Diver dead after accident in Lake Ontario near Oakville

[Hatul]

While I would agree that experiencing a CNS hit at a ppO2 of around 1.5 is not that common, you need to realize that there are many other factors that can come into play in addition to just the ppO2, and considering the ppO2 alone is really just a general guideline. There is a lot we still don't know about the mechanics involved in CNS toxicity, and given this and that there are still details about the dive that we do not know, experiencing a CNS hit at a level one may not normally expect is still quite possible.

Presumably that's what happened, which means we need to rethink the margin of safety when using nitrox.

 

[Warren_L]

Presumably that's what happened, which means we need to rethink the margin of safety when using nitrox.

I doubt we would accomplish much by doing that in this instance since there is a lot that we do not know in way of specifics.

 

[Blue Sparkle]

I doubt we would accomplish much by doing that in this instance since there is a lot that we do not know in way of specifics.

What sort of additional specifics would influence this? I'm curious along the same lines as Hatul after reading the report.

Thanks,
Blue Sparkle

 

[Warren_L]

What sort of additional specifics would influence this? I'm curious along the same lines as Hatul after reading the report.

Thanks,
Blue Sparkle

Given that there is a lot we do not understand about the onset of CNS hits, there are things that may come into play that we cannot even account for since we either don't know what they are or do not understand the specific effect they have. For starters, do we know how much physical stress the diver was under at the time of the hit? Or how much work he was under leading up to the hit? Do physical factors come into play and how? Factors such as state of hydration, was he too warm/cold? These things can affect circulation which would affect delivery of oxygen to his tissues. Even if it were possible to determine these things quantitatively do we know what kind of effect they may have on the onset of CNS?

 

[swamp diver]

Presumably that's what happened, which means we need to rethink the margin of safety when using nitrox.

I would have to agree with you on this on Hatul and that is the direction both DAN and many other agencies and jurisdictions are going.

There are many risk factors for increased oxygen toxicity that we do know of such as cold water (below 49 F) or excessively warm (> 87 F), exertion, and CO2 retention (increases cerebral blood flow). Other possible factors are where an individual is in their circadian rhythm, gender, certain drugs such as decongestants (pseudoephedrine), and exposure to darkness.

On the dive in question we do know the diver was in the ppO2 range of 1.4 to 1.6 ata where the risk of oxygen toxicity is increased for some people particularly if the dive involves exertion and exposure to cold water.

Here is an excerpt from an DAN article on the subject:

Proceeding With Caution

"Between 1.4 ata and 1.6 ata (this is 99 feet / 30 meters on a 40-percent mix) is the "yellow light" region. The possibility of oxygen toxicity at 1.6 ata is low, but the margin of error is very slim compared to 1.4 ata. Individual variation, the likelihood of an unplanned depth excursion causing an increase in oxygen partial pressure, and the possibility of having to perform heavy exercise in an emergency put the possibility of oxygen toxicity at levels where caution should be exercised. Thus, levels of 1.5 to 1.6 ata should be reserved for conditions where the diver is completely at rest, such as during decompression. Again, as noted previously, the dive team must still be prepared for the possibility of an oxygen convulsion at these levels."

DAN | Medical

In this quarter's issue of Wreck Diving magazine there is a DAN article on oxygen toxicity where the author goes further and says "If you intend to dive in the 1.3 to 1.6 ata range, you may want to consider using a full face mask to mitigate the potential consequences of a seizure underwater."

I think you'll find that many divers are now using 1.2 to 1.3 ata as their maximum exposure with 1.4 ata for deco or for dives with no exertion in tropical warm water. The USN and NOAA are also using 1.3 ata and I believe in the province of BC working divers are limited to 1.2 ata.

Personally in cold Ontario waters I'd keep exposures to 1.4 ata maximum and lower if tired or if going to be exerting oneself at any level. The overall risk of oxygen toxcity may be low at 1.5 ata, however the consequences of a tonic-clonic seizure or syncopal incident underwater are unforgiving as this tragic incident shows.

For further reading I'd have a look at this Rubicon list particularly the articles by the late Dr. Bill Hamilton and that by Bitterman.
Oxygen Toxicity | Rubicon Foundation

 

[Blue Sparkle]

Given that there is a lot we do not understand about the onset of CNS hits, there are things that may come into play that we cannot even account for since we either don't know what they are or do not understand the specific effect they have.

Okay, I understand that. I thought you were speaking of the specific dive, not the "unknowns" of the CNS in general.

For starters, do we know how much physical stress the diver was under at the time of the hit? Or how much work he was under leading up to the hit? Do physical factors come into play and how?

Although we don't know for sure, wouldn't it likely be not that much since they had just got in the water and descended down a line, and nothing else in particular? I realize that is not a 100% worthy assumption, and they could have exerted on the boat or getting in -- but there was no mention of unusual exertion, current, or etc. and it was the beginning of the dive.

Factors such as state of hydration, was he too warm/cold?

True, we don't know that, and he could have been dehydrated and/or chilled from the boat ride out to the site.

Even if it were possible to determine these things quantitatively do we know what kind of effect they may have on the onset of CNS?

Again, this part I understand - that being that we don't have an exact handle on the CNS. But I see that as different from whether or not we know details about the dive, and I thought you were only referring to the latter in your previous post (but I wasn't sure so that is why I asked for clarification).

I guess that's what "seemed" unusual and made me do a mental double-take. (but granted, I'm not very experienced). Just that it was the beginning of the dive, they had only just got to the bottom of the line, and there was no "unusual" stress reported by the buddy. It seems that unless there was something unusual (which it would seem the buddy probably would have reported) there would have been as little exertion as you can have while actively diving.

 

[Hatul]

Based on this accident and the DAN info posted above by Swampdiver I've set my computer's max ppO2 to 1.3 bar from 1.4.

Adam

 

[The Chairman]

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