When to worry about heart palpitations?

[ABt8]

I have had 4 echocardiograms done over the last 2 years, numerous EKGs, 3 Holter Monitors, and blood work. I do have a "low end of normal" 50-55% heart function but my cardiologist not concerned....The last Holter Monitor showed 10 isolated ventricular events, 0 couplets, 0 runs, 1 isolated supraventricular event, 0 couplets, and 0 runs. There were no pauses recorded. This was all measured on a 24-hour monitor. I am currently wearing a 48-hour monitor because the "flutters" are still happening 5-10 times a day after my beta blocker was increased to twice a day. Yesterday, while sitting, I experienced a flutter and blacked out for a split second. I am 31-year-old normally healthy female....no caffeine, exercise 3 times a week.....The "flutters" are causing me to have extreme anxiety for the fear of dropping dead. They come out of nowhere...and are not going away.