Gas toxicity

[The Iceni]

Hi Madmole (and Erwin),

I fully appreciate what you say about the dangers of Nitrox to the ill informed, but could I ask why you suggest Erwin should be required to track his pulmonary oxtox exposure?

As an OC diver for now, I for one, never keep a track of my pumonary oxygen exposure. If you care to look at the NOAA tables you will see that the average nitrox OC diver seldom even approches a quarter of the limit for pulmonary oxygen toxicity on a normal recreational day's diving.

I would seldom do more than two dives a day, three at most. Say these were an hour each at 1.4 bar for a total of three hours.

kp at 1.4 bar = 1.63 UPTD per minute.

A total of three hours exposure would give 180 x 1.63 = 293.4 UPTD.

The maximum accepted pulmonary toxicity allowance is 1440 UPTD in 24 hours. (One UPTD per minute in 24 hours and 24 x 60 mins is 1440)

I would therefore be exposed to 293.4/1440 or 20% of the safe limit of pulmonary exposure at the end of that day's diving with 1,146.6 UPTDs to spare.

This will give me a very safe margin for the necessary hyperbaric oxygen treatment should it be needed. The aggressive US Navy treatment table 6 uses about 400 UPDTs.

On the other hand, of course, I would not normally allow myself to be exposed to 1.4 bar of oxygen for three hours in any single day. Even then this is still equal to the safe maximum suggested by the NOAA for CNS oxygen toxicity at 1.4 bar, which happens to be 2.5 hours per dive and 3 hours in any 24 hour period.

This brings me back to the odd BSAC recommendation that 100% surface oxygen should not always be used for the victims of a CNS oxtox hit. If such a victim with DCI is treated with surface oxygen for two hours before he reaches the chamber his rescuers will have used only 1 UPTD per minute over that one hundred and twenty minute period. Another 120 UPTDs.

Still plenty of spare capacity! -

293.4 + 120 = 413.4. (1440 - 413.4 = 1026.6 UPTD to spare).

What if I used 1.6 bar for my dives?

kp for 1.6 bar O2 is 1.93 UPTD/min.

180 x 1.93 = 347.4 UKPD for the dives

347.4 + 120 = 467.4 UPTD, dives plus surface oxygen.

1440 - 467.4 = 972.6 UPTD spare.

This will still allow TWO standard treatments using table 6 that same day without producing any pulmonary problems whatsover.

The same cannot be said for CNS oxygen exposure. With the current state of knowledge it would be a very foolish diver indeed who ignored the MOD of his mix and regularly exceeded 1.6 bar.

To my mind the NOAA CNS exposure tables are not very helpful because, unlike pulmonary oxygen limits, CNS oxygen toxicity is so variable and may even produce fitting in suscepible individuals at oxygen pressures of less than 1.4 bar.

In summary, as I see it, if an average open circuit Nitrox or Trimix diver keeps his pp O2 at or below 1.4 bar at all stages of his dive, as is recommneded by most agencioes, I suggest he will seldom have any need to worry about pulmonary limits.

One hopes that those technical divers who undertake long dives with extended periods of accelerated decompression, including rebreather divers, are aware of and are advised by the NOAA recommendations but doubt Erwin is in that category.

Erwin, All Nitrox divers must be able to calulate the Maximum Operating Depth of their Nitrox mix, or chose the mix for the proposed target depth to give a pp O2 of no more than 1.4 bar;

The MOD is the depth at which the pp O2 is 1.6 bar;

In metres the MOD (1.6 bar) = ((1.60 / mix) -1) x 10.

Thus 32% Nitrox is safe to 40 metres (130 feet).

To get 1.4 bar at 40 metres, where the pressure is 5 bar ((depth /10 )+1) the required mix is 1.4/5 = 28%.

Of course the MOD of 28% Nitrox is 47 metres or 153 feet.

I hope this helps, and apologise if I am telling you something you already know but this is something that is ESSENTIAL for you to know!

Regards,

 

[Erwin Kodiat]

Dr Paul Thomas & Madmole,
I really appreciate Dr Paul Thomas' concern and madmole's suggestion to take another course while I've already logged more than 50 Nitrox dives and all of the dives adhere to PADI Nitrox manual. I don't see that I need to argue about my own knowledge and skill in Nitrox diving.
I must tell you that I'm not in purpose to exceed my maximum depth as suggested by PADI Table or my computer. It was happened because I have to help a beginner fellow diver that uncontrolledly descending into 40 meters. Should I just stay if I can give him a hand? Just forget it, it's not the matter.
I only curious, why my OLF in Suunto Stinger reach 6 bars, while when I calculating my Oxygen Exposure using PADI Nitrox Table, I'm still within safe figure. I know that this OLF is related with the CNS and not pulmonary, the second one is mostly happen in the hospital when a patient get a pure oxygen for more than 2-3 days. It's beyond the recreational diver, right?

 

[The Iceni]

Hi again Erwin,

I agree, you have no need to justify your reasons for asking the questions you do. As I understand it that's what this forum is for. I have learned a hell of a lot by asking "stupid" questions. At least it gets contributors to think! This is a public forum, after all.

I think you do remarkalbly well for someone for whom English is obviously not their first language. You did say you "unintentionally" exceeded your MOD and have now explained why. It could have been very nasty and I am glad you both came out of it OK. It sounds like you may have saved a life by risking your own. However, you have proven that you can expose yourself to 1.6 bar pp O2 for a short time without ill effects.

You also said

I know that this OLF is related with the CNS and not pulmonary, the second one is mostly happen in the hospital when a patient get a pure oxygen for more than 2-3 days. It's beyond the recreational diver, right?

My original Nitrox training was with IANTD and they supply very comprehesive course material (some of which is overly complicated in my humble opinion). However, the manual provided, as a part of the course, includes NOAA tables for later reference, which is where I obtained my figures for my last post. I heartlily recommend that manul to you.

In summary partial pressures of oxygen of less than 0.5 bar do not cause any pulmonary (or CNS) toxicity. As I said in my early post the maximum recommended daily exposure for pulmonary toxicity ia 1440 UPTDs. This is what you get if you are on 100% surface oxygen continuously. 1 bar o2 = 1 UPTD per minute and is what you would be exposed to in a hospital ITU on pure oxygen.

So yes, this far exceeds the exposure of recreational Nitrox divers.

The kp values show that at 1.4 bar it is 1.63 taimes as toxic and at 1.6 bar nearly twice as toxic.

As for your Suunto computer, it is programmed to record the deepest depth and pp O2 and, it seems, will err on the side of caution.

 

[madmole]

As the others have pointed out OLF on the Suunto is a CNS figure so you dont want to exceed it

Also rescuing your buddy I agree is a VERY valid reason for exceeding the MOD. I take back my comments about not doing it if you dont know the consequences. Sinking and floating trainees is one of the reasons I'm a strong believer in buddy lines

Pulmonary O2, I agree is not important on a single days dive if you're on open circuit. It does however become an issue if you're on a livaboard and are doing multiple dives for a longer time period. the NOAA limits for multiday exposure reduce for each day of diving. Its easy to hit the limit after 7 days. Or if like me you are an instructor doing 8 or more dives in a single day and using 50% or 100% on safety stops to increase the safety factor

Also I'm a rebreather diver and I can easily exceed the NOAA 3 hour limit at 1.3 bar PO2 on a single dive and very easily exceed the 3.5 hour daily limit

As far as I'm aware the BSAC teach 100% O2 for an incident, do now and always have done. I'm only a national instructor for them so I would probably have noticed and made a fuss if it was otherwise as I'm also a qualified paramedic/Ambulance driver and probably use the stuff theraputically a bit more than most people. Their Nitrox training even covers DCI treatment after using high O2 mixes and why it makes no difference (there are still a few misguided facilities that wont treat a diver who has used a "fancy" gas)

By the way the BSAC have quite a nice and easy set of O2 tox tables which make it easy to calc both O2 tox's. They set an 80% CNS limit with a 2 hour half life (conservative) and set 800 pulmonary units a day max on day one. The 80% CNS limit works well as it alows the longest US recompression tables to be used immediately after a dive without too much risk

 

[rcohn]

Madmole:

The Suunto computers compute both the CNS and pulmonary limits, they display whichever one is closer to its
limit.

Erwin:

If you read the Suunto manuals you will find that the algorithm follows the 1991 NOAA CNS table for partial pressures less than or equal to 1.4 atm. At higher partial pressures the computation becomes much more conservative than the NOAA table.

Ralph

 

[The Iceni]

Dear Madmole

You said

As far as I'm aware the BSAC teach 100% O2 for an incident, do now and always have done. I'm only a national instructor for them so I would probably have noticed and made a fuss if it was otherwise as I'm also a qualified paramedic/Ambulance driver and probably use the stuff theraputically a bit more than most people.

I have tried to download the detailed notes of the BSAC Oxygen administration course and the Nitrox and Advanced Nitrox Diver's course without success.

For your information, Madmole, for sevaral years the recommendations given to students on all three of the above BSAC courses read something like this.

"The only time oxygen must not be given to a diving casualty is if CNS oxygen toxicity is suspected."

This is probably the only fact all students on the O2 admin course remember. I was certainly taught this when I attended such a course last year.

Following a considerable fuss made by myself and a number of medically qualified BSAC instructors and other interested parties, (and my recent letter in the BSAC's "Dive" magazine on this very subject) the recommendation was recently amended to read something like this;

"Oxygen must be given to all diving casualties unless they continue to show the effects of CNS oxygen toxicity."

I apologise if the wording is not exact but the clear inference from BSAC is that they consider surface oxygen is harmful or at least of doubtful benefit to a diver who is suspected of having previously suffered an oxygen induced convulsion underwater. I still take issue with the modified wording, since this suggests to the non-medical diver, that if such a casualty is still convulsing on the surface (many minutes after rescue) he will still be suffering oxygen induced fits so oxygen will make matters worse.

However, as I am sure you are aware it is far, far more likely that any such fitting or unconciousness is due to HYPOXIA induced by the sequelae of his underwater convulsion and near drowning, if indeed CNS oxygen toxicity was the original problem.

In addition, in all likelihood such an unconcious casualty will have missed all necessary stops and will need oxygen treatment for the DCI from which he will inevitable be suffering, although quite clearly he will be in no position to describe any symptoms.

Also, of cousre, oxygen at 1 bar does not cause CNS (or pulmonary) oxygen toxicity.

I know the NHS is a shambles but I am sure NHS paramedics are taught the adage "The victim of trauma is always hypoxic."

Because the enzymatic mechanisms of respiration still require oxygen it makes little sense to reduce the inspired oxygen to well below 1 bar following a fit induced by oxygen toxicity. This will only serve to generate further tissue hypoxia, as in the "oxygen off effect" and actually make matters worse.

:doctor: Madmole, as a medical practitioner who is formally qualified as such being registered with the general medical council, with more than twenty years experience [fifteen of which have been in immediate care i.e trauma and emergency medicine (and with a background of human physiology)] my considered advice to the BSAC is that their recommendation to students should be rewritten as follows;

On surfacing 100% oxygen must be administered to all casualties who have suffered diving related injuries, however trivial those injuries may initially appear, and even if those injuries are believed to have their origins in CNS oxygen toxicity.

I can assure you Madmole, I would not have finished my letter in such terms if the BSAC teach 100% O2 for an incident, do now and always have done

Regards,

 

[madmole]

OK I see what you mean. Yep I looked it up last night to and your right (I was never taught it and I just never taught it with the emphasis you use). I always said to folks to use their common sense and administer O2 as much as possible and pointed out the relevent seriousness of each condition. I'm used to dealing with EP's etc so having to deal with a O2 convulsion doesn't frighten me, but I can see it could terrify some

I agree whole heartedly with your comments and wound second them. Not aware of too many side effects of O2 convultions other than associated trauma when falling or fitting and as you point out its theorietcally impossible to induce at the surface anyway.

I'd much rather have a O2 hit on the surface than DCI

Never heard the Victim is hypoxic saying before but of course its correct. Of course being the NHS means that we tend to get it emphasised to only use supplies if we have to!!!!!!! (The bosses dont like how much we use anyway, it costs money). Personally I would stick 100% on any diver for any condition and I know most others crews would do so (Pity UK Ambulances only carry delivery masks that will give a max of 40-60%), unless you use an Ambu bag

So UK divers, if an Ambulance turns up to collect your diver make sure YOUR 100% O2 kit goes with him.

Also remind the crew NOT to give Entonox for the pain as although DCI is mentioned during training its normally forgotten

Actually thats something the Docs can clear up for me. I've never heard the real reason why Entonox (a mix of 50% O2 and 50% Nitous Oxide) is bad for divers, after all it has a lower PP of Nitrogen than air and a raised O2 content, it also relieves pain but is very short term so it doesnt impair CNS assesment

 

[The Iceni]

Morning Madmole,

I came on the forum this morning, intent on modifying my rather agressive post. Too late. I am glad we didn't fall out.

As you say . . " being the NHS means that we tend to get it emphasised to only use supplies if we have to!!!!!!! (The bosses dont like how much we use anyway, it costs money)." Don't I know it, but I won't go on. I could write a book!

You are of course right to spot the fact that British ambulance crews can only give up to 60% oxygen by a so-called rebreather mask. It would be a very brave diver rescuer indeed who insisted that his buddy continues using their DV oxygen on the ambulance journey, contrary to the wishes of the "professional" who will simply be following orders. I carry an oxygen DV for my acute medical and trauma cases, which is not supplied by the NHS, and also carry a pin- index - DIN connecter so I can use their on-board O2 supply. (You should the puzzled looks I get!)

As for nitrous oxide.

There are at least two very good reasons not to use it. Remember the mechanisms of gas transfer.

All divers will have many micronuclei, if not small bubbles, on board none of which contain any nitrous oxide, which will simply diffuse into the bubbles making the pressure in them greater and their size larger and the DCI worse.

In addition, of course, the same applies to pulmonary barotrauma. A tension pneumothorax is worsened by entonox for the same reason. (I know ambulance crews are told not to administer it in thoracic trauma - for the same reason)

In addition its anaesthetic effects will mask insipient symptoms of DCI and there may be other reasons that do not immediately spring to mind.

Best wishes,

 

[madmole]

Thanks for the explanation. Dont worry I dont take offense easily and I know how easy it is to come over in a way you dont intend

Actually I think most crews would be perfectly happy if a diver produced his own O2 set, as long as you explain the need for 100% O2. A pin index adapter to use the vehicles plumbed in supply or and adapter to allow plug in to the quick releases would be a bright idea. Crews by the nature of the job have to be flexible and open to new ideas. In fact about the only people crews habitually will ignore are GP's and nurses who seem to appear like magic at incidents and proceed to tell you how to do your job. (I've had a GP declare one of my patients and put a blanket over his head, shame really as I had him talking to me about 5 mins later)

Personally I'd like to see some research on the effects of divers continuing to breath High O2 mixes post dive. As this is the time where we bubble the most it would seem sensible to continue to breath the shallow deco mix for a period after the dive. Now I'm on a rebreather I could do this easily with very little gas usage

Thanks for the Entonox explanation. Is this theory or has it ever been tried. I would have assumed the ingasing of Nitrous was less than and slower than the increased N2 outgassing as you are now breathing a No Nitrogen gas. In severe type 1 bends the pain relief could be benificial. As you know, if you stop breathing the Entonox, full CNS symptoms would return in seconds so assesment is easy. Reduction of pain would help treat the patients shock. I'm not saying I think the Entonox is a good idea, I'm just curious.

Also I'd like to know (and understand) the exact mechanism's of Entonox on the patient compared with Narcosis. My crew and I are both divers and we both feel that the feeling we get on Entonox is very similar to Narcosis (well some days anyway). You can build up a tolerence to Entonox with repeat exposure and I've often wondered if this would work towards Narcosis tolerence

 

[The Iceni]

Madmole,

As a lowly GP !!!!!!!!. . . . I am no anesthetist but I am aware of the Meyer- Overton principle of the anaesthetic properties of gasses.

It is my understanding that all inert gasses have anaesthetic properties which are dose dependent and related to the molecular weight of the gas and it's relative aqueous/lipid solubility. A surgical plane of anaesthesia is produced by inhalation of the heavier anaesthetic gasses, such a halothane, at lower than surface pressure. Nitrogen, a smaller molecule, will produce the same effects but only at pressures exceeding 6 bar.

At lower pressure there is a "twilight zone" where they are analgesic, soporific and "mind altering" - they become "narcotic" and also addictive I might add .

Nitrous oxide N2O (MW 22) is a heavier molecule that either oxygen O2 (MW 16) or nitrogen N2 (MW 14) and I believe is more lipid soluble than both, so produces narcosis at pressures of only 0.5 bar.

I think it was during WWII that the problems with entonox and divers were first discovered. It is most certainly not just theory!

On another post we were considering exon and as a possibly ideal diluent for accelerated decompression, because, unlike helium it is slow to diffuse, however also unlike helium it is highly narcotic and would cause unconciousness in any diver!

As for O2 post-dive, I think the answer is already known. As long as pulmonary limits are observed I can see only benefit, particularly after multiple dives over multiple days.