Blood-oxygen saturation (O2 sat) meters have become ubiquitous, but how much good do they actually do in determining whether an individual is fit to continue a climb?

The recent proliferation of affordable blood-oxygen saturation meters has made them commonplace in high-mountain base camps worldwide. But what do the numbers you read on your meter mean in terms of your preparedness to move higher on the mountain?

Not long ago, we fielded a question from one of our coaching clients, Giselle, about what she had seen on her O2 sat meter while climbing in Ecuador. Giselle sent this photo taken at 5,500 meters/18,050 feet on Chimborazo (6,263 meters/20,550 feet). She was feeling great, sleeping well, and had a good appetite. She was moving strongly, exceeding her guide’s time estimates on all her climbs. But when she saw her O2 sat, she wasn’t sure whether or not to be concerned. So she reached out to us to find out more.

As a response, we gathered the thoughts of Training for the New Alpinism co-author Steve House and high-altitude medical authority Dr. Monica Piris. Steve is not a medical authority, but with his extensive high-altitude climbing and guiding experience he has amassed a wealth of altitude experience, on top of having consulted with several of the world’s top altitude medical authorities while researching the “Altitude” chapter in Training for the New Alpinism. Monica, meanwhile, has accompanied a dozen high-altitude expeditions to some of the world’s highest peaks as an expedition doctor, including Everest, Lhotse, Cho Oyu, and Manaslu. Her work on these expeditions ranges from monitoring team members’ health and treating locals and visitors for common illnesses to treating frostbite and potentially fatal altitude-related illnesses and offering diagnostic and therapeutic advice over the radio for unwell climbers being rescued above 8,000 meters. Some of you may remember her from the Discovery Channel reality show Everest, Beyond the Limit.

From Steve and Monica’s replies, we have harvested the following take-home concepts regarding the use of O2 sat monitors on high-altitude climbs. We hope this will be useful in helping you determine how much stock to put in your O2 sat monitors in the future.

Steve House’s Thoughts on O2 Sat Meters

• A reading of 72–74 percent at the Chimborazo hut (I have been there many times) is not so unusual when you arrive quickly from sea level. Especially if you have low resting heart and respiration rates due to your good cardiovascular fitness.
• This number, on its own, is not a diagnosis of acute mountain sickness (AMS), high-altitude pulmonary edema (HAPE), or high-altitude cerebral edema (HACE). These are serious medical conditions that require separate attention and diagnosis. While a low O2 sat number in conjunction with other medical symptoms can corroborate a diagnosis of the above medical problems, that O2 sat number alone does not constitute a diagnosis. But I would be watching for trends.
• Acclimatization is a complex process and O2 sat numbers are only one snapshot of one factor.
• I now only allow my guiding clients to see their O2 sat numbers if they get sick and we need to make sure that their O2 sat is not getting lower while they are staying at the same altitude. I found that people become too obsessed with the numbers, which very often don’t relate to who feels strong and who feels weak.
• The more important information is how you eat (appetite) and how you sleep (recovery). If these two things are good, and no other significant AMS, HAPE, or HACE symptoms are present, then it doesn’t matter what the O2 sat number is. You’ll perform well.
• I have a theory: Well-trained athletes have a lower resting heart rate and respiratory rate than “normal people.” This low heart rate and low respiratory rate causes them to have low O2 sat levels when they first arrive at altitudes. (This makes sense, because the heart is not pumping as fast.) But over time (about two weeks), fitter folks will catch up to others and acclimatize better in the long run, because their bodies have more energy “left” to carry out the changes of acclimatization.

Dr. Monica Piris’s Thoughts on O2 Sat Meters

• How the client feels is the most important thing and the most valuable indicator of her health.
• It’s important to note that the heart rate in this case is not elevated. If her body were struggling because of hypoxia, her heart rate would be high and trying to compensate.
• Her numbers are actually not dissimilar to what I see in Sherpas at similar altitudes. They almost always have lower saturations than Caucasians!
• The O2 sat meters merely show the percentage of total hemoglobin (Hb) in the blood that is saturated with oxygen (and not particularly accurately). So if your total body Hb is 150 grams and you are saturating at 50 percent, 75 grams of Hb are delivering oxygen to your tissues. If my total Hb is 100 grams and I’m saturating at 75 percent, I am delivering exactly the same amount of oxygen to my tissues! Of course, we don’t have the same oxygen requirements, but this illustrates how useless it is to compare sats between individuals.
• The client can use this episode to help her build experience for other high-altitude climbs. She knows now that at 5,500 meters her sats are in the low 80s and she feels great. She can use that as her baseline and only worry when her sats fall below her baseline expectations.
• Not all oxygen saturation monitors are calibrated well enough to accurately measure sats below about 90 percent. In fact, many of the ones on the market appear to have undergone little or no testing. I would always recommend that if people buy their own pulse oximeter, they should get one from a reputable manufacturer and not simply purchase the cheapest one they find. I’ve been using the Nonin GO2 for years; I think it’s one of the OK ones.
• Expedition guides often (wrongly) put a lot of weight on the O2 sats and use the numbers completely out of context. I’m not sure if this is because of a basic lack of understanding of what those numbers actually mean, or simply because it’s an easy way of stopping people from going higher. Either way, it’s a shame and just propagates the myth that the number is an accurate reflection of acclimatization and can be used in isolation. It can’t. It’s my absolute pet hate when the O2 sat monitor comes out at the dinner table and I see people getting psychologically crushed before they’ve even started!

O2 Sat Meters and High-Altitude Expeditions: Uphill Athlete’s Conclusion

The takeaway from this seems pretty clear:

• Do not judge your preparedness to ascend on one number in isolation. Using the O2 sat meter out of context or to compare between individuals may lead to a wrong conclusion.
• How you feel, your appetite, and your sleep quality are the most important indicators of how you are doing.
• Fit people with low resting heart rates and low ventilation rates may initially appear to be acclimatizing less well.
• It’s useless to compare one person’s O2 sat against another’s as a sole indicator of who is in better shape to continue the climb: different-size people have different blood volumes and different oxygen needs.
• Finally, in our fairly extensive experience observing ourselves and others at altitude, we can state with no hesitation that the fitter you are, the less of your aerobic capacity you need to utilize to climb high. This allows you to handle the additional stress of acclimatizing to altitude more easily.