It may be that you are especially sensitive to high altitudes. Some people experience acute mountain sickness (AMS) at elevations above 9000 feet. That results in nausea, headaches, palps, etc. just a few hours after ascending. Perhaps you should try trips to moderate elevations, say 3000-5000 feet, and see how that works out before going higher.
Come to think of it, I first experienced severe palps when I lived in New Mexico at 7000 feet elevation. Later it was diagnosed as PACs/afib. I have mitral valve regurgitation, which is similar to MVP. Both eventually can enlarge the left atrium and lead to afib. But this is likely not your problem since afib begets afib and once you suffer from it, it would follow you to lower altitudes (as in my case).
Good luck
Tony
I've had arrhythmias all my life (PAC's, PVC's and tachy). For about 5 years we lived in Denver. My heart was very "active" there and was a big bother. I didn't make a connection to the altitude business until we happened to move to Oregon and a lower altitude. Then I realized that my heart wasn't acting up as much. Very nice. But I lived at the higher altitude with no injury. Guess it didn't matter that much for my heart at least because I'm still alive 29 years later. Oh, and I do have a heart murmur and a "floppy valve" according to my cardio. Not a problem.