I would first say you need to find out the cause of the pre-atrial fibrillation. I get it because I have a very hard time getting my magnesium levels up. I take 900mg-1200mg a day of Magnesium and my blood test comes back .1 above the bottom of the scale. I just dump magnesium. My mother has the same issue. Low magnesium levels can cause a-fib, OR, do you have an actual heart condition.
Either way, in general I would say stay away from anything that stresses the cardiovascular system, like Tren. If you're just taking test, and you keep estrogen under control, you really shouldn't have any issues.