Anti depressants often don't work for anxiety. Whole different drug set. You say she is having heart rhythm issues. It is not at all uncommon for this to cause anxiety. People vary in how much they are "aware" of their own heart beat. My partner is VERY aware of his atrial fib, and very aware of any changes, and speeding up or slowing down. I on the other hand have chronic atrial fib and honestly don't notice a difference at all for all these years. We each have had this arrhythmia for more than a decade. Some heart rhythm issues are more profoundly disconcerting, and these are usually the PATs, or paroxysmal atrial tachycardias. The heart can do these racing beats, beating easily 120 to 140 a minute, and many patients are very much bothered by this. You don't mention what she has or what is being done for it, but they will come up the right drug cocktail undoubtedly and she will be more comfortable. Meanwhile reassurance to her that this is the electrical of the heart, not the pump of the heart, and while they will try to scare you to death about them, people in general live good long lives with them. Reassurance may be better than drugs. But mild anti-anxiety medication may help as well; remember they are all habit forming and they can all send you down to the floor dizzy, in danger of breaking things. Good luck. Let us know what works.
I feel that a low dosage of a medication for anxiousness could help but it might be best if it could be administered to her. You don't mention her living circumstances. I don't care for the thought process in the medical community that feels an aged person might become addicted. Really their age and suffering should be taken into consideration. Hopefully she is under the care of a doctor who would take into consideration her existing conditions. That would help with what they feel would suit her best and as I stated is there a way for medication to be in control of someone else. Prior to my mother entering into AL she was not clear about her medications and took one she was supposed to have stopped for a few days before a procedure and that caused excessive bleeding which caused her to pass out and led to her no longer living on her own. Good luck with this.
We, as adult children, are often not the best judges of whether our parents need meds.
Is a doctor suggesting an antidepressant? I would give it a try, if yes.