What's the best way to calm your racing heart when having a panic attack?

Different ways work better for some than others, though personally - I find myself laying down, giving myself the challenge of counting to 1000! Obviously, I never get to 1000; but it gives me something else to put my mind on.. Something else to concentrate on. Obviously, I focus on my breathing too - But the counting thing really works for me. :)

Well, the best way can be different for every person, all I can say is breathing helps me slows down the pulse. The exercise hours like this - take a deep breath and then breathe out (significantly) more slowly than you breathed in, do that at least 10 times.

Take a deep breath in through your nose, hold it a moment, and then exhale out of your mouth. Close your eyes, if in a safe position to do so, and imagine ocean waves sweeping in and out across the beautiful, peaceful sand. Imagine this with your eyes open it is not a good time to close your eyes. As you imagine the waves slowly coming in and out, breathe in and out with them. In with the wave, hold it slightly, and out with the wave. Focus on the feeling of air coming in and going out. Empty your mind of all but the breaths and the peaceful waves. Let any and all other thoughts, especially chaotic thoughts, fall away as you focus on the slow, deep breaths and each wave. Over time, I add more detail to the waves and the sands: the play of light and shadow, the white foam that trails the waves slightly... sometimes I will study videos of waves on YouTube. Later, when I need it, I can recall the scene and replay it in my mind in tune with deep, slow breathing. If ocean waves are not relaxing to you, instead picture a scene that you find highly peaceful and rewarding, such as holding a soft, warm, purring cat, and stroking it's fur slowly from its head all the way down its back. On the first stroke, breathe in. On the second stroke, breathe out. Feel the warmth if the cat as it snuggles into you. It's softness. The smooth, soft fur. The rumble of its purr. Whatever brings you great joy and is peaceful, find a rhythm you can repeat in your head. Sometimes is it helpful to remind yourself that you are in a (physically) safe place by thinking things like. "I am safe. I am well. This is a safe place." If these things are true. If not, seek safety or help in finding safety. If you are not in a safe place, you may be experiencing a specific fear instead of anxiety.

The best way that I have found are breathing exercises. Generally it will relax the whole body and steady your breathing. There are several around but a common one is 4 seconds in, hold for 4 seconds (best you can), out for 4 seconds. You repeat until you can do the whole 12 second routine without a break for hyperventilation (if you are hyperventilating) then continue until you feel calmer.

Think of happy thoughts and know that you aren't dying. Assess the situation and find something, may it be a person or an object which helps to calm you down. Maybe a friend who understands or a string or object you can fiddle with and focus all your energy on :)

The best way to calm your racing heart when having a panic attack is by taking breaks when they occur, taking a shower, or even other ways too.