Theory behind the half-life of CNS toxicity?

[Lorenzoid]

I have really found this thread enlightening and plan to re-read some of what has been said.

Seeing where the discussion began, who chimed in, and where it is now, reminds me of the old parable about what causes decompression sickness, where the non-diver answers "I don't know" when asked to explain it, the dive instructor gives a detailed answer when asked to explain it, the hyperbaric expert gives an even more detailed answer, and the world's foremost scholar on the subject answers "I don't know."

 

[scubadada]

Unfortunately, the majority of divers probably do not care about their decompression algorithm or tracking their CNS O2 exposure. It's all done by my computer

 

[Kevrumbo]

Some "practical" observations from the field:
_________________________________________________ _____
From Simon Mitchelll, MD, Ph.D ; quoted from CCR Explorer Board:

Hello,

This is an interesting topic which is open to a huge amount of misunderstanding.

First, let me make one thing very clear: I am not proposing any change to the CNS limits as currently taught to technical divers. For a start I have no data to justify such a proposal, and any change would be replacing one set of limits that are largely data free with another set that are largely data free. What I was trying to do in the presentation was to put some context on the current NOAA limits from the perspective of expedition level technical diving where we are frequently forced to exceed them during long decompressions. Let me try to explain.

One of the most common questions I get asked at high-end technical diving shows is "how do I deal with the fact that my CNS% frequently exceeds 200 towards the end of a long decompression from deep dives. Am I going to tox, or are the limits wrong"? The truth is that many such dives have been done, and adverse outcomes (seizures) seem rare so where does this leave us? The current limits were originally proposed by brilliant diving scientist Chris Lambertson, and first published by NOAA. They have become known as the NOAA limits. As best we can tell, Lambertson based them on his experience and observations, rather than on a formally accumulated database of known outcomes. Morevoer, they probably reflect his experience of working exposures / or experiments in which physical activity was maintained at a fixed depth, rather than of dives or experiments with a short period of activity deep and a long tail of shallow exposure during rest (like a real technical dive).

This is where it gets interesting because activity status and depth have significant implications for risk of oxygen toxicity; largely because these factors influence the tendency to retain CO2, and CO2 retention in turn is a major risk factor for CNS oxygen toxicity. When we exercise at depth during breathing of a dense gas, there is a tendency (stronger in some people than others) to retain CO2. I have written at some length about this elswehere (the RF3 Proceedings freely available on line are a good place to start) and so I won't explain this in detail here. However, the increased work of breathing imposed by dense gas, the respiratory apparatus, and exercise blunts the normal increase in ventilation of the lungs which is how we get rid of CO2. If we don't ventilate the lungs enough, and therefore don't get rid of the CO2 we are producing, then body CO2 levels rise (CO2 retention). When CO2 levels rise, this markedly increases the circulation of blood through the brain, and therefore results in a higher delivery of oxygen to the brain tissue. Not surprisingly, this increases the toxicity of oxygen.

This brings me back to the question I am often asked at dive shows about high CNS percentages during decompression. The tendency to retain CO2 should be less when resting at shallow depths breathing a less dense gas. This would imply that the NOAA CNS limits are less likely to be strictly relevant to a diver resting on deco than a diver exercising at depth. On this basis, I have always been tempted to answer the question about risk during deco along those lines, and to suggest that having a CNS percent of "200" is probably not as significant in the resting deco phase of the dive as it would be if you were exercising at depth. Indeed, there is likely to be a substantial difference in risk between these two scenarios, even though the CNS% is the same. However, I was never confident in giving this answer simply because no one had ever checked for CO2 retention during resting decompression during long rebreather dives. In theory, it should be much less likely than at depth because the divers are at rest and not breathing a dense gas. But one or two of the risk factors for CO2 retention are still present during deco; not least of which is breathing through a device that does increase the work of breathing.

Thus, on a recent expedition to Bikini atoll we monitored 18 divers during two dives by measuring their end tidal CO2 immediately on arrival at the surface and before they left the water. Unfortunately there is currently no means of monitoring end tidal CO2 during the decompression itself which is why we chose this strategy. To cut a long story shortish, there was no obvious tendency for the divers to be retaining CO2 during resting shallow decompression. Their surfacing end tidal CO2 values were not different to control values made at rest on the boat 2 hours after the dive. One diver surfaced with a slightly higher than normal value after one dive.

This study suggests that there is no general trend to CO2 retention during shallow resting decompression, and this would support the notion that limits primarily targeted to oxygen exposure during activity at depth are probably not as relevant to the shallow resting decompression phase of the dive. This provides some reassurance that allowing the limits to exceed 100% during deco is not overtly irresponsible, but it does not of course, provide any guarantees that seizures will not occur under these circumstances. Clearly such events are still possible.
[Study Abstract: End tidal CO2 in recreational rebreather divers on surfacing after decompression dives. - PubMed - NCBI ]

Thus, my answer to the question "how do I deal with the fact that my CNS% frequently exceeds 200 towards the end of a long decompression from deep dives. Am I going to tox, or are the limits wrong" is now slightly more informed. I can say that there is evidence to believe that the limits are almost certainly less relevant to shallow resting decompression than they are to the deep working phase of a dive. That does not mean that divers are "tox proof" during decompression because they aren't. But this does provide some measure of reassurance that the high CNS percentages we see on deco do not have the same implications they would have if we were working at depth with the same percentages. I hope you can understand the subtleties of this.

We should continue to teach the currently limits because they have provided a useful benchmark, and entry level technical divers do need some guidelines. Moreover, the NOAA limits may well have sharper predictive power of problems during activity at depth. For example, you probably don't want to be exercising hard when breathing 1.3 ATA for more than 180 minutes (CNS 100%). But there is now reason to believe that the risk (for most people most of the time) of a problem at the same CNS% during resting shallow deco is probably less.

Hope this helps.

The paper has been accepted for publication in Aviation Space and Environmental Medicine. I will let you know when it is available.

Simon M

http://www.ccrexplorers.com/showthread.php?t=17617

 

[scubadada]

I have really found this thread enlightening and plan to re-read some of what has been said.

Seeing where the discussion began, who chimed in, and where it is now, reminds me of the old parable about what causes decompression sickness, where the non-diver answers "I don't know" when asked to explain it, the dive instructor gives a detailed answer when asked to explain it, the hyperbaric expert gives an even more detailed answer, and the world's foremost scholar on the subject answers "I don't know."

I have enjoyed and learned from this thread also and appreciate the comments from the participants.

---------- Post added December 3rd, 2015 at 11:06 AM ----------

It seems like most SB participants would rather participate in a thread with fluffier content