Purely theoretical question about hypoxic gasses

[Akimbo]

If you must test breathe a hypoxic mix, a shallow single breath, with a quick exhale would be sensible, and certainly not repetive breaths from that mix.

Depending on how hypoxic, that can be a very high risk test. See Post #7 above.

 

[MaxTorque]

Absolutely!

Hence the "if you must" bit in my post Recommendation clearly is you don't !


if we are talking about why you might want to test breath a mix, i guess we are talking about the difference between breathing a second and simply pushing the purge button? Which is another interesting quesiton in itself!

Clearly pushing the purge does almost the same as breathing the reg, but not quite! It certainly doesn't test the reg for negative pressure leaks, (split diaphram, leaky exh flapper valve, cracked or distorted housing, split mouth piece) and i guess the range of lever movement is most likely different between pushing the purge button onto the diaphram and then onto the lever as opposed to sucking the diaphram right in against the lever (most purge buttons do not have the same total travel as the diaphram itself)

Pushing the purge certainly tells you that your 1st is turned on and working, and that the 2nd can flow air, and hopefully stops flowing air when you let go. And you can test the sealing by trying to breath the reg with the cylinder valve off (making sure there are no other potentialnegative pressure air entry points, ie QD's or drysuit/inflator schrader valves etc)

Is that enough to be sure to be sure? good question!!!

 

[Centrals]

Tried it once and never again
It was 10/60 OC and nearly passed out after just one breath! I was sitting down next to my expensive mix on the last dive of the trip. So I thought I will give this hypoxic mix a try!
Good experience.

 

[Akimbo]

if we are talking about why you might want to test breath a mix, i guess we are talking about the difference between breathing a second and simply pushing the purge button? Which is another interesting quesiton in itself!

It is a good question. My suggestion is to test breathe all your regulators on an air or Nitrox cylinder before departing for the dive site. A quick predive test purge on a hypoxic cylinder is adequate at that point. Another thing you can do is test your mix through the second stage. That will test the flow, purge, and the mix at the same time, which wastes less gas for the dive.

This phenomenon was clearly demonstrated to me during pre-dive checks that included testing every BIBS masks -- the oral-nasal masks hanging in every lock for emergency and treatment gasses. The plumbing systems in this particular chamber still contained deep mix; 1% in this case for a 0.3 PPO2 at 850'. I was going through the checklist, pressed the first BIBS mask against my face, and that is the last I remember until I woke up flat on the deckplate with a knot on the back of my head.

This life lesson (and the knot on my head) convinced me to test BIBs masks using the purge and O2 analyzer before putting one my face.

 

[Dr Simon Mitchell]

Tried it once and never again
It was 10/60 OC and nearly passed out after just one breath! I was sitting down next to my expensive mix on the last dive of the trip. So I thought I will give this hypoxic mix a try!
Good experience.

Hello Centrals,

I cannot deny that you may have perceived this, but it is almost certainly not correct. In a study we published 3 years ago, we conducted 50 human exposures to breathing 5.5% oxygen. It took just over 2.5 minutes (on average) for the subjects to reach an end-point of severe cognitive impairment, and no one passed out.

I have attached a pdf of the paper.

Simon M

 

[ginti]

Hello Centrals,

I cannot deny that you may have perceived this, but it is almost certainly not correct. In a study we published 3 years ago, we conducted 50 human exposures to breathing 5.5% oxygen. It took just over 2.5 minutes (on average) for the subjects to reach an end-point of severe cognitive impairment, and no one passed out.

I have attached a pdf of the paper.

Simon M

In this post:
Children die playing with scuba gear left in pool - Jensen Beach, Florida

@Duke Dive Medicine suggested that "pure helium will act like an oxygen vacuum because of the large diffusion gradient between the bloodstream and the lungs". The result is that a couple of breaths would result in a person passing out.

I thought that was true for any hypoxic mix, just at a different rate. Indeed, I understand that the diffusion gradient between the bloodstream and the lungs depends on two factors:
{1} the size of the molecule of the gas mix;
{2} the percentage of oxygen in the mix.
Do I understand correctly?

 

[MaxTorque]

Hello Centrals,

I cannot deny that you may have perceived this, but it is almost certainly not correct. In a study we published 3 years ago, we conducted 50 human exposures to breathing 5.5% oxygen. It took just over 2.5 minutes (on average) for the subjects to reach an end-point of severe cognitive impairment, and no one passed out.

I have attached a pdf of the paper.

Simon M

Very interesting study!

Does Pulse Oximetry have any merit in a rebreather environment as an independant measure of blood oxygenation? Is it feasible to do underwater? It seems the tech is now very much cheap enough?

 

[Duke Dive Medicine]

In this post:
Children die playing with scuba gear left in pool - Jensen Beach, Florida

@Duke Dive Medicine suggested that "pure helium will act like an oxygen vacuum because of the large diffusion gradient between the bloodstream and the lungs". The result is that a couple of breaths would result in a person passing out.

I thought that was true for any hypoxic mix, just at a different rate. Indeed, I understand that the diffusion gradient between the bloodstream and the lungs depends on two factors:
{1} the size of the molecule of the gas mix;
{2} the percentage of oxygen in the mix.
Do I understand correctly?

It's the partial pressure differential that drives the diffusion in and out of the bloodstream at the gas/fluid interface in the lungs. @Dr Simon Mitchell and colleagues' research is interesting and relevant to this conversation because their inspired pO2 roughly duplicates the partial pressure of dissolved O2 in the pulmonary arterial side of the pulmonary capillaries, so there would be little to no diffusion of O2 in or out.

Compare this to the partial pressure of O2 on the summit of Mount Everest. The barometric pressure there is 250-260 mmHg, which would make the inspired O2 partial pressure around 53-55 mmHg, the equivalent of 6-7% O2 at sea level. There are people (e.g. Ed Viesturs) who can summit Mount Everest without supplemental O2. The diffusion gradient into the pulmonary capillaries from the lungs is extremely low, but some people are able to better extract and utilize it. Part of it is individual physiology, and part of it is physical conditioning.

Once the percentage of O2 in a gas mix drops below 4-5%, the diffusion gradient reverses and O2 will exit the bloodstream from the pulmonary capillaries and into the lungs. This results in rapid hypoxemia. How rapid, exactly? It takes a minute or less for blood to make a complete circuit at rest, so in a completely anoxic mix, unconsciousness would likely occur at around that point in an individual at rest, maybe even before since one physiologic response to hypoxemia is an increase in cardiac output.

Best regards,
DDM

 

[ginti]

Thanks a lot!

@Dr Simon Mitchell and colleagues' research is interesting and relevant to this conversation because their inspired pO2 roughly duplicates the partial pressure of dissolved O2 in the pulmonary arterial side of the pulmonary capillaries, so there would be little to no diffusion of O2 in or out.

I have never doubted that his research was relevant and interesting. I questioned my comprehension of the topic.

The diffusion gradient into the pulmonary capillaries from the lungs is extremely low, but some people are able to better extract and utilize it. Part of it is individual physiology, and part of it is physical conditioning.

Sorry for the next question, it may be silly; I have just started learning all these things Do you believe that a (tech) diver would gain diving-related benefits from an improved extraction capacity? If yes, would standard aerobic training (e.g. run long distances) be the right tool to improve this capacity, or is there anything better?

 

[Duke Dive Medicine]

Thanks a lot!



I have never doubted that his research was relevant and interesting. I questioned my comprehension of the topic.



Sorry for the next question, it may be silly; I have just started learning all these things Do you believe that a (tech) diver would gain diving-related benefits from an improved extraction capacity? If yes, would standard aerobic training (e.g. run long distances) be the right tool to improve this capacity, or is there anything better?

Oh, I didn't mean to imply that you doubted it at all, sorry if it came across that way. I was just drawing a comparison to the topic at hand.

Anything that you can do to improve your cardiovascular condition can only benefit you.

Best regards,
DDM