Do you already have a heart condition? If not, you really have no worries. I don't think most doctors even realize this, but my psychiatrist told me the last time I saw her that people on Celexa are supposed to have their potassium levels checked and an EKG done annually. She said the odds of them affecting a "normal" person are quite slim, but since I already have a rapid heartbeat, she didn't want me going on the Celexa (I would have been on 60 mg, though). As long as you don't already have heart issues, you should have no qualms about taking it. I took 20 mg for a month on two occasions and I had no trouble at all.
While it's good to educate ourselves on the risks and side effects of meds, sometimes it can scare the crap out of us!
I can assure you that severe adverse reactions like not breathing are RARE. When studies are performed with meds, and something negative occurs, it has to be included in the warnings. So, to give you an example...if 15,000 people were studied on Celexa, and ONE person stopped breathing, it would have to be included in the warnings.
Try not to worry!