I have a relatively high tolerance for stims so I don't really have any issues with using a good PWO before doing cardio. I like to use a PWO before cardio if I'm not really feeling it that day or if I'm going to be running because I hate running and the PWO can help give me the focus and drive to get through it. When I do something like P90X or INSANITY, the workouts are fun and motivating enough for me on their own that I don't need a PWO, but occasionally I will take one. My biggest concern with taking a PWO and doing cardio isn't my heart, it's my stomach. I hate having a PWO that is harsh on my stomach or makes me burp too much while all that liquid is sloshing around. I found Jack3d to be horrible for cardio and it even has made me puke a couple times doing P90X. CTD Labs' Noxipro has been by far the easiest on my stomach and their isn't too much caffeine if you only take one scoop. I've also taken Betanox and 1.M.R. before cardio because unlike some people, I do enjoy that cracked out feeling once in awhile and it can help knock out an intense cardio session. In the end, it's all about knowing your body and testing your limits...hope this helps...best of luck!