Afib can be symptomatic causing those that have it to feel their hearts racing and palpitations. This can be horrible. However, afib episodes are not dangerous in themselves. The dangers come from clots forming and causing a stroke and a very high heart rate. Your afib issues are being managed with an anticoagulant (blood thinner) to prevent clots and a beta blocker to keep the heart rate from going too high.

Your other heart problem is qt prolongation which can be much more serious than afib and is detected through an ekg. On an ekg read out, heart beats appear as long vertical lines. There are much shorter lines between the beats that look like waves. One of those waves is the qt and yours is abnormal. Some drugs including some antibiotics may affect the qt
prolongation making it worse.

Your doctors need to balance the risk of antibiotics for MAC affecting your heart. If you don't have severe MAC symptoms or a lot of MAC and especially if you don't have cavities, the decision may be to keep your MAC levels stable or even reduce it through nebulizing 7% saline and doing airway clearance and not doing antibiotics.

I hope this helps and I hope you hear your doctors' recommendation soon.