Outdoor

How does your mind react while you maintain your breath?

We all know what it feels like when we run out of oxygen – or at least what it feels like when we run out of oxygen. In reality, the breathlessness we experience during hard training or at high altitude or when we simply hold our breath has more to do with too much carbon dioxide in the blood than too little oxygen. As demonstrated by the achievements of elite freedivers – like taking a single breath for 11 minutes and 35 seconds – our limits are not what they seem.

I have long been intrigued by studies of what happens in freedivers when they hold their breath, what defines their limits, and how those skills translate to other environments like high altitude. But their skills are so unusual that it feels like studying a different species. So I was particularly keen to see a study recently published in the European Journal of Applied Physiology that looked at holding breath in normal people without prior training. The study is very straightforward and measures heart rate and oxygen levels while volunteers hold their breath. It provides a telling picture of how the body deals with a lack of oxygen – and what can go wrong.

The research was carried out at Ghent University in Belgium by Janne Bouten, Jan Bourgois and Jan Boone. (I assume that scientists in Belgium are assigned to different departments in alphabetical order.) They asked 31 volunteers (17 men, 14, women) to hold their breath three times in a row for as long as possible, each with a two-minute break. As a rule, people get better and better with repeated respiratory arrests, also because their spleen pushes more oxygen-carrying red blood cells into the circulation. During the third and final breath hold, they took continuous measurements of parameters including heart rate, oxygen levels in the brain, and oxygen levels in the leg muscles.

Humans, like other mammals, have a “diving response” that kicks in when you hold your breath to make sure your brain always has enough oxygen. As the researchers point out, if your circulation stops abruptly, if your circulation stops abruptly, you pass out in 30 seconds and suffer irreversible damage within two to ten minutes. The diving response is improved when your face is submerged in water, but this also happens on dry land. Your heart rate drops, and the blood vessels that lead to non-essential parts of the body like your leg muscles constrict to redirect critical blood (and oxygen) to the brain.

The subjects held their third breath for an average of two minutes and 37 seconds, which is incredibly good for normal, untrained people. Perhaps the secret is to take three breaths in a row. or maybe i’m just weak. Anyway, this was the average heart rate response. The data is only displayed for the first 60 seconds (left) and the last 60 seconds (right), so that you can plot all the data together, even though they took different lengths of time. The gray area shows when you started and stopped breathing.

(Image: Courtesy of the European Journal of Applied Physiology)

On the far left, you can see how the blue dots (which represent the average value) increase as subjects prepare to hold their breath. This may be because you are excited or concerned, and it may also be the result of taking deep breaths in preparation. Subjects were specifically prohibited from hyperventilating (which blows away a lot of carbon dioxide so you can hold your breath longer) before holding their breath, but they were given a 30-second warning and 10-second countdown and prompted to do so Just before you start, take a deep breath, but not a maximum. Heart rate will drop within about ten seconds of starting to hold your breath. It drops 27 beats per minute and hits its low point after an average of 83 seconds. This is pretty similar to what you see with elite freedivers, except that they hit their minimum heart rate within 30 to 60 seconds.

You will notice a row of red dots and another row of white dots. There are two people who quit early; one of them passed out, the other became dizzy and on the verge of fainting. More on this below.

The next parameter is the oxygen supply to the tissues in the leg muscles, as measured by near-infrared spectroscopy, which essentially radiates infrared light through the skin and measures how much of the oxygen-rich hemoglobin is absorbed. Here the picture is pretty simple: the oxygen levels in the muscles begin to decrease within five seconds and continue to decrease until the subjects start breathing again. This is what you would expect as the blood vessels narrow to shift blood flow from the extremities to the brain.

Respiratory arrest-two_h.jpg(Image: Courtesy of the European Journal of Applied Physiology)

The final piece of the puzzle is where things get interesting. The oxygen supply to the brain was also measured using near-infrared spectroscopy:

Breath-hold-three_h.jpg(Image: Courtesy of the European Journal of Applied Physiology)

Here you can see an initial decrease in oxygen levels in the brain, possibly related to the sudden drop in blood pressure associated with beginning breath-holding. But within about five seconds the drop is reversed and the oxygen levels in the brain begin to rise – and after about a minute it actually reaches values ​​that are about four percent higher than the initial value. This is a pretty good indication of how powerful the brain’s self-protecting cabling is: you hold your breath, and it gets more oxygen than less.

However, this happy state does not last forever. Even as more and more blood is directed to the brain, as you hold your breath, that blood carries less and less oxygen, so your brain oxygen levels gradually decrease. This decline continues until you finally give up. On average, brain oxygen decreased by about five percent when the subjects gave up. Interestingly, that’s roughly the same level you see in elite freedivers after two and a half minutes. This means that the freedivers cannot maintain the oxygen levels in their brains much better. Instead, the difference seems to be that they are willing to sustain the uncomfortable urge to breathe longer. Other research has found that freedivers are able to hold their breath until their brain’s oxygen levels are so low that they lose consciousness – a very dangerous situation if this happens underwater.

That brings us back to the two issues that passed out or were approaching her. If you look again at the graph of oxygen levels in the brain you can see that their data has gotten completely out of hand compared to everyone else’s. They drop off steeply, then manage to compensate for a while, but the drop continues and very soon their oxygen levels in the brain are so low that they reach the limit of consciousness. For the red dots, the muscle oxygen data indicates that this subject had a weak response to the narrowing of blood flow to the muscles. This means that he or she kept pumping blood to the extremities and not enough getting to the brain. For the white dots, the data gives no indication of what went wrong, but the result was the same: not enough oxygen for the brain.

One of the reasons for the study was that some researchers and coaches have advocated various forms of respiratory arrest training to improve athletic or altitude performance. Since most previous respiratory arrest exams used trained freedivers, it was not clear whether the brain’s self-defense mechanisms would be effective for beginners. The new data shows it’s okay, but the two fainting spells also show that caution should be exercised: the researchers suggest that everyone should be familiar with the warning signs of fainting (especially dizziness) and not do respiratory arrest training on their own.

If you’d like to learn more about Sweat Science, visit me on Twitter and Facebook, subscribe to the email newsletter, and read my book Endure: Mind, Body, and the Strangely Elastic Limits of Human Achievement.

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