Minimum depth change required for an AGE?

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Antarctic-Adventurer

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We all know not to hold one's breath when ascending while diving, it's one of the most important rules in Scuba Diving taught right at the beginning of an Open Water course.

We also know that due to Boyle's Law, the risk of Arterial Gas Embolism from air entering torn (pulmonary) capillaries is actually greatest in shallow water/closer to the surface rather than deep water (as the volume change is greater the shallower you are).

However one question that I've been wondering is just how little of a pressure (depth) change would it require to get AGE? E.g. is 1 metre enough if shallow? 2 m? 3 m? The most extreme example would be if you took a full breath on scuba at say only 1 or 2 metres under, held your breath and surfaced would you get an AGE? Or is it more like ascending from 5 metres while holding one's breath before that would happen?

Curious to hear from anyone with a better understanding of this than me.
 

KenGordon

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1m as below.

“Acute exacerbation of reactive airway disease, anatomic anomalies such as bullae or blebs, or holding the breath can entrap air in the lungs of a diver. [1] Subsequent ascent of as little as 1 meter (approximately 3 feet) may cause an overpressurization sufficient to rupture lung alveoli and introduce gas into the surrounding tissues and/or blood vessels. [2] [3][4] This is referred to as pulmonary overinflation syndrome and results in one or more overexpansion injuries: pneumomediastinum, pneumothorax, subcutaneous emphysema, or arterial gas embolism.”

from Diving Gas Embolism - StatPearls - NCBI Bookshelf

It is easy to imagine a full fragile and slightly stretchy thing having an extra 10% of volume might lead to a tear. I also like to think about how fast local pockets of gas might empty. Lungs are not like a single balloon, but very many balloons linked together. They might mostly be venting fine but the odd one not, so don’t push it. This is what worries me about fast ascents. These are just my mental models and not anything I’d push as “truth” though.
 

ginti

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As you mentioned, one of the key factors is the relative (percentage) pressure change, not the depth change. The difference between 51 and 50 meters in terms of relative pressure is way smaller than the one between 3 and 2 meters. Another key factor is how full the lungs are before changing the depth. If lungs are almost empty, they can tolerate a higher depth change.

What is interesting is to know how much overextension the lungs can tolerate. I once read that the maximum volume overextension tolerable is in the range of 30%. In other words, if you fully inhale and you change depth inducing more than 30% pressure decrease, lungs will have mechanical damage. Assuming a pressure/depth ratio of 1bar/10m, the pressure difference that brings a 30% volume overextension is equivalent to a depth change:
- from 5m (pressure 1.5bar) to 0.5m (pressure 1.05bar);
- from 50m (pressure 6bar) to32m (pressure 4.2bar).

Unfortunately, I cannot find anymore the reference I read, and looking at my numbers, I believe that "30%" could be too much... better ask some expert @Duke Dive Medicine

EDIT: after reading the reference in the previous post, probably the number was "3%", not "30%"... More precisely, assuming the same pressure/depth ratio, the depth change mentioned in the article, from 10m to 9m, induces a pressure change of 5%
 
OP
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Antarctic-Adventurer

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Thanks for your replies @ginti @KenGordon. OK so it seems that the answer is, a small pressure change could lead to lung rupture in theory. What surprises me then is that lung over-expansion injuries don't happen far more often? For example, in heavy seas a diver could easily be pushed up and down several metres near the surface due to swell/wave action, for instance when surfacing or performing a safety stop. If taking a full breath happened to coincide with the upward movement from a swell you could easily imagine AGE occurring, and yet if it was that common/risky, one would think we would hear about it much more frequently; and yet we don't.

That makes me wonder in practice how easy it really is... not planning on finding out of course!
 

Akimbo

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The difference between 51 and 50 meters in terms of pressure is way smaller than the one between 3 and 2 meters.

I believe you meant volume instead of pressure. The change in PRESSURE in a one meter depth change is constant regardless of depth. The change in VOLUME of a gas varies depending on the depth, being much greater closer to the surface.
 

ginti

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I believe you meant volume instead of pressure. The change in PRESSURE in a one meter depth change is constant regardless of depth. The change in VOLUME of a gas varies depending on the depth, being much greater closer to the surface.

You are somewhat right :) I meant the relative (percentage) change in pressure, which is responsible for volume changes. The absolute pressure change is irrelevant.

I edited the post to make it clear.
 

ginti

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Thanks for your replies @ginti @KenGordon. OK so it seems that the answer is, a small pressure change could lead to lung rupture in theory. What surprises me then is that lung over-expansion injuries don't happen far more often? For example, in heavy seas a diver could easily be pushed up and down several metres near the surface due to swell/wave action, for instance when surfacing or performing a safety stop.

If your lungs are half empty, you could tolerate a pressure decrease of 50% without inducing any overexpansion of the alveoli. Indeed, with a 50% decrease in pressure, the gas volume doubles: if at the beginning it was half the maximum allowed by the lungs, now it is the maximum allowed.

To give an idea, at a depth of 6m, the pressure is roughly 1.6bar, and a 50% decrease would bring the absolute pressure to 0.8bar... so in theory, you could go from 6m to the surface with half-empty lungs without incurring in any damage.

That said, do not try it :D

If taking a full breath happened to coincide with the upward movement from a swell you could easily imagine AGE occurring, and yet if it was that common/risky, one would think we would hear about it much more frequently; and yet we don't.

How many times do you keep your lungs full of air? Personally, I never ever take full breaths when I dive

That makes me wonder in practice how easy it really is... not planning on finding out of course!

Usually, we hear about it on uncontrolled ascents when people panic and hold their breath - besides this situation, I believe it is almost impossible to get an AGE.

Now, I am not an expert at all, so I would be glad if people with more experience add something here.
 

Akimbo

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Expanding a little more, you also have to consider how inflated the diver's lungs are when they start their ascent. I find that I can voluntarily increase my lung volume about double between the top of a normal relaxed breathing inhalation cycle and the hyperinflated volume typical for freediving.

For example, a entanglement emergency where a diver has to ditch their gear and takes a deep breath before ascending would be very likely to embolize in the last meter or two if they forget to exhale on ascent. Making the same ascent from a sudden OOA (Out of Air) situation where the lungs were near their normal exhalation cycle would not result in an embolism because the volume change would be less than the normal voluntary hyperinflation level.

This is an important distinction because you will hear stories where untrained people didn't embolize themselves in a swimming pool when holding their breath. The probable reason is they were fortunate enough to have enough hyperinflation volume left to reach the surface before embolizing themselves. The critical factor to remember is a human alveoli is only one cell thick, and a relatively tiny cell at that.

full.png

It is amazing that we don't embolize ourselves by sneezing given that the alveoli wall is far thinner than a piece of paper!

Edit: Simulpost with @ginti's above.
 

Bob DBF

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What surprises me then is that lung over-expansion injuries don't happen far more often?

As long as the airway is open, there is no differential pressure so no problem. The issue is that when divers panic, the training to keep the airway open is most likely to be forgotten, whether a new diver or more experienced.
 

Duke Dive Medicine

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We all know not to hold one's breath when ascending while diving, it's one of the most important rules in Scuba Diving taught right at the beginning of an Open Water course.

We also know that due to Boyle's Law, the risk of Arterial Gas Embolism from air entering torn (pulmonary) capillaries is actually greatest in shallow water/closer to the surface rather than deep water (as the volume change is greater the shallower you are).

However one question that I've been wondering is just how little of a pressure (depth) change would it require to get AGE? E.g. is 1 metre enough if shallow? 2 m? 3 m? The most extreme example would be if you took a full breath on scuba at say only 1 or 2 metres under, held your breath and surfaced would you get an AGE? Or is it more like ascending from 5 metres while holding one's breath before that would happen?

Curious to hear from anyone with a better understanding of this than me.

I've personally seen it happen in less than a meter. A colleague of mine was on surface-supplied air at about 25 feet, squatted down to pick up a heavy object on the bottom, held her breath when she stood up with the object, maybe 2 1/2 feet of depth change, and suffered a gas embolism. That was in relatively shallow water, which makes a difference as @Akimbo pointed out.

Best regards,
DDM
 
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