Well, I used to suffer from panic attacks - sometimes three or four a day - and the worst aspect of them, for me, was the fear of my heart going so fast that it would stop, as well as, of course, the feeling of going mad.
A friend of mine gave me a little tip which still works for me to this day, twenty years later: close your eyes and gently press with one fingertip on each eyeball whilst holding your breath for a few seconds. Feel a little bit of tension building up in your throat, then release it. Apparently it interrupts the vagus nerve or something which is what can cause those frightening palpitations. Another thing which works for me is firmly squeezing my upper lip between thumb and forefinger, just under the middle of the nose. Apparently that also regulates the heart beat. Whether or not they work at all, on sound scientific principles, is another matter, but I think that even just having the security of knowing you have things under control is a great medicine!
Try these and let me know if they work for you!