G.Egstrom: On your graphs, were those outliers at the far end in the 300-foot range? I know you had some data points that were down within the very low border, and then the line would go way up and there'd be one datum point.
W.Gerth: Bottom line is no, they're not outliers. We're getting into some issues here regarding the state of the art of our modeling capability. I could go on for hours about that. If I were to show you the estimated risks of the exceptional exposure schedules in those spots, you would see the risk rapidly going up to 60, 70, 90%
with this model. This model is not a strict bubble bind model. This DCS risk is not a function of model bubble volume compartments. We do have such a model in development that does a lot better on the higher risk dives under the LEM model.
The reason for the difference is that the LEM model handles oxygen as it contributes to DCS risk in a way much different than the bubble volume models that we now have. The way LEM handles it is kind of a rubber band approach whereas our bubble volume models actually incorporate the helium/oxygen saturation curve and lets oxygen exchange with tissues as bubbles grow in the tissues according to that curve. LEM doesn't. So, we have better technology. This technology works well up to PO2s in the order of 1.7. Higher than that, which you do get in some surface- supplied dives, it starts over striving a risk of DCS from oxygen, something that doesn't happen in the bubble volume model. A long-winded answer to your question.
JP. Imbert: In the revised table, do you still carry the option of the switch to pure oxygen at 50 feet?
W.Gerth: No.
JP. Imbert: You dropped that one? Replaced by?
W. Gerth: I'm not imminently familiar with the emergency procedures. You say do we have the option? Operationally no. One of the main objectives was to get rid of the 50- and 40-foot oxygen breathing.
JP.Imbert: How do you exactly define your decompression stress, and how is it related it to the symptoms?
C.Gutvik: Currently we just use the total volume of excess gas as a measure of decompression stress. We have the ability to weigh this between different tissues.
We can say that bubble growth in the muscle is worse than getting bubbles in the fat tissue for instance because it's most likely that bubbles growing in muscles are more likely dislodged from the endothelial layer and then are washed over to the arterial side. But bubbles don't grow in the fat tissue, they just stick there and do not cause serious problems.
K.Shreeves: Do you recall what setting you had Voyager on when you did your analysis, the conservatism setting?
K.Huggins: That was set at zero because I didn't see anything in the software that stated that there was any level of conservatism over the Bühlmann model.
K.Shreeves: The reason I ask is I've got quite a number of dives on the NiTek Aegis and what you presented wasn't lining up with my experience. I find that the algorithm of the Abyss 120, which is more conservative than Voyager set at zero, lines up more
with the NiTek Aegis., which in my experience, is more conservative than Bill Hamilton's DCAP program. For those who are evaluating computers, you might get a different result than what Voyager gave. That was certainly a good basis for it, but we probably needed the conservatism.
K.Huggins: It would have been nice to have it.
K.Shreeves: I'm surprised Lamar Hires didn't get back to you on that. You actually can raise the conservatism in the NiTek Aegis. It's a bit of a workaround, but you can go to the altitude mode and select an artificially high altitude, which will raise its conservatism.
A.Brubakk: Regarding the use of computers in trimix diving, the point is that if we talk about dive computers actually attached to the diver, they're probably right.
However, if the algorithm is reasonably right, a topside computer would give you much more flexibility. For a surface-supplied diver, you have all the computing power that you need if you move the dive computer from the arm of the diver to the surface. I agree with the fact that no one has actually evaluated these procedures, which is a serious drawback. On the other hand, the validation of the dive tables, particularly in the deeper range, is also not very impressive. I am, therefore, not sure if tables actually are much safer.
M.Lang: You're referring to the US Navy tables?
A.Brubakk: Any tables. Listen to what was said earlier today, there is very little experience with deep trimix and mixed-gas diving and the experience that we have indicates that the number of decompression incidents is pretty high.
M.Lang: Now I'm very interested to hear of the commercial diving operational experience.
Many of the commercial tables are proprietary and have been tweaked, to meet their specific objectives, but certainly as far as experience level and number of dives in the 300-foot range, it's got to be tremendous, right?
G.Beyerstein: Literally thousands, even to the 300-foot level and not by analyzing on the dive computer. The DCIEM tables are good tables. It needs to be remembered that Ron Nishi made that choice not to use 50/50 nitrox for logistical reasons because their diving vessels had no room for 50/50. Otherwise, he would've used it. The Navy is another matter altogether. We've heard several discussions about risk here and speaking now as a safety manager for the last 20 years, conventional wisdom is that risk is a combination of two factors: the likelihood of something happening (from statistical analysis) and the consequences of that happening. In order to properly evaluate risk, you have to take both of those into consideration. The consequences of something happening are twofold. In our business a DCS bend, for instance, we can treat if you have a chamber and the ability to treat on site. There is another consequence that has assumed orders of magnitude more importance in the last few years, the cultural emphasis. Glen Egstrom stated zero risk not being something that we could achieve and yet in our business, that's our aim. It has become intolerable to hurt anybody to any degree now. One bend with unresolved serious symptom equals one lawsuit equals two to three million dollars. It depends on what level of risk, what level of consequence that you can accept in the scientific community. When I hear the military people talking about two to four percent DCS, that to me is abhorrent. If any commercial company tried to function in that realm,
they'd be out of business in an instant, whereas the military can accept that risk.
They have a mission and can tolerate that level. Any attempt to transpose military experience to commercial or even scientific experience is fraught with difficulty and certainly should be considered very carefully.
M.Lang: In the science community’s attempt to implement a new 300-foot seawater mixed gas program, one major incident or fatality will shelve the effort for the next 50 years.
W.Gerth: The issue of two to four percent, as I had tried to stress and Mike Gernhardt re- emphasized, those are risks of schedules in the tables dived as written. If you look at the actual incidents of DCS in the U.S. Navy diving operations, overall that's really very low.
M.Lang: You're talking about the outer envelope of the square-wave profile’s shell, right?
G.Beyerstein: The actual lab quality tests that produce those kind of numbers don't live in the real world. It's true that we don't dive square profile dives, but when you get sheer volumes of numbers of decompression dives over a long period of time, then you have data to consider. Rather than talk about percent, we usually prefer to talk about the number of undeserved bends hits. If you have a bend where the dive profile is right, without table compromise, and no succession of susceptibility factors all occurring at once, we call that an undeserved bend. On standard air tables that are modified in commercial situations, we get maybe one bend in 1,500 dives. On the final iteration of the SubSea tables after they were changed we had over 4,000 decompression dives. Probably triple Zs with only one suspected case of bends.
Since then other companies have been pirating and using them we're getting even more experience, whereas on gas, maybe one bend in 800 dives is as good as you're going to get.
M.Lang: Jack Reedy mentioned ‘acceptable standard of practice’ for a community when he summarized the commercial diving section of the repetitive diving workshop in 1991. For the commercial, military, scientific, and recreational diving communities, that is different, almost by orders of magnitude. The data published at the time was one DCS hit per 1,000 dives for the commercial diving community. That rate reflects the severity of dive exposure and the type of diving performed, but more important is their risk mitigation. As does the military community, the commercial divers have technicians and chambers on site, which the recreational and scientific diving communities do not. There are thus different levels of acceptance of risk for a particular communities, and it remains important to distinguish them when you're comparing incident rates because of the nature of the activity and very different exposures.
P.Ruden: The Navy experienced difficulties on the TWA Flight 800 dives that they attributed to decompression sickness as a result of using hot water suits. Was there a definitive conclusion on that issue?
J.Wilkins: I was at the Experimental Diving Unit at the time the TWA Flight 800 dives were happening. We were looking at that long and hard to try and figure out exactly what was going on. We experienced a higher degree of decompression sickness than we thought we were going to, even when we substantially jumped tables and still were having difficulties. We subsequently dug into the thermal effects
associated with decompression sickness. I didn't see much opportunity to insert the thermal effects into the decompression models, whether you're talking a gas content model or a bubble volume model. We have reason to believe that thermal influence is a substantial factor. We ran some warm/warm, cold/cold, warm/cold, cold/warm dives at the EDU anticipating that we'd find what everybody suspects and intuitively knows that one approach is better than another. Over hundreds of dives we found a substantial difference, a surprisingly high correlation of the fact that by keeping the diver cold on the bottom and warm during decompression the incidence of bends was greatly reduced. We’re trying to make a final determination of which decompression model is the way of the future. We want to find a model that also ultimately allows for thermal contribution if you just measure skin temperature. We found in the most recent series of tests thermal contribution to be a substantial input as to whether or not the bends incidence increased or decreased. We're also looking at long duration SEAL opportunities and swimmer delivery vehicles, where the hands and feet drive the divers out of the water. Dale High at NEDU has gone to a great lengths to actually map the thermal mass of the hand, where it loses heat, where you can apply heat, and how much heat you need to apply over what schedule so that you can actually schedule the energy application to the hand in an active heating element manner. You can then drop the energy requirements down to something that's manageable, batteries that are the size and about the same density as lead that could be used for weight belts on or around the body, a double contribution. We're working with a couple of manufacturers to take a look at some materials that can apply the heat as mapped against the human hand and are hoping that we're on the edge of a breakthrough here for hands, and then ultimately, feet D.Southerland: There were several things that went on during TWA 800 dives. Several
factors were changed throughout the dive. People suspected difficulties were due to thermal problems or changes in thermal status of the diver on the bottom during decompression. Changes were made to what they were wearing, but also to the bottom time. This raised the questions that resulted in the studies at EDU. Leffler, a Diving Medical Officer there published those studies.
W.Gerth: Exercise affects decompression, and thermal factors are equally, if not more, important. You have to keep in mind where the heat or cold is being applied to the diver. In the TWA cases, and the dives looked at by Tom Shields for the HSE, he made a similar conclusion that was reached in the TWA 800 dives, and that was that use of a hot water suit increased DCS incidents. This is consistent with the other information we've gotten from experimental studies. Going to a hot water suit will get you warmer on the bottom, but all these dives were sur-D-O2. Whether you were cold or warm on the bottom, they were going to sur-D-O2 under equivalent conditions. The difference between the higher DCS and the lower DCS cases discussed had to do with just going warm on the bottom.
P.Ruden: I remember conducting diving operations out of San Francisco, surface-supplied, mixed gas, 240-260 feet from open bells off the ASR9 using a band mask and hot water suits. We spent two and a half weeks diving every day, all day long, working hard under heavy sea conditions. We never bent anybody and did sur-D-O2 for all the dives.
D.Long: In the 1970s when they first started opening up the gas fields off of Great Yarmouth, Taylor Diving was out there trying to lay a pipeline. They couldn't keep a guy on the bottom in a wet suit more than about 20 minutes, and he couldn't talk when he came up. They were doing in-water decompression followed by a surface decompression. The guys were so cold they were afraid they were going to die during the in-water decompression, so they started pulling them out and it actually got to the point where they were skipping two decompression stops, pulling them out, putting them in surface decompression and the guys decompressed fine. They said our tables are off, they could do that. They got to the point where it was not possible to do the job and finally brought hot water suits up there. As soon as they put hot water suits on the divers, they went to the same decompression profile of missing the two stops and bent many of them. The report came back that hot water suits give guys the bends, but they’re a lot more comfortable. Thermal factor is every bit an issue with the diffusion of gas, as is the gas mixture itself, and related to the level of work. The harder he works, the more gas he takes on. We have seen cases where with a thermal component, whether it be hot water or surface decompression, the body changes such as when guys get inside of a chamber up against the cold wall. If we keep that diver in good thermal condition all the way through the cycle, we're going to be okay.
A.Brubakk: This all boils down to the fact that perfusion places a significant role in what's going to happen. You have to have a prescription for the decompression that takes into account that perfusion is different, and you have to adjust your decompression requirements accordingly. That's the temperature, the exercise, and at what time in the cycle it is applied. In the North Sea, deep bounce dives are hardly ever done.
The reason for this is that all procedures as were shown by several studies, have had a too high incidence of decompression sickness. We need to do something radical about the procedures if we're going to use bounce dives using trimix or heliox. You probably should not do these dives, at least not until we have much better procedures, without having a chamber on site so you can treat problems. You probably can’t do it with the level of safety that you want to have in the scientific diving community without some kind of treatment capability.
B.Stinton: Around 300 feet respiratory heat loss is about 25 percent of metabolic heat production, so we can put somebody in a hot water suit and keep their skin in a thermally neutral environment. The cooling is taking place directly out of the core of the body. Go to 300 meters and keep somebody in an electric suit and then watch them breath 40 degree hyperbaric helium and they're shivering within minutes. The fact is we can keep the skin at perfect temperature, but if we don't protect the respiratory channel through gas heating, then possibly that's where we're getting some problems. If you take nitrogen-specific heat and density it is worse, therefore nitrox is has a high heat loss. Trimix is higher than heliox so we need to look at that fact because at 300 feet we're basically at 25 percent of metabolic heat production as respiratory heat loss and at 600 feet it's about 50 percent. If you work more and do generate more heat, you're just losing more because you have to breathe more. That has to be taken into consideration.
K.Kohanowich: Exercise has a significant effect on incidents of decompression sickness.
Have any recommendations come out of that work?
M.Gernhardt: With respect to our altitude decompression, we have a validated procedure that is being used on the space station now. That involves exercise during oxygen pre-breathe prior to decompression. From the commercial diving background on our multilevel dives that we did, I don't remember any DCS incidents in 30,000 dives per year. There's also research that shows that exercise after decompression actually increases the decompression stress. There's some confounding effects, but the indication from our laboratory research in the altitude case and the commercial diving experience is that exercise with relatively low levels of supersaturation is actually a good countermeasure for decompression stress.
A.Brubakk: This is so complicated. We have data from experimental air dives that shows that actually exercises after the decompression, even when they had a considerable number of gas bubbles, reduced the amount of gas bubbles. It is complicated. The whole thing we learn from all of this is that these relationships are not simple.
K.Huggins: Both Vann and Jankowski published studies looking at mild exercise during decompression that reduced stress.