Heart, gastric, anxiety? Exercise?

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One doctor says that my symptoms cannot be heart-related as my thallium scan in April was negative, while another is pretty certain it IS because of the nature of the pain and my risk factors.  I do have GORD/GERD. but the pain is very different.

A fortnight ago I started getting central, pressing chest pain, radiating up into my neck and relieved by GTN.  Usually it was short-lived, but last Wednesday I had a much worse bout lasting 25 minutes and needing 3 sprays to touch it.  I should have gone to ER but didn't want to.  Since then I've just had a few minor episodes lasting less than 5 minutes and not requiring GTN.

I'm 61, on medication for hypertension and high cholesterol, with a very strong history of coronary heart disease.  My radionuclide test was negative.  I've had an unidentified murmur all my life and an echocardiogram in April showed slight left ventricular hypertrophy from the hypertension - now pretty well-controlled.

Am I being neurotic in being 90% sure it is cardiac?  

I'm hoping to go back to the gym tonight (still very new to it.)  I find it very hard to keep my heart rate within the desirable range.  If I let it get higher, is it likely to do any harm?  Does pushing myself to the point of ? anginal pain do any harm if I then stop?

Ann