Hello @otiswinston and welcome to the group. Good idea for reaching out to this site, you'll get a lot of good feedback.
I, too, have afib, having been diagnosed quite a few years ago. And have had hypertension for over 30 years. I've been on Verapamil for years and various BP meds, then just started Xarelto a few months ago. I've lived in a city for over 30 years that's at 5000' (Denver is not the only "Mile High City"). We're close to CO so have gone up there frequently. I've never had breathing problems or increased problems with afib until last summer. (I was at 7000'). I was short of breath and my feet and legs swelled significantly. I was 73 and worked out pretty consistently. It was disconcerting.
So, will you be trying a long hike at a higher elevation in the US prior to committing to Nepal?
I totally understand the frustration that comes with having to make changes in your life as you get older and have more health problems, but regardless of other's opinions, only you can decide how you're going to handle those changes. Best of luck in making this decision.