I ve had similiar issues. What is happening is you are having a freak out. You shocked your system with too much THC and too fast and if you re a new user, your body probably wasn t used to it. Anxiety started to build in your system causing your heart to beat faster. The act of your heart beating faster actually built even more anxiety, so it becomes a snowball effect with you being unable to calm yourself down. I ve had two of these episodes myself. If you start to freak out like this, the best thing you can do is eat something bready like a tortilla. I also find that chewing on ice helps with this a lot. But you basically need to start eating something not only to bring your system back down but the act of methodically eating food tends to distract you and calm you down a bit. As for pot causing a heart attack, it stands to reason that if you have too many of these episodes you will do permanent damage to your heart.