Heart rate at altitude

[Oldibtgdy]

I'm getting my aerobic base back after an illness, so there may bee a couple of things happening besides altitude. Anyhow, in Sept. My resting rate (at awakening) was Round 62bpm and my VMax was around 178 (I'm 67 years old). From a lot of work in the boat and the erg, I figured my AT was around 145. Just before I went to our house at 8,500' I was doing a lot of volume particularly in the boat, preparing for a couple of head races. My resting rate dropped to the low 50s, my AT was in the same ballpark as before and my VMax was around 168/170. After a week or so at altitude, doing 30 min pieces on the erg, I found that my resting rate was basically unchanged, but my AT appeared to have dropped to around 130 and I wasn't able to push my HR much beyond 145. I stayed at 8500' for three weeks with not much change in these numbers. The precipitous drop in VMax confused me, but recovery of HR following exercise confused me more. Following exercise, my HR stayed pretty flat (unchanged) for several (about 15 to 20) seconds and then started down -- slower than at sea level, but it was going down. When I got back to sea level a couple of days ago, I did a couple of exercises on the erg and, while the resting HR was still in the v low 50s, I found AT to be back up to around 145 and VMax appears to have returned to the (low) 170s. And recovery of HR started nearly immediately following the piece, implying a reversible saturation effect at altitude. Does anyone have any idea what's going on here? It's as if whatever determines the climb from AT to VMax somehow went on vacation at 8500', truncating my dynamic range, but returned when I got back to sea level. Probably a dumb question, but i haven't come up with an answer -- and the effect seems to be fairly robust.

[jamesg]

Air pressure at that height is about 75% sea-level pressure.