Hyperoxia or Hypoxia - Formal Definitions?

[Mr Walker]

Mt 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

Yes I did that without getting hit, no symptoms at all . Aborted the dive after being notified by diver coming in after me. It had been noticed that my o-ring and spacer were left on the bench. Back to surface after 12 mins @ 25 m, swimming against weak current.

No pride in that. Makes you think...

 

[matt]

Your definition of hyperoxia is written as CCR whereas your others are OC and CCR.

Otherwise looks ok, your boundaries are pretty much personal choice, though.

I would think you will get a lot of unscientific noise in the hypercapnia question - how will you screen that out (I.e. I out breathed my scrubber....).

Matt.

 

[Chris Brown]

I think PADI, TDI and IANTD have it defined in CCR classes.

Hypoxic / Hyperoxic scale or 'oxygen parameters' or whatever you want to call it / them:

.16 to 1.6

The lacking definition on CCR for me is at what level(s) the diver is supposed to take remedial action and at what should that action look like.

For example, the classic 'Three H's' drill where the instructor will say, "OK, when I give you the signal I want you to react to a hyperoxic loop" or something similar. What is the p02 that we are simulating exactly? Above high set point or above 1.4 or above 1.6?

I think Peter DenHaan has a good stab at it with his 'three H's" videos of on and off the loop. Although he doesnt set the exact p02 levels in the scenarios, the idea is to get the diver to understand that you can stay ON the loop (at least initially) in the vast majority of scenarios that present themselves on the Evolution rebreather, and simply validate the situation:

-Cell warning
-High Oxygen Alarm
-Low Oxygen Alarm

Most of the alarms are due to a dynamic event (Ascent / descent) and are a temporary alarm. Some can be qualified by a dil flush or a check of the cylinder pressures / valves. (02 off or out of 02 for example)

Then, he moves on to the three H's OFF the loop where the diver bails out to off board.

Just exactly at what point do you make that decision and should it be up to the diver and their level of experience / comfort or should we mandate certain p02's for certain action? Again, unit specifics come in to play too.

PADI has also done a good job of splitting the reactions to hypoxic / hyperoxic events into experience levels. Recreational divers its "BAILOUT" every time, whereas for tec divers its "Not necessarily, look at the situation and react accordingly" This is good advice because low oxygen on an air dil no deco dive on an eccr should never be seen, whereas it may well be on a tech dive with hypoxic dil or an mccr.

Personally I dont feel we need a set criteria, but more definition would be advantageous. In class on the AP rebreathers for example, I advise the student what I personally feel comfortable with (which MUST include a set response of checking cylinder pressures and valves to rule out shut off and out of gas)

Hyperoxia (No guarantee of oxtox symptoms at a set pressure of oxygen)-

• Po2 is above High set point to 1.6 - STAY ON LOOP, breathe down / flush

• 1.6 - 1.8 exhale loop add dil. P02 GOOD - YES (carry on) NO (Get off loop)

• above 1.8 - get off loop

Hypoxia (unconsciousness guaranteed at a set pressure of 02 after probable euphoria and ignorance to severity of the situation)

• below low set point (.7) to .3 Inject 02 and FIND OUT WHY.

• below .3 (get off loop)

Again, this is specific to the Evo and the characteristics of its electronics and solenoid. Particularly with low 02 pressures. It simply shouldnt be that low as the solenoid is so quick. Likely you are out of 02, have it shut off or you just made a dynamic move in the water column whilst firing the adv with hypoxic trimix that did something very temporary.

It would be nice to have some wider agreement as to what is 'manageable low and high oxygen' Perhaps we shouldnt call it Hypoxia and Hyperoxia at all except in certain circumstances?

 

[ebt]

To me, Hypoxia, Hyperoxia, hypercapnia all refer to physiological events/levels, so their definition should stem from that.

Anything else is just a 'deviation from desired setpoint', who's urgency will vary depending on the individual/circumstances.

Whilst the agency definitions have varied a little, the commonly accepted norms seem to put hypoxia in the <0.16-0.19 range, hyperoxia at >1.6 and hypercapnia at >0.5% SEV (all measured in loop).

There may be circumstances where people choose to operate outside these, but they're not the 'norm' (eg dry habitat deco).

 

[emmbee]

Hi,

Nobody's arguing (much) about those values as the "danger zone", but you need to consider the context. And the context is important: only in diving would one consider .7 "low" and set the threshold for hyperoxia at 1.6 bar.

The context here isn't to try to establish the cause of death. Or to set the never to be exceeded limits - it's in all the manuals, that's why we all know them. As I understand it, it's to look for potential root causes, "violations or errors". One way to do that would be to look at the fatalities, but that data can be fragmentary - the ouija board is not a scientifically valid method of data collection . Another way is to ask people who got close, or possibly a bit beyond. Maybe by asking an audience. Or by sending a survey.

At this point, there's two issues:
- false positive: the guy who says he was hyperoxic because he switched to the high setpoint 10m above the wreck, got 1.4 when he reached the bottom and breathed it down. If you ask for "> 1.3", it'll qualify. But it's noise. Eliminating false positives requires stringent criteria.
- false negative: unit calibrated, the loop evens to maybe .7, quick pre-breathe and in with the controller on a .19 setpoint, trusting it to switch automagically. Usually that'll be fine, but one day it won't. Another scenario is being a bit slow, but still fast enough to catch a dead solenoid battery. If you say "< .18", you may not catch either, but that's precisely what you're looking for. Eliminating false negatives requires loose criteria.

It's important to note that these are to some extent contradictory, so you need to strike a balance. In this case, the criteria should be stringent enough that by-the-book dives, or common practice (O2 rebreather at 6, maybe) are out. But loose enough that you don't only get the cases where pure blind luck avoided ending up on "the list" - that's sort of the point.

Of course once you get past the basic criteria, there needs to be a more information, and a more manual process, to evaluate the individual cases.

As it is, divers tend to dive close enough to the edge, so there isn't much choice: .18, 1.6, that sort of thing. I offered 1.4 on the bottom because I don't know too many who go past that. For hypoxia on CCR, however, we're in luck. There's a massive margin between what's supposed to happen (>=.7, always, pretty much, AFAIK) and where it gets dangerous. So you can draw the line quite far from danger and still not get false positives. Hence why I suggested 0.5. To me if it gets that low, there was a problem. Might be just a zip not completely closed followed by a real quick and cold ascent from 6 , but a problem anyway.

Cheers,

Matthieu