Theory behind the half-life of CNS toxicity?

[scubadada]

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 based on qualitative & observational phenomena anyway. . .

I have no trouble, in fact, find intellectually intuitive, that there should be an exponential elimination of oxygen or metabolites/intermediates. I'm just trying to reconcile the 90 minute half life of the Seiko Epson model with the fast compartments you put forth. Personally, I do not understand the choice of the 90 min half life and find your argument reasonably compelling. Out of curiosity, do you use a dive computer to monitor your O2 exposure in addition to other things? If yes, would you mind sharing which one(s)

Come on, let's have a few more interested SBers chime in here, sure I'm not the only one interested in oxygen exposure monitoring and ox-tox.

---------- Post added November 30th, 2015 at 12:05 PM ----------

Suunto and Mares both have insufficient information in their manuals (Eon, Matrix) to know what the hell they are doing for O2 tracking. Of course, they are both running proprietary RGBM decompression algorithms that we don't know much about either.

Much that goes on in scuba is relatively secretive, hopefully/probably pretty safe

 

[Kevrumbo]

I have no trouble, in fact, find intellectually intuitive, that there should be an exponential elimination of oxygen or metabolites/intermediates. I'm just trying to reconcile the 90 minute half life of the Seiko Epson model with the fast compartments you put forth. Personally, I do not understand the choice of the 90 min half life and find your argument reasonably compelling. Out of curiosity, do you use a dive computer to monitor your O2 exposure in addition to other things? If yes, would you mind sharing which one(s)

Come on, let's have a few more interested SBers chime in here, sure I'm not the only one interested in oxygen exposure monitoring and ox-tox. . .

I can't reconcile with the "90min half-life" either, unless you look at it arbitrarily in analogous terms of a Fast Tissue Compartment with a half life time of 15min:

6 half life times of 15 is 90 minutes which would clear or reset this Tissue Compartment along with the faster 5min and 10min Tissue Compartments. And there are five of these Fast Tissue Half-life Compartments in Buhlmann's ZHL-16 model which represent Brain & Neural Tissues and are considered critical & controlling (5min; 10min; 15min; 20min and 30min Half-life Times). So 90min surface interval recovery time clears 3 of these 5 Fast Tissues, or in other words 50% of this set of Tissue Compartments representing the Brain & Central Nervous System as a group. Therefore in a loose analogy, the 90 minutes surface interval recovery can be construed as a half-life time for clearing the most sensitive brain and neural tissues of reactive oxygen precursor molecules thought to cause OxTox seizures at depth.

Btw, I use a Petrel Computer to track O2 exposure. (See also: http://www.scubaboard.com/forums/te...pest-you-have-been-pure-o2-2.html#post7556892 )

 

[scubadada]

From the abstract:

For recovery we used exponential decay, letting the OLI decrease with a half time of approximately 90 minutes when the PO2 is less than the established threshold of 0.5 atm. This allows 50% recovery in 1.5 hrs, 90% in 5, and full recovery in 9 hrs. Precise data to support this recovery rate is lacking, but it is consistent with the practice of intermittent oxygen breathing during hyperbaric therapy, and the delay between repetitive treatments.

From the Dive Rite blog:

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.

So....I think the 90 minute half life used for oxygen/metabolite/intermediate elimination is not based on any high science, but, was a relatively conservative estimate used for the development of the Seiko Epson dive computer oxygen exposure tracking algorithm. It does not appear to be related to any of the nitrogen compartments we know from decompression algorithms. The oxygen exposure algorithm was implemented in 1992 and has been used successfully by countless divers with many different brands of computers, similar to experience with decompression algorithms.

Getting back to my personal concern of frequently exceeding 80% of my O2 exposure limit and, more than occasionally, exceeding 100% using my Oceanic computers. My computers track O2 exposure using the NOAA tables with a rolling 24 hour window and no credit for surface intervals. This mechanism of tracking is very conservative. Oxygen tracking using the NOAA tables with a 90 minute elimination half life is considerably more liberal but also appears to be safe as demonstrated by the method's wide use over more than 20 years.

After doing 4 dives in a day, I frequently go into my 1st dive of the next day with an O2 exposure of 80+%. Using the elimination half life, after an overnight surface interval, I would be going into my 1st dive with no O2 exposure. I feel better about the safety of my diving practices but the O2 tracking mechanism Oceanic uses is a nuisance. My primary VT3 dive computer's main screen is controlled by exceeding the O2 limit and no deco time must be monitored with the 8 segment bar graph and/or by accessing an alternate screen, cylinder pressure is always displayed. My Geo2 backup locks you out of no deco time and you must follow the 5 segment bar graph.

I've been diving my VT3 for more than 5 years and more than 600 dives. I've been diving my Geo 2 backup (along with a SPG) for more than 4 years and more than 500 dives. They have been 100% reliable, I know them very well, and they have helped keep me safe. I also know that my next computer(s) will have an algorithm that will allow me to control my no deco and deco profiles and will track O2 utilizing the elimination half life.

---------- Post added December 1st, 2015 at 07:59 AM ----------

Theory behind the half-life of CNS toxicity? I would be very grateful if anyone could provide any info on this question.
Much obliged.

I think we may have finally addressed the OP's question

 

[Kevrumbo]

From the abstract:

For recovery we used exponential decay, letting the OLI decrease with a half time of approximately 90 minutes when the PO2 is less than the established threshold of 0.5 atm. This allows 50% recovery in 1.5 hrs, 90% in 5, and full recovery in 9 hrs. Precise data to support this recovery rate is lacking, but it is consistent with the practice of intermittent oxygen breathing during hyperbaric therapy, and the delay between repetitive treatments.

From the Dive Rite blog:

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.

So....I think the 90 minute half life used for oxygen/metabolite/intermediate elimination is not based on any high science, but, was a relatively conservative estimate used for the development of the Seiko Epson dive computer oxygen exposure tracking algorithm. It does not appear to be related to any of the nitrogen compartments we know from decompression algorithms. The oxygen exposure algorithm was implemented in 1992 and has been used successfully by countless divers with many different brands of computers, similar to experience with decompression algorithms.
. . .

The practice of Air Breaks during Hyperbaric Oxygen Therapy (HBOT) is to mitigate the inflammatory & vasoconstrictive effects of prolonged Oxygen exposure on Pulmonary Lung Alveolar Tissue --NOT as a qualitative justification to employ an exponential decay algorithm to relieve Neural Tissues of Reactive Oxygen Species (ROS) molecules (and this point IMO is where the articles above are incorrect in their interpretation).

A true recovery from prolonged ppO2 exposure or HBOT is to breath normal ambient air at 1ATA, just as you would during a Surface Interval where coincidentally you would be off-gassing residual inert N2 at the same time anyway. . . a 90 minute SIT, a conservative time interval for repetitive diving in general, is just as convenient as labeling it a "90min half life time" for recreational Nitrox diving as well.

 

[scubadada]

The practice of Air Breaks during Hyperbaric Oxygen Therapy (HBOT)is to mitigate the inflammatory & vasoconstrictive effects of prolonged Oxygen exposure on Pulmonary Lung Alveolar Tissue --NOT as a qualitative justification to employ an exponential decay algorithm to relieve Neural Tissues of Reactive Oxygen Species (ROS) molecules (and this point IMO is where the articles above are incorrect in their interpretation).

A true recovery from prolonged ppO2 exposure or HBOT is to breath normal ambient air at 1ATA, just as you would during a Surface Interval where coincidentally you would be off-gassing residual inert N2 as well. . .

I didn't make it up, it's just what's out there. Like it or not, it's proven to be successful, you're using it on your Petrel.

O2 elimination, in this model, begins when the pO2 falls below 0.5, most of the time is spent breathing air during the surface interval

---------- Post added December 1st, 2015 at 11:17 AM ----------

So, I finally agree with all the folks who have been telling me that you will not reach your O2 limit in recreational diving (as long as you do not dive PPS or other computers using NOAA limits without an O2 elimination half life)

 

[Kevrumbo]

I didn't make it up, it's just what's out there. Like it or not, it's proven to be successful, you're using it on your Petrel.

O2 elimination, in this model, begins when the pO2 falls below 0.5, most of the time is spent breathing air during the surface interval

---------- Post added December 1st, 2015 at 11:17 AM ----------

So, I finally agree with all the folks who have been telling me that you will not reach your O2 limit in recreational diving (as long as you do not dive PPS or other computers using NOAA limits without an O2 elimination half life)

A 90 minute SIT, a conservative time interval for repetitive diving in general -breathing ambient air at 1ATA & ppO2 0.21- it's all just as convenient as labeling it a "90min half life time" for recreational Nitrox diving purposes as well.

A dive computer that can track this is fine just so it doesn't lock you out if exceeding CNS exposure limits (I've had the Petrel Computer max'd out at "999" during an IWR last year, and 200 to 300 on extended O2 deco stops on a Truk wreck trip this past Jan. So how does this info help me? Well. . . despite such egregious realtime CNS values calculated & tracked by the Petrel, "qualitatively" I did not suffer any OxTox seizures).

---------- Post added December 1st, 2015 at 09:54 PM ----------

From GUE Founder Jarrod Jablonski:

A subject near and dear to my own heart;-). I have invested considerable energy into this subject and long considered it the most nebulous and challenging risk we face in very long exposures at depth. What we know (or don’t know) about CNS toxicity is startling. What we don’t know about CNS toxicity as it relates to a tracking, multi-exposure et al is even more breathtaking.

The 90min is largely derived from work done by Bill Hamilton for the release of the Bridge Nitrox computer (if I remember right sometime in the late 1990s). I pressed Bill pretty hard a couple times because I thought “recovery” was much faster and because I was equally curious about numbers that just “appeared” and had been inaccessible to me despite significant research on the subject. My best assessment is they are a WAG based upon a conservative (probably quite conservative) assessment of what will insulate most people (the overwhelming majority) from CNS toxicity. Like the “clock” this is mostly useless for aggressive technical diving. But it is not hard for most divers to use (average tech dives stay close) and because it has some measure of industry support/precedent we felt it was necessary to introduce. Just like decompression tables which may or may not be derived from an algorithm that is really telling us what happens, precedent and/or frequency of use creates some measure of reliability.

Because CNS toxicity is so hopelessly divergent with respect to risk (inter and intra dependency is massive) and because there are precious few studies (and ever fewer that are reliable) we (GUE, industry etc) had to start somewhere. Regrettably we are not likely to see any meaningful or useful studies on this subject so we had to build our own assessments and procedures to find the best compromise. The lack of knowledge on this subject is why GUE procedures are cautious with respect to oxygen exposure, breaks etc.

Best Wishes,
Jarrod

Jarrod Jablonski

CEO
Halcyon Manufacturing
Global Underwater Explorers
Extreme Exposure

 

[scubadada]

Thanks Kevrumbo, I appreciate your post including the information from Jarrod Jablonski. It appears that my understanding of the 90 min elimination half life was pretty close, very conservative estimate based on meager information.

In the meantime, I ran a 10 dive series on MultiDeco in which I had exceeded the Oceanic computer O2 exposure limit (>100%) on 6 of the dives. MultiDeco uses the 90 min half life and none of the dives reached even 50% of the CNS O2 exposure limit. The difference between these 2 methods is startling.

 

[Doppler]

If the OP can get hold of a copy of the NOAA diving manual, the half-life was originally proposed by NOAA for exposures of 1.6 bar ONLY.

In later editions, the manual recognized that technical divers applied the half-life concept to ALL hyperoxic exposures.

There is no "science" supporting the practice... at least not from NOAA, and, as already mentioned in this thread, Bill Hamilton called methods common in the technical diving community "bull****." It is easier and better supported to use the NOAA tables... INCLUDING THE 24-HOUR LIMITS!

I don't dispute that there are some data to support a half-life and yet more that suggests NOAA's tables need to be redressed, but the science, peer-reviewed studies et al seem to be lacking.

 

[scubadada]

If the OP can get hold of a copy of the NOAA diving manual, the half-life was originally proposed by NOAA for exposures of 1.6 bar ONLY.

In later editions, the manual recognized that technical divers applied the half-life concept to ALL hyperoxic exposures.

There is no "science" supporting the practice... at least not from NOAA, and, as already mentioned in this thread, Bill Hamilton called methods common in the technical diving community "bull****." It is easier and better supported to use the NOAA tables... INCLUDING THE 24-HOUR LIMITS!

I don't dispute that there are some data to support a half-life and yet more that suggests NOAA's tables need to be redressed, but the science, peer-reviewed studies et al seem to be lacking.

Agreed, entirely, I respect that you took this stand in your blog. However, most dive computers use the Seiko Epson algorithm and have proven to be safe/not proven to be unsafe, including the Shearwater Petrel. It would seem like the limits are relatively conservative. The rolling 24 hour period used by my Oceanic computers would appear to be ultra, ultra conservative. The Seiko Epson algorithm was 1st implemented in 1992. It would be great if there was more quality research on O2 exposure, or decompression as far as that goes, but who is going to fund it. If it's not the US Navy, who? We may be stuck where we are.

Maybe the computer manufacturers will fund the research, that's the way it works for new medical devices. Yeah, right, that's the way it's going to work. Decompression algorithms generally work, O2 exposure algorithms generally work, I doubt we're going to make much progress here. A sorry state of affairs, but that's it.

 

[Doppler]

... It would be great if there was more quality research on O2 exposure, or decompression as far as that goes, but who is going to fund it. ... good stuff snipped... Decompression algorithms generally work, O2 exposure algorithms generally work, I doubt we're going to make much progress here. A sorry state of affairs, but that's it.

Agree totally on the funding comments... be nice to see some definitive work and studies... rather than the WAG stuff we use, but...