There are
rare reports of tech diver 02 convulsions at P02 => 1.3. As far as the early 1990s, its been suggested that tech divers maintain <= 1.2, as no recorded convulsions have occurred here. Many studies with low P02 convulsions have occur on USN rebreathers trials rather than open circuit.
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Nevertheless, the rare but anecodatal reports of convulsions and the small difference in P02 have always led me to recommend planning not exceed p02 of 1.2, but strictly when on trimix.
In addition, the risk of 02 convulsions
appears [ no true controlled studies] higher:
Among Rebreather versus OC divers for the same P02
trimix versus nitrox versus air for the same P02
certain classes of divers
In OC, the P02 is maximum only at the planned maximum bottom depth. For Rebreather divers, it can be sustained throughout the dive. The changes in p02 with depth in OC simulates 'air' breaks depending on depth, and the variability in P02 seems to provide increase tolerance to 02 convulsions.
The amount of c02 buildup that can trigger 02 convulsions is unknown, except that its a strong culprit in initiating convulsions and the dose can be very small. In dry chambers, convulsions are rare even during the peak exposures of 2.8. Ivan Montoya in Miami commented that when divers are told to remain absolutely still, no reading talking or any activity, during this phase, he has reduced dry chamber convulsions to near 0% [personal communication.]
PN2 provides some protection against 02 convulsions. When this is replaced by He, the risk for oxtox increases. The risk in increasing order is ~ air > nitrox > trimix > heliox.
Without formal testing, its not possible to determine if a diver is oxtox prone, but USN trials suggests there were such folks. It was largely abandoned when studies suggested that tox was not dose related, as suggested by CNS clock, but more consistently related to the maximum p02 inhaled. Convulsions can occur
anytime. The CNS% clock has little value.
Although the risk of tox remains low at the PO2 1.3 level, the consequences of tox, as in this report, are very harsh.
The above is not 'new' information and can be found in
Bennett and Elliott's Physiology and Medicine of Diving. You can find further discussions linked to my pen name at The Deco Stop and Scubaboard.com.
There are no known relationships between scopolamine and convulsions, when other factors [such as those mentioned here] are considered.
Dive Safe.
Quote: (Originally Posted by jhaaja)
I have been doing those 40-60m mix dives for 2,5 years now. First on OC and now on CCR. I have never earlier had any problems with quite high CNS% loads. I think something just made me more prone to toxing on that particular dive. I think the only explanation is that the previous days near 100% CNS did not count-down as it normally does (90mins halftime).
The chamber doc was more or less out from the situation. I think I knew better and my dive partner also. They were really surprised when I jerked the mask off from my face the first time in the chamber. They asked what I am doing and I just stated that I don`t want to convulse alone here in the chamber, I am taking a break. The first onset in the chamber almoust got to the point that I would have lost consiousness, I could stop it by hyperventilating the chamber air and the diaphgram contractions stopped. The doc was really surprised. I have heard that convulsions in chamber treatment are really rare.
I was lucky that I did not start to breathe on my own. That prevented me from drowning. I guess normally breathing reflex starts after the convulsion, but in my case it did not do that. My breathing started with mouth-to-mouth.
I have also been thinking of getting a full face mask. Anyway you are not able to give your mouthpiece to your dive partner. Of course I need a good CC/OC switch also.
JH
Finland