The good news here...is you seem to have had a pretty thorough work-up..especially when it comes to your heart...and so far...everything has turned out okay...that's definitely good!
The BAD news is...you're unfortunately not "too young" to be going thru perimenopause. While it isn't AS common in women in their 40's...it certainly happens. And hormone fluctuations such as those that come with menopause can cause some pretty significant anxiety-related symptoms.
It COULD also be "just" panic disorder without being related to hormones...but your description would make me curiious if I were you. My advice to you would be...to talk to your OB/GYN in depth about looking into menopause as a possibility...so that if it IS related to that...you can approach the panic symptoms from that angle. If not...you can start addressing the anxiety simply as it is. Make sense?
Keep us in the loop...and in the meantime....there are a LOT of great resources here, both in the health pages ("anxiety")...and via others' posts/journals. It may help you with coping techniques while you sort this out. Best of luck.
You most certainly could be going through perimenopause which by the way, was worse for me than actual menopause. Having your blood levels taken can really fluctuate during this time of your life so it may not show any concrete reference to perimenopause but that doesn't mean you aren't going through it. I don't think you are too young to be going through it anyway. I was even younger when I went through it and it lasted for a very long time. Even up to the time that I was in menopause, my gp was still telling me I was not menopausal. Then when it was all over, he said maybe some of your anxiety was related to menopause...he's always great after the fact. I went to an endocrinologist who confirmed that I was menopausal and was treated for it. Without question, hormonal changes can effect your anxiety especially if you have an anxiety disorder to start with. It definitely will enhance it. So, give yourself a break, this can be a tough time for you. I charted my cycle with my anxiety attacks and it became blatantly obvious there was a correlation. I charted everything. When I had a panic attack, and the level from 1 - 10. My menstrual cycle, what other stressors were in my life and I put it on a monthly chart that I could show my psychiatrist. It made everything very clear and easy to see where my anxiety came from and how we could work to resolve it. Each page was one month so we could put the charts for each month side by side and see the progression or regression as my therapy continued. I really liked this idea because there it was right in front of me and it was an eye opener too that really was instrumental in helping me get through my panic.
Thank you ladies for your helpful comments. I suspected that I needed to see my OB/GYN again since it had been 6 months since my last visit with her. She had denied that these attacks could have something to do with perimenopause. This time I was prepared to show her my detailed calendar of menstrual cycles and attacks and how cyclical they were. Half way through the appointment she said, "This has to be hormonal." She then told me something that I have been wondering about and researching for some time. She said that a woman will suffer three main symptoms in perimenopause, hot flashes, migraines and PANIC ATTACKS. Aha! Now they tell me :) Anyway, she has placed me on a low dose estrogen gel that I only take the first ten days at the onslaught of my menstrual cycle. We'll see if this makes a difference :) Thanks again, ladies, for confirming my suspicions.