Before I share my personal experience, my #1 advice: Check with your Cardiologist or other Medical Professional. We're ALL like snowflakes, No 2 Of Us Are Alike!
I too have A-Fib. Last Christmas my daughter & family invited me to Steamboat Spring, Colorado. Home for me is Springfield, Ill. Altitude of Spfld. = 600'. The base of the Mt. & town of Steamboat Springs = 6,700' . I found I just didn't have the endurance to take long walks like I take at home, just didn't have the energy ... however it didn't keep me from going for shorter walks around the village at the Mt. base, or into town. On our Last full day my daughter was Insistent I ride the Gondola to the top of Mt. Werner with her, walk around for +- 5 minutes & ride back down. (elevation 10,560') . For the next two days & nights my heart-beat was erratic, I couldn't sleep, it took 5 days for me to recover.
After I returned home I googled "A-Fib & Elevation" ... it recommended A-Fib patients not visit places with an elevation above 5,600', (I was "pushing-it" going to Steamboat Springs at 6,700'.)
Hope this helps ... but I'd like to repeat, Your Best Resource is A Medical Professional.