Hyperoxia or Hypoxia - Formal Definitions?

[drtommi]

The shearwater HUD also increases brightness of the led blink as it gets further from 1.0

 

[Mark Chase]

It's interesting that everyone agrees <.21 is hypoxic. And their reasoning is that is below normal air conditions at sea level. I live at altitude and normal for me is .18. I still would call <.21 hypoxic but because it is below normal for most divers not because it is below normal for me.
And at the same time when refering to trimix mixtures they use the term "Normoxic" (Sounds a lot like normal) with a 16% rule.

Not every one.

I said 0.16 which is what i was taught for OC trimix.

I have jumped in OC mix with slightly less but id only do that if conditions were very easy as any kind of physical effort on a sub 0.16 PP02 I'd consider dangerous.


Going back to the upper limits for hyperoxia...


Is there a specific upper limit in terms of PP02? I thaught it was more down to exposure time?

I know I have been exposed to well over 3.0 pp02 for short bursts. Luckely without issue.

My big referance point on this is deep air diving.

The deep air depth record is 155m set by Dan Manion in 1994.

Thats a PP02 of 3.47

Asuming he hit 1.6 at 66m and had to decend to 155m lets say at 5m / min and ascended at 10m / min. Thats a total exposure above 1.6 of 27mins

I know divers who have switched to 100% 02 at 21m (PP02 3.1)and have staid on 100% all they way to their next gas switch depth and time. They toxed (well at least the ones I herd abouit did) but only after a prolonged exposure.

So whilst I consider anything below 0.16 to be dangeroulsy close to a serious incident. I don't consider anything above 1.6 to be a significant danger but i do consider it "an incident" as the risk of tox on prolonged exposure above 1.6 is greatly increased.


Thats why I consider the prime dangers of CCR as C02 and Hypoxia.

Everything else is managable


A couple of my infamous Chasey graph below. Sadly The 3.0bar + incidents (four of them) were all on a Hammer Head CCR that started injecting pure 02 on reboot after multiple power interuptions at depths between 65 and 75m. As a result i couldent down load the graphs.

Both dives MCCR KISS


Hypoxia in a cave Min 34 in the dive I hit a PP02 of 0.12-0.15. The little spike was I think hitting ADV on min loop and sucking in Air. Even this didnt alert me as I was too task loaded. Patric tapping me on the head and alerting me to my HUD saved me.

PP02 Spike of 2.3bar min 2 of dive imediatly on decent.Problem with 02 inject on decent to 65m wreck (diver error)

ATB

Mark

 

[matt]

It's interesting that everyone agrees <.21 is hypoxic. And their reasoning is that is below normal air conditions at sea level. I live at altitude and normal for me is .18. I still would call <.21 hypoxic but because it is below normal for most divers not because it is below normal for me.
And at the same time when refering to trimix mixtures they use the term "Normoxic" (Sounds a lot like normal) with a 16% rule.

Long drive to the coast I definitely think there's a lot of grey areas here, this for me is a good example of one of them.

Matt.

 

[AD_Ward9]

. I was just curious to know if you were referring to an analysis of a case I saw some time ago where the inspired PO2 had been calculated based on a number of assumptions that were inaccurate.

If you PM me the case, we may have the data for it.

I have seem mistakes in an Excel simulation one rebreather company used, and others where the metabolisms were unreasonable (again in Excel). Another mistake we have seen in a third party's file is where they assumed an O2 flush brought the PPO2 to 0.99 on the surface (even three lung flushes only get 94% on that unit due to the rigid volumes in the unit).

We don't use Excel for rebreather accident simulations: MatLab or LabView are the tools we use to simulate rebreathers, and we publish the model. I have never seen any Excel simulation cover the detail in that MatLab model: they make approximations even in how they treat the environment.

Any inspired PPO2s I have quoted in public have come either directly from a rebreather log or from a Matlab simulation of a dive profile where the outcome is known. In the latter case we use a window of 0.065 to 0.10 as the range in which a dive would become unconscious while active on a rebreather loop. If the dive is inactive, we increase the upper limit to 0.12 atm.

Hyperoxia
Mark Chase raises an interesting hazard, that can lead to over-confidence for rebreather divers. In an O.C. dive, the inhaled CO2 is very low with a good regulator (with low dead space). The Volume Weighted Inspired CO2 is typically 3 or 4 times higher on rebreathers at depth, and can be 8 or even 10 times higher. The higher level of CO2 on a rebreather means that running a PPO2 over 3 atm for even a fairly short time, can result in serious problems. The inhaled CO2 comes from the larger dynamic volume (dead space) in the DSV, any flapper valve bypass due to turbulent flow, and the reduced efficiency of the scrubber at depth. Many divers are not aware just how badly some scrubbers reduce efficiency with depth.

Alex

 

[emmbee]

In the latter case we use a window of 0.065 to 0.10 as the range in which a dive would become unconscious while active on a rebreather loop. If the dive is inactive, we increase the upper limit to 0.12 atm.

I must be misunderstanding something - you're saying an inactive diver will not tolerate as low a ppO2 as an active one? Seems odd.

Cheers,

Matthieu

 

[Simon Mitchell]

Not every one.

I said 0.16 which is what i was taught for OC trimix.

Hi Mark,

I agree with most of what you say. Part of the problem here is the difference between an appropriate physiological definition vs an operational definition for the purposes of incident classification. I think 0.16 is a better physiological definition than 0.21, but proving that it existed in the absence of a PO2 data logger might sometimes be tricky whereas being certain that the PO2 was less than 0.21 might be less tricky. Moreover, from an operational perspective, 0.21 would be a very conservative definition of hypoxia in a rebreather because operationally you could argue that even a PO2 just below set point is "hypoxia" (which is obviously physiological nonsense).

There is another operational perspective in relation to comparing OC and CC. Just as Alex pointed out in the debate about hyperoxia, there are some important differences between these diving methods. If you are breathing 0.16 on OC then that is because the combination of ambient pressure and the fixed inspired oxygen fraction in your OC gas supply produces an inspired PO2 of 0.16, and so long as your depth isn't decreasing, you are in a stable (and probably "safe") condition. If you are breathing 0.16 on CC it is almost certainly because of some sort of oxygen control or addition problem that is not likely to be stable at all. Indeed, your PO2 is probably falling quickly and there is not far to go to disaster. The same could be said for 0.21 of course, but you have a slightly greater margin before disaster.

Thus, "<0.21" appeals as a definition of hypoxia because it is a compromise between operational and physiological definitions. Specifically, although it is higher than the point where consciousness is lost, it is at least lower than the normal inspired PO2 breathing air at atmospheric pressure (the physiological side); and, although 0.21 is lower than would be necessary to suggest some sort of problem with inspired oxygen control in the rebreather (the operational side), it is not so high as to be a ridiculous physiological definition of hypoxia.

I hope you can see what I am getting at. You could probably argue that 0.16 satisfies these criteria also.... it just shifts the weighting toward a physiological definition.

There is no easy answer to this (and I don't pretend to know the correct one for Gareth's purposes).

Thats a PP02 of 3.47

Mark Chase raises an interesting hazard, that can lead to over-confidence for rebreather divers.

I agree with Alex about this. We need to be very careful about extrapolating OC experience to CC in respect of hyperoxia. I do completely agree with your suggestion that a hyperoxic spike does not impose the same need for immediate action as a hypoxic dip. Nevertheless, given what we know about the unpredictability of oxygen toxicity (and the likelihood of increased risk on closed circuit loops), I must say I feel uncomfortable leaving my inspired PO2 higher than set point for very long.

Simon M

 

[DrHarry]

Hi Gareth, it seems to me you are looking for a definition that will fit more with an incident monitoring style of study (i.e. something that had the potential to cause harm if not detected, as well as something that actually caused harm). For a definition like that, you want to know why and how often the PO2 falls outside of what the diver, the training agencies or the manufacturers consider to be a safe range.

So if an eCCR wanders outside of range according to the various standards, then that could constitute an incident. However this is so common during a dive as to be "normal" e.g. a small spike on descent, a small spike due to over enthusiastic manual addition, a small drop during ascent etc. So you could widen that range considerably to consider CLEAR breaches of equipment function or diver procedure. e.g. <0.3 and > 1.8. If you only look at a range that constitutes a major physiological threat then I think you will be missing a lot of cases which can contribute to your understanding of root cause analysis.

mCCR use is I believe, associated with more frequent digressions from safe PO2 ranges (that has certainly been my personal experience, usually as a result of task loading and inattention) and the two low PO2 incidents I have personally suffered certainly reflect that (once surface swimming into current before descent with hypoxic dil and one being harassed by sharks in fairly shallow water). eCCR would not have allowed these incidents to happen if functioning properly.

It is impossible to put a high or low number as a cut off for physiological responses e.g. seizure or loss or consciousness, for all the intra- and inter-individual as well as contextual variations (alveolar CO2, work rate and metabolism, cerebral CO2, serum green tea levels!) that exist. So as I said, I'd look at defining safe operational levels rather than physiological ones.

 

[GLOC]

Harry, that is what I was looking for in the training manuals rather than, as you say, physiological limitations. It is a 'rule' which is being broken; I just need to know the 'rule' but trying to define that is hard given all the different viewpoints/opinions/guidance that are out there.

It has given me some food for thought for this and the other violations I want to test.

Regards

 

[Mark Chase]

Thanks for the input Simon,

I'd stress again (in case I wasn't clear in my post) that for me anything over 1.6 is an incident yes, but for me seeing a PP02 of below 0.16 would not only be an incident, but it would also regester as a near miss in my personal scale of near death experiances on CCR.

In the last 9 years on CCR i have had floods, caustic coctails, multiple electronics failures and cell issues and a few incidents of very high PP02 (2-3bar+) I even had three incidents of total loss of dill due to freeflow. One very deep in a cave.

In all these years the two incidents that had me waking up in a cold swet weeks after, were the low PP02 incident in the graph above and both handsets shutting down on my Inspo classic early in my CCR experiance. (I was saved by an aftermarket HUD I had installed which carried on working.)

Both were in shalow water and both were low PP02 events and scared me so much because no level of fight reflex or skill can save you if you pass out.

I grasp wood firmly when I say i haven't had a C02 event yet but it scares me for the same reasion.


Possably for the purpose of incident reporting, taking in mind the issues of depth changes and P02 on CCR, any drop below low set point of 0.7 should qualifie as an incident?



ATB

Mark

 

[Simon Mitchell]

Possably for the purpose of incident reporting, taking in mind the issues of depth changes and P02 on CCR, any drop below low set point of 0.7 should qualifie as an incident?

Hi Mark,

Yes, you could certainly make an argument for <0.7 (or whatever your low set point is) as an operational definition.

Simon

 

[Sambo]

Hi Gareth, it seems to me you are looking for a definition that will fit more with an incident monitoring style of study (i.e. something that had the potential to cause harm if not detected, as well as something that actually caused harm). For a definition like that, you want to know why and how often the PO2 falls outside of what the diver, the training agencies or the manufacturers consider to be a safe range.

So if an eCCR wanders outside of range according to the various standards, then that could constitute an incident. However this is so common during a dive as to be "normal" e.g. a small spike on descent, a small spike due to over enthusiastic manual addition, a small drop during ascent etc. So you could widen that range considerably to consider CLEAR breaches of equipment function or diver procedure. e.g. <0.3 and > 1.8. If you only look at a range that constitutes a major physiological threat then I think you will be missing a lot of cases which can contribute to your understanding of root cause analysis.

mCCR use is I believe, associated with more frequent digressions from safe PO2 ranges (that has certainly been my personal experience, usually as a result of task loading and inattention) and the two low PO2 incidents I have personally suffered certainly reflect that (once surface swimming into current before descent with hypoxic dil and one being harassed by sharks in fairly shallow water). eCCR would not have allowed these incidents to happen if functioning properly.

It is impossible to put a high or low number as a cut off for physiological responses e.g. seizure or loss or consciousness, for all the intra- and inter-individual as well as contextual variations (alveolar CO2, work rate and metabolism, cerebral CO2, serum green tea levels!) that exist. So as I said, I'd look at defining safe operational levels rather than physiological ones.

Harry what are "Serum Green Tea Levels"? Should divers be drinking or NOT be drinking green tea?

 

[DrHarry]

Just a joke about the current fascination with antioxidants :glug:

Harry

 

[Randy Thornton]

GLOC,

If I am understanding what you are after here, it would seem that you are looking more for operational abnormalities of PO2 set points. It would seem to me that you are basically looking for a consensus of how far under a low set point is acceptable before classifying the drop as an "incident". The same would apply to the high set point. The difficulty with this approach is that different units have differing set point options. Different agencies teach differing approaches to set point management and different protocols are occasionally used dependent upon altitude. (For example, it is difficult to maintain a .7 PO2 at the surface where we do much of our training at 6,000 - 9,000 feet elevation.)

Perhaps a more useful and universally agreeable approach would be to establish a guideline for incident classification as any variation of X% outside of the planned and expected PO2.

Just a thought,
Randy

 

[emmbee]

It is impossible to put a high or low number as a cut off for physiological responses e.g. seizure or loss or consciousness, for all the intra- and inter-individual as well as contextual variations (alveolar CO2, work rate and metabolism, cerebral CO2, serum green tea levels!) that exist.

Not to mention the actual duration of exposure (CNS, and google up "time of useful consciousness"). And there's also the CCR aspects, e.g. the question is not so much "how long will you stay conscious at that ppO2" (like it would be on OC), but rather "how long will you stay conscious if the ppO2 of this limited gas volume is that, and O2 is added at that rate (insufficient, possibly 0)".

Perhaps a more useful and universally agreeable approach would be to establish a guideline for incident classification as any variation of X% outside of the planned and expected PO2.

I'm not sure that's a good idea. You don't want the submitter's opinions and ideas to impact that decision, especially considering that those potentially wrong ideas could be the problem being studied. You'd take out an entire class of issues: bad practices executed in good faith.

The fixed threshold seems a better idea. But if it's too strict, you'll get a lot of noise (like: at 9000ft, .7 is pretty much pure O2). If it's too slack, you'll miss on data. For hypoxia, fortunately, there's a lot of room to play with... 0.3... (.2 (air)+.7 (low SP))/2=0.45... .7 (low SP for 1atm)*.72(~p atm at 9000ft)=0.5... 0.7... For hyperoxia, between those running 1.4 on the bottom, those running 1.5 for the ascent, and those running O2 at 6m...

The problem is that the better answers will come out of the data itself

Cheers,

Matthieu

 

[GLOC]

Thanks for all the comments. There will be some qualifying questions to be able to identify the potential outliers such as those who do high altitude diving but the 'rules' will be 'operationally' focussed and not physiologically.

I am going to have the same 'fun' defining OOG for OC diving and why people go OOG! How many rules are out there for that one!

Regards

 

[GLOC]

Thanks for all of the feedback.

I am now putting things together and am using the following definitions to define whether a diver has had an incident to understand the distribution of HFACS categories within the sample population - the survey will be based in the UK so I am not too bothered about high altitude environments and looking to collect responses from 175 CCR users and 25 CCR instructors (plus 800 OC divers).

Hyperoxia: Where the diver ends up with an Oxygen toxicity (OxTox) event/seizure, or the CCR has a pO2 above the planned maximum set point which was not demanded by the diver nor recognised that it would happen. e.g. a diver may push the pO2 above 1.6 to check the cells or when descending fast down a shotline - neither of these are incidents because they are expected and/or demanded.

Hypoxia: Where an OC diver ends up breathing a gas with a pO2 less than 0.18 underwater, or the CCR has a pO2 less than .7 which was not demanded by the diver nor recognised that it would happen. e.g. on the ascent the pO2 may drop as the ambient pressure drops but this should be controlled by the diver or the CCR electronics to be at a level which is sustainable and controlled or the diver may demand a dil flush which may bring the pO2 below 0.7 depending on the dil gas.

Hypercapnia: Where the diver had an event where they have a high confidence that excess CO2 was to blame for outcomes, or was formally diagnosed as having a hypercapnic event. This could be on OC due to dense gas and/or high workloads, or CCR due to a variety of mechanical (scrubber, mushroom valves, missing o-rings etc) issues and/or physiological issues such as being a CO2 retainer or skip breathing.

Any thoughts/comments/feedback would be appreciated before I get formal endorsement from a consensus of 'named individuals'.

Regards

 

[Mr Walker]

Thanks for all of the feedback.

Hypoxia: Where the diver ends up with an Oxygen toxicity (OxTox) event/seizure, or the CCR has a pO2 above the planned maximum set point which was not demanded by the diver nor recognised that it would happen. e.g. a diver may push the pO2 above 1.6 to check the cells or when descending fast down a shotline - neither of these are incidents because they are expected and/or demanded.

Hyperoxia: Where an OC diver ends up breathing a gas with a pO2 less than 0.18 underwater, or the CCR has a pO2 less than .7 which was not demanded by the diver nor recognised that it would happen. e.g. on the ascent the pO2 may drop as the ambient pressure drops but this should be controlled by the diver or the CCR electronics to be at a level which is sustainable and controlled or the diver may demand a dil flush which may bring the pO2 below 0.7 depending on the dil gas.

Any thoughts/comments/feedback would be appreciated before I get formal endorsement from a consensus of 'named individuals'.

Regards

Unless I'm misreading you've mixed up hypoxia with hyperoxia.

If you dive with the o-ring and spacer missing in the scrubber and have no symptoms of hypercapnia whatsoever, what type of incident would that be?

 

[emmbee]

Thanks for all of the feedback.

I am now putting things together and am using the following definitions to define whether a diver has had an incident to understand the distribution of HFACS categories within the sample population - the survey will be based in the UK so I am not too bothered about high altitude environments and looking to collect responses from 175 CCR users and 25 CCR instructors (plus 800 OC divers).

Hypoxia: Where the diver ends up with an Oxygen toxicity (OxTox) event/seizure, or the CCR has a pO2 above the planned maximum set point which was not demanded by the diver nor recognised that it would happen. e.g. a diver may push the pO2 above 1.6 to check the cells or when descending fast down a shotline - neither of these are incidents because they are expected and/or demanded.

Hyperoxia: Where an OC diver ends up breathing a gas with a pO2 less than 0.18 underwater, or the CCR has a pO2 less than .7 which was not demanded by the diver nor recognised that it would happen. e.g. on the ascent the pO2 may drop as the ambient pressure drops but this should be controlled by the diver or the CCR electronics to be at a level which is sustainable and controlled or the diver may demand a dil flush which may bring the pO2 below 0.7 depending on the dil gas.

Hypercapnia: Where the diver had an event where they have a high confidence that excess CO2 was to blame for outcomes, or was formally diagnosed as having a hypercapnic event. This could be on OC due to dense gas and/or high workloads, or CCR due to a variety of mechanical (scrubber, mushroom valves, missing o-rings etc) issues and/or physiological issues such as being a CO2 retainer or skip breathing.

Any thoughts/comments/feedback would be appreciated before I get formal endorsement from a consensus of 'named individuals'.

Regards

Hi,

Couple of points:
1) you seem to have mixed up hypoxia and hyperoxia.
2) these read like my credit card contract. I appreciate the need for precision in definitions, but I would imagine you'd get more and possibly more accurate answers with clear and simple ones.
3) I still don't like the mixing of fact and intent. With your definitions, I could go in with my SW set to .19, because that's the way I do things, the computer will change it (note: I don't), and I'd be a "no problem" diver.

Personally, I would have went with <.18 / <.5, >1.4 (dive) or >1.6 (deco) or Oxtox, and what you said. Reason/intent in a separate question "if you answered yes". You're the expert, tho

Cheers,

Matthieu

 

[GLOC]

D'oh.

Problems with Cut and Paste and not reading!! The original document is correct!

MB, thank you. I hadn't thought about breaking the dive down that way, and also covering off the intent in a separate question.

The definition of a 3H incident isn't to see how many incidents occur within each domain, but rather to screen in those who have had an incident (which I am trying to define) to then conduct some further analysis on a model.

Mr Walker, that will be covered by another 'catch' which defines an incident as having equipment problems/failure and will use mis-configured/missing equipment such as o'rings or poorly packed scrubber. Of course, you would need to know you had a missing space/o-ring to know you had an incident

(As an aside, did you manage this and not get a hit or did you not jump in the water when someone held it up? )

Regards

 

[Philippe GERIN]

Hi,
I have difficulty to grasp the concept of a gas mix of PO2 @ 0.7 being hypoxic on a ccr.
For me hypoxia = life threatening event. Your definition refers more to "a less than optimum PO2 management" that can lead to an event during the dive or even deco accident rather than being directly life threatening.
Did I made myself clear on the idea I want to convey?
You might want to invent another word to define your incident limit on a ccr as hypoxia might lead to confusion for the reader.
That said, you're the expert and if this is your definition for the purpose of your work that's fine and I'll keep that in mind when reading it.
Philippe