Your concern about tachycardia is understandable, @warpedtrekker. As your explain it, a few questions arise that you may wish to discuss in detail with your cardiologist or another cardiologist willing to give you second opinions. First, the diagnosis of "inappropriate sinus tachycardia" is specific to a set of conditions recognizable by a good cardiologist. Has your doctor wondered whether your tachycardia is related to surges of adrenalin — or some other seemingly separate condition — that might cause your heart rate to leap up? What role does Bystolic (a drug related to adrenalin) play in your situation? Second, Multaq is advertised as a treatment for patients who once had A-fib. Is that true in your case, but you no longer show signs of it? This question is suggested by your recent EKG and Echocardiogram results, which both show "normal" results.
Your resting heart rate of 70-80 is much like mine; mine also used to be 50-60 until my A-fib diagnosis four years ago. I don't have tachycardia symptoms like yours, but if I did, I would have detailed discussions with two qualified physicians — separately.