IST and pregnancy

Hello. I am a healthy non smoker aged 32 who was diagnosed with IST in May 2010. After trying a few different drugs I have ended up on bisophrolol (bate blocker) to control the fast heart rate. Mostly my 'episodes' happen when I am at rest but under cardiologist advice I can happily complete a high impact dance class twice a week without any problems?! The episodes seem to be relatively under control touch wood.
People who haven't experienced an episode don't seem to really understand how worrying one is. A recent example was when I was on a train on my own and the train manager had to make a call to see if there were any doctors on board. Luckily there was so I was looked after.
I am hoping to start a family so am in the process of coming off the betablocker slowly over a number of weeks as it is one you cannot be on when pregnant. Does anyone know a safe one that has worked for them when pregnant and controlling the IST symptoms?
Tanks and look forward to talking to you all