Emergency swimming ascent 33ft depth limitation?

[Subcooled]

I want to understand some physiology...

In the NAUI Scuba Rescue Diver book it is said that an emergency swimming ascent is increasingly difficult from depths beyond 33ft / 2 ATA. This is said to be due to loss of consciousness as the partial pressure in the brain drops. I recall that unconsciousness results from something like 12% x 1 ATA = 0.12 ATA ppO2.

I don't really understand the dynamics of this...

- I can hold my breath from 1½ minutes to 2 minutes while resting at surface, room temperature.

- An emergency ascent using a drysuit tends to become a boyant one, so no need to work hard.

- Ascending at 18m/min (PADI recommendation) I could do an emergency ascent from about 18m in one minute, without air.

So... what makes the ascent so difficult?

ppO2 is decreasing as oxygen is metabolized, and the drop in ambient pressure also cuts ppO2... but how is this different from the breath holding at the surface? I inhaled oxygen at higher pressure at the bottom. Doesn't that compensate for the pressure drop effect? Did I not get more of it down there?

 

[danvolker]

I want to understand some physiology...

In the NAUI Scuba Rescue Diver book it is said that an emergency swimming ascent is increasingly difficult from depths beyond 33ft / 2 ATA. This is said to be due to loss of consciousness as the partial pressure in the brain drops. I recall that unconsciousness results from something like 12% x 1 ATA = 0.12 ATA ppO2.

I don't really understand the dynamics of this...

- I can hold my breath from 1½ minutes to 2 minutes while resting at surface, room temperature.

- An emergency ascent using a drysuit tends to become a boyant one, so no need to work hard.

- Ascending at 18m/min (PADI recommendation) I could do an emergency ascent from about 18m in one minute, without air.

So... what makes the ascent so difficult?

ppO2 is decreasing as oxygen is metabolized, and the drop in ambient pressure also cuts ppO2... but how is this different from the breath holding at the surface? I inhaled oxygen at higher pressure at the bottom. Doesn't that compensate for the pressure drop effect? Did I not get more of it down there?

This is pretty much nonsense.
a bouyant ascent with your drysuit from 90 feet would be easy, as long as you exhale all the way up, and don't freak out. Back in the late 70's we all did free swimming ascents from 90 feet routinely, and no one died or had problems with this. As the adventure divers of the 70's got replaced by couch potatoes and video gamers of the last decade, physical abilities of the "group" have changed, and what should be really easy may not be, for some people( for some people who maybe should not be divers

 

[Doubler]

In 1988 I had a regulator failure at 65 ft which resulted in me performing a CESA. Difficult did not even cross my mind, survival did. I don't understand the 33 ft thing. In Sub school escapes where practiced in the tower from 100+.

 

[Crowley]

Are you talking about loss of consciousness during the controlled emergency swimming ascent (CESA, in PADI speak), using compressed gas and scuba or shallow water blackout whilst skin-diving?

My basic logic: you breath a pp02 at the surface of .21 ATA - you hyperventilate excessively, perform a breath hold dive to 10m, where the pp02 in air is doubled to .42ata - your body continues to metabolise the 02 in your body, and there is a 60 (ish) second window where the pp02 remains relatively stable and after which the amount of 02 in your body starts to drop rapidly. The excessive hyperventilation is the key - it fools your brain into ignoring the breathing cycle trigger, reducing the urge to surface for a breath of fresh air. Let's assume for the sake of mathematics that you stay at depth long enough to metabolise half the 02 in your body - so now you have a pp02 of .21ata in the system at 10m. You ascend, meaning at the surface this will drop to .105 ata. The human body requires a pp02 of approximately .16ata to remain conscious - you will reach this level at 3 or 4 metres and black out. This is shallow water blackout, and happens during breath-hold diving - but you have to stay underwater for a fair amount of time for the body to metabolise enough oxygen to do so.

If you suddenly run out of air at 20m on scuba, the pp02 from the tank you are breathing is at .21 x 3 = .63ata - and if you could skin dive to 20m the pp02 in your body would be roughly the same - .63ata, so you don't get more 02 because you are on scuba. If you metabolise at the same rate (assuming a constant work effort), the pp02 will drop as you ascend in the same way as if you were making a breath-hold dive. If you ascend at 9m per minute as per NAUI recommendations then yes, it would take you two minutes to ascend, and I suppose you could metabolise enough 02 during this time for the partial pressure to drop significantly enough that you would black out on the way up, but I think you'd really have to work at it. It would depend, I think, on the amount of energy you expend swimming, and the length of time it takes to reach the surface. One thing you're not going to do is hyperventilate excessively, hold your breath and hang around underwater before you head for the surface, which is the key to shallow water blackout. You're also unlikely to maintain such a slow ascent rate! Addendum - I am not familiar with NAUI training standards regarding emergency ascents.

Several older dive training techniques required CESAs from 30m/100ft so for sure it is possible, as other posters here have and will testify.

So short version - it's possible, but highly unlikely, that you would black out from a deep CESA.

Is my logic correct (if simplistic)? I am sort of asking a question here also - and always happy to learn.

Cheers,

C.

 

[Zippsy]

Well it IS more difficult to do it, the deeper you are but there is nothing magical about 33ft. It certainly can be done from significantly deeper depths by most people, IMHO.

 

[Hank49]

I want to understand some physiology...

In the NAUI Scuba Rescue Diver book it is said that an emergency swimming ascent is increasingly difficult from depths beyond 33ft / 2 ATA. This is said to be due to loss of consciousness as the partial pressure in the brain drops. I recall that unconsciousness results from something like 12% x 1 ATA = 0.12 ATA ppO2.

I don't really understand the dynamics of this...

- I can hold my breath from 1½ minutes to 2 minutes while resting at surface, room temperature.

- An emergency ascent using a drysuit tends to become a boyant one, so no need to work hard.

- Ascending at 18m/min (PADI recommendation) I could do an emergency ascent from about 18m in one minute, without air.

So... what makes the ascent so difficult?

ppO2 is decreasing as oxygen is metabolized, and the drop in ambient pressure also cuts ppO2... but how is this different from the breath holding at the surface? I inhaled oxygen at higher pressure at the bottom. Doesn't that compensate for the pressure drop effect? Did I not get more of it down there?

12% of 1 ATA air is .12 but the pp O2 is only .0252 since air is only 21% O2.
You would have the same pp 02 at 10 meters as a free diver who took a breath at the surface but you would have double the volume since you filled your lungs at depth. The Free diver filled his a the surface but his compressed to half that volume at 10 meters.
You would have a much higher pp 02 as you neared the surface as the freediver in most cases (assuming metabolism and lung capacity is roughly equal) Not likely you'd black out. The risk in a CESA is not dumping air fast enough and injuring your lungs.

 

[Tortuga68]

12% of 1 ATA air is .12 but the pp O2 is only .0252 since air is only 21% O2

The OP was perhaps a little confusing, but the PP02 at 1ATA is 0.21

 

[danvolker]

I want to understand some physiology...

In the NAUI Scuba Rescue Diver book it is said that an emergency swimming ascent is increasingly difficult from depths beyond 33ft / 2 ATA. This is said to be due to loss of consciousness as the partial pressure in the brain drops. I recall that unconsciousness results from something like 12% x 1 ATA = 0.12 ATA ppO2.

I don't really understand the dynamics of this...

- I can hold my breath from 1½ minutes to 2 minutes while resting at surface, room temperature.

- An emergency ascent using a drysuit tends to become a boyant one, so no need to work hard.

- Ascending at 18m/min (PADI recommendation) I could do an emergency ascent from about 18m in one minute, without air.

So... what makes the ascent so difficult?

ppO2 is decreasing as oxygen is metabolized, and the drop in ambient pressure also cuts ppO2... but how is this different from the breath holding at the surface? I inhaled oxygen at higher pressure at the bottom. Doesn't that compensate for the pressure drop effect? Did I not get more of it down there?

AT 90 feet, the pp O2 is high enough that you could actually have a longer breathhold at 90 feet, than one originating on the surface--the danger whether originating at the surface, or from a breath at 90 ( and then a 2 minute fun swim breath hold--not smart, just explaining) would be that as you approach 30 then 20 then 10 feet, the PP O2 drops so much that if you have been dawdling or sightseeing too long, you may suddenly have insuffcient ppo2 to maintain consciousness...shallow water blackout ensues....while this is the theoretical issue you asked about, the real use of a free swimming ascent is with a fairly rapid ascent, one typically with very little effort involved, and where PPO2 is just fine when you reach the surface. The only way I can imagine a person not being able to do this is if :

1. they have a huge heart rate due to extreme fear
2. a huge heart rate due to their body being very negative and having to swim upward and performing a high workload at the same time ( heavy tanks, bc failure, carrying gold bullion and treasure found on the bottom
3. fitness level so low they were really badly impaired even on the surface, and have no business diving.
4. Went ridiculouslyt slow on ascent due to unfounded fears of DCS ( OOA death is way worse than DCS) , even to the point of trying to do a safety stop.. aND/OR , blew out air way too fast at the bottom, and the rest of the way up--maybe from fear of lung overexpansion injury....you really need to just purse your lips and make like a whistle--as long as a little stream of air keeps coming out, and you don;t feel your lungs getting too full, you are fine....you DO feel how full your lungs are....from 90 feet, I'd be ascending quickly, and keeping my lungs around 1/3 full....and just keep it with that feeling, slowing down or increasing exhale volume to maintain this feeling.

 

[Bubbletrubble]

If you suddenly run out of air at 20m on scuba, the pp02 from the tank you are breathing is at .21 x 3 = .63ata - and if you could skin dive to 20m the pp02 in your body would be roughly the same - .63ata, so you don't get more 02 because you are on scuba. If you metabolise at the same rate (assuming a constant work effort), the pp02 will drop as you ascend in the same way as if you were making a breath-hold dive. If you ascend at 9m per minute as per NAUI recommendations then yes, it would take you two minutes to ascend, and I suppose you could metabolise enough 02 during this time for the partial pressure to drop significantly enough that you would black out on the way up, but I think you'd really have to work at it. It would depend, I think, on the amount of energy you expend swimming, and the length of time it takes to reach the surface. One thing you're not going to do is hyperventilate excessively, hold your breath and hang around underwater before you head for the surface, which is the key to shallow water blackout. You're also unlikely to maintain such a slow ascent rate! Addendum - I am not familiar with NAUI training standards regarding emergency ascents.

@Crowley: Is this the explanation that PADI or NAUI gives for the possible mechanism of a diver blacking out during a fast ascent (CESA or other)?

The reason I ask is that there's a distinction between the ppO2 in the lungs and the concentration of O2 in the blood. Cerebral hypoxia is presumably the cause of blackout in the discussed scenario. It's possible that other physiological mechanisms play a role (carotid body pressure sensors, baroreceptor reflex, fluid shifts in water, blood chemistry changes causing shifts in the hemoglobin-O2 curve, etc.). There's also the possibility of inadvertent Valsalva leading to lung-overexpansion injury ± cerebral gas embolism.

 

[Crowley]

@Crowley: Is this the explanation that PADI or NAUI gives for the possible mechanism of a diver blacking out during a fast ascent (CESA or other)?

The reason I ask is that there's a distinction between the ppO2 in the lungs and the concentration of O2 in the blood. Cerebral hypoxia is presumably the cause of blackout in the discussed scenario. It's possible that other physiological mechanisms play a role (carotid body pressure sensors, baroreceptor reflex, fluid shifts in water, blood chemistry changes causing shifts in the hemoglobin-O2 curve, etc.). There's also the possibility of inadvertent Valsalva leading to lung-overexpansion injury ± cerebral gas embolism.

No it's more of an over-simplification on my part - lumping together the amount of oxygen in the system as a whole, rather than each particular component, in order to think my way around the possibilities and simplify into a more "layman's" perspective. It is the general reason given for shallow-water blackout during breath-hold diving and whilst my own physiological knowledge is perhaps a little better than the average instructor, it's really not my field of expertise and the OP's question was a novel one.

Describing the terms you put forward to the average open water student would bounce right off their grey matter, so I'd actually like to get a handle on whether or not what I theorised is "broadly accurate for a person without in-depth physiological knowledge", in terms of the relationship between shallow water blackout and a possible similar phenomenon during a deep CESA... or complete cobblers!

We're all learning. At least, I hope we are! And I know somebody's going to say "cobblers", shortly... so pretend you've already said it and offer an explanation. I'm not afraid of big words, I have the internet handy!

Cheers

C.