Considering your other posts and PMs, you're probably trying to do this on the console... But I'm just going to ignore that and explain how I'd do it (have already done it, actually):
Reverse GTAIV.exe -> find the function which gets called when a ped is hurt.
Find out who or what is causing the ped to get hurt.
Code a hook to detour the native code to your own code, then do the things needed.
Retour back to native code, so code execution can continue.
Have a look at my latest youtube vid, you can see it's working (though I don't put explosions, I play a sound upon hit).