I didn't have a-fib but I had other cardiac arrhythmias while on Prednisone. I didn't get on blood thinners until I had multiple, bilateral and extensive pulmonary embolisms of "unknown etiology." After spending a a couple of days in intensive care, I eventually went home on "warfarin for life." My cardiac abnormalities probably stemmed from elevated blood pressure in excess of 200/100. I developed left ventricular hypertrophy (LVH) from high blood pressure.. A cardiologist started three blood pressure medications. I didn't have much hope that I would ever be able to stop the blood pressure medications and warfarin. Being short of breath with minimal exertion all the time was the worst of it.

Things started to improve after I was able to discontinue Prednisone. At first, I was dizzy all the time and I had to be careful so I wouldn't faint. My blood pressure was getting too low and I was diagnosed with orthostatic hypotension.
https://www.mayoclinic.org/diseases-conditions/orthostatic-hypotension/symptoms-causes/syc-20352548
My blood pressure medications were stopped one by one. Now my blood pressure is a healthy 110/70 with no blood pressure medications. I have bradycardia but that was normal for me when I didn't take prednisone.

After 10+ years on warfarin, the anticoagulation clinic offered Eliquis so I wouldn't need to have my INR checked so often. I was reevaluated for my heart health. When it was learned that I was no longer on Prednisone, it was decided that I no longer needed any blood thinner. Now I'm off warfarin and all my cardiac medications including Atorvastatin in addition to being off prednisone.