My heart rate goes to ~260 at the gym. Walking today it went to 224. I’m a 20 year old normal weight female. Have a normal ECG, doctor gave me a referral to get an echo and holter test but it’s been months and only one is scheduled. Any thoughts? Can maintain HR at 240-260 for about twenty minutes and not feel ill. Get some chest pain/dizziness/shortness of breath after an extended period of time