Hello there. I am currently pregnant with my first child, and I suffer from tachycardia which I have postponed being on beta blockers for.
If your pregnancy is anything like mine, you will find that the first trimester is fine as far as the problem goes, and mine picked up and got worse around 16 weeks when blood volume is increasing. It doubles by week 24 so keep that in mind.
You might want to talk to your doctor, and consider whether or not you want to be on medication during the pregnancy. It's recommended to wait to be on a beta blocker until the first trimester is over if at all possible- the drugs can cause a bit of a stunt in growth so you might just have a smaller baby if you stay on the meds. Also make sure your OB is aware of the situation, and have a plan together for the pregnancy and the delivery.
Good luck!
regarding having a baby....dont let them give you pitocin!!! read the thread about it causing arrythmias! are you on a low dose beta blocker like atenolol? i dont know what dilzem is?