Dive planning and OTU's

[nakatomi]

Ollie, you probably already have it, but in case you don't. You should get the "Ehm", he has got really good sections on oxtox that were backed up by actual testing. Don't think it is available in English though. You can borrow it if you want. It's the best book on diving physiology I have come across.

Thank you Bent! I believe I have that book in my collection, although I can't access it right now as I'm out of the country.

There must be a bunch of people on here that have done dives with the cns clock going through the roof. Where are you guys?

I fear that some of them may be on that Excel sheet over on that .uk web site and that's exactly what worries me.

[bent]

I'd just check on CCR boards like rbw. There should be threads on that for sure.

[RN]

There you go. This is what I meant. From what you guys are posting you seem to confuse those two. That is what you should know from a basic Nitrox course. From exceeding your clock % over extended periods of time you will screw your lungs. Getting cramps you will get ONLY from high ppo2 at a point in time, not because of a high clock %.

Pulmonary toxicity is really difficult to get. You need hours upon hours of exposure to high levels of O2. In the ICU we'll keep ventilator patients on 100% O2 for days if necessary. It's not something we like to do but oxygenating the brain is more important than protecting the pulmonary tissue. And it takes days for that to be an issue. Under pressure you have less time but even a few hours shouldn't be an issue. It's the neurological toxicity that is a concern then.

[jebernot]

Here is the biggest problem with OTUs and multiple days of exposure to high levels of Oxygen. This is especially applicable to cave and rebreather divers who come to cave country and dive, dive, dive. I see divers who make multiple dives over a period of days and unlike CNS you do not get a surface interval credit with pulmonary O2 tracking unless you let the clock completely reset. So here we have a diver for instance that loads up day one to 600 or 700. That is probably fine. IF THEY DO NOT DIVE THE NEXT DAY. The problem comes in when they do, and then they dive the day after, and the day after that. Then they get bent that last day of a week long trip and have to go to the chamber and get hit with additional O2 for hours on end. Now they are frying their lungs. A big part of the OTU limits was keeping in mind the ability to be able to survive a chamber ride without significant damage to the lungs due to high Oxygen exposure. Many OC divers do not pay as close of attention to OTUs as they should in part because most OC divers do not ever get up to those limits. The average boat trip is two dives at 45 minutes each. The average vacation cave diver may do 2 or 3 times that in water time every single day. I see divers go back and forth to the dive shops into the eveneing to refill tanks over and over again. With cave divers doing multiple long dives in a day for a week straight or CC divers doing 4-6 hour dives over a period of days this becomes a very real issue. Believe me I have seen divers go to the chamber after having reached that OTU limit and they experienced the signs and symptoms of pulmonary O2 toxicity not upon exiting the water, but upon being treated for the hit. Food for thought.

[Bobby]

Pulmonary toxicity is really difficult to get. You need hours upon hours of exposure to high levels of O2. In the ICU we'll keep ventilator patients on 100% O2 for days if necessary. It's not something we like to do but oxygenating the brain is more important than protecting the pulmonary tissue. And it takes days for that to be an issue. Under pressure you have less time but even a few hours shouldn't be an issue. It's the neurological toxicity that is a concern then.

Rob,
I disagree with your statement. What works in a hospital has no relationship to what is happening or works in the water. My experience is that even on short shallow decompression dives of a few hours pulmonary can and will become an issue. In hospitals the pressure is not above 1.0 and the patient does not have a substantial inert gas load that requires off loading within a short limited time. On dives 2-2.5 hours at less than 100 feet I would commonly get skin bends. After dropping my working PO2 down to 1.0 I have not had a single case of skin bends even with trimix dives running double digit run times. This advise was given to me by Simon who does work extensively in the field of recreational decompression theory.

My experience is that on single long dives CNS load well beyond the standard accepted limit are not an issue. Multiple dives over multiple days however can and will be an issue within the presently accepted CNS teaching. When Dr. X was Basic Cave and we lived up North it was common for us to do multiple dives over multiple days. From my past experience including working on live aboard dive boats with divers doing no decompression dives over multiple days with elevated PO2's I have seen many CNS issues.

On single extended run dives my primary concern is pulmonary, CNS is not a concern (for me). When doing multiple dives in a day we track CNS & do not give surface credit for 24 hours. Just like years ago we would use higher than 1.6 PO2 for decompression I believe that the present teaching of Advanced Nitrox & Decompression Procedures needs to updated to our present understanding. Those courses were developed from the NOAA standards which are very limited in their application & do not relate well with present time types of dives that have become common.

By lowering my PO2 for the working part of the dive I feel better getting out of the water, which is intuitive data. I also have not had a single case of skin bends, which is empirical data. Before I made a concerted effort to lower my PO2's I would occasionally dive places like LR that pushed my PO2's back up to 1.4 on the working part of the dive on OC. The skin bends would then come back, since lowering them to 1.0 on all dives both OC & CCR I have had zero incidents. This is a small data set however it is enough for me to believe that it is spot on.

[Burke]

Rob,
I disagree with your statement. What works in a hospital has no relationship to what is happening or works in the water. My experience is that even on short shallow decompression dives of a few hours pulmonary can and will become an issue. In hospitals the pressure is not above 1.0 and the patient does not have a substantial inert gas load that requires off loading within a short limited time. On dives 2-2.5 hours at less than 100 feet I would commonly get skin bends. After dropping my working PO2 down to 1.0 I have not had a single case of skin bends even with trimix dives running double digit run times. This advise was given to me by Simon who does work extensively in the field of recreational decompression theory.

My experience is that on single long dives CNS load well beyond the standard accepted limit are not an issue. Multiple dives over multiple days however can and will be an issue within the presently accepted CNS teaching. When Dr. X was Basic Cave and we lived up North it was common for us to do multiple dives over multiple days. From my past experience including working on live aboard dive boats with divers doing no decompression dives over multiple days with elevated PO2's I have seen many CNS issues.

On single extended run dives my primary concern is pulmonary, CNS is not a concern (for me). When doing multiple dives in a day we track CNS & do not give surface credit for 24 hours. Just like years ago we would use higher than 1.6 PO2 for decompression I believe that the present teaching of Advanced Nitrox & Decompression Procedures needs to updated to our present understanding. Those courses were developed from the NOAA standards which are very limited in their application & do not relate well with present time types of dives that have become common.

By lowering my PO2 for the working part of the dive I feel better getting out of the water, which is intuitive data. I also have not had a single case of skin bends, which is empirical data. Before I made a concerted effort to lower my PO2's I would occasionally dive places like LR that pushed my PO2's back up to 1.4 on the working part of the dive on OC. The skin bends would then come back, since lowering them to 1.0 on all dives both OC & CCR I have had zero incidents. This is a small data set however it is enough for me to believe that it is spot on.

I've haven't heard of skin bends being associated with PO2 at depth. The few cases I've experienced involved air dives and insufficient decompression.

[Bobby]

It is not something I read. I was chatting with Simon at a conference about my experience. After establishing that I had tested negative for a PFO he explained how & why lowering my PO2 on the working part of the dive. It took a while to get my head around it because, at the time, my line of thought was higher PO2 = less loading of inert gases. Now it is lower PO2 = less pulmonary stress allowing more efficient off gassing on decompression.

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[apitkin]

Pulmonary toxicity is really difficult to get. You need hours upon hours of exposure to high levels of O2. In the ICU we'll keep ventilator patients on 100% O2 for days if necessary. It's not something we like to do but oxygenating the brain is more important than protecting the pulmonary tissue. And it takes days for that to be an issue. Under pressure you have less time but even a few hours shouldn't be an issue. It's the neurological toxicity that is a concern then.

Not sure I'd agree. During the research phase of development of saturation diving pulmonary toxicity was a real problem that had to be quantified, so most of the research you see in diving medicine textbooks comes from that era. Essentially the maximum PO2 that will be tolerated indefinitely without evidence of toxicity is about 0.5 ata. Above that the rate at which manifestations develop rises quite steeply. At 1 ata (100% in your ICU) even people with normal lungs will have objective evidence of decreased vital capacity after 6 hours.

In diving the effect may be harder to discern but I have only experienced subjective symptoms of pulmonary toxicity during and after dives of 8 hours or more, and only troublesome symptoms over about 10 hours, with PO2s always exceeding 1.0 (usually up to 1.3 during decompression).

The CNS clock is a reasonable measure of total oxygen exposure. It has no demonstrable correlation with risk of CNS toxicity, which is not surprising given the quasi-random nature of CNS toxicity. There is definitely a threshold PO2 below which CNS toxicity is just about unheard of regardless of time of exposure (say 1.2 ata), yet it is possible on a long dive in which the PO2 never exceeds 1.2 to push the CNS clock up to 200-300+%. OTUs and the CNS clock are measuring the same thing in different ways.

[mstueven]

Bobby I think that's a fascinating point. It seems backwards that lowering the PO2 on the working part of the dive will result in LESS decompression stress, but the way you just reasoned it out really piques my interest - to the point I'm going to alter some diving habits and see what happens.

I've had what I assume are skin bends on a few decompression dives, and always connected it with not being conservative enough. But I also know that extending the decompression sometimes leaves me with a dry cough which I suspect is a symptom of some level of pulmonary toxicity. I have extremely minor non-exercise induced asthma, so I just wrote it off as being a little more susceptible to the Oxygen and dry tank air.

Now thinking of it counter-intuitively, I can really see how lowering the PO2 during the working phase could minimize the initial pulmonary stress, which in turn makes the decompression more efficient, and could actually decrease the likelihood of getting bent, even though I'm loading a little more inert gas for a given amount of bottom time.

I never would have connected the dots like that. Thanks.

[RN]

Rob,
I disagree with your statement. What works in a hospital has no relationship to what is happening or works in the water. My experience is that even on short shallow decompression dives of a few hours pulmonary can and will become an issue. In hospitals the pressure is not above 1.0 and the patient does not have a substantial inert gas load that requires off loading within a short limited time. On dives 2-2.5 hours at less than 100 feet I would commonly get skin bends. After dropping my working PO2 down to 1.0 I have not had a single case of skin bends even with trimix dives running double digit run times. This advise was given to me by Simon who does work extensively in the field of recreational decompression theory.

My experience is that on single long dives CNS load well beyond the standard accepted limit are not an issue. Multiple dives over multiple days however can and will be an issue within the presently accepted CNS teaching. When Dr. X was Basic Cave and we lived up North it was common for us to do multiple dives over multiple days. From my past experience including working on live aboard dive boats with divers doing no decompression dives over multiple days with elevated PO2's I have seen many CNS issues.

On single extended run dives my primary concern is pulmonary, CNS is not a concern (for me). When doing multiple dives in a day we track CNS & do not give surface credit for 24 hours. Just like years ago we would use higher than 1.6 PO2 for decompression I believe that the present teaching of Advanced Nitrox & Decompression Procedures needs to updated to our present understanding. Those courses were developed from the NOAA standards which are very limited in their application & do not relate well with present time types of dives that have become common.

By lowering my PO2 for the working part of the dive I feel better getting out of the water, which is intuitive data. I also have not had a single case of skin bends, which is empirical data. Before I made a concerted effort to lower my PO2's I would occasionally dive places like LR that pushed my PO2's back up to 1.4 on the working part of the dive on OC. The skin bends would then come back, since lowering them to 1.0 on all dives both OC & CCR I have had zero incidents. This is a small data set however it is enough for me to believe that it is spot on.

I did mention that the time limitations under the pressure experienced in diving does play a factor. My comments are more in response to the statement that you will "screw your lungs" with high CNS. Yes, there are other issues involved but I think the effect on the pulmonary tissue will be the least of the issues.

Not sure I'd agree. During the research phase of development of saturation diving pulmonary toxicity was a real problem that had to be quantified, so most of the research you see in diving medicine textbooks comes from that era. Essentially the maximum PO2 that will be tolerated indefinitely without evidence of toxicity is about 0.5 ata. Above that the rate at which manifestations develop rises quite steeply. At 1 ata (100% in your ICU) even people with normal lungs will have objective evidence of decreased vital capacity after 6 hours.

In diving the effect may be harder to discern but I have only experienced subjective symptoms of pulmonary toxicity during and after dives of 8 hours or more, and only troublesome symptoms over about 10 hours, with PO2s always exceeding 1.0 (usually up to 1.3 during decompression).

The CNS clock is a reasonable measure of total oxygen exposure. It has no demonstrable correlation with risk of CNS toxicity, which is not surprising given the quasi-random nature of CNS toxicity. There is definitely a threshold PO2 below which CNS toxicity is just about unheard of regardless of time of exposure (say 1.2 ata), yet it is possible on a long dive in which the PO2 never exceeds 1.2 to push the CNS clock up to 200-300+%. OTUs and the CNS clock are measuring the same thing in different ways.

I think we're saying the same thing. I just didn't go into detail. My response was specifically geared to bent's statement. I wasn't trying to say high CNS is not an issue on pulmonary tissue but rather that it's the least of the issues.