Emergency swimming ascent 33ft depth limitation?
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kanonfodr
Contributor
I'm curious as to how many posters on this thread have actually seen someone have a either a Loss of Motor Control (LMC or also Samba in freediving circles) or Shallow Water Blackout (properly: Loss of Consciousness or LOC). Having seen one personally, as well as being a PFI trained freediver via this lady, I hope I can shed a bit more light on why CESA becomes more problematic from deeper depths:
First of all, the problem isn't just ppO2: it's ppCO2. An apnea freediver will go through a breathing pattern that lasts for several minutes to rid themselves of as much CO2 as possible, in addition to oxygenating the blood. They are also focusing on being as relaxed as possible, since relaxation discourages CO2 buildup.
If you were to think of the prelude to a freedive, it could be broken down into three stages: Ventilation, Purging, and Peak Inhalation. Ventilation is about the same as what we scuba divers do: long, deep breaths to oxygenate the body. Purging is 5 full, hard breaths (not hyperventilation which is shallow, rapid breathing) to try and clear as much CO2 as possible from the lungs (think HOO! HOO! HOO! HOO! HOO! vs huff huff huff huff huff). Finally, right before the dive is a Peak Inhalation: filling the lungs as efficiently as possible starting from the diaphragm below the lungs, to the intercostals around the lungs, then finally scalene muscles on top of the lungs. This should give you an idea.
After peak inhalation, the dive is begun. The freediver focuses on streamlining his body and moving as efficiently as possible. As they descend, absolute pressure is rising (just like scuba diving) so while ppO2 is climbing, so is ppCo2 as the body turns Oxygen into Carbon Dioxide. Also, gas volume in the lungs is shrinking as the freediver descends due to Boyle's Law so the body is having a harder time replacing CO2 with O2. This occurs all the way down to the diver's turn-around depth.
The ascent:
As the freediver ascends, ppO2 is once again dropping, however ppCO2 is rising pretty fast since the diver is still working until they reach 10 meters where, if properly weighted, exposure suit bouyancy takes over and they can relax their swimming. As we all know, Mr. Boyle's law really favors the uppermost 10 meters: that's where the largest shift in volume occurs per depth change. So pp02 is REALLY dropping, and ppCO2 is REALLY rising. This occurs all the way to the surface, where the diver can begin recovery breathing.
The physiology of the final 10 meters: Breathing is triggered not by a lack of oxygen but rather an abundance of CO2. However there are limits to what the average human is capable of withstanding, in regards to hypoxia (lack of oxygen content in the blood). One of the most accepted numbers is 18% O2, but I have seen 16% and, today, 12%. At the surface these are ppO2s of .18, .16, and .12 respectively. It is very likely that when the freediver breaches the surface, his actual ppO2 will be well below these numbers.
What happens when ppO2 gets this low is the brain goes into self-preservation mode, shutting down lots of the external systems (nerves, senses, even muscle groups) in attempts to conserve oxygen for itself. This can lead to either LMC or LOC, depending on severity. Also, the brain will instruct the lungs to try and breathe again, whatever it can get. This is felt as a contraction (and sometimes seen, as in this video after the 4:00 mark). This could very well be water, if you aren't careful.
What makes a CESA different?
A lot. More than likely you will be stressed, a little CO2 loaded, and you are carrying a heck of a lot more stuff that causes a lot more drag than a freediver so you will have to work harder for less motion. An average freediver can swim at a speed of about 60m/min, twice probably what you are capable of and they are expending a lot less energy to do so. Also, due to Boyle's law the gas in your lungs will be expanding so to avoid a Lung Embolism we are taught to breathe out, hum, sing, whatever. Well, when you do so you are letting out perfectly good O2 which you need instead of the constantly-building CO2 which you don't.
Finally, who will be waiting at the surface for you?? This is the biggest killer of apnea freedivers in the world: either diving solo or with un-trained or un-disciplined buddies. The most dangerous part of a freedive is the 30 seconds after a return to the surface: that's where LMC and LOC will strike. Recovery breathing techniques give the diver the largest possible chance of avoiding LMC/LOC. Most freedivers practice and dive in buddy teams, and the diver who just surfaced is watched for 30 seconds, sometimes coached through recovery breathing, and often is holding onto a surface flotation device that can support them. There are many stories of freedivers making amazing dives, then passing out on the surface, inhaling water, and sinking to their doom. In the presence of other divers.
Hope this helps you understand why CESA from deeper depths isn't advised.
Peace,
Greg
First of all, the problem isn't just ppO2: it's ppCO2. An apnea freediver will go through a breathing pattern that lasts for several minutes to rid themselves of as much CO2 as possible, in addition to oxygenating the blood. They are also focusing on being as relaxed as possible, since relaxation discourages CO2 buildup.
If you were to think of the prelude to a freedive, it could be broken down into three stages: Ventilation, Purging, and Peak Inhalation. Ventilation is about the same as what we scuba divers do: long, deep breaths to oxygenate the body. Purging is 5 full, hard breaths (not hyperventilation which is shallow, rapid breathing) to try and clear as much CO2 as possible from the lungs (think HOO! HOO! HOO! HOO! HOO! vs huff huff huff huff huff). Finally, right before the dive is a Peak Inhalation: filling the lungs as efficiently as possible starting from the diaphragm below the lungs, to the intercostals around the lungs, then finally scalene muscles on top of the lungs. This should give you an idea.
After peak inhalation, the dive is begun. The freediver focuses on streamlining his body and moving as efficiently as possible. As they descend, absolute pressure is rising (just like scuba diving) so while ppO2 is climbing, so is ppCo2 as the body turns Oxygen into Carbon Dioxide. Also, gas volume in the lungs is shrinking as the freediver descends due to Boyle's Law so the body is having a harder time replacing CO2 with O2. This occurs all the way down to the diver's turn-around depth.
The ascent:
As the freediver ascends, ppO2 is once again dropping, however ppCO2 is rising pretty fast since the diver is still working until they reach 10 meters where, if properly weighted, exposure suit bouyancy takes over and they can relax their swimming. As we all know, Mr. Boyle's law really favors the uppermost 10 meters: that's where the largest shift in volume occurs per depth change. So pp02 is REALLY dropping, and ppCO2 is REALLY rising. This occurs all the way to the surface, where the diver can begin recovery breathing.
The physiology of the final 10 meters: Breathing is triggered not by a lack of oxygen but rather an abundance of CO2. However there are limits to what the average human is capable of withstanding, in regards to hypoxia (lack of oxygen content in the blood). One of the most accepted numbers is 18% O2, but I have seen 16% and, today, 12%. At the surface these are ppO2s of .18, .16, and .12 respectively. It is very likely that when the freediver breaches the surface, his actual ppO2 will be well below these numbers.
What happens when ppO2 gets this low is the brain goes into self-preservation mode, shutting down lots of the external systems (nerves, senses, even muscle groups) in attempts to conserve oxygen for itself. This can lead to either LMC or LOC, depending on severity. Also, the brain will instruct the lungs to try and breathe again, whatever it can get. This is felt as a contraction (and sometimes seen, as in this video after the 4:00 mark). This could very well be water, if you aren't careful.
What makes a CESA different?
A lot. More than likely you will be stressed, a little CO2 loaded, and you are carrying a heck of a lot more stuff that causes a lot more drag than a freediver so you will have to work harder for less motion. An average freediver can swim at a speed of about 60m/min, twice probably what you are capable of and they are expending a lot less energy to do so. Also, due to Boyle's law the gas in your lungs will be expanding so to avoid a Lung Embolism we are taught to breathe out, hum, sing, whatever. Well, when you do so you are letting out perfectly good O2 which you need instead of the constantly-building CO2 which you don't.
Finally, who will be waiting at the surface for you?? This is the biggest killer of apnea freedivers in the world: either diving solo or with un-trained or un-disciplined buddies. The most dangerous part of a freedive is the 30 seconds after a return to the surface: that's where LMC and LOC will strike. Recovery breathing techniques give the diver the largest possible chance of avoiding LMC/LOC. Most freedivers practice and dive in buddy teams, and the diver who just surfaced is watched for 30 seconds, sometimes coached through recovery breathing, and often is holding onto a surface flotation device that can support them. There are many stories of freedivers making amazing dives, then passing out on the surface, inhaling water, and sinking to their doom. In the presence of other divers.
Hope this helps you understand why CESA from deeper depths isn't advised.
Peace,
Greg
Bubbletrubble
Contributor
@Crowley: For what it's worth, I don't necessarily think that your simplified explanation is 100% cobblers!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!
I'm not an expert on dive physiology either. I was using some of that terminology to keep the post succinct. I think all an "average" OW student needs to know is that the closer he/she is to the surface the greater the probability that the CESA will be successful. This is a great reason to monitor the SPG vigilantly, have an appreciation for gas planning, and stay close to one's buddy. If the student wanted to know more, I'd probably give the noncommittal answer: "It's complicated."
If you want to get a better handle on the physiological explanation as to why breath-hold divers are susceptible to shallow water blackout (SWB), read this essay on latent hypoxia on the scuba-doc.com website. Pay particular attention to the section "On Ascent to the Surface." You can then apply some of the same reasoning to a scuba diver conducting a CESA, although there's almost certainly more to the story. As you correctly pointed out in your previous post, hyperventilation probably plays a central role in SWB (at least for intermediate/experienced skin divers), but it does not necessarily play a role in the CESA scenario.
I maintain that the most likely cause of unconsciousness at/near the surface following a CESA is probably inadvertent breath-holding with a closed airway (panicky diver?) which leads to lung-overexpansion injury and cerebral arterial gas embolism.
Laryngospasm could also lead to lung-overexpansion.
If any of you out there find this stuff interesting, I highly recommend reading a book entitled "Respiratory Physiology: The Essentials" written by John B. West. I didn't think much of his teaching style (no sense of humor at all) or his stories about climbing Mt. Everest (we all suspected sherpas carried him up to the base camp), but his book is very well-written.
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I had a student (not mine) talk to me once, between her confined and open water training. She was looking for an instructor to do her OW dives and she recounted at great length a tale of her first attempt at completing the OW referral. I say "attempt" because on OW dive 2 she apparently blacked out during her ascent and required assistance to the surface where she regained consciousness. She told me that her instructor advised her that she had suffered a "shallow-water blackout." I couldn't wrap my head around how that would have happened, as my understanding of a loss of consciousness in that situation was as Crowley stated. Due to the reduction in CO2 that had been achieved, your urge to breathe was not triggered in sufficient time to get you to the surface prior to your PPO2 falling to a level to low to sustain consciousness. At DEMA this year, I asked one of the DAN doctors about this incident, and he didn't have anything to say other than that he agreed that he couldn't see a way to suffer a "shallow-water blackout" on open circuit scuba.
@bubbletrouble, when you say "in this scenario" can you explain what scenario you mean?
Thanks in advance,
kari
p.s. I recommended to the student that she have a full medical evaluation before continuing on with her diving, as in my mind she had unexplained LOC in the water, and that was a concern to me. She advised me she is a nurse, and is "best friends" with the head doctor in the ER at the hospital where she worked, and she had mentioned it over dinner and he wasn't concerned. I declined the referral.
really???
kanonfodr
Contributor
Bubbletrubble
Contributor
@Karibelle: For my previous post in this thread (Post #9), "in this scenario" = a scuba diver performing a CESA from depth
Hmmm. It's curious that the student didn't follow-up with a formal appointment with one or more physicians immediately after her incident. Given what happened to her, there are several important medical issues that any dive-savvy physician would want to rule out. In my experience, pleasant dinner conversation is no substitute for a physical exam and cardiovascular workup.
FWIW, I think you gave that student good advice. I don't blame you for declining the referral.
Hmmm. It's curious that the student didn't follow-up with a formal appointment with one or more physicians immediately after her incident. Given what happened to her, there are several important medical issues that any dive-savvy physician would want to rule out. In my experience, pleasant dinner conversation is no substitute for a physical exam and cardiovascular workup.
FWIW, I think you gave that student good advice. I don't blame you for declining the referral.
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Mr Carcharodon
Contributor
Your hemoglobin is pretty much saturated even at a ppO2 of .21 ata. So you cannot put signifcantly more oxygen into the hemoglobin by increasing the ppO2. Some more oxygen does dissolve into solution as you would expect from Henry's law. But that is only a couple percent, of the total volume of oxygen being transported, per ata.
While emergency ascents from deeper than 33 fsw maybe more difficult they are possible. LA County's Instructor course used to do CESAs from 100 fsw with everyone in the class for over 20 years. Several thousand divers overall with no issues that I am aware of.
I thought so too, but she was a nurse, and she knew A LOT.
Thanks for the clarification; I'll reread and if I have further questions, will of course ask!
kari
Correct. While breathing air at one atmosphere, one's hemoglobin is about 90-95% saturated with oxygen. Increasing ambient pressure won't change that significantly, and it is the hemoglobin-bound oxygen that matters to the brain and other tissues.
When one stops breathing -- either through holding a breath (free dive) or through a prolonged exhale (CESA) -- then the tissues extract oxygen from the hemoglobin so that it becomes desaturated. If there is no respiration going on to reload oxygen onto hemoglobin, oxygen-dependent body processes can start to fail.
Where scuba and freediving differ is that when a scuba diver breathes air at depth, he is actually inhaling more oxygen molecules than the freediver inahles at the surface. This provides the scuba diver with more oxygen to continue loading onto hemoglobin prior to and during an emergency ascent.
So it seems to me that a CESA should be easier than a free dive with respect to oxygenation concerns. The amount of oxygen inhaled will vary proportional to the depth, but the percentage in the lungs does not change with depth.
What changes over the distance traveled to the surface is the amount of oxygen consumed by swimming to the surface, and the amount of residual oxygen left in the lungs may or may not be enough to get you there. You can drown at any depth.
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