Atrial Fibrillation

["Julia Thom" <thomlabs@netbistro.com>]

I went to a fun ride today with my seasonal endurance
mare.   She is 11 yrs old and has 4 yrs of endurance
riding to her credit.

Today after approx 12 miles we came in to see the vet 
and the vet said my horse had atrial fibrillation.  She
explained that she could not hear her heartbeat and
therefore could not do CRI on her therefore she was not
fit to continue.  Even four hrs later she still had atrial
fibrillation.

My mare has never had this problem before and she has
been in numerous 50 mile rides without problem.  She
is usually a bit energetic for the first 37 miles.

Is this something to be concerned about, what is the cause,
is she prone to this type of problem???

Thanks
Julia