I have asthma as well but only light, I was put on Lanoxin along with other meds with my first afib, but the lanoxin gave me so many arythmias that I was allways out of breath, since I am not in afib and since I am off Lanoxin I have no more breathing problems, just a comment , hope it helps.
It can sometimes be difficult to sort out which of two problems that cause identical symptoms is the primary problem. I am suspicious that the heart is the culprit but your cardiologist will have to make that determination.