It could also be a thyroid condition that's making these panic attacks attacks last so long. I'm going through a similar situation only worse and I have suspected for years that it could be something in the glands that's causing these unusual attacks that last for hours and cause me fear all day long. I was told to get my thyroids,parathyroid,pituitary gland and the hypothalamus checked through bloodwork and a good visit to an endocrinologist. You have nothing to lose. Here's something that explains all about how these can effect the nervous system:

http://en.wikipedia.org/wiki/Neuroendocrinology
And:
http://en.wikipedia.org/wiki/Hypothalamus