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Since your Cardiologist has cleared you of anything heart-related, I think it is time to look into other directions. I would suggest you see an Orthopedist and have them order a MRI of the left shoulder that is causing you the pain. The shoulder pain can be radiating into your chest and your anxiety about the whole ordeal can be making your heart rate vary.
The heavy lifting you sometimes do and this 'pull' you feel make the shoulder even more so seem like the culprit.