Your doctor described your condition as "borderline anemia," and as such, it would be unlikely to be contributing to your development of atrial fibrillation. A mild or borderline degree of anemia also would not be expected to bring about heart failure. It would be important to make sure that the anemia does not get worse, since that could indicate the presence of some other underlying medical problem. The causes of anemia are complex, but generally derive from blood loss (for example, from the gut), or from decreased production or increased destruction of red blood cells inside the body. Your doctor can order tests to help sort out the cause.