Ask the doctor. I'm 64 and have had lone atrial fibrillation for about a decade. I have an echocardiogram every year to make sure the rest of my heart is okay. (It is.) My doctor hasn't prescribed and medications for me, but she wants me to take a blood thinner when I hit my 65th birthday. I'd rather not do this. Should I follow her recommendation? Also, is it possible that the endurance-type exercise regimens I have performed over many years led to my developing lone atrial fibrillation?