I have the exact same reaction to novacaine that you are describing and it scares the daylights out of me. I always tell the dentist before any procedure what my fears are and they have always been extremely kind about it. I've had one dentist (for root canal) who administered the novacaine very slowly and in two doses. I got a tiny bit of the fast heart rate, which only lasted a second on the first dose, and then nothing on the second dose. Another dentist (oral surgeon to have wisdom teeth pulled) used what he called a "short acting novacaine", which I had absolutely no negative heart effect to at all. These could be a couple of possibilities for you as well. After I explain my fears, I also ask for a cup of water, so after the shot I can take some small drinks, it keeps me calm since another fear of mine will be that once I get the novacaine, I will not be able to swallow, which of course has NEVER happened. Try to remember that the feeling of the novacaine only lasts a few seconds and there is a reason outside of your panic that is causing it, I keep that thought running through my head each time...it's just a short acting reaction.
I agree with kitt that you should tell your dentist about your MVPS and the meds you are on, that's always important to let them know any health issues and meds! I so understand your fears of the dentist, and I really, really hope that your appointment will go very smoothly tomorrow! I'll be thinking of you and hoping for the best!