Long lasting panic attacks-how fast does heart increase?

Hi again,
I'm sorry to bother everyone again with my neurosis, but I need some help/reassurance...
I have been through all the cardiology tests- EKGs, stress, Echo (have had a number of eKgs this year due to trips to ER) and also a 24 hour monitor.  My heart races at unbelievable rates and I have had panic disorder for the past 18 years.  REcently started up again and I'm wondering how fast other's hearts go during these periods and when it's considered dangerous.  My heart rate can keep increasing and go up to 180 or 200.  The cardiologist had caught on my 24 hr monitor 2 instances when my heart went to 150bpm.   HE said it was normal and not coming from my heart or any problems there, but from the signals  from my brain.  I know I've had this forever and should be used to it and comforted by surviving my past million panic attacks, but I'm really distressed about this.  I felt as if the cardiologist didn't really answer my questions and wanted to speak with my shrink instead.  But I need to know how to control this when it breaks through my meds (i am now on Klonopin .5 2x a day and starting Prozac again...got off xanax because i had to use it too often and it wasn't controlling attacks the way it used to.).
so my questions are this...have any of you experienced these ridiculously high rates during panic attacks that other doctors have determined are 'safe' (i was told it is sinus tachycardia)...
do you stay in these panic/anxiety states for as long as i do (i can spend a few hours going through waves where the meds seem to kick in and out)...
when is it time to go to ER?  When is the heart rate dangerous??? It feels so scary.  I'm crawling out of my skin.
I have been on every drug out there.  Would like to hear how the rest of you deal with fast heart rate.  My shrink also recommended Inderal which I do think helps, but the cardiologist disagreed with him and said it wouldn't do anything since the problem is not my heart.
I did post this on heart forum as well. I apologize for the repetition. I would like to get more information if possible from those of you who suffer from this as well since according to the cardiologist and shrink- there is nothing wrong iwth my heart, but everything wrong with my brain. :-(
thanks much in advance,
k

[MrGreen]

Here. Read all this. Might help you. Might make you worry more.

http://kidshealth.org/parent/medical/heart/arrhythmias.html

[tinkerbelle713]

Hello. I do have a fast heart rate when I get my episodes, but nothing like you do. I can relate though. It is always scary, each and every time.
There was a thread on here about how we, people with anxiety always go so deep into what we are afraid of, that it makes things so much worse.
Do you trust your doctors? Do you feel you are being heard?
We need to understand that the worry alone will make the symptoms more intense and worse. It is the REAL fear that keeps all of the symptoms alive.
In no way am I saying : JUST RELAX. I know how you feel, but you have to try somehow to trick your mind into a different state somehow. There are techniques, such as breathing exercises and such, that can really make a difference. Talking about it helps as well, so if you ever need to talk to someone, don't hesitate, write me a note. I will listen. My heart goes out to you.  :)

[bubbles_830]

For the past year i have been going in and out of hospitals, because of my heart and my head.......   Im very scared,  My heart beat will reach up to 190 i feel dizzy and i can feel my heart pounding through my skin no doctor has put me on anything they said it was just anxiety.....    But everydayis becoming more of a hassle all of these doctors say the same thing..... I just turned 21 3 days ago .................   please someone i need advice and some help out here