I am beginning to wonder if this (i.e., susceptibility to AMS, etc.) is static in people. It does not seem entirely static to me, but I only have my own anecdotal experience to go on.

In the 1990s and early 2000s, I regularly got headaches and weakness at 8-9,000 feet. I recall that, for the first 2-3 days on trips to Snowbird or Alta (in Utah), I always felt like crap (headaches, malaise, weakness), and then it would all suddenly clear and I would feel great. I assumed I was someone with a poor ability to adapt to altitude.

Now, I regularly go from sea level to 10,000 feet in a day, on my trips to Tioga Pass, the East Side, etc. So far, I have not experienced the headaches, weakness, malaise, etc., on any of my trips. While I sometimes use Diamox for Cheyne Stokes, I only do that for trips with sleeping altitude above 11,000. I don't use anything for day trips, or for overnight trips with sleeping altitudes below 11,000.

I wonder if a combination of some things intrinsic (age) and some things not (behavior, unconscious adaptation) has made the difference for me. My suspicion is that, with more experience at altitude, I now drink more, and breathe more deeply, without thinking about it. I will be closing in on 50 in a few years, so maybe that is also a factor.

These issues were vividly brought home to me on a backpacking trip last weekend in Yellowstone. Five clients, two guides, and all four of the other clients got sick at 7,000-9,000 feet -- indeed, two had to be walked out and finish the trip early after suffering through two days of sickness and misery. Watching them, I remembered what 8,000 feet used to feel like to me. But now, with all the Whitney experience, 8,000 feet no longer feels like high altitude to me. I'm not about to get cocky ... will continue to be careful and build in acclimatization ... but I am finding that there is real difference in how I adapt in the present.