The hidden link between freediving and mountaineering

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Since reading James Nestor’s book in 2014 Deep, I was fascinated by the barely believable exploits of freedivers. Dive 335 feet below the ocean’s surface and come back in one breath, or just hold your breath for 11 minutes and 35 seconds, clearly requires a very special set of skills and traits.

But until a recent conference, I never considered whether these same features might be useful in other contexts where oxygen is scarce, such as the thin air of trekking and high altitude mountaineering. At Medicine in Extremes Conference in Amsterdam last month, Erika Schagatay from Mid Sweden University gave a presentation that summarized over two decades of freediving research. The twist that caught my eye: Understanding what makes a good freediver could be helpful in predicting and possibly even alleviating altitude sickness.

Schagatay’s initial research interest was what she calls “professional” freedivers, as opposed to recreational or competitive freedivers. These are people who dive for fish and shellfish, just as their ancestors did for countless generations: like the Ama pearl divers in Japan and the Bajau subsistence fishermen in the Philippines and Malaysia. The latter group performs repeated dives to about 50 feet and sometimes goes up to 130 feet deep, with recoveries so short that they spend about 60% of their time underwater. In a nine-hour day, they can spend up to five hours underwater, don’t breathe.

These populations of divers, Schagatay and others have discovered, share three distinguishing characteristics with successful competitive freedivers, who compete in competitions sanctioned by AIDA, the international freediving authority:

  • Big lungs: In a study of 14 world championship freedivers, vital capacity – the maximum amount of air you can expel from your lungs – correlated with their scores in competition. The top three divers in the group had an average vital capacity of 7.9 liters, while the bottom three averaged only 6.7 liters. And it’s not just genetic: Schagatay found that an 11-week stretching program increased lung volume by almost half a liter.
  • Lots of red blood cells: Divers tend to have higher levels of hemoglobin, the component of red blood cells that carries oxygen. This is probably the direct result of their diving. Even if you only do a series of 15 breaths, you will have a natural EPO surge an hour later, which will trigger the formation of red blood cells.

    But there is a more direct and immediate way to increase your red blood cell count: squeeze your spleen, which can store around 300 milliliters of concentrated red blood cells. Seals, which are among the most impressive divers in the animal kingdom, actually store about half of their red blood cells in their spleen, so they don’t waste energy pumping out all that extra blood when they’re not. are not needed. When you hold your breath (or even just do hard training), your spleen contracts and circulates blood rich in additional oxygen. Not surprisingly, the size of the spleen is correlated with snorkeling performance.

  • A robust ‘mammalian diving response’: When you hold your breath, your heart rate drops by about 10% on average. Immerse your face in the water and it will drop by about 20%. Your peripheral blood vessels will also constrict, carrying precious oxygen to the brain and heart. Together, these oxygen-conserving reflexes are known as the mammalian diving response – and again, the strength of this response correlates with competitive diving performance.

These three factors help you cope with a complete stop in breathing for a few minutes. Do they have to do with prolonged exposure to a slight decrease in oxygen, as is the case in the mountains? This is what Schagatay and his colleagues have explored in a series of studies involving Sherpas, trekkers and Everest peaks in Nepal.

In a study published last year, they followed 18 hikers to Everest Base Camp at 17,500 feet (5,360 meters). Indeed, hikers with the largest lungs, largest spleen, and the greatest reduction in heart rate during apnea were the least likely to develop symptoms of acute mountain sickness.

The size of the spleen isn’t the only thing that matters – its benefits depend on a strong compression response to remove all red blood cells. In a 2014 study on eight peaks of Everest, they found that three repeated breaths before the ascent increased the volume of the spleen, on average, from 213 milliliters to 184 milliliters. After the ascent, the same three apneas lowered the spleen to 132 milliliters. Prolonged exposure to altitude enhanced the spleen’s response while diving. In fact, there is also some evidence that just getting to a moderate altitude will cause a sustained slight contraction of the spleen, as your body has a hard time coping with low-oxygen air.

Some of these adaptations are clearly genetic. Freedivers Sherpas and Bajau both have greater spleen than other closely related populations, likely from generations spent either in the high mountains or underwater. But Schagatay does not believe that it is all genetic. After all, Sherpas who no longer live at high altitudes have larger spleen than Lowland Nepalese, but not as big as Sherpas who still live at altitude. Along with other traits like the diving reflex, it’s something that improves with training, she believes.

What can you do with this information in practice? Here is some data from the Everest Base Camp study, showing the percentage decrease in heart rate during a one-minute apnea. Participants are divided into three groups, based on their scores on the Lake Louise Questionnaire (LLQ), a measure of acute mountain sickness during the hike. Those with the highest scores – the sickest, in other words – barely have a reduction in heart rate; those with the lowest scores were on average about 18% lower:

Data from the Everest Base Camp study, showing the percentage decrease in heart rate during a 1-minute apnea (Frontiers in physiology)

To test your own heart rate decrease during a one minute apnea, you would need an appropriate heart rate monitor, as the relevant data point is the lowest instantaneous rate. you reach by the end of the minute. This is just one factor among many, but it can give you an indication of your risk of suffering from altitude sickness while trekking, which could help you decide on aggressiveness. a route to follow or if you wish to take Diamox prophylactically. . (This particular study was done in Kathmandu, at 4,800 feet, so it is possible that the predictions are different at sea level – water for a future study.)

Even more intriguing is the possibility that you can form these responses. For example, in a 2013 study, Schagatay and colleagues found that two weeks of maximum 10 breaths per day enhanced the diving response, producing a faster and more pronounced drop in heart rate. The next step: determine if this type of improvement would make a practical difference for hikers.

The biggest takeaway, for me, is the idea that snorkeling is not as crazy and unnatural a hobby as I initially thought it was when I first read Deep. The mammalian diving reflex goes back to our evolutionary history – it’s what Per Scholander, one of the first scientists to study it, called “the main switch of life.” And if Schagatay is right, the circuit that allows us to go deep is also what allows us to reach the top of Mount Everest, because, as she says, we were born to dive.

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