CNS exposure and deep diving

[Zaixon]

I have been planning deep OC dives lately and have been running into the issue of CNS exposure exceeding the recommended 80% value. (Shearwater and the CNS Oxygen Clock - Shearwater Research) Now in a dive say to 260fsw for 35 min using gasses 15/55, 25/25, 50%, and 100% this puts the CNS exposure over 100%. Dropping that 100% to 80% gives a CNS exposure to 80%. Now is there any reason to not plan the dive this way? I have heard multiple opinions on the topic and none have been wrong just different theories on the subject. What would be your preferred way to doing a dive as such and why. It seems the longer you plan for deeper dives on OC the limiting factor comes to CNS exposure, or am I wrong?

 

[PfcAJ]

I'd just do the dive. backgas breaks before switches and a 12 on 6 off routine on the oxygen stop.

 

[dreamdive]

Ask yourself, where is the DATA for the NOAA CNS limits coming from? Is there other data, i.e. US Navy......and

 

[kensuf]

I would rock the 100% instead of 80% and do gas breaks like AJ said.

 

[Kevrumbo]

Some "practical" observations from the field:

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 [Surface Interval/CNS % half-life] 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

_________________________________________________ _____
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

 

[dreamdive]

As best we can tell, Lambertson based them on his experience and observations, rather than on a formally accumulated database of known outcomes. They have become known as the NOAA limits.

So here we are: based on very limited scientific information the "world" has adopted standards that are......based on the physiology of one diver?!

Anecdotally, there is evidence that suggests that constant PO2 vs variable PO2 (OC diving) has different O2 toxicity profiles. That would be a worthwhile study. However, please remember that due to inter and intra-variability of O2 toxicity, large numbers are required.

Meanwhile, at least regarding constant PO2, the US Navy numbers are more 'generous' than NOAA.

 

[rjack321]

Well they have (by and large) kept divers from toxing on deco, so they aren't all bad. I would just do the dive with decent gas breaks onto your 15/55

 

[scubadada]

Title: [abstract] A PROVISIONAL METHOD OF OXYGEN EXPOSURE MANAGEMENT FOR A RECREATIONAL DIVE COMPUTER
Author: Bohrer, CR; Hamilton, RW
Abstract: BACKGROUND: Increasing the fraction of oxygen in a breathing mixture effectively improves the efficiency of decompression. Unfortunately, oxygen is toxic in varying degrees depending upon the partial pressure and duration of the exposure. Various techniques have been suggested for managing a single oxygen exposure, but there is not yet available a mathematical algorithm usable in a dive computer (these have limited processing power and memory) or when developing tables that can account for multiple and multi-level exposures and for recovery between exposures. Even so, we needed a practical algorithm for use in a recreational dive computer that would warn the user when accepted limits were approached. METHODS: For limits we selected those in the 3rd edition of the NOAA Diving Manual. To deal with non-square exposures we used the interpolation method proposed by Kenyon and Hamilton (1989, XVth EUBS) which interpolates both O2 level and exposure time; this method tracks an Oxygen Limit Index (OLI), which is incremented at discreet time intervals by the ratio between the exposure time at the PO2 and the time limit for that PO2. A warning is issued when the ratio reaches 1.0, equivalent to the allowed time at a specific level. 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. RESULTS and CONCLUSION: The algorithm has been programmed into a new recreational dive computer, the Bridge, which has been developed by Dive Rite and Seiko. Random trials of "typical" exposures show that it appears to warn conservatively when presumably acceptable exposures are exceeded, but it allows dives that are regarded as having an acceptable oxygen exposure to be displayed without a warning.
Description: Abstract of the Undersea and Hyperbaric Medical Society, Inc. Annual Scientific Meeting held July 7-10, 1993. World Trade and Convention Centre, Halifax, Nova Scotia, Canada. (Home - Undersea & Hyperbaric Medical Society)
URI: http://archive.rubicon-foundation.org/8193
Date: 1993

Most dive computers use the Seiko Epson method to track CNS O2 exposure using the NOAA tables and the 90 minute O2 elimination half life. Pelagic Pressure Systems uses an even more conservative method, a rolling 24 hour window of exposure using the NOAA tables with no O2 elimination half life.

I, not infrequently, violate the O2 exposure limit doing 4 no stop dives per day (5 dives in the 24 hour window), on my Oceanic VT3 and Geo2 computers, especially when using EAN36. These dives would never exceed the O2 exposure limit if the O2 elimination half life was used.

See Theory behind the half-life of CNS toxicity? for another relatively recent discussion on this topic

 

[Kevrumbo]

As best we can tell, Lambertson based them on his experience and observations, rather than on a formally accumulated database of known outcomes. They have become known as the NOAA limits.

So here we are: based on very limited scientific information the "world" has adopted standards that are......based on the physiology of one diver?!

Anecdotally, there is evidence that suggests that constant PO2 vs variable PO2 (OC diving) has different O2 toxicity profiles. That would be a worthwhile study. However, please remember that due to inter and intra-variability of O2 toxicity, large numbers are required.

Meanwhile, at least regarding constant PO2, the US Navy numbers are more 'generous' than NOAA.

PPO2 Exceptional Exposure Table

We removed the Exceptional Exposure Oxygen tables from the NOAA diving manual 4th editon because there was fear that if the general public saw them printed that they might take it as an endorsement to use them.

The NOAA exceptional exposure limits are set for extreme emergencies only and are not for routine use. IE: should be used for life saving only.

These are for a working dive meaning with light exertion. Remember that there are a variety of factors that come into oxygen toxicity, and crossing the 1.6 atm 45min line does not guarantee convulsion, it also does not guarantee it won't.

NOAA OXYGEN
EXCEPTIONAL EXPOSURE LIMITS

PO2/Minutes:
2.8 5
2.4 10
2.0 30
1.9 45
1.8 60
1.7 75
1.6 120
1.5 150
1.4 160
1.3 240

As you can see the exceptional times allow you a fairly large margin to use this method for an "escape." The table is NOT linear. Note that exceptional exposures are DANGEROUS and can only be done once in a day. . .

Joel Silverstein
------
Elevation of arterial CO2 levels during exertion increases the risk of cerebral oxygen toxicity. However it is plausible that going beyond oxygen exposure limits during resting decompression is less hazardous than equivalent excursions when exercising at deep depths. (See post #5 above).

Here are some extreme examples of incidents, in which the victim tragically & unknowingly was breathing a high ppO2 deco mix past MOD, and did not tox into seizures until after some minutes:

An infamous tragic accident, as told by Dan Volker:
Then, there was the Jane Orenstein death... In it, tech instructor Derrick McNulty, had his buddy and student with him on ascent from a 280 foot dive. He did not watch her switch gas from bottom mix to travel gas at 100 feet, and missed that she went to her O2 bottle. Jane breathed the O2 from 100 feet, through the 50 foot, 40 foot, and 30 foot stops....at the 30 foot stop, she signalled she was low on gas, and McNulty waved her up to the 20 foot stop direction--she ascended by herself, and then McNulty watched her stop ascending, and begin a plunge downward.....He just watched as she began falling, and the 2 tech students below saw her dropping at their 50 foot stop, and one tried to chase after her, but could not equlalize, and had to stop....The instructor later said that he could not follow her, because he did not have enough gas to go after her and to rescue her.

Fatality at WKP
I'm very sorry to report to you that our friend and fellow explorer Jim Miller died today during a dive in the WKP.

It's too early for us to report on the dive in great detail, but what I can tell you is that he seized and drowned in the cave after breathing a 70ft deco bottle [Eanx50] for an extended period of time on his way into the cave. The bottle was marked and analyzed correctly. The depth was approximately 200ft and the incident occurred soon after the team turned and began their exit. He was brought back to the basin by his buddies following an unsuccessful attempt to revive him at depth, and then to the surface by other team members.

[Comment: Interesting that the victim did not OxTox-seizure immediately, but after nearly an hour on a RB80 Rebreather inadvertently gas switched to a 21m/70' MOD tank (Nitrox 50) at approx 7 ATA. . .]