Man this type of shit usually happens to me when Ive stopped smoking for a while. I end up hittin it so hard and smokin it so fast that like 5 minutes later i'm fucked outta my mind thinkin i'm gonna die. Shit i've felt like I was gonna black out at times everything started gettin dark and I was just paranoid as fuck.

the best thing that works for me is to just make sure your completley relaxed before you smoke. Make sure your in a good environment and take it slow and easy. That way it wont just creep up on your ass and scare the shit outta you lol.

And another thing If your thinking about the rapid heart beat before you toke your more than likley gonna feel while high and its gonna fuck off your high, so just try to take your mind off of it and you should be alright.