Hyperoxia or Hypoxia - Formal Definitions?

[GLOC]

It may appear to be a bit of a numpty question but please bear me out!

In another thread I mentioned the difficulty in trying to determine whether violations or errors had occurred when you don't have formal rules. Furthermore, because each diver could have undertaken training through a number of instructors and/or agencies and their units are configured differently (out of the box and/or modified), then the definition of right or wrong is a little difficult!

As many know I am undertaking a PhD looking at the role of Human Factors and the scope of the study is to develop a model in a similar style to Human Factors Analysis Classification System (HFACS) which covers the spectrum of error development from organisation to individual. The first part of the study will be to look at causal factor categories in diving incidents (which is also an interesting term to try and define) along the lines of this

I recognise that some of these factors or categories will not be applicable to recreational diving (sport diving) and I will need to create some new ones. I will develop the categories by looking through as many of the incident reports (formal or anecdotal) as I can and then validate the model with a number of raters. I had a first go nearly 2 years ago, but wanted to use the PhD to provide rigour and structure to the model so that it wasn't just an opinion.

The next step will be to work out why people make the mistakes. I started to look at the causes for some incidents and realised that it was easy to put a mistake into knowledge error, rule error or routine violation depending on the context and knowledge of the diver. I believe this is a second order effect of not having 'rules' within the activity (not that there necessarily should be across the whole activity!). However, to make the most of my research I want to focus on certain areas of interest. In OC diving, 41% of fatalities had OOA as a trigger (Denoble et al, 2007) with entrapment the trigger for 21% and equipment problems (using, not equipment failure) 15%. I'd like to see why divers went OOA, or low on gas, as this is the major trigger factor in OC diving, then maybe move to entrapment. Obviously this only works for non-fatals!

I asked around some notable characters in CCR diving to let me know what they thought the major factors in CCR incidents were and they all pretty much came up with the 3 Hs. So in CCR, I'd like to address these. From what I have read, hypercapnia is somewhat binary when it becomes severe, but at lesser levels are harder to determine. Hypoxia and Hyperoxia can be covered more easily through the controllers/O2 cells, but what values would be considered excessive (high or low) to use in a survey as the trigger question? (This is of course assuming that the O2 cells are working and the displays are correct - I am not going to go into detail in my paper about this other than to say that there are potential issues but that they are unlikely to influence a diver into making a mistake, or continue to knowingly make a mistake).

So, thoughts? Are there formal definitions within training manuals (agency or manufacturer)of hypoxia and hyperoxia when operating a CCR, including durations? If not, I will need to conduct some sort of survey to get a consensus opinion across the community.

Regards

 

[Peter L]

Each agency has their own definition of Hyper / Hypo but they all come down to a level that causes loss of conciousness. Haldane is your man and his papers are the grand-daddy of all root knowledge in this area and are a great read for insights on cause and effect for inspired O2 and some of the variance between cause and effects on individuals. I know through events and dry dives that I seem to have a high tolerance to extremes, such that there was never an incident within published ranges but likewise the Italians allow combat divers to twice our accepted depths and we have become more conservative over time.

I do think if you stick to the three H's your missing a trick as more than ever CCR diving is the study of alcoholic circus clowns who have trained to ride unicycles while inebriated on scotch, narcotic effect from all inspired and retained gas has a huge variable to play on incident causality and outcome and although reflexes are in many cases the saving grace, you can't plan for everything when intoxicated.

 

[GLOC]

Thanks Peter, it isn't so much about the outcome of the incident that I am looking at. There is plenty of evidence to show what happens at the general population level of hyperoxia and hypoxia, and the subsequent impact on the diver.

What I am trying to understand is why people break the 'rules' of hyperoxia and hypoxia which they have been trained to i.e. what pO2 (however it is displayed to the diver - need to define) is acceptable for hyperoxia or hypoxia? For that to happen, I need to have a value which I can put into a survey which asks the participant why they went hyperoxic or hypoxic, maybe using the 'five whys' to determine that. As I have posted elsewhere, it is easy in hindsight to spot the 'single root cause', it isn't necessarily so easy for the diver to spot it at the time. But by collecting a statistical significant amount of data to show why those poor decisions were made, it may influence the community to behave differently by having credible evidence (or as good as I can get given the biases and limitations in using retrospective/voluntary surveys)

It is an interesting point regarding the acceptable levels for hyperoxia or hypoxia being different across agencies; do you have evidence for this Peter? i.e. quotes from training materials which I can track down?

Regards

 

[Peter L]

I am not in the instruction field so am not a training manual owner / fanatic but acceptable Hyper PPO2 has been dropping from the high 2's to the current 1.6 that we all know and love. Most tech agency education tout CNS tables but even in those tables short exposures above 1.6 are routinely stated, in example you can accept 2.0 for a specific duration aka breathing it down rather than having a fear response to it which as you know fear is more likely to produce convulsions.

On hypoxia the normoxic tickets from different agencies state .18, .17 and .15 that I have seen depending on who and when. I consider anything equal or below .14 to be hypoxic but even then it's duration that counts.

 

[DeepUnderground]

Watched a good documentry called doctors in the death zone..

A load of doctors climbed everest and experimented on each other amongst other things on the effects of living and working in a low oxygen environment.

Some seemingly very fit people suffered very badly some got to the top without oxygen and were able to carry out bike tests etc

Seems the low threshold for hypoxia is also very personal rather than black and white 0.16 or what ever you have been taught

I think it was around 10% or less at the top of everest

Sent from my BlackBerry 9900 using Tapatalk

 

[Peter L]

Seems the low threshold for hypoxia is also very personal rather than black and white 0.16 or what ever you have been taught

Working from recall alone I remember Haldane could inspire .12 for 4-5 mins before starting to feel the grey out yet his assistant would be down in 1-2 minutes. I have played with similar gas for the education and know I was fine after 4 minutes and stopped at that point.

P

PS. They took volunteers with them on that trip and my friend went with them but they only took the tourists to the base camp.

 

[Mark Chase]

When I first started diving CCR my big three risk factors were Hyperoxia Hypoxia and Flooding.

As knowledge progressed I began to add C02 to the list.

Nine years later I only have two things that scare me on CCR. C02 and Hypoxia.

For me Hypoxia is dropping below 0.16 pp02 an "incident" for me is dropping below my normal low set point of 0.7 on anything but a dill check.

I have had one Hypoxic near miss on my Cave course doing a blind exit on a KISS Whilst the event was a genuine event I don’t consider it likely to happen in "Normal diving" as my viz is never going to be so bad I cant see my HUD.


My fear is of a situation where my HUD falls off ( this has happened several times) and because you develop a blind spot for the HUD lights you don’t notice (This has happened as well) Combine this with a failed 02 injection event and I could simply fall asleep and die.

Hyperoxia doesn’t worry me so much. I have had so many high PP02 events (well over 2bar) and got away with it I feel I have a natural ability to cope with high PP02s. I understand from history they tested the mini sub divers on high PP02s and got rid of the applicants who toxed early so threes obviously some natural susceptibility

High PP02 doesn’t result in disorientation or lack of cognitive thinking so it’s something that can be managed. For me a high PP02 event is anything over 1.6 pp02 that’s not a cell checking exercise.

Co2 on the other hand results in severe narcosis and it’s not something that can be quickly fixed. Again in the early days for me C02 was something I related to bad scrubber packing and overusing the scrubber. Today I worry far less about that and much more about retained C02 from work of breathing issues.


I disagree with the assumption that different CCR demand different response.

To me the response to all CCR issues is go OC and either stay there or for technical diving capable CCR, fix the issue and go back on to CCR if possible.

This would allow for recreational units like the Poseidon which will force bailout and offers no on board fix. Tec units that can be flushed flood recovered and managed without fully functioning hand sets, offer greater options to the more experienced CCR diver. Regardless of how the unit is designed this boils down to flood recovery, manual diluents’ flushing and running manual PP02.

What button you have to push to achieve this may vary but the end result is the same.




In terms of safety procedures I don’t think divers should be given the option not to have a BOV. They should be standard equipment on all CCR and training should be geared to having the BOV connected to a suitable off board supply.


ATB

Mark

 

[AD_Ward9]

So, thoughts? Are there formal definitions within training manuals (agency or manufacturer)of hypoxia and hyperoxia when operating a CCR, including durations? If not, I will need to conduct some sort of survey to get a consensus opinion across the community.

From a manufacturer viewpoint, one works to standards. In theory the agencies should also. The standards define what is what, and accident studies then add in the detail.

Hypoxia:
CE requires a Warning if the PPO2 is below 0.4 bar, and Alarm if below 0.3 bar, and treats anything below 0.21 bar at sea level as hypoxia.

From accident studies, the situation is:
Both PPO2 and duration are relevant. From rebreather accident studies somewhere between 0.07 atm and 0.065 atm will cause sudden unconsciousness when the PPO2 falls at the rate in a rebreather due to metabolism without O2 being injected. In one case where the PPO2 fell very fast due to a flush with the wrong gas, it was 0.10 atm for one individual. There is very noticeable impairment below 0.12 atm and reports of unconsciousness when 0.12atm is breathed for longer periods than present in a rebreather when the O2 injection stops. From this, we take 0.16 atm as an emergency level as there is a variation from person to person, and depending on their activity level. I watched one diver on a test as his PPO2 fell, and he did not bail out when he saw the PPO2 was 0.14atm.

Hyperoxia:
CE is moving to warning above 1.5 bar, but EN 14143:2003 requires a warning above 1.6 bar, and alarm is above 1.6 bar for 1 minute or above 2.0 bar at any time.

From accident studies:
A great deal depends on the retained CO2 loading of the diver. One accident had an hyperoxic convulsion leading to death with the diver at a PPO2 between 1.2 or 1.3 bar. At high CO2 one has to reduce the limits. A PPO2 of over 2 is not seen to be a real immediate hazard if the CO2 is low. However from accident studies, a PPO2 much above 3 tends to cause hyperoxic events quickly. The highest I could find in the accident database is a PPO2 of around 4.5 atm. There is also the widely reported "off effect", which can complicate things further.

A well known medical professor, with considerable expertise in diving and who pioneered use of hyperbaric treatment (who does not use dive forum), confirmed these levels for both suitability as a standard and accident effects.

Alex

 

[emmbee]

Hypoxia:
CE requires a Warning if the PPO2 is below 0.4 bar, and Alarm if below 0.3 bar, and treats anything below 0.21 bar at sea level as hypoxia.

Hyperoxia:
CE is moving to warning above 1.5 bar, but EN 14143:2003 requires a warning above 1.6 bar, and alarm is above 1.6 bar for 1 minute or above 2.0 bar at any time.

?

14143 says the ppO2 shall remain between .2 and 1.6 bar (but allows for up to 2 bar for up to 1 min on descent), and that the "active warning device" should warn the diver if it's outside limits defined by the manufacturer, which have to be within [0.27 ... 1.6].

In industry, as far as I remember, the limit for hypoxia is .15 bar.

Cheers,

Matthieu

 

[Simon Mitchell]

Hello Gareth,

Tricky questions.

Hypoxia:
CE ... treats anything below 0.21 bar at sea level as hypoxia.

I agree with the suggestion that anything less than an inspired PO2 of less than 0.21 atm should be the definition of hypoxia. You don't need Alex's reference to "sea level" in there because this is a partial pressure. Depth is irrelevant. There is little point in setting the definition higher because it simply won't be physiologically harmful (even if it can be considered poor practice to run a circle circuit at a PO2 of 0.3 atm). Put another way, if a diver has a problem while the inspired gas has a PO2 of 0.3 atm, then the problem was not caused by hypoxia.

The problem with this definition is that between 0.21 and whatever the inspired PO2 is when the diver becomes unconscious, there is a zone of ambiguity. For example, it is very unlikely someone will be significantly compromised at an inspired PO2 of 0.19 or even 0.16. But as the PO2 falls further the likelihood of compromise increases, and becomes more context sensitive so that factors such as exercise level and even CO2 could start to influence things. (One of the adaptations that helps preserve consciousness at the top of Everest breathing air is hyperventilation and massive lowering of the alveolar PCO2 - and the opposite is more likely to be true in diving). I would suggest that in many cases you are not going to have the level of detail in the accident data to derive the exact inspired PO2 (and other relevant factors such as oxygen consumption) at the time the diver became incapacitated. So it makes sense to me that if there is reasonable evidence that the inspired PO2 was less than 0.21 atm at the time of incapacitation, then you would be justified in designating hypoxia as the disabling injury. I could probably be convinced that 0.16 was a suitable threshold, but if you set that as your criterion, then there may be some accidents were you lack solid evidence that the PO2 was below that.

From rebreather accident studies somewhere between 0.07 atm and 0.065 atm will cause sudden unconsciousness when the PPO2 falls at the rate in a rebreather due to metabolism without O2 being injected.

While I don't dispute the suggestion that a diver would likely become unconscious suddenly if inspiring a PO2 of 0.07 atm or less, I would be very interested in detail of (and references to) the "rebreather accident studies" that led to the implied formal derivation of this conclusion.

Hyperoxia:
CE is moving to warning above 1.5 bar, but EN 14143:2003 requires a warning above 1.6 bar, and alarm is above 1.6 bar for 1 minute or above 2.0 bar at any time.

A definition of hyperoxia is even more tricky and context sensitive (duration of exposure and CO2) as Alex implies. I would probably also opt for an inspired PO2 of > 1.6 atm as your working definition of hyperoxia. Seizures are very rare (but not unknown) at lower PO2s, but I suspect the risk increases quickly (in diving) at higher levels. There will, of course, be accidents where this definition of hyperoxia is met, but in which the disabling injury was not a seizure. Another approach might be to look at Dick Vann's paper in the Technical Diving Workshop Proceedings, specifically page 53, Figure 15 in his paper and choose a PO2 where the risk isopleths are all starting to steepen. Just eyeballing it, that looks about 1.8 to me, but I would still probably go for 1.6 for your work.

There is also the widely reported "off effect", which can complicate things further.

Alex, "widely reported and completely unproven" would be a more accurate description. I suspect you are aware that I discussed this issue with your buddy Ann Marie a few years ago when she was on line advising people to lower their inspired oxygen levels slowly if they developed symptoms of oxygen toxicity (to avoid the off effect); advise that was contrary to that provided by every major diving authority. It is completely unclear whether seizures attributed to the "off effect" were simply events that were going to happen anyway... in other words, it was too late to stop them by inspiring a lower PO2. The "off effect" may therefore be nothing more than a myth, but it must be said we are not certain of that. One compelling piece of contrary evidence in my view is that the widespread practice of technical divers imposing air breaks during extreme oxygen exposures does not seem to provoke seizures (which you might expect if the "off effect" were real).

Simon M

 

[GLOC]

Thanks for all the feedback. This requirement isn't to go through the accident data and try to work out what the pO2 was, it is to provide a threshold which I can then use in a question as to why they ended up in either a hypoxic or hyperoxic situation.

If the question was 'Have you had a hyperoxic situation whilst diving a CCR?' the first question likely to come back is 'what is defined as hyperoxic?', same goes for hypoxic.

I then want to understand why they were in the hypo/hyperoxic situation? Task loading, HUD, old O2 cells, weren't monitoring (why?) and so on. But without a threshold defined, it becomes harder to isolate the causality.

Regards

 

[Randy Thornton]

Thanks for all the feedback. This requirement isn't to go through the accident data and try to work out what the pO2 was, it is to provide a threshold which I can then use in a question as to why they ended up in either a hypoxic or hyperoxic situation.

If the question was 'Have you had a hyperoxic situation whilst diving a CCR?' the first question likely to come back is 'what is defined as hyperoxic?', same goes for hypoxic.

I then want to understand why they were in the hypo/hyperoxic situation? Task loading, HUD, old O2 cells, weren't monitoring (why?) and so on. But without a threshold defined, it becomes harder to isolate the causality.

Regards

So if I understand your original question, you are asking for the definition from an operational standpoint and not a physiological standpoint. Is that correct?

 

[GLOC]

If they are the same, then yes.

'Rules' which could be broken are either stated in standards (alarms are a good start) or by best practice (which is why I was asking about training agencies). Operators (users) will exceed those for a variety of reasons. Lack of knowledge, not following rules (operational, routine or exception violations), task loaded, lack of situational awareness, equipment failure, poor equipment assembly, lack of pre-dive checks, lack of training...

Some of the reasons will have other reasons why they happened. No pre-dive check, why? Busy. Why? Late to the drop point. Why? Poor planning on skipper's part. Why? Misjudge tides? And so on. Each one of those "Whys" provides an insight as to why the incident developed, rather than just say 'hypoxic because they were stupid'.

I do know that trying to put a useful survey together is going to be a challenge and something I am working with my supervisor to develop. The same will go the OC questions so I do have my work cut out!

Regards

 

[Peter L]

If they are the same, then yes.

'Rules' which could be broken are either stated in standards (alarms are a good start) or by best practice (which is why I was asking about training agencies). Operators (users) will exceed those for a variety of reasons. Lack of knowledge, not following rules (operational, routine or exception violations), task loaded, lack of situational awareness, equipment failure, poor equipment assembly, lack of pre-dive checks, lack of training...

For the sake of your survey to avoid false positives impacting your datasets you need to ignore manufacturer limits, they are not relevant to your intention.

The best example of the measure your seeking is present in the DAN insurance clause which states something akin to "known to have inhaled a gas mix that could cause impairment, in example lower than .15 or above 1.6 PPO2 or above 3.16 PPN2 or .05 PPCO2 by either intent or accident during a dive"

The issue you face is that most normoxic CCR divers will surface test breathe their BOV or bail out reg for a breath or two and this will always be lower than .21 which is the measure suggested above and not helpful for your data.

 

[matt]

0.21-1.5 is normal range for me, 1.6 is high, lower than 0.21 is too low.

I'd be uncomfortable below 0.16 and above 1.6, so there's my grey area.

Matt.

 

[Clare]

You are right that it differs. I see less than 0.18 as hypoxic and more than 0.5 as hyperoxic. The former because that is the least amount of O2 available on surface naturally and thus we can function and the latter as this is the level we can start monitoring symptomatic long term Pulmonary exposure. Matt's less than 0.16 and greater than 1.6 makes sense but clearly 1.6 is way past what the body would consider hyperoxic.

 

[AD_Ward9]

You don't need Alex's reference to "sea level" in there because this is a partial pressure. Depth is irrelevant.

Simon, some rebreathers are designed to be able to be used in mountain lakes. Therefore pressure is relevant, and the 0.21 figure reduces with height. If a diver at 2,000m is diving in a lake, he needs to be able to check his sensors and get onto the loop.

The 0.3 atm is a CE alarm level, because a rebreather should not be operated below that level: with typical set points of 0.7 atm upwards, a 0.3atm level indicates a problem.

While I don't dispute the suggestion that a diver would likely become unconscious suddenly if inspiring a PO2 of 0.07 atm or less, I would be very interested in detail of (and references to) the "rebreather accident studies" that led to the implied formal derivation of this conclusion.

There are many examples, and would many more if rebreather controllers did not have a tendency to lose their logs after an accident ;-). One 0.065atm example is a US diver who swam to his buddy when his O2 ran out, instead of bailing, or at least injecting his make-up-gas, then a couple of yards away from the buddy he passed out and his mouthpiece then fell out. The accident list is published, so everyone should be abe to find that one easily. On many dives we have a full log, but no permission to publish. We also publish a Matlab simulator so investigators can determine the range of PPO2 profiles from known events such as a dive profile and spot readings.

... I discussed this issue with your buddy Ann Marie a few years ago ...

So people whom I met once at an exhibition and once briefly when some equipment was dropped off to me, and with whom I have never dived, are now my buddy. I am glad that on your definition, the whole world is now "my buddy"! Making side points means that people skip reading the rest.

Alex

 

[Simon Mitchell]

Simon, some rebreathers are designed to be able to be used in mountain lakes. Therefore pressure is relevant, and the 0.21 figure reduces with height. If a diver at 2,000m is diving in a lake, he needs to be able to check his sensors and get onto the loop.

Alex, I have no idea what you are talking about. We are trying to agree on the PO2 in the inspired gas that defines "hypoxia". Whatever PO2 we choose, be it 0.1, 0.16, 0.21 atm or something else, the altitude or depth at which a diver respires the gas mix producing this PO2 is completely irrelevant.

There are many examples ...snip...many dives we have a full log, but no permission to publish.

Fair enough. 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.

Simon M

 

[Michael Thornton]

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.

 

[Michael Thornton]

I have had one Hypoxic near miss on my Cave course doing a blind exit on a KISS Whilst the event was a genuine event I don***8217;t consider it likely to happen in "Normal diving" as my viz is never going to be so bad I cant see my HUD.


My fear is of a situation where my HUD falls off ( this has happened several times) and because you develop a blind spot for the HUD lights you don***8217;t notice (This has happened as well) Combine this with a failed 02 injection event and I could simply fall asleep and die.

That's why when I teach a CCR cave class I use a neoprene mask cover that allows them to stick their HUD in and monitor their PO2. This is after all number one rule of CCR diving.

With the blind spot this is why I feel so strongly the need for a vibrating alarm. It's also interesting that the HH Diva is the only one I know of that varies its brightness during a blink.