Theory behind the half-life of CNS toxicity?

[dreamdive]

It will be very interesting once we get the data back on the ketotic study. I hope that we behave similar than the rats during ketosis and oxygen tox. See the study:

D’Agostino P, Pilla R, Held HE, Landon CS, et al – Therapeutic ketosis with ketone ester delays central nervous system oxygen toxicity seizures in rats
Am J Phys 2013; 304(10): 829-36
Therapeutic ketosis with ketone ester delays central nervous system oxygen toxicity seizures in rats | Regulatory, Integrative and Comparative Physiology

There is also some anecdotal evidence that a steady PO2 is tolerated better and longer than gas switches leading to O2 spikes.

There is a lot of inter- and intra variability when it comes to O2 toxicity. WHat I mean is that one day you tox much faster than on other days. And that one person may be very different than others in the group.That is one reason the Navy suspended its O2 tox testing in divers.

Sudafed has been linked to increased O2 toxicity, as has Viagra and Cialis.

Demchenko IT, Allen BW, Vann RD, Piantadosi CA - Phosphodiesterase-5 inhibitors oppose hyperoxic vasoconstriction and accelerate seizure development in rats exposed to hyperbaric oxygen
J Appl Physio 2009; 106: 1234-1242
http://jap.physiology.org/content/106/4/1234

A list of various oxygen toxicity studies conducted can be found in our Add Helium Reference Library in the Article section of the website.

C

 

[packrat12]

Who started the practice in the beginning?
And most of us just followed it blindly!!
And some do get O2 tox even within the "safety limit"!
And how many computer manufacturers are corporating this unproven exponent into their algorithms in tracking O2 exposure?

I know that Shearwater uses 1/2 life on their computers - was using it Saturday for my 3 dives . I think I got out at 30% on my last dive (roughly 40% dive 1, 30% dive 2, 25% dive 3) Still here - not O-Tox'd.

I know Multi-Deco program also uses it. I assume v-planner (same programmer) also uses it.

https://www.shearwater.com/news/shearwater-and-the-cns-oxygen-clock/


As for following it blindly. I think most of us do this anyway with any algorithm. I know I cannot follow the process, even as an engineer, well enough to find faults or errors. I must accept the process of tried and true algorithms and the anecdotal data achieved from their successes, or failures. If people were O-toxing all the time while using Shearwaters, I would assume there was an issue with their algorithm. I do not see this, have used it myself and as a result I am willing to trust it.

With few exceptions, most of the algorithms used today are theoretical anyway, not based on empirical data. They may get tweaked as empirical data comes in but the core is based on modeling only. The old story of dropping 100 Navy divers in the water until they bend just do not apply well to todays computer models.

 

[scubadada]

I have posed this very question on SB on several occasions and have not gotten a satisfactory answer. The NOAA table uses a 24 hour rolling window of O2 exposure as do my Oceanic computers. Many of us have heard of the 90 minute half life for O2 exposure and some of us find that inherently believable, kind of like nitrogen exposure and elimination. Some manufacturers, Shearwater and Uwatec, for example, use the O2 half life in their exposure calculations. I wish there was a critical review on this topic we could all refer to. It would be great to hear from one of the manufacturers who have incorporated O2 half life into their computer to explain how and why they chose to due so. I would imagine major risk taking aversion would limit this choice to reasonable.

 

[Kevrumbo]

Think of it in terms of Surface Interval off-gassing in a set of Fast (neural) Tissue Compartments. For instance, consider the five theoretical half-time Buhlmann tissue compartment values: 5min; 10min; 15min; 20min and 30min compartments.

Remember in six half times, a particular tissue compartment (TC) is 98% desaturated:

So in a 30min Surface Interval, the 5minTC is cleared (6 multiplied-by 5 is 30min);
60min Surface Interval, the 10minTC is cleared (6 times 10 is 60);
90min Surface Interval, the 15minTC is cleared (6 times 15 is 90);
120min Surface Interval, the 20minTC is cleared (6 times 20 is 120);
180min Surface Interval, the 30minTC is cleared (6 times 30 is 180).

This is why if you're gonna do a lot of Deco diving exposure time to ppO2 1.6 ATA, you should do no more than two such deco dives a day with at least a three hour (180min) Surface Interval in between, to effectively "clear" (or neutralize those reactive oxygen free radical precursor species hypothesized to induce OxTox symptoms at depth)- in all five critical theoretical Fast Neural Tissues Compartments. Anything higher than 1.6 ATA (like in an IWR session), you should take the rest of the day off and the next day off as well. (See also: http://www.scubaboard.com/forums/te...pest-you-have-been-pure-o2-2.html#post7556892 )

For recreational NDL Nitrox diving, a maximum of 90min SIT should be ok. . .

 

[Rooster59]

I'd bet that in reality your body recovers from a lesser exposure to elevated PPO2 with a shorter "half life" than it recovers from an exposure that approaches a max limit.

 

[Kevrumbo]

I'd bet that in reality your body recovers from a lesser exposure to elevated PPO2 with a shorter "half life" than it recovers from an exposure that approaches a max limit.

Yes . . .but what about three to five dives a day on a typical week-long liveaboard on recreational Eanx32 NDL's??? I would start increasing SIT between dives to 90min nearing the end of the week's dive itinerary, and at least unload the 5min, 10min & 15min TC's of CNS exposure time as well as residual N2. . .

 

[scubadada]

Think of it in terms of Surface Interval off-gassing in a set of Fast (neural) Tissue Compartments. For instance, consider the five theoretical half-time Buhlmann tissue compartment values: 5min; 10min; 15min; 20min and 30min compartments.

Remember in six half times, a particular tissue compartment (TC) is 98% desaturated:

So in a 30min Surface Interval, the 5minTC is cleared (6 multiplied-by 5 is 30min);
60min Surface Interval, the 10minTC is cleared (6 times 10 is 60);
90min Surface Interval, the 15minTC is cleared (6 times 15 is 90);
120min Surface Interval, the 20minTC is cleared (6 times 20 is 120);
180min Surface Interval, the 30minTC is cleared (6 times 30 is 180).

This is why if you're gonna do a lot of Deco diving exposure time to ppO2 1.6 ATA, you should do no more than two such deco dives a day with at least a three hour (180min) Surface Interval in between, to effectively "clear" (or neutralize those reactive oxygen free radical precursor species hypothesized to induce OxTox symptoms at depth)- in all five critical theoretical Fast Neural Tissues Compartments. Anything higher than 1.6 ATA (like in an IWR session), you should take the rest of the day off and the next day off as well. (See also: http://www.scubaboard.com/forums/te...pest-you-have-been-pure-o2-2.html#post7556892 )

For recreational NDL Nitrox diving, a maximum of 90min SIT should be ok. . .

For oxygen elimination, where did all these fast tissues and half lives come from? What does it have to do the most often used oxygen elimination half life of 90 minutes?

I'd bet that in reality your body recovers from a lesser exposure to elevated PPO2 with a shorter "half life" than it recovers from an exposure that approaches a max limit.

That's generally not how it works, elimination half life is simply time dependent

---------- Post added November 30th, 2015 at 06:05 AM ----------

I have posed this very question on SB on several occasions and have not gotten a satisfactory answer. The NOAA table uses a 24 hour rolling window of O2 exposure as do my Oceanic computers. Many of us have heard of the 90 minute half life for O2 exposure and some of us find that inherently believable, kind of like nitrogen exposure and elimination. Some manufacturers, Shearwater and Uwatec, for example, use the O2 half life in their exposure calculations. I wish there was a critical review on this topic we could all refer to. It would be great to hear from one of the manufacturers who have incorporated O2 half life into their computer to explain how and why they chose to due so. I would imagine major risk taking aversion would limit this choice to reasonable.

Perhaps it is not critically important because there is safety leeway built into the recommendation, but, nobody has attempted to answer the simple question of whether to follow the NOAA oxygen exposure limits with a rolling 24 hour exposure or to employ a 90 minutes half life of elimination of oxygen during the surface interval (or when the pO2 is below 0.5). The support for a 90 minute half life, as posted by turisops, does not appear to be scientifically strong [abstract] A PROVISIONAL METHOD OF OXYGEN EXPOSURE MANAGEMENT FOR A RECREATIONAL DIVE COMPUTER I don't believe this abstract was ever published as a full paper. Then again, most of the oxygen exposure data is more observational than rigorously scientific. Steve Lewis is one who has taken a stand on this topic and recommends the more conservative 24 hour window rather than using an elimination half life https://decodoppler.wordpress.com/2009/02/10/daily-limits-for-cns-oxygen-toxicity/

I am personally interested in this topic due to my usual diving habits in SE Florida. I frequently dive 4 times per day on nitrox. With 5 dives in my 24 hour window of oxygen exposure, I am frequently above 80% exposure, even after 4 dives, and, not infrequently, exceed 100% exposure. If I used a computer with the 90 minute elimination half life, I would not come close to the NOAA limits. I do try to limit my oxygen exposure by opting for EAN32, whenever available, over the more commonly offered EAN36.

 

[Kevrumbo]

For oxygen elimination, where did all these fast tissues and half lives come from? What does it have to do the most often used oxygen elimination half life of 90 minutes?
. . .

Simply as an conceptual analogy and easy mathematical construct (see below post from Dr. Deco). And IMO, a 90 minute SIT parameter clears the three fastest half times of the five controlling Fast Tissue Compartments -i.g. 5minTC; 10minTC and the 15minTC. In other words, 90 minutes SIT clears over half (3 out of 5) of these critical Buhlmann Fast Compartments representing Brain and Central Nervous System Tissue. . .

. . .

. . .My first guess is that it related to the halftimes in terms of how they were assigned by Professor Buhlmann. That is, brain is 5 minutes, skin is 100 minutes, bone is 250 minutes (or something like that). These are nitrogen halftimes but are based on blood flows.

This would give the perfusion. However, oxygen toxicity appears to be related to oxygen utilization. The utilization by bone and fat is small and the partial pressure would become quite high in this tissue. The utilization by brain is very large and the partial pressure is never very high. The utilization rate is in Bennett and Elliott’s book (Dick Vann’s chapter, I believe). The CNS is a target and bone is not.

Brain and the CNS respond very profoundly to oxygen poisoning and bone does not. I would posit that some metabolic product is involved, and that it is associated with reactive oxygen intermediates. Free radicals are an example.

The data on the oxygen tolerance test (in Bennett and Elliott’s book) shows that the response is extremely variable from day to day. Pulmonary effects are apparently more regular. All in all it would not be an easy problem.

The final result is that I do not have the perfusion numbers handy but do not think that is the best way to get the half times.

:doctor:

 

[scubadada]

Interesting background for computer oxygen exposure tracking related to the abstract cited above. This comes from the Dive Rite Blog Dive Rite - Dive Rite's online journal exploring the latest in technical diving

It appears that dive computers use 2 major methods for tracking oxygen exposure, the Seiko Epson method, implemented in 1992, or a relatively straightforward use of the NOAA 24 hour exposure limits.

July 1st, 2013 Nitek Q Oxygen Tracking

By Lamar Hires
Dive Rite introduced oxygen tracking on a dive computer in 1992 with the launch of the Bridge, the first user programmable computer from 21% to 50%. It has since become the standard for any Nitrox computer. Oxygen tracking was developed by Randy Bohrer and the late Dr. Bill Hamilton for SEIKO EPSON. SEIKO EPSON went on to patent the technology of graphing oxygen tracking on dive computers, but decided it was too important not to be on all dive computers to enforce the patent. This led to the birth of the OLI (oxygen limits index), which was based on hyperbaric treatment studies. See this article for more information: A PROVISIONAL METHOD OF OXYGEN EXPOSURE MANAGEMENT FOR A RECREATIONAL DIVE COMPUTER
In the oxygen tracking algorithm, we don’t track OTUs at all. Hamilton’s OTU approach was used to manage long exposures, such as saturation type dives. For the dive computers, we track one value – the oxygen limit index. This index accounts for both CNS and chronic/whole body oxygen toxicity (or oxygen tolerance). The 90 minute half time for recovery is very conservative for CNS toxicity, since we know that it is possible to breathe oxygen at high partial pressures if a short air break is taken periodically. It is adequate for chronic/whole body toxicity, since it allows for full recovery in an amount of time that is sometimes used in respiratory therapy.
The Nitek Q has nine bars for tracking oxygen and nitrogen. At 80% of calculated maximum oxygen exposure the bar graph will start to flash once the 8th bar starts to fill up. The Nitek Q will assume oxygen loading for one hour after the dive. This is our conservative approach to oxygen tracking. With a 90 minute half life, the 8 bars lit will drop to 4 in 90 minutes, then drop to 2 bars in another 90 minutes, and to 1 bar in another 90 minutes. If your oxygen loading exceeds 100% the graph will flash until it is back into range.
Remember, tracking is only as good as the information supplied. Properly analyzing and entering the correct gas mixture is a good start.

---------- Post added November 30th, 2015 at 08:49 AM ----------

Simply as an conceptual analogy and easy mathematical construct (see below post from Dr. Deco). And IMO, a 90 minute SIT parameter clears the three fastest half times of the five controlling Fast Tissue Compartments -i.g. 5minTC; 10minTC and the 15minTC. In other words, 90 minutes SIT clears over half (3 out of 5) of these critical Buhlmann Fast Compartments representing Brain and Central Nervous System Tissue. . .

Hi Kevrumbo,

Thanks very much for the reply. I'm not sure nitrogen compartments are directly relevant to the discussion of oxygen exposure, metabolism, and reactive intermediates but it has given me one more thing to think about.

Would be great to hear from others who have spent time pondering these questions. It appears that Shearwater, Sea Bear, Uwatec, and Dive Rite all employ the oxygen half life of elimination method whereas Oceanic and Hollis (? Pelagic Pressure Systems) uses the 24 hour NOAA exposure. I did not have the time or patience to look up other brands. It would be interesting to hear why manufacturers chose their O2 tracking method.

Good diving, Craig

 

[Kevrumbo]

. . .
Hi Kevrumbo,

Thanks very much for the reply. I'm not sure nitrogen compartments are directly relevant to the discussion of oxygen exposure, metabolism, and reactive intermediates but it has given me one more thing to think about. . .

Good diving, Craig

From the OLI article link above (A PROVISIONAL METHOD OF OXYGEN EXPOSURE MANAGEMENT FOR A RECREATIONAL DIVE COMPUTER)
, an exponential decay algorithm was used for recovery. So why not use N2 exponential representative Fast Tissues off-gassing as a "loose" conceptual analogy? Everything else regarding CNS exposure & OxTox is just as anecdotal or referenced to statistically uncorrelated raw data, based for the moment on qualitative & observational phenomena anyway. . .