Hi -
It could also be a magnesium deficiency. I had heart palpitations - wore the Holter Monitor - had an echocardiogram, blah, blah, blah - all negative.
I looked up vitamins and minerals and the symptoms of deficiency....yep, not enough magnesium in your system can also cause palpitations! Since I have been taking it for the past few years I only get the heart palps when I have slacked off taking my calcium and magnesium. I am FAST reminded and start taking them again, and the palps go away.
Now this is just *my* experience mind you....and only a suggestion for something you might want to look into.
Make sure to research magnesium first, and to take calcium and vitamin D with it (they all work together in the body) if you do decide to start taking it.
Best Wishes, and I sympathize!
Cara