I was going to say holy crap batman when I saw that 300! Usually people aren't conscious at 300, I think I actually have pulled out paddles when I have seen numbers like that (depends on the rhythm)- old ICU nurse in me. Even 200 is pretty high...has the doctor considered putting what is called a Holter monitor on you (or an event monitor for that matter?) The Holter measures the heart activity for a 24 hour period, you get a diary and write down what you are doing at the time of any symptoms (ie dizzy, faint, palpitations etc...). Afterwards, you give the device to your doc and it is then read by a cardiologist. If you have arrythmias, they will show up. I have SVT, supra ventricular tachycardia, caught by an event monitor. My heart rates would go to 200 and I would feel like crap, like fainting. My blood pressure would fall dangerously low, 70's systolically. I have even had IV cardiac drugs to slow the damned thing down. I am now on meds to supress those rhythms. The point is, if it weren't for that monitor, I may never have had an accurate diagnosis. An EKG when you are resting usually won't show much. That's why the Holter because it is a continous EKG for 24 hours, shows all of the changes when you exert yourself, sleep, eat etc..you get my point.
As alot of folks have said here, perhaps looking into mineral deficiencies would be a good idea (potassium, mag, calcium are the big cardiac ones)
I wish you luck and please take care of yourself!
Hugs and best wishes, Jenn <img src="i/expressions/face-icon-small-smile.gif" border="0">