OTU's. Where do the limits come from?

[BigJetDriver]

OK, BJ, just what is, by definition, "lung vital capacity"?
Didn't know I'd end up going to med school, but there it is . . . .

Sorry, but I had to run out in the middle of the discussion. I suspect that, by now, everyone has checked the reference on vital lung capacity.

The clinincal manifestations of pulmonary oxygen toxicity appear to be caused by a tracheobronchitis that starts in the substernal or carinal area and spreads throughout the tracheobronchial tree as observed by Clark and Lambertsen in 1971. (Bennett and Elliott 5th)

Using the Repex (so-named for Repeated Exposure) chart, developed by R. W. "Bill" Hamilton, et al, the one day dose is 850 units in total. If you plan to dive for many days in a row, it drops to 300 units per day.

The effect is sometimes described as "Whole Body Oxygen Toxicity", due to the range of effects that can occur due to longer term exposure to high PPO2's. This can be somewhat confusing, since some of the effects are not easily observed or measured, and/or have not been studied as extensively. None the less, the pulmonary effect is the most rapidly and easily noticed, and can be tracked through the UPTD/CPTD method.

Bennett and Elliott point out that: "It is important to recognize that any mathematical expression used to describe the rate of development of oxygen poisoning is dependant upon and limited by the empirical data set upon which the analysis is based." This applies to CNS toxicity as well. The formulae are expressions of the data sets.

 

[BigJetDriver]

Along the same thread, how does this compare to the % CNS/min ???

Kracken,

In direct answer to your question, tracking the level of exposure to CNS toxicity is perhaps more important initially than tracking the CPTD, due to the rapid nature of its onset, and the likely effect on the diver if he or she were to convulse (or to use the technical term, "Do the Chicken!") underwater. BOTH must be tracked, however, in multi-day diving situations.

Cheers!

 

[wedivebc]

List,

I'm wondering if anyone knows how the OTU times are derived. I'm not speaking about the pulmonary "clock" used to determine your percentage of O2 dose over time. For example:

PPO2...Max Single dive exp..........24 hour Exp..........OTU p/Min
.6.................720..........................720...................0.27
.7.................570..........................570...................0.47
.8.................450..........................450...................0.65
.9.................360..........................360...................0.83
1.0...............300..........................300...................1.00
1.1...............240..........................270...................1.16
1.2...............210..........................240...................1.32
etc.

How do we get the OTUp/min numbers? Ie. Where does the 0.27, 0.47 etc come from?

Thanks,
Harrison

Wait I'm confused. When we talk about CNS toxicity we refer to a CNS% clock such as the chart above. When we talk about OTUs we are now dealing with whole body toxicity which is an entirely different animal. Or am I missing something?

 

[BigJetDriver]

Thanks for the quick reply.
If one did not have access to the OTU time chart, is there a math formula known to create them, lets say, based on PPO2 and something else?

Thanks again,
Harrison

Harrison,

As you have seen, there are formulae for both CPTD and CNS exposure, but it is far easier to utilize the pre-prepared tabulated data to effect your tracking. (We pilots like to do it the easy way! )

Cheers!

 

[BigJetDriver]

Wait I'm confused. When we talk about CNS toxicity we refer to a CNS% clock such as the chart above. When we talk about OTUs we are now dealing with whole body toxicity which is an entirely different animal. Or am I missing something?

Dave,

No, you are not confused. They are two different animals. The CNS clock refers to "central nervous system poisoning by oxygen" and the UPTD clock refers to "lung tissue poisoning by oxygen", to use a looser but still accurate description.

 

[wedivebc]

Dave,

No, you are not confused. They are two different animals. The CNS clock refers to "nervous system poisoning by oxygen" and the UPTD clock refers to "lung tissue poisoning by oxygen", to use a looser but still accurate description.

We used to call CNS the Paul Bert effect and whole body Lorrain Smith effect. I don't know if those terms are still used.

 

[BigJetDriver]

List,

I'm wondering if anyone knows how the OTU times are derived. I'm not speaking about the pulmonary "clock" used to determine your percentage of O2 dose over time. For example:

Thanks,
Harrison

In regard to the first question, I was trying to do several things at once, and did not read Harrison's question accurately.

In any case, the formulae for both CNS toxicity, and Pulmonary toxicity, are simply mathematical representations of observed data sets from many studies.

Better???

 

[BigJetDriver]

We used to call CNS the Paul Bert effect and whole body Lorrain Smith effect. I don't know if those terms are still used.

You are quite correct. They were named after the men who did the first effective studies of the conditions; i.e. Paul Bert for CNS work, and Lorraine-Smith for pulmonary toxicity studies. We have drifted away from those descriptions in average use, however, and just use OTU and CPTD.

 

[harrison]

Harrison,

As you have seen, there are formulae for both CPTD and CNS exposure, but it is far easier to utilize the pre-prepared tabulated data to effect your tracking. (We pilots like to do it the easy way! )

Cheers!

I agree with you, I'm all about easy. I just wanted to know more about OTU's. I hear them referenced, they're tracked on my computer, we even had to do exercises with them in our trimix class. I knew that OTU’s were a method of O2 exposure tracking, but I didn’t know the relationship between pulmonary “CNS %" tracking and OTU tracking. They sound like they do the same thing. One thought that came to mind was, if we’re already tracking the dose (PPO2), and the exposure as a CNS percentage, what is OTU tracking.
Thanks to everyone that responded.
---Harrison

 

[BigJetDriver]

I agree with you, I'm all about easy. I just wanted to know more about OTU's. I hear them referenced, they're tracked on my computer, we even had to do exercises with them in our trimix class. I knew that OTU’s were a method of O2 exposure tracking, but I didn’t know the relationship between pulmonary “CNS %" tracking and OTU tracking. They sound like they do the same thing. One thought that came to mind was, if we’re already tracking the dose (PPO2), and the exposure as a CNS percentage, what is OTU tracking.
Thanks to everyone that responded.
---Harrison

Harrison,

The tracking charts are referenced to different data sets. The whole exercise is to keep our intentional exposure to high oxygen PPO2's from going into the red, so to speak, and causing bad symptoms to occur.

The CNS tracking table is referenced to data sets in which people, and animals such as goats, swine, and so on, either intentionally in experiments, or un-intentionally, displayed the signs and symptoms of oxygen poisoning in the nervous system. This is of more immediate concern to us as divers because we don not wish to convulse (Technical Term: "Do the Chicken!") underwater, for obvious reasons.

The CPTD tracking table is referenced to those data sets in which the signs and symptoms were those of the longer term Pulmonary or "whole body" type of poisoning.

BOTH are important to us. Track the CNS numbers FIRST. Then, if you are doing multi-day exposures, track your CPTD.

Hope that helps!

Cheers!

Rob

P.S.---Bennett and Elliott's "Physiology and Medicine of Diving" has several chapters on the subject, but perhaps an easier read is the coverage in "Diving and Subaquatic Medicine" by Edmonds, Lowry, Pennfather, et al. (Both are available from Best Publishing.)