C.,
I was about 22 when I had my first pregnancy, and I had a history of SVT. For me it was not an ongoing issue (as is true for you) however, my triggers were high stress levels... not that labor is stressful ;) I had not been on meds before prgenancy and choose not to use them durring. I had absolutely no real problems. On the few occations that I did have an "issue" I would sit quietly for a couple minutes, remind myself what my priorities were, and breathe! For me, just figuring out what triggered the attack was enough to settle it down and stop it. Honestly, I was so releaved to FIANLLY be in labor that i didnt' have a single episode during, or after delivery!
And the one trick that my cardiologist taught me as a child that is my all-else-fail-free-go-to: cold cans (of soda or anything) on the pulse points on the neck. It literally 'tricks' your brain into thinking it is being frozen and automatically slows your heartrate. Not my favorite feeling in the wolrd (I HATE COLD) but deffinately a good deal better then medicating!
Good luck and enjoy your pregnancy!