Spent the pass two summers in CO at about 8,400 feet. Could not breath. Stress test showed all is OK with heart and lungs. Stayed 3 months and loss 15 pounds. Could not eat, much. Yea. The second summer I purchased a OX concentor (you can make that happen in CO) (look on Craig list or the ads in news papers).
Would use the machine whenever resting, day or night, just sit a relax and take the anxerity (can't spell) out of breathing. Helped enough to be able to go out and play, some.
Back at sea level but still use my machine to relax and un-stress. Good luck.