Mom's at the hospital.  Dad will be staying at Cooper at least another 2 days.  The atrial fibrillation is not normal.  It's not common but it's also not uncommon.  They are treating it and monitoring it.  He was sleeping when Mom called.

Here's some links on Atrial Fibrillation:
American Heart Association | Mayo Clinic | Dept of Health and Human Services (includes animation) | WebMD