I would give it some time. and don't keep checking your heart rate. Cipralex and this class of drugs are long acting and not something that would just drop your heart rate. But as your stress and anxiety decrease, your heart rate in the resting mode may go back to normal. A resting heart rate of 50 is in the normal range. You are on a low dose. Hope it helps your anxiety/panic. I have found that often anxiety takes a bit higher dose to really work well.
I'm a 74 yo guy. In high school my family doctor told me I had an athlete's heart, which meant a heart rate around 60. The basketball coach really got us fit for the season. So, I guess my heart enlarged to survive the practices. Now my heart rate is around 40 to 45 generally. I've seen a cardiologist and he says I won't keel over dead.
I would recommend CBT for the panic attacks. It's a proven therapy for dealing with them and anxiety. I've worked with it over the last 24 years. I highly recommend it.