. Email me if you wish.