How conservative is the PPO2 Limit

[tamas970]

From wiki: "...oxymetazoline may produce profound central nervous system depression due to stimulation of central alpha-2 receptors and imidazoline receptors, much like clonidine" - okay, it also says this applies for children, in case of overdose.
However, those children are breathing ppO2=0.2 bar at ambient temperature...

Tamas970. I do not know it as fact but I don't think that oxymetazoline has the theoretical risk of increasing oxtox susceptibility that pseudoephedrine does. True, they are both decongestants but pseudoephedrine has CNS stimulate (amphetamine) characteristics that oxymetazoline does not have. I believe that it is the stimulate aspect of pseudophedrine that theoritically poses the enhanced oxtox risk, not the decongestant affect.

 

[uncfnp]

From wiki: "...oxymetazoline may produce profound central nervous system depression due to stimulation of central alpha-2 receptors and imidazoline receptors, much like clonidine" - okay, it also says this applies for children, in case of overdose.
However, those children are breathing ppO2=0.2 bar at ambient temperature...

If oxymetazoline can indeed act as a CNS depressant, similar to clonidine, then it should RAISE the seizure threshold (at least in theory) thereby reducing the risk of seizure.

The alpha 2-agonist clonidine suppresses seizures,... [Brain Res. 1994] - PubMed - NCBI

 

[Foxfish]

Interesting that the last few posts highlight some of the factors that may or may not affect a divers response to oxygen toxicity. Some chemical seem to increase the divers sensitivity to oxygen toxicity, others suppress the sensitivity. It is theorized that the narcotic effect of nitrogen can inhibit oxygen toxicity.

There are still a lot of uncertainties regarding how we respond to oxygen toxicity which is why caution is encouraged.

 

[Centrals]

Would PPO2 1.4 good enough for you for recreational dive?

 

[drrich2]

It is theorized that the narcotic effect of nitrogen can inhibit oxygen toxicity.

Seems an uncertain, unknowable and dangerous-in-its-own-right thing to use, of course.

Given that some of the diver susceptibility research was done in relation to military uses, I wonder whether anyone ever took a look at using anti-convulsants (e.g.: Dilantin, Depakote) to lower the risk of seizure from oxygen toxicity?

It's worth noting that not all anti-convulsants are equally good at preventing all types of seizure, and I have no idea whether such a medication would lower the risk of ox-tox generated seizures.

I'm just wandering whether anyone's ever checked into it.

Richard.

 

[Foxfish]

Would PPO2 1.4 good enough for you for recreational dive?

Yes, I use 1.4 b, for recreational diving. I'd recommend doing the Nitrox course and confirming in your own mind that it is a suitable limit.

I've read posts in the past that gave the impression this limit is overly conservative. The cases provided at the start of the thread show that is not the case. All the evidence I've seen to date support the recommended limits taught by the diving agencies. Until there is convincing evidence to demonstrate otherwise, I don't think that the limits are going to change to allow greater exposure.

If you are doing multiple dives a day near the MOD as happens on say a live-aboard, there has been some concern expressed about exceeding the 24 hour exposure limit. In that case I would be cautious about surface intervals and monitoring exposure times.

SDI recommend a maximum time limit of 45 minutes for a single dive and 150 minutes for a 24 hour period but that is for a PPO2 of 1.6 b. At PO2 of 1.4 b you've got a lot more time on a single dive. As you move away from the MOD the PPO2 decreases and the allowed exposure time increases dramatically. I still follow the time limits for PPO2 = 1.6 b even though I'm using PPO2=1.4 b for the MOD which is again conservative. I'd probably review that if the time limits started to become and issue.

 

[Doppler]

Seems an uncertain, unknowable and dangerous-in-its-own-right thing to use, of course.

Given that some of the diver susceptibility research was done in relation to military uses, I wonder whether anyone ever took a look at using anti-convulsants (e.g.: Dilantin, Depakote) to lower the risk of seizure from oxygen toxicity?

It's worth noting that not all anti-convulsants are equally good at preventing all types of seizure, and I have no idea whether such a medication would lower the risk of ox-tox generated seizures.

I'm just wandering whether anyone's ever checked into it.

Richard.

Richard: when my buddies and I first began to conduct deepish, staged decompression dives using nitrox et al, we stuffed ourselves with anti-oxidants for a few days pre-dive... modified diet mostly... anyhow, not sure if it did ANY good or if that strategy has any measurable effect since there was no control to compare our results to. HOWEVER, we often approached and sometimes exceeded the cumulative CNS single-dive NOAA limits during our exposures. Did handfuls of blue-berries help to keep us from convulsion? Do you think they may have? For my part, I have no clue.

 

[markmantei]

You will also notice that as well as a SINGLE exposure time limit for each oxygen depth, there is a DAILY LIMIT or 24-hour limit. These time limits vary too. MANY, MANY instructors and a couple of agencies DO NOT TEACH divers about these limits and this is a HUGE mistake in my opinion.

THESE 24-hour limits have NOTHING TO DO with Oxygen Toxicity Units. These are used to track...


#2... Pulmonary or Whole Body toxicity. This is the second type of issue related to breathing elevated levels of oxygen. Pulmonary tox is NOT an issue for divers doing recreational diving... and I include most technical divers in that caveat.

Steve, are you saying that the CNS clock is indeed tracking CNS toxicity. In all of my training, including my admittedly limited technical training, I have been told that the term CNS clock is a misnomer and is actually tracking pulmonary o2 toxicity (as do OTU's). Granted this is a bit of an educational issue as opposed to a safety issue, if you are tracking both CNS % and OTU's and obeying the prescribed limits (both single dive and cumulative). If I read/understood you correctly, I would assume that this is one of those myths you are referring too.

Mark

 

[tracydr]

Seems an uncertain, unknowable and dangerous-in-its-own-right thing to use, of course.

Given that some of the diver susceptibility research was done in relation to military uses, I wonder whether anyone ever took a look at using anti-convulsants (e.g.: Dilantin, Depakote) to lower the risk of seizure from oxygen toxicity?

It's worth noting that not all anti-convulsants are equally good at preventing all types of seizure, and I have no idea whether such a medication would lower the risk of ox-tox generated seizures.

I'm just wandering whether anyone's ever checked into it.

Richard.

I've wondered this a few times myself. Although, I'm not too sure I'd want to be diving on phenobarbitol or Dilantin, given the side effects. It would certainly be an interesting hyperbaric study.

 

[uncfnp]

I've wondered this a few times myself. Although, I'm not too sure I'd want to be diving on phenobarbitol or Dilantin, given the side effects. It would certainly be an interesting hyperbaric study.

If you need a volunteer for the valium arm of the study, I willing...