As far as heart-rate goes, it speeds up a lot, but it doesn't come to the front of your notice until you go too far or begin to crash.
Desire to live... Maybe "desire" is a wrong word to use. Perhaps "drive" is more fitting here as well. You are moved to... do things. It's not a coke-like "Yeah! I'm the king of the fucking world!" sort of feeling, but more of an automotion to act, perform. Hard to explain, when the meth is gone, so is your will to do anything at all - thus the universal feeling of "life is not worth living" when you suddenly can't score more for whatever reason.
This is really not helpful, I'm realizing. I hope someone with better literary abilities helps explain.