Why Would One Cardiologist Say I Shouldn’t Take Multaq While In Atrial Fibrillation?

My cardiologist says I should not be taking Multaq while in a-fib. It was prescribed by another cardiologist after I was in the ER for a-fib which was resolved by a shock. I thought it was supposed to keep me from a-fib. Why would two cardiologists have a differing opinion on Multaq?