In myself, it seems to be a cumulative effect. I had very low BP during my first 2 pregnancies, then had mono, then had a bad fall flat on my bottom which herniated 4 discs in my neck. Shortly after my fall the HR issues began, and 14 years later, I can say it has progressively gotten worse. I think without all those other problems, I would not be as bad. My 19 yo son has flare ups of tachycardia at times when he is tired and the usually IBS an sleep disorders. I am hoping that his will not progress as mine has.