The symptoms of a panic attack ( rapid heart beat, shaking, wobbly legs, feeling of panic etc ) are all caused by a release of adrenalin into your blood stream.
You body thinks its in danger so the adrenalin is released, for you to run from the situation.
If you saw a bus about to run you over, then this adrenalin is a lifesaver, enabling you to run fast.
However, the body can only store so much adrenalin at a time, so after it has flooded your body, you cannot phyically have another one until you have build up another supply of the adrenalin.
The fear of having a panic attack makes you release the adrenalin,
It is absolutly harmless.
What you need help with is how to stop the thoughts and the fears that bring on these attacks.
I recommend you buy a book written by a
Dr. Clare Weeks, called Self Help For Your Nerves.
She has written several books on this subject, and is very very good.
Try Amazon for the book.
You can beat this.
Good Luck