Hello, I am a little worried about my 24 year old girlfriends heart rate, it's over 100 - 110 bpm even at rest or asleep, constantly, all her life. Easily over 200 while exercising. Her blood pressure is low. She does'nt feel any discomfort, just gets a little dizzy while doing heavy exercise, probably from low blood pressure. The doctors say she is healthy apart from the rapid heart rate. She says it's fine but I don't like it. What can cause this? Should I be worried? Your help is much appreciated. Thank you!