While anecdotal evidence may not always beat controlled studies, in the absence of those studies and when there is an overwhelming body of anecdotal evidence supporting a conclusion, those observations warrant some credence.

The coaches at Evoke Endurance have helped many mountaineers successfully prepare to climb 8000-meter peaks. We have been able to come to some general conclusions through our observations. Most of these climbers have used normobaric-hypoxic methods to help pre-acclimate at home. The most popular and cost-effective way of achieving this is with the Hypoxico tent system. In this case, your bed is placed in a “tent” connected to the Hypoxico generator, which reduces the amount of oxygen in the air inside the tent. Unlike real high altitude, the air pressure inside the tent will be the same as whatever altitude your bedroom is. That’s why this is called normobaric (normal barometric pressure). Because the pressure does not change as it does at high altitudes, there are probably certain adaptations that do not take place. More on this later.

Most of us have an oversimplified understanding of the acclimatization process that goes something like this: In response to hypoxia (reduced oxygen in your blood), your system compensates by producing more red blood cells (that carry the oxygen). This allows for more oxygen to be transported with each heart beat, partially compensating for the lowered oxygen concentration in the air you breathe at high altitudes. While this process is incredibly complex, this simple explanation does a decent job of explaining the layperson’s understanding of altitude acclimatization.

Or we used to think it did. Recent research indicates that more than one-quarter of the genes that make up the human genome are affected by changes in altitude and play a role in determining: The rate at which you acclimate, who adapts well, and who adapts poorly to exposure to increased altitude. Many of these adaptions take place during the first two weeks of exposure to hypoxia BEFORE your red blood cell count has had a chance to increase. It stands to reason, then, that there is a lot more going on during the acclimatization process than just increasing red blood cells.

It is worth noting that this might explain why, within our small sample size, exposure to lowered atmospheric pressure and hypoxia in a natural high altitude setting seems to provide better acclimatization than the artificial “altitude” created inside a Hypoxico tent.

This is not an argument against using Hypoxico tents to pre-acclimate before heading off to a high elevation. Most people will not have the opportunity to undergo a 3+ week natural acclimatization before they spend another six weeks on a climbing expedition. That’s unrealistic for most of us.

So what have our coaches observed over the years that can be helpful for you?

• Using a Hypoxico tent is most beneficial, almost essential when the climber needs to hit the ground running at an altitude he or she is not acclimated to. Such as a Florida-based climber headed to Denali or flying into Lukla (or higher) for a trek to Everest base camp.
• Everyone sleeping at simulated altitude will see less complete recovery from training sessions due to poor quality sleep. This is compounded because the ramp-up in training load during the final weeks before an expedition also corresponds with the weeks when the simulated altitude will be the highest and the recovery the most impacted. Thus, monitoring your recovery status each morning is critical.
• Monitoring recovery status will be critical in order to balance the training and acclimatization protocols. Checking morning resting heart rate (wrist-based) and blood O2 saturation level will be useful. Establish a baseline before you begin the ‘acclimatization’. O2 sats in the mid-80s combined with a Resting Heart Rate (RHR) rise of more than 10% will almost always be accompanied by obvious fatigue. Those days should have light aerobic training in Zone 1 only. Consider sleeping 1000ft/300m lower that night. After a particularly tough workout (TSS > 20% higher than the last four weeks running TSS average), drop 3000ft/1000m to allow for better sleep.
• Hypoxico offers input to their clients on the rate of ascent, but you will also want to be monitoring your recovery.

The use of a Hypoxico will be of less benefit when the climber does a normal acclimatization schedule on the mountain and is already acclimated to the starting altitude. For example, a Colorado climber headed to the West Buttress of Denali or flying to Lukla on the way to climb Everest with the normal acclimatization rotations. In a case like this, there would be minimal benefit accrued from the Hypoxico tent when the climber finally gets to those higher altitudes two weeks later. Any pre-acclimatization would have ‘worn off’ by that time.

Intermittent Hypoxic Training (IHT) Observations

This term means exposure to simulated altitude by breathing a hypoxic air mixture during some training sessions lasting only an hour or so, compared to 8 hours of exposure when sleeping in the Hypoxico tent

We have coached athletes who include IHT ni their training program by using the Hypoxico generator and face mask while running and hiking on an incline trainer or cycling on a stationary bike for an hour 2-3 times per week.

• There is minimal to no pre-acclimatization benefit from spending an hour three times a week on a treadmill at a simulated altitude. The time exposure is too short to trigger acclimatization adaptations.
• There does seem to be a noticeable performance benefit, however. We hypothesize that the benefit comes from increasing the aerobic capacity and economy of the ventilatory muscles. This group of muscles can require about 20% of the heart’s output when working hard. The conventional way to work those breathing muscles hard is to do high-intensity work. IHT involves exercising at a moderate intensity, producing a very high ventilatory rate and depth as if one were doing a high-intensity session. The difference is that during IHT, the legs will not be working hard. Hence, there is much less global fatigue generated, but those all-important ventilatory muscles will get a strong training stimulus for aerobic adaptation.
• Even low work rates at high altitudes will have a climber breathing like she is doing an interval work out of 400m repeats on a track. With the above-mentioned hypoxic treadmill hiking session, the climber is simulating the demands of breathing at high altitudes but without the high intensity that would be required to get the same high ventilation rate.

In conclusion, we think it bears repeating that observations of hundreds of mountaineers preparing to go to higher elevations that they benefit significantly from sleeping every night in a Hypoxico tent for 6-8 weeks. This can be shortened to 4 weeks when the goal is a 6000-7000 meter peak.  They have fewer altitude-related problems when they first get to higher elevations. This is especially true for those who are planning rapid ascents from very low altitudes to high elevations.

Without contradictory evidence, the benefits of using a Hypoxico tent and monitored pre-acclimatization protocol make sense for almost all climbers going to elevations over 5000 meters. The only significant problem other than the discomfort of sleeping in a rather stuffy little tent at a simulated altitude is the poor recovery from training. Balancing the training load with adequate recovery is essential and warrants attending to those recovery monitor methods mentioned above.

–Scott J.