Welcome to HCM,
Your mother is 87 years and having Atrial Fibrillation
now she has Atrial Flutter
which is more dangerous, treatment depends on underlying cause if there is atrial flutter and fibrillation which can precipitate cardiac failure, and cardioversion
is treatment of choice if possible your cardiologist knows best.
2 D echocardiography
is required to see any clot in side heart where blood thinner are required to be given.
If suitable then cardioversion should be done as it convert to sinus rhythm
and improves cardiac function.
Once sinus rhythm is restored there are very good drugs which can be given to maintain it and if required even cardioversion can be repeated.
Please follow advice of your cardiologist and hope she recovers soon.
Please take care