I monitor my heart rate during exercise and I check it periodically when idle. I try to every few days check my resting heart rate as well, and watch how fast my HR recovers after exercise. The faster my HR drops after exercise, the better...the lower my RHR is, the better.
Interesting that you mentioned that. If you keep track of your resting heart rate, you'll see that it will elevate when you first arrive at your hunting spot. That's because there's less oxygen in the air (not a lower percentage, just fewer molecules within a given volume) so your heart has to pump faster to get the same amount of oxygen to your muscles and organs.
Your body reacts to this or acclimates by producing more red blood cells, raising the amount of oxygen your blood can transport. So at the end of your trip, your resting heart rate at altitude will have returned to somewhere close to what it was back at sea level, since you'll have more red cells.
The masks make you work harder to breathe but they don't help your body acclimate by making more red cells. Making more red cells requires iron, so eat some iron-rich foods the first few days you're up there, it can't hurt.
