Lack of oxygen during a free ascent?

[Akimbo]

Start at 10' and add 10'. You will get to the point that where 10' increase seems like nothing. It is all builds skill and experience, which results in the self-reliant confidence you are ultimately seeking. Repetition at this point is as or more important than the depth. When you get to around 60', consider limiting them to two cycles at the beginning of a diving day. Maybe one cycle if/when you choose to try 100'+.

Keep your airway open and enjoy yourself.

 

[Akimbo]

As I go up should I be merely saying "ahhh" or should I be yelling "AAHHHHHH"?

Whisper cheese if that keeps your airway open. One of the Navy divers I worked with had recently come from duty at the submarine escape tower at New London. He described instructions to boat sailor candidates more like:

• thrust your lower jaw forward
• look up at about 45-60°
• gently purse lips like blowing your lover a kiss (I like that part, which is probably the only reason I remember this guy)

The idea is to find a physical position that makes it difficult, awkward, or uncomfortable to close your airway. Yelling requires too much conscious effort at a time where you may have more important issues demanding your attention. It probably would expel air faster than necessary as well. You will feel it after a few cycles even from 20'. Besides, you would probably choose different single syllable word if it really hits the fan.

It would be fun to have one of these babies to practice in and climbing the steps will keep you in shape. I hear the elevator only goes up in this thing... really fast

Submarine escape training facility - Wikipedia

 

[Moerby]

Whisper cheese if that keeps your airway open

Hey I really like that one. For reasons I fail to comprehend I have this reflexive need to say ahhh LOUDLY, whereas saying cheese presents no such problem.

 

[Moerby]

What is the height on that tank? I'm horrible at estimating.

 

[TSandM]

Okay, let me back up and deal with some misinformation here.

One -- exhaling alone does not decrease the CO2 in your blood. The blood is coming back to the lungs and CO2 is diffusing into the gas that's in the alveoli. As the CO2 does this, the CO2 concentration in the alveoli increases, and the rate of diffusion slows. When you exhale, all you do is reduce the volume in your bronchial tree -- you do nothing to change the concentration of the gases there. In the absence of fresh gas to inhale, the CO2 will continue to climb. However, there is a psychological relief to exhaling, and it is necessary because excess volume will cause trauma to the lungs, as the gas in them expands on ascent.

Second, you can black out from CO2, but it takes high concentrations to do it. I have seen patients with three times the normal amount of CO2 in their blood, who were still conscious. A normal person without lung disease would be in violent panic far before they got to those kinds of CO2 levels. Rebreather divers have the risk of getting there, because no matter how much they breathe, if the scrubber isn't removing CO2, neither will they. Very deep divers combine an element of narcosis with CO2 retention, and can and do tolerate enough CO2 to lose consciousness. When you are talking about recreational scuba depths, that's just not going to happen.

Shallow water blackout from low O2 levels, however, IS a real phenomenon. It is unlikely to occur, however, during any length of time when the average, untrained person is able to go without inhaling. One of the exercises I did during my rebreather tryout was to sit and breathe the loop down from a ppO2 of 1.0 to .21. A ppO2 of 1.0 corresponds to approximately air at 130 feet, so this isn't a ridiculous comparison to normal scuba, although admittedly, the volume of the counterlungs increases MY lung volume by quite a bit. Still, it took over 20 minutes to drop the ppO2 that much -- so you certainly have several minutes of not breathing before your ppO2 from 100 feet is going to get to where you aren't conscious any more.

It is quite possible to do CESAs from 100 feet, and any number of people on this board have done them. I maintain strongly, however, that practicing to do a CESA from that depth is like rearranging the deck chairs on the Titanic. The proper question is not, "How do I CESA from 100 feet?" but "How do I avoid ever having to CESA at all?" And it isn't necessarily a team question, because providing one's own redundant equipment is another answer. But not planning, and practicing a procedure with risks attached to it that you should never need to perform, seems to me to be the wrong way to approach the question.

 

[Akimbo]

Okay, let me back up and deal with some misinformation here.

One -- exhaling alone does not decrease the CO2 in your blood. The blood is coming back to the lungs and CO2 is diffusing into the gas that's in the alveoli. As the CO2 does this, the CO2 concentration in the alveoli increases, and the rate of diffusion slows. When you exhale, all you do is reduce the volume in your bronchial tree -- you do nothing to change the concentration of the gases there. In the absence of fresh gas to inhale, the CO2 will continue to climb. However, there is a psychological relief to exhaling, and it is necessary because excess volume will cause trauma to the lungs, as the gas in them expands on ascent...

Nobody said that exhaling alone decreased CO2 blood levels. Exhaling as atmospheric pressure reduces also reduces the number of CO2 molecules in the in the lungs compared to a static breath holding condition. Therefore the diffusion rate increases over what it would have been at higher concentrations.

... Second, you can black out from CO2, but it takes high concentrations to do it. I have seen patients with three times the normal amount of CO2 in their blood, who were still conscious. A normal person without lung disease would be in violent panic far before they got to those kinds of CO2 levels…

Do you think those patients might black out at say 99' with 12 times normal PPCO2 -- 4 ATA x 3x normal CO2? Blackout aside, panic induced drowning dooms far more recreational divers anyway, long before anoxia does them in.

... Rebreather divers have the risk of getting there, because no matter how much they breathe, if the scrubber isn't removing CO2, neither will they...

True, but they rarely do (blackout from CO2). Generally speaking, they are well trained to recognize the symptoms and react accordingly -- going open circuit on their bailout. It is not like their symptoms are subtle! Hypoxia and anoxia nails a lot more of them since detection is entirely instrument dependent.

... Very deep divers combine an element of narcosis with CO2 retention, and can and do tolerate enough CO2 to lose consciousness. When you are talking about recreational scuba depths, that's just not going to happen...

That is a highly questionable hypothesis. Several diving disciplines develop a mental and physical tolerance to CO2. Narcosis also numbs people's senses in many and varying ways. The phenomenon is most studied, at the moment, among world class apneists (deep freedivers), but was recognized in military and commercial divers in the 1960 before widespread use of helium essentially eliminated severe narcosis from diving profiles. CO2 blackout was also the "presumed" cause of several deaths before the mid-1960s when regulator performance was marginal and dead spaces in full face masks and regulators was huge (especially double hoses before inhalation & exhalation valves were introduced). Sure the PPCO2 concentrations have to be high for blackout, but it is not that difficult under heavy workloads and more than 6 atmospheres in this old gear to reach that point. I have experienced it myself on an instrumented test dive in a wet pot.

... Shallow water blackout from low O2 levels, however, IS a real phenomenon. It is unlikely to occur, however, during any length of time when the average, untrained person is able to go without inhaling....

You're right, shallow water blackout is not a factor here. It IS a very big deal for freedivers who are not well oxygenated when they leave the bottom and are skilled at suppressing CO2 symptoms.

The real point here is every sane diver agrees that it is stupid to attempt an emergency free ascent when you are not skilled at it. This discussion is about developing the skill and indirect benefits that will, IMHO, make anyone a more competent diver.

There is nothing unsafe about practicing free ascents as long as you keep the airway open, don't get bent, and have your regulator ready to stuff back in your mouth. It is silly to discourage or deny this self-rescue tool to the diving community. It just is not as hard as people try to make it out to be, as anyone who actually has tried it can attest.

Start slow, gradually increase depth until you are satisfied or have found your personal limits.

 

[knotical]

... and have your regulator ready to stuff back in your mouth.

An alternate (in my opinion, preferable) technique is to keep your regulator in your mouth. In a real-world out of air situation, you will likely be able to inhale more gas as ambient pressure decreases during your ascent. In the bad old days of J-valves / pre-SPGs, I did just that.

 

[TSandM]

Sure the PPCO2 concentrations have to be high for blackout, but it is not that difficult under heavy workloads and more than 6 atmospheres in this old gear to reach that point.

More than 6 ATAs is not typical "recreational" diving, and I did say that this can occur to technical divers.

I remain firmly in the camp that spending a great deal of time thinking about this technique, worrying about it, or practicing it, is wasting time MUCH better spent on dive planning, gas management, improving buddy skills, and doing gear maintenance.

 

[Moerby]

More than 6 ATAs is not typical "recreational" diving, and I did say that this can occur to technical divers.

I remain firmly in the camp that spending a great deal of time thinking about this technique, worrying about it, or practicing it, is wasting time MUCH better spent on dive planning, gas management, improving buddy skills, and doing gear maintenance.

TSandM, I agree with that but I think there is room for practicing and perfecting the basics - AND - practicing extreme emergency measures such as CESA (if only once in a blue moon). For me, doing a CESA in a controlled setting is simply about having the confidence that I can do it when a real emergency comes around. So it's a mental thing, maybe that's just the way I'm wired.

 

[Akimbo]

An alternate (in my opinion, preferable) technique is to keep your regulator in your mouth. In a real-world out of air situation, you will likely be able to inhale more gas as ambient pressure decreases during your ascent. In the bad old days of J-valves / pre-SPGs, I did just that.

The most important thing is to develop automatic reflexes to keep the airway open when ascending at higher rates. Embolism is by far a greater risk in an emergency free ascent than anoxia or CO2 blackout. I fear that there may be too great a tendency to momentarily block your airway while attempting to inhale, especially in that critical last 10-15'. Any technique that keeps that airway open is a good one and your point is well taken.

This circles back to some of my other posts advocating for the characteristic of unbalanced first stage regulators; which increase breathing resistance to noticeable levels long before the tank is virtually empty. A regulator that does not allow the diver to breath cylinders down to the last few breaths before noticing would eliminate most of the completely out of gas accidents that might lead to an emergency free ascent.

Your technique is undeniably best for the situation where you notice breathing resistance and still have 300-400 PSI left, as you would on an unbalanced first stage. I would not consider that a free ascent; more like a low gas ascent that justifies skipping safety stops. The bad old pre-SPG and J-valve days were more like this, especially since the vast majority of regulators were unbalanced and the J-valves were too often accidently already pulled or didn’t activate until the pressure was closer to 300 than 500 PSI.