I think you really need to try and discover the cause of your tachycardia in the first place and it is only by trying to find that cause that you will find the cure. You do not just get tachycardia for no reason, so there must be a cause. Did you have hypertension before the tachycardia or not?
Unfortunately, your doc is just treating your symptoms, so I would suggest that you try either acupuncture or homeopathy and see what these do for you. Acupuncture may be better in this instance.
Lots of docs frown about the alternatives, but it is a fact that they do work and could really help you, so I would say go for it. You have nothing to lose at all and they could only help you to feel better. Most alternative therapist also have more time for you, whereas an MD's time is limited.