I usually get rapid heartbeats when starting a new anxiety med, or any med for that matter, want to know why? lol cause i'm always so nervous when starting a new med! You sure thats not why your heart is beating fast? Even if it is the medication, i'm surprised your doctor wants to take you off it already, how long have you been taking it? side effects like that usually go away after a few days. Unless you been on something else and didn't have the rapid heart beat then?
Not to sure how long it stays in your system, but when i went off lexapro, i went right to another SSRI the next day and had no problems.