Question Hypercapnia and oxygen toxicity

[Diver With Altitude]

hypercapnia increases blood flow to the brain, and therefore increases the amount of high oxygen po2 that is delivered to the area that triggers a convulsion

That is the prevailing theory but is far from proven or fully understood. We don't even understand why immersion makes OxTox so much worse.

I would think it’s also related to the shift in the oxyhemoglobin dissociation curve, isn’t it? Hypercapnia shifts it to the right, the direction that allows easier release of oxygen by hemoglobin. Hypocapnia shifts the curve in the opposite direction. That’s why breath hold divers do not recommend hyperventilation before a dive. It induces hypocapnia, reducing the urge to breath, while at the same time making it harder for oxygen to be released, decreasing the time to loss of consciousness.

 

[Akimbo]

I would think it’s also related to the shift in the oxyhemoglobin dissociation curve, isn’t it?

Maybe, or maybe that is a gross over simplification. Hyperbaric research is very expensive and there isn't much motivation to understand such unique problems that can easily be worked around. There are much bigger problems for science to spend limited resources on.

We wouldn't understand half as much about hyperbaric physiology if the world's navies didn't make the investment — largely combat swimmers for OxTox and submarine rescue for DCS. The offshore oil industry contributed a little via commercial diving firms, but nothing compared to the US, UK, and French Navies.

To be honest, DCS was a bigger concern for caisson workers building bridges and tunnels the than divers in the early days of research. You might find the history of the US Navy Experimental Diving Unit interesting.

US Navy Experimental Diving Unit

It's hard to overstate the profound impact that NEDU has had on recreational Scuba diving. NEDU stands for the US Navy Experimental Diving Unit and is often referred to as EDU or just "The Unit". Everything from diving physiology, equipment, testing, and safety is influenced by their work. It...

scubaboard.com

 

[lowwall]

To be honest, DCS was a bigger concern for caisson workers building bridges and tunnels the than divers in the early days of research. You might find the history of the US Navy Experimental Diving Unit interesting.

US Navy Experimental Diving Unit

It's hard to overstate the profound impact that NEDU has had on recreational Scuba diving. NEDU stands for the US Navy Experimental Diving Unit and is often referred to as EDU or just "The Unit". Everything from diving physiology, equipment, testing, and safety is influenced by their work. It...

scubaboard.com

I haven't read your article yet, but I'm giving you a like just for the correct use of "overstate."

 

[Dr Simon Mitchell]

That is the prevailing theory but is far from proven or fully understood. We don't even understand why immersion makes OxTox so much worse.

Hi Akimbo and others.

I think it is fair to say there is pretty strong evidence for the mechanism by which hypercapnia could increase the risk of cerebral oxygen toxicity. It is proven that, for a given inspired PO2, hypercapnia can markedly increase brain tissue PO2 by causing cerebral vasodilation, thus increasing brain blood flow and oxygen delivery (see the attached paper by Lambertsen et al).

That, in turn, provides us with a strongly plausible explanation for the reason why immersion / diving increases the risk of oxygen toxicity. Immersion and diving with its combination of exercise and increased work of breathing through equipment resistance and dense gas is proven to provoke CO2 retention (see the attached paper by David Doolette and myself).

Simon M

 

[Akimbo]

That, in turn, provides us with a strongly plausible explanation for the reason why immersion / diving increases the risk of oxygen toxicity.

Plausible under normal diving conditions for sure but it doesn't fully explain why immersion alone, under very relaxed conditions, results in a marked increase in OxTox. Several of us were discussing ideas to optimize IWR with a physiologist at NEDU. He recounted a series of wetpot tests in warm water where subjects were told to relax as much as possible. They still exhibited OxTox symptoms at a significantly higher rate than sitting at rest in a dry chamber.

The reasoning behind the tests was that immersion in warm water should have produced less stress due to the effect of weightlessness when mental and physical exertion were minimized. Subjects were even falling asleep. The work was funded by the Teams instead of SUPSALV. I got the impression that the purpose was to develop techniques to minimize OxTox when special forces operators on pure O2 rebreathes are forced into deep water.

Another interesting piece of the puzzle was that SEALs did a little better than Navy divers, who would have comparable levels of comfort in the water. SEALs are better conditioned and have more training in stress management but he believed that there was more going on.

He didn't want to go into detail about how they quantified OxTox symptoms when we asked about monitoring options. I don't know if they had some classified technology or the methods have not be validated.

 

[Duke Dive Medicine]

Plausible under normal diving conditions for sure but it doesn't fully explain why immersion alone, under very relaxed conditions, results in a marked increase in OxTox. Several of us were discussing ideas to optimize IWR with a physiologist at NEDU. He recounted a series of wetpot tests in warm water where subjects were told to relax as much as possible. They still exhibited OxTox symptoms at a significantly higher rate than sitting at rest in a dry chamber.

The reasoning behind the tests was that immersion in warm water should have produced less stress due to the effect of weightlessness when mental and physical exertion were minimized. Subjects were even falling asleep. The work was funded by the Teams instead of SUPSALV. I got the impression that the purpose was to develop techniques to minimize OxTox when special forces operators on pure O2 rebreathes are forced into deep water.

Another interesting piece of the puzzle was that SEALs did a little better than Navy divers, who would have comparable levels of comfort in the water. SEALs are better conditioned and have more training in stress management but he believed that there was more going on.

He didn't want to go into detail about how they quantified OxTox symptoms when we asked about monitoring options. I don't know if they had some classified technology or the methods have not be validated.

Was this head-out immersion or full immersion with diving gear?

 

[Akimbo]

Was this head-out immersion or full immersion with diving gear?

My understanding is it was full immersion in NEDU's giant wet pot in Panama City. I'm assuming that divers were in half masks rather than FFMs or hats since it was funded by the Teams instead of SUPSALV.

He alluded to some instrumentation on the diver that really peeked my interest, but didn't want to say any more. I got the impression the instrumentation was to detect OxTox symptoms before any perceptible VENTIDC symptom but maybe that is wishful thinking on my part. I can imagine a device SEALs can wear to warn them about OxTox symptoms but you can be sure would be heavily classified.

A related issue I have been meaning to discuss with you: I just finished reading Frogmen First Battles about the Italian frogmen during World War II. The book makes it sound like they routinely dove to 20-40m (66-130') on O2 rebreathers. They mentioned OxTox symptoms quite often but barely even mentioned convulsions.

Maybe red wine improves resistance to OxTox since they drank plenty of it even on the subs and ships. Great read.

 

[Dr Simon Mitchell]

My understanding is it was full immersion in NEDU's giant wet pot in Panama City. I'm assuming that divers were in half masks rather than FFMs or hats since it was funded by the Teams instead of SUPSALV.

Hi Akimbo,

That's the point. Even at rest, being immersed and breathing through UBA (both of which increase the work of breathing) and presumably at increased depth / pressure breathing denser gas (which also increases the work of breathing), will provoke CO2 retention in some subjects. I think that CO2 elevations remain a plausible explanation for the observations reported to you.

A related issue I have been meaning to discuss with you: I just finished reading Frogmen First Battles about the Italian frogmen during World War II. The book makes it sound like they routinely dove to 20-40m (66-130') on O2 rebreathers. They mentioned OxTox symptoms quite often but barely even mentioned convulsions.

I have not read that book, but I think you are right that WWII era divers often used oxygen at deeper depths than would be considered appropriate today. There may be 'reporting' issues associated with the apparent lack of convulsions. For example, my understanding is that many of those servicemen simply never returned and no one knows what happened to them - those may have been the ones who had convulsions. Donald's work suggested that there is quite a lot of variability in convulsion latency within and between individuals, even at inspired oxygen pressures of 4 ATA. There are probably some very lucky Italian divers described in that book.

Maybe red wine improves resistance to OxTox since they drank plenty of it even on the subs and ships.

Ah! A useful excuse if I get called out at dinner on our next trip to Truk. (Disclaimer - I only ever have one glass!)

Simon M

 

[lowwall]

Maybe red wine improves resistance to OxTox since they drank plenty of it even on the subs and ships.

Ahh, you speak in jest, but in vino veritas.

"Currently, resveratrol is sold as a nutritional supplement with a wide range of pharmacological effects, including cellular defensive action against oxidative stress."

Resveratrol: A Double-Edged Sword in Health Benefits - PMC

Resveratrol (3,5,4′-trihydroxy-trans-stilbene) belongs to polyphenols’ stilbenoids group, possessing two phenol rings linked to each other by an ethylene bridge. This natural polyphenol has been detected in more than 70 plant species, especially in ...

www.ncbi.nlm.nih.gov

 

[Akimbo]

I have not read that book, but I think you are right that WWII era divers often used oxygen at deeper depths than would be considered appropriate today.

It is interesting that the Royal Navy had a limit of 3ATA near the start of the war but this book implies the Italians didn't have any strict limit. Maybe @Angelo Farina can tell us more?

Very few of the different groups (one I think) that were ultimately merged into the UDT (predecessor to the SEALs) used O2 rebreathers during the war. It is interesting that the only reference to Oxygen Poisoning I could find in the 1943 US Navy Diving Manual refers to exposure on air in a chamber.

NEDU was doing a great deal on OxTox but Dr Behnke's work in 1935 and Doctors Donald and Yarbrough's work during the war didn't make it into the diving manual until the 1950s.

Even Dr. Christian Lambertsen had to "rediscover" Oxygen Poisoning the hard way, but his work was compartmentalized for security. As an Army officer, his work was not well received by the US Navy diving community.

(Disclaimer - I only ever have one glass!)

Isn't that a disqualification for the Italian Navy?