Why on low HP?
You “hear” your hearth pumping when you can’t no longer run, not when you get injured very bad. Why do you guys say it should be on low HP?
On injury the body releases adrenaline which makes the hearth pump faster but that is for action speed and reaction time, reflex, not to tell you that you are hurt, the pain tells you you are hurt :P, reaction which I really hope will not be depicted in any way in this game.
Low HP could be associated with noises and maybe window effects, which in this game and the fact that there are (or will be) a lot of kids playing, that kind of effects I do not recommend.
Hearth pounding is/should be a signal of too much effort, and for the need of rest.