Well, that's the problem , isn't it? What is this "DCS stress" you speak of?
Although the occurrence of VGE might be a relatively poor predictor of DCS,
the absence of VGE is a good indicator of decompression safety, and can be used to estimate
a level of decompression stress
Clearly it's not DCS, VGE being a "poor predictor" of that. Best I can figure from the above is it's some kind of "decompression unsafety".
"a close relationship between the number or load of VGE present and DCS cannot be derived"
I believe in these last few posts you are mixing up two possible ways in which VGE might be used. In an earlier posts you mentioned predictive power and specificity.These are measures of the utility of a binary classification test, for instance a diagnosis of DCS in an individual based on VGE. VGE are very poor for this purpose. This is the "1:1" relationship that Ross knows is poor, and therefore wants to use to discredit the real utility of VGE. But no one else on any of these threads is talking about using VGE to diagnose DCS. The real utility of VGE is that in a large enough sample of repetitions of a dive profile, the median (for instance) VGE grades are indicative of the decompression stress or probability of DCS.
Copied from the other thread:
As some background, VGE are graded using ordinal scales typically ranging from 0 to IV, that correspond to the number of bubbles heard in Doppler blood flow signals in large veins or seen in 2-D echocardiographic images of the heart chambers. These VGE grades have a general correlation with the incidence of DCS in large compilations of data. The biggest of these compilations is from the development of the Canadian Forces decompression tables and some of these data have appeared in several places, including Bennett and Elliott’s Physiology of Medicine and Diving, and were analyzed by David Sawatzky in his M.Sc thesis. If you separate the dives into those in which the maximum grade detected following diving was 0 or I or II etc, there is an increasing incidence of decompression sickness (DCS) in the successive higher VGE grades. For instance in air diving: grade 0 (0 DCS/ 819 dives); grades I (3 DCS/287 dives); grade II (2 DCS/183 dives; grades III (27 DCS/365 dives); and grade IV (9 DCS/72 dives). I do not have heliox and trimix data in front of me, but it contains the same correlation. This correlation arises in part because VGE can cause some manifestations of DCS, but also, because VGE form in response to the same stresses that form bubbles elsewhere in the body, VGE are probably correlated with an increase risk of bubble formation at other (extravascular) DCS site.
So, on to the possible uses of VGE.
1) Can you use the VGE grade in an individual diver to diagnose DCS or predict if they will develop DCS? No, the relationship is not strong enough. This is use for which the poor “positive predictive power” of VGE, which has been mentioned, is relevant. To illustrate this, let us suppose that we decided to diagnose DCS in divers on the basis of a VGE score of IV, i.e. any diver who shows a score VGE grade of IV gets tossed in the chamber and treated whether they have symptoms or not. The data above suggests that we would be correct about 13% of the time and wrong about 87% of the time. These latter 87% ‘false positives’, without going into details of the calculations, are the essence of poor positive predictive power.
2) Do the VGE grades measured in a group of divers, all who have performed the same dive profile, provide an estimate of risk of DCS of that decompression schedule? Yes. To illustrate, instead of an individual diver, imagine 50 divers conducted an identical dive profile (dive profile 1) and all 50 divers manifest VGE grade I and another 50 divers conducted dive profile 2 and all 50 divers had VGE grade III. We still would not know if any particular diver would go on to develop DCS, but we could estimate, with considerable confidence based on the data above, that profile 2 had a greater risk of DCS than profile 1, and even (with less confidence), that dive profiles 1 and 2 had risks of DCS of about 1% and 7%. This is an oversimplification, but again this is the essence of the use of VGE to estimate the risk of DCS.
2a) Are VGE used to validate decompression schedules? Yes. The gold standard for validating decompression schedules – and the method still used by the U.S. Navy - is the incidence of DCS, i.e. you test a schedule and accept it if it results in a low incidence of DCS and reject it if it results in a high incidence of DCS. However, many other navies now use VGE as well as, or instead of, DCS. The best example is the validation of the Canadian Forces (DCIEM) decompression tables, which produced the data mentioned above. In this approach, the maximum VGE measured after diving is considered a measure of “decompression stress”, and a schedule is accepted if it consistently produces low VGE grades and rejected if it consistently produces high VGE grades. The DCIEM criteria for heliox schedules is to reject a schedule if in 20 dives the median VGE grade is III or higher.
David
