how well do theoretical tissue compartment compare with real life?

[anton115]

Hi Dr Deco,

I was hoping you could help me answer a few questions.
I am a PADI open water diver and have been diving on vacations for 10 years only maybe 2 dives a year so am very inexperienced. I have never dived with a dive computer instead staying with the instructor under 18m well within no deco limits. A few months ago I was in the Solomon Islands where I explained my inexperience and on my second day of diving, the instructor told me I would be fine diving with some other divers to 30+ meters and the dive master leading the dive would keep an eye on my dive profile keeping it within the limits (looking back at this I feel foolish to accept this). This did not happen. According to the dive computer of the person I was buddied up with, the first dive was for 49 minutes to a max depth of 31.4m and average depth of 18.3m. This included a 3 minute safety stop and my buddy said we dived right up until the no deco limit when at depth. We had a surface interval of 2hours 10 minutes and the second dive was (according to her computer) for 51 min to a max depth of 32m average depth of 16.8m including a 10 minute deco stop because we had passed the no deco limit. I had constantly been 1 to 2 meters deeper than her for both dives so would have been passes no deco limits for both dives. Due to lack of knowledge at the time I did not know I had put myself in danger and the dive master did not think it was an issue. Personally I felt fine until the next day where I felt very tiered which continued for a week but there was no access to a decompression chamber and was told it is too late for it anyway if it was DCS. Since the incident I have had subtle symptoms of Cerebral DCS but these could also be because I am anxious after the incidence.

I wanted to know is this dive profile puts me at an increased risk of cerebral DCS?

From what I have read, the brain is a very fast tissue compartment that would be close to saturation on a dive to 32m inside no deco limits so the dive profile I followed would not put me at an increased risk of cerebral DCS. I wanted to know how well theoretical tissue compartments compare with real life? Ie is the brain is said to be associated with a tissue compartment with a 5min half time then do all tissues in the brain load nitrogen with a half time of around 5 min or is there more variability with some areas more comparable to a 10 min halftime compartment?

Finally I wanted to ask if cerebral DCS is associated with subtle symptoms or do they tend to always be obvious?

Thanks

 

[Crush]

This link: http://www.scubaboard.com/forums/5559679-post32.html might help to show how much we don't know.

 

[Dr Deco]

Hello Anton115:

Tissue “Compartments”

How well do compartments correspond? Not at all. This is actually the reason the various halftimes are no long called “tissues” but rather “compartments. The Haldane model is based on a variation in blood flow to load and unload inert gas. Tissues with a very good blood supply can load and unload quickly and are termed “fast” compartments [or “tissues” in the Haldane model]. Those that have a poor blood supply are termed slow compartments [or “tissues” in the Haldane model].

Blood flows as actually measured by various experimental means do not produce the very long “compartments” needed for tables to generate useful decompression schedules. Something is occurring in the model that is not mirrored in the physiological world.

Blood flows will give “compartments” of from one minute to about sixty minutes. To generate useful tables for decompression it is necessary to have “compartments” as long as 400 minutes. That is a very small blood supply, and a tissue with such a slow flow would not be able to live. It would not have sufficient oxygen and nutrients.

What is Happening?

One possibility is that capillaries open and close such that the blood supply is reduced and a “slow” compartment results. This is not the complete answer since capillary closure cannot be that long in living tissue. It is an attempt to reconcile the Haldane model with real-life physiology.

Another possibility [and the one I favor, since I thought of it] is that there is very slow exchange of dissolved nitrogen with gaseous nitrogen sequestered [trapped] in a bubble. Some “compartments” will be only “virtual” in that they do not necessarily exist if there are no microbubbles in the volume. In saturation decompression, divers are moving in the deco chamber. This allows bubbles to form in tissues, and these bubbles will trap nitrogen. Very slow decompressions [4 ft/hr] is the result.

What About Your CNS Symptoms?

You probably do not have CNS bubbles at this late stage. Good. CNS symptoms are not always obvious; some divers simply feel tired. This may be or not be cerebral DCS.

Dr Deco :doctor:

 

[Mr Carcharodon]

Anton,

As far as I can tell what changes the time constants from nominal rates is mostly changes to perfusion (blood flow) of peripheral tissues. The big drivers of blood flow seem to be how warm you are, and activity levels. I imagine that reduced circulation due to getting cold is not much of an issue in the Solomon Islands. High work loads during the dive could lead to higher than normal loading, and during the the period after the dive could lead to faster than normal offgassing both of which are potentially bad. Based on your post it is hard to say if this was an issue or not.

Personally I would focus more on the average depths and times rather than the max depths since that relates more closely to gas loading. As such I would say those were two low risk dives because you spent a significant amount of time shallow, and you had a long surface interval. The CNS probably does load up and unload relatively quickly due to being well perfused.

It is worth noting that some computers, RGBM in particular, will insert lengthly stops where other models think none are needed. There is simply a huge variability in recommended stop times. So it is a mistake to put too much faith into a dive computer. Almost all of them will keep you safe, but that is not the same as saying going beyond their limits is unsafe. It is just that the manufacturer will not say if it is safe or not. Where you at increased risk of DCS. Well compared to what? Your risk was low in my imperfect view of the world.

DCS symptoms can be diffuse since there are multiple mechanisms and parts of the body that can be effected. The DAN paper on the pathophysiology of DCS is good reading on this. If you have specific symptoms you would be well advised to discuss them with a physician.

 

[KLD]

Anton,

As far as I can tell what changes the time constants from nominal rates is mostly changes to perfusion (blood flow) of peripheral tissues. The big drivers of blood flow seem to be how warm you are, and activity levels. I imagine that reduced circulation due to getting cold is not much of an issue in the Solomon Islands. High work loads during the dive could lead to higher than normal loading, and during the the period after the dive could lead to faster than normal offgassing both of which are potentially bad. Based on your post it is hard to say if this was an issue or not.

Personally I would focus more on the average depths and times rather than the max depths since that relates more closely to gas loading. As such I would say those were two low risk dives because you spent a significant amount of time shallow, and you had a long surface interval. The CNS probably does load up and unload relatively quickly due to being well perfused.

It is worth noting that some computers, RGBM in particular, will insert lengthly stops where other models think none are needed. There is simply a huge variability in recommended stop times. So it is a mistake to put too much faith into a dive computer. Almost all of them will keep you safe, but that is not the same as saying going beyond their limits is unsafe. It is just that the manufacturer will not say if it is safe or not. Where you at increased risk of DCS. Well compared to what? Your risk was low in my imperfect view of the world.

DCS symptoms can be diffuse since there are multiple mechanisms and parts of the body that can be effected. The DAN paper on the pathophysiology of DCS is good reading on this. If you have specific symptoms you would be well advised to discuss them with a physician.

All,

There's a little more to the RGBM computers than just manufacturer inputs. The algorithm is
correlated with published data from Profile Data Banks so the stops are not just "lengthy inserts".
See, for instance,

"Statistical Correlations And Risk Analysis Techniques For A Diving Dual Bubble Model And
Data Bank, Comp Biol Med 38 (2008) 583-600.

"Diving Decompression Models And Bubble Metrics: Modern Syntheses, Comp Biol Med 39 (2009)
309-331.

and especially,

"Computer Validation And Statistical Correlations Of A Modern Diving Decompression Algorithm"
Comp Biol Med 41 (2010) 230-252.

Plus, not just "time constant" for perfusion are affected by workload and temperature, but also
related visco-elastic properties of bubbles. Just FYI. Best.

 

[anton115]

Thanks a lot for your replies. There is no dought that in the future I will be diving with both a dive computer and using my brain.

It has occured to me that the training I recieved on decompression was minimal and before this incident I knew nothing about it. From What I have observed, This is the case with many amature divers who do not dive with computers or tables and rely on the diving instructor to keep them safe. In effect they are sharing the instructors computer. The instructor would stay well within the no deco limits keeping it safe.

This does have the potential risk of what happened to me where the limit is passed and the amature diver ends up using their buddies computer for decompression guidance. The dive computer companies obviously say this should not be done because of the variation in dive profiles between two buddies. I was wondering if this realy did put the amature diver at a massive amout of greater risk of DCS. If the 2 divers were generaly within 2m depth of each other, would it lead to a large change in the deco requirement?
my understanding of the theory is it could not lead to much of a difference and the computers used today use conservative algorithms with a risk of around 1 in 10,000 so the diver sharing the computer is still going to be in a relativly safe risk catagory.

 

[TSandM]

anton, I'll tell you a true story.

My husband and I both did a dive up in Saanich Inlet. Planned max depth was 120, average of 110. We were using a non-computer method of calculating decompression obligation, but each of us was wearing a Liquivision X1 computer with VPM software running. At the end of the dive, my X1 felt the deco I did, which was according to our non-computer-generated plan, was entirely adequate; my husband's unit demanded 7 more minutes of deco at 10 feet.

The only explanation we could come up with, for why the two computers, running the same software and doing essentially the same dive, said such different things was that, through a good portion of the deep time, Peter was a few feet below me, shooting up through the cloud sponges with me above them, because that gave him a more dramatic photograph. The difference in depths was never more than five or six feet, at most.

Was that seven minutes really necessary? No way to know, because we did it. I think it's likely that we could have surfaced according to our original plan (whose parameters we had followed) and been fine. Or one of us could have developed symptoms. One of the most baffling and frustrating things about diving is that decompression is NOT fully understood, and although it is possible to make pretty strong statements about a population, it is NOT possible to make those same statements about an individual.

So are you likely to get hurt sharing a computer? Probably not, if you stay close together. But small differences in depth can yield quite different results in computation, and unless you are willing to second-guess the algorithm, you had probably better have your own results.

 

[Mr Carcharodon]

One of the current limitations of current training on decompression is that it scares the heck out of students without giving them a lot of practical knowledge of how to deal with dives that do not go perfectly to plan. The current half time groups, and limits, seem to do a pretty good job a making DCS a rare occurrence. While there still are unexplained DCS cases it seems disingenuous to have a system that works almost always and then say that it poorly represents what is going on. All the hand wringing over the occasional outlier detracts from dealing with risk factors that we know about already. Those should be discussed as well as what should be done if those factors apply to a given dive. The general advice going into a dive is to treat stressed dives as if they were 10 feet deeper than they actually are. That corresponds to 5 minutes less no decompression limit time than nominal. Also the 5 minutes per 10 feet relationship tells you what to do if your dive is a bit deeper than your buddies and you only have their computer’s plan to work with. If you are within 10 feet of your buddy and their computer is 5 minutes or more inside NDL then you can surface. But if they are at zero NLD then you need to add 5 minutes of decompression. That is not a situation you would want to be in if you are unsure about having enough gas, or about holding the stops, or do not know where to do the stops. Finally one of the biggest risk factors is doing a rapid ascent from 15 or 20 feet to the surface. Conditions permitting there is no downside to spending an extra 5 minutes at 15 feet if you are cold, or worked hard, or are doing repetitive dives, or felt tired after previous dives. Then after that stop spend a minute or two getting to the surface.

 

[Mr Carcharodon]

One of the current limitations of current training on decompression is that it scares the heck out of students without giving them a lot of practical knowledge of how to deal with dives that do not go perfectly to plan. The current half time groups, and limits, seem to do a pretty good job a making DCS a rare occurrence. While there still are unexplained DCS cases it seems disingenuous to have a system that works almost always and then say that it poorly represents what is going on. All the hand wringing over the occasional outlier detracts from dealing with risk factors that we know about already. Those should be discussed as well as what should be done if those factors apply to a given dive. The general advice going into a dive is to treat stressed dives as if they were 10 feet deeper than they actually are. That corresponds to 5 minutes less no decompression limit time than nominal. Also the 5 minutes per 10 feet relationship tells you what to do if your dive is a bit deeper than your buddies and you only have their computer’s plan to work with. If you are within 10 feet of your buddy and their computer is 5 minutes or more inside NDL then you can surface. But if they are at zero NLD then you need to add 5 minutes of decompression. That is not a situation you would want to be in if you are unsure about having enough gas, or about holding the stops, or do not know where to do the stops. Finally one of the biggest risk factors is doing a rapid ascent from 15 or 20 feet to the surface. Conditions permitting there is no downside to spending an extra 5 minutes at 15 feet if you are cold, or worked hard, or are doing repetitive dives, or felt tired after previous dives. Then after that stop spend a minute or two getting to the surface.