Nitrox workshop dings - Divers Alert Network

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Rubicon Foundation Archive (http://rubicon-foundation.org)//. Nitrox Physiology DiscussionII. NITROX PHYSIOLOGYA. Discussion of Kerem paperB. Wienke: From your experiences, what percentage of divers would you estimate is in theextreme CO2 retention category?D. Kerem: Below five percent.B. Wienke: What would you estimate for the moderate category?D. Kerem: For the professional navy divers, that would be approximately 25%.M. Wells: Dan, could you comment on the NOAA oxygen limits, for example, 1.6 atm for 45minutes? Could you give us some insight into what time duration dives are common for yourdivers?D. Kerem: These guys we're asking to work with nitrox using just one scuba tank. Going by thetimes that we allow for pure oxygen, the combat divers use the same limits for several hoursof strenuous swimming underwater. We didn't actually specify time limits on that.M. Wells: I understand then in practice your combat divers go 1.6 atm for several hours is thatcorrect?D. Kerem: Affirmative.D. Richardson: Would you say that, for an individual whose CO2 retention level isn't known, thelimits of 1.4 up to 1.6 atm are reasonable?D. Kerem: On the whole, I would say so.R. Moon: Dan, I think those are very nice data. In using those data to construct guidelines, it'simportant to note that the end-tidal CO2 may both overestimate and underestimate the arterialPCO2. Whereas your divers were probably relatively young with no lung disease, there is anaging component in recreational divers for whom pulmonary dead space increases.Therefore I would hypothesize that it would be more difficult to detect CO2 retainers usingend-tidal CO2 groups.D. Kerem: Your point is well taken.T. Mount: In your opening slide you mentioned CO2 limits being affected by both helium andnitrogen. However, none of your data reflected the use of helium. I'm curious what you mayhave found out as far as helium is concerned because it's always been my understanding thatthe use of helium reduces CO2 retention.D. Kerem: The only mention I made about helium is that test that examined the influence of aninert gas on oxygen toxicity, not on CO2 retention. Those were in vitro, not in people. Thiswas on pure oxygen on top of which you added a few atmospheres of either nitrogen orhelium that would exacerbate the oxygen toxicity. Helium should be better (CO2 retentionwise)in people because of its lower density.D. Vann: Can you comment on the increase in CO2 retention that occurs as depth and densityincreases?87
Rubicon Foundation Archive (http://rubicon-foundation.org)Lang (ed.): DAN Nitrox Workshop, Divers Alert Network, November 2000D. Kerem: Not much more than what I have shown. There were people for whom, at 1 atm ofair, the CO2 retention was not even in existence. When they were taken to 30 meters, theysuddenly showed either a moderate or an extreme retention. You would probably want to saythat it was the increased density caused mainly by depth.D. Vann: Richard, aren't there data from Swedish workers that address that question?R. Moon: Well, almost all the data in the literature have been based on end-tidal CO2. I thinkit's the easiest measurement to make, but it's also fraught with a lot of difficulty.A. Marroni: There is a growing interest in Europe for deep air diving and our multiple agenciesare about to start certification at the 50 and 60 meters level on air for deco diving. Do youthink that there is a role for a research project to screen for carbon dioxide retention in thiskind of deep air diving?D. Kerem: Probably not, unless you want to start consideration of wash-in and wash-out ofgases that could be different with CO2 retention. You have increased CO2 during your diveand less when you are decompressing, nor a narcosis effect. There could be some synergism,but not regarding oxygen toxicity.B. Hamilton: Dan, if you drew a best fit line through your data points, it would be very close toa 45-degree slope, or slope of one, which is what you would expect. For this particularmeasurement, air and enriched air, you would expect to see an end-tidal PCO2, wouldn't you?It's going to affect them both in the same way isn't it, as they are the same densities?D. Kerem: No, the air is at 1 atm and the enriched air at 4 atm. We just wanted to see if wecould do a simple air test and predict who is going to be the CO2 retainer. Most of themcontinued to show the same degree of retention when you took them to depth, but some ofthem only appeared as retainers at depth.R. Moon: Dan, what was the PO2 in the group breathing nitrox?D. Kerem: For this particular one it was 40 percent, so at 4 atm, 1.6 atm.D. Vann: Alessandro, regarding your question about deep air diving, there are a number ofreports in the literature. I have also seen a case in the lab of people just going to sleep at 150to 180 feet and there have been a number of fatalities there. That's a mighty difficult topic tostudy in the laboratory for ethical reasons. But I think there are certainly anecdotes.D. Kerem: Were they resting or exercising?D. Vann: They were exercising. The one that I'm familiar with personally, divers wereswimming in open water. At some point, it might be reasonable to think about switchingfrom nitrogen to helium to get rid of the narcosis and also the increased density, which wouldappear to increase CO2 retention.B. Maximum PO2 Limits DiscussionM. Lang: The next topic under physiology actually provides for a discussion session. It wasincluded because it surfaced repeatedly from several of you as a topic in need of discussion,i.e., the continually shrinking maximum PO2 limit.88
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