If you’ve visited altitude, you’ve noticed that it makes you uncomfortable. If you live up here, you might remember your first month in the sky (bloody noses, constantly itchy skin, headaches, constant dehydration, worse), and you’ve probably noticed that when you leave, even for a week, you’ve already lost that magical adaptation called acclimatization, and you have to work your way back into living the hard but sweet thin air life. We all know that altitude puts a strain on our body, and those of us who live here are always telling tourists not to underestimate it, but how big of a strain is it really? And why?

Altitude affects oxygen saturation, which is the amount of oxygen the hemoglobin in your red blood cells is carrying at any given time. At sea level, the atmospheric pressure and concentration of oxygen in the air is highest, which is optimal for oxygen saturation in humans. As altitude increases, atmospheric pressure decreases exponentially, while the fraction of oxygen remains the same, which leads to lower saturation. According to Princeton, it takes 1-3 days to begin acclimatizing, and months to adapt fully. The biggest change is the increase in hemoglobin specifically, and red blood cells in general, which is an adaptation that we lose quickly when we lose altitude. Other adaptations include: increased depth of breathing, increased pressure in pulmonary arteries (which increases blood flow in the lungs, and utilizes a higher portion of the lungs than you’d use at sea level), and increased production of enzymes that release oxygen from hemoglobin.

So the reason this is all top of my mind right now is that I’ve just returned to Ouray 7,792ft, after two weeks in southern Missouri, at approximately 1,004ft. You may remember that I’m now wearing a Whoop device 24 hours a day, and the data it’s given me around this trip is astonishing. Obviously, we know that altitude is a big physical stressor, but seeing it quantitatively, it’s bigger than I ever imagined.

For simplicity, let’s start with the Whoop’s overall recovery metric. They use an algorithm that takes into account my average resting HR, heart rate variability, and respiratory rate while I slept, and then other stuff like the previous day’s HR variations and the amount of physical strain from previous days, that results in a percentage. I don’t strictly adhere to this number, but it paints a clear picture of my journey from 8k to 1k and back. Here’s three weeks worth of my recovery scores:

The first photo is a full week spent in Ouray (all fairly low, which is average). The second includes my drive to Missouri and a full week there, the third is a full week just in Missouri, then the last photo I started driving on Saturday and arrived home in Ouray on Sunday night. The four days of red, then, were all spent in Ouray. Once I left Ouray, I had subpar recoveries on Saturday, after a stressful day of snowy mountain driving, then there’s two particularly intense cycling days that resulted in low recovery that quickly bounced back, another one of those on 12/22, then you can see the decline starts again after the drive back to CO.

140 million people live above 8,000ft. People who were born and raised in the Andes and Himalayas show particularly impressive evolution, being born with larger lung volumes and excellent oxygen saturation even as high as 16,000ft. High altitude dwellers in Ethiopia are less evolved, and show the same type of adaptations as people from low elevations that moved to high elevations during their life, and of all the high altitude populations studied, only Himalayans can move between altitudes without losing any of their altitude adaptations. Which means a Sherpa could go to Missouri for a month and they would not lose any red blood cells or have a change in stroke volume or cardiac output or anything, then on their return to the Himalayas, they would not need to re-acclimate. While someone born in the Andes would go to Missouri and have a similar reaction to me, then on their return to the mountains, they would have an increased HR and lower HRV and begin the process re-acclimatization. Himalayas also have another leg-up on us, with a sustained increase in cerebral blood flow, low concentration of hemoglobin, and an obvious resilience to chronic mountain sickness (CMS).

In studies of permanent high altitude residents, those that are born into it and those that live high by choice share a decreased instance of all types of cardiovascular disease and obesity, but an unexplained increased rate of suicide, even with controls in place for known suicide risk factors.

Let’s dive deeper into the data. Most people measure resting HR by taking their pulse the moment they wake up in the morning, while the Whoop measures your HR every second while you’re sleeping then takes an average. My resting HR at altitude before OTS was 50. Since OTS, I haven’t seen it get lower than 55, and it’s generally somewhere in the 60’s. Let’s take a look at the trends for the same four weeks:

For the week in Ouray, it was 63-68. It went up during the drive, then decreased steadily over the next two weeks, going all the way down to 50. It starts going back up after a day of driving, 57, then 65-74 over the next four days of being at home in Ouray. This is interesting, because an increase in HR is to be expected, as one of the first short-term adaptations to altitude is a decrease in overall blood volume, my body is trying to increase the ratio of red blood cells, and a decrease in blood volume corresponds to an increase in HR to pump it. My average HR, taken throughout the day, and including any exercise, actually didn’t change that significantly during the trip. It’s generally 70-80, and while I was in Missouri it was 67-78.

Another reason HR increases at high altitude is that hypoxia activates the sympathetic nervous system. This isn’t great news for me, as my tendency towards sympathetic dominance is one of the biggest driving factors for how I got Overtraining Syndrome in the first place, but it’s possible to do things to counteract it. Basically, you can expect your HR and blood pressure both to increase while at altitude, and not just within the first couple of days, because of pulmonary vasoconstriction, particularly during exercise, is caused by sympathetic nervous system excitation (which happens because hypoxia, which is lowered oxygen saturation, which happens because of the change in pressure at altitude). This link, from altitude to nervous system imbalance, was exactly what I hoped to find in my research today, because it is obvious from the Whoop data that I’m having cardiovascular effects, but they’re likely nervous system related, as we’ll discuss in this next section about HRV.

Now, here’s Heart Rate Variability (HRV). It’s a controversial metric, mostly it seems like because since it’s been introduced to the wider world, folks put a little too much stock in it and it’s true that you can’t base all your decisions around a number that’s so squirrelly. What I really like about it though is that it reflects the health of your nervous system specifically. Mostly when you’re training, you’re thinking about your cardiovascular system (resting HR) and the health, feel, and fatigue of your body itself (neuromuscular). Both your parasympathetic and sympathetic nervous systems are exerting control over your heart rate at any given time, and if both sides are healthy, that competition results in a high HRV. Parasympathetic is what’s damaged in OTS, so while you’re in a state of overtraining, your HRV is low because the sympathetic system is dominating the decision making. There’s still plenty we don’t know about OTS, and even in healthy folks it fluctuates rapidly. But these graphs are still telling:

To sum up these graphs, before I left it was typically pretty low. I forgot to mention above that HRV is highly individualized and part of what makes it a squirrelly and controversial metric is because it’s only relevant compared to extended data in the same person. So my HRV is only comparable to my HRV at other times. Moving on, before I left it was low. During my trip, it fluctuated a lot (mostly in relation to exercise) and occasionally skyrocketed, much higher than it’s ever been since I got the Whoop device. Most interesting I think is the steady decline since I’ve gotten back to altitude. When I got up this morning, I thought, it’s so low (19) that am I going to survive the day? (Just kidding, that’s not how HRV works).

So this was all so interesting, I thought I’d look back on our UT/AZ trip in October, and the results at first were uninteresting, there was a period where everything was slightly better for a couple days, probably associated with the relaxation and fun of the trip, but mostly it all was similar to when I’m in Ouray. Then I realized, we were mostly at 7,000ft which isn’t any great improvement to 7,992. 🤷

Thanks so much for reading and following along. If you’re interested in mountain running and coaching for adventure, learn more on my website or check out coaching options and training plans here or on Training Peaks, there’s training plans for the Grand Canyon, general base building, and 14ers up now. Also, Pippa Climbs Rainier is available in paperback, check it out on Amazon.