0 1 1 1 2 2 2 3 3 3 4 5
Bubble count bubbles/cm^
0 0.01 0.05 0.1 0.15 0.2 0 3 OS
1 2 5 10
Even if this obviously is an approximation, it now becomes possible to convert the bubble grades to a linear scale. Initial analysis of data from dives where both Doppler grade and bends incidence are known indicates that this method may have considerable power for discriminating between "safe" and "unsafe"
dives.
Use of pulmonary artery bubbles and animal experiments to evaluate reverse diving profiles The previous findings indicate that counting the gas bubbles in the pulmonary artery can be used to compare two different profiles. There is at present no method for doing this accurately in man, thus for an initial comparison, animal experiments are needed. When evaluating reverse dive profiles, the question asked will be, "how will the depth-time combination of repeated dives influence the outcome, e.g., the safety for the diver?" As demonstrated at this workshop, the usual approach to this is to test different depth-time combinations using mathematical models. There is probably no other field in biology or medicine where one relies so strongly on mathematical models for determining outcome.
The advantage of the mathematical model is its simplicity and the ease with which many combinations can be tested. The use of models combined with statistical techniques, e.g., maximum likelihood, has been a powerful tool for developing decompression procedures (Thalmann et ah, 1997).
However, the model's strength is also its weakness; it may not be easy to test all the assumptions made.
We would argue that a biological model, using a quantitative endpoint, is better suited for determining which combination of dive depth and time actually produces less gas bubbles. This approach also overcomes another problem, namely how to correlate gas bubbles and decompression sickness. Animal experiments can actually give additional insight, as organs can be studied after the experiments have been terminated. It is important to remember that the proposal made here focuses on the comparison of the number of bubbles produced by two profiles; this does not require any information about the relationship between bubbles and injury to the organism.
Brubakk and Eftedal: Evaluation of Reverse Dive Profiles
One has to exercise caution when extrapolating from the results obtained in any model system to man. In designing animal experiments, a large animal model should preferably be used (Brubakk, 19%).
However, if two procedures are to be compared, then the most "stressful" one will produce the largest amount of separated gas both in man and in any model, mathematical or biological.
Comparing decompression procedures in this way will obviously give considerable input to the mathematical modelers, who can use this approach for improving and calibrating the models and for simulations to suggest additional experiments. Thus, a combination of both approaches may be the most profitable way for comparing different profiles and for developing decompression procedures in the future.
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Brubakk and Eftedal: Evaluation of Reverse Dive Profiles
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REVERSE DIVE PROFILES IN THE NMRI AIR AND N2O2 DIVING DATA BASE PauUCWeathersby
5 Ferry View Drive Gales Ferry, CONNECTICUT 06335 U.S.A.
Wayne A, Gerth Duke University Medical Center Box 3823 Durham, NORTH CAROLINA 27710 U.S.A.
A central issue in the advisability of diving "reverse" profiles is whether such profiles incur DCS risk by different mechanisms from those governing DCS risk in other types of diving. Primary empirical information for DCS incidence and time of occurrence in a large number and variety of repetitive dive profiles has been obtained in U.S. Navy man-trials since 1974. Over 1200 repetitive and multilevel exposures are present in the published database (NMRC Technical Report 99-02, 1999). However, only a few dozen are "reverse" since Navy developers never considered "reverse" profiles worthy of special study. Nevertheless, all of these "reverse" data were included in the Primary Data used to calibrate the USN93, JAP98 and Duke BVM(3) gas and bubble dynamics probabilistic models. All the models provide estimates of DCS incidence and time of occurrence for these reverse profiles that are within the 95% binomial confidence limits of the observations. Available empirical data for reverse profiles has thus been successfully combined with data for other profiles under each probabilistic model. This ability is consistent with the view that DCS risk accumulates by the same mechanism in both types of profiles and that reverse profiles are not "special".
Introduction
In the history of U.S. Navy development of decompression tables and procedures, we can find no concern over "reverse" profiles. The investigators have always calculated and tested profiles now labeled as "reverse" as part of the repetitive or multilevel range needed at the time. Thus, the Navy would simply never have seen the need for this Workshop. However, the Navy has acquired some data directly relevant to "reverse" dives.
As part of a major effort to control decompression based on valid statistical treatment of human diving observations, the Naval Medical Research Institute (NMRI) amassed a collection of thousands of well controlled dives and relevant DCS outcome symptoms and onset timings. The compiled high quality modern data (since 1974) was published as "Primary Data" for decompression modeling (Weathersby et ah, 1992). More recently the data base has been doubled in size to over 8000 dives and expanded to include earlier controlled decompression trials (Temple et al., 1999). Almost 1600 repetitive and multilevel exposures are present in the newer data base, and at least 1200 of them qualify as modern
"Primary Data." Table 1 is an excerpt from a summary chart in the newer NMRI report. All of these profiles and outcomes are available for anyone desiring high-quality diving data. Several of the published profiles identified as "reverse," or illustrative of issues pertinent to consideration of DCS risk in reverse profiles, are illustrated in Figures 1-5.
Profiles
The profile shown in Figure 1 is a very long experimental profile (11 hours) in which 9 subjects made three downward excursions to 81.5,615, and 81.5 fsw, respectively, but spent most of the day at 16.5 few.
The downward excursions were made on air, while an oxygen-enriched gas (0.7 ata O& balance N2) was
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October1999.
breathed during the 16.5 fsw portion to simulate a dive on one of the Navy's rebreathers. Note that the dives to the deepest depth, 81.5 fsw, were made at the beginning and end of the profile, qualifying the profile as a "reverse" profile. Final surfacing was preceded by staged decompression prescribed by a controlled-risk real-time decompression algorithm, but the decompression time was modest by Navy standards. Very similar profiles following the same real-time algorithm were dived by an additional 52 subjects (Thalmann a cd., 1999). Overall, only 2 subjects reported decompression-related symptoms, but neither of them required recompression therapy according to the judgement of the on-scene Diving Medical Officer. Minor symptoms, referred to as "marginals" in the data, are rather common in closely monitored decompression trials, but are routinely ignored in field dives.
Table L Sections III, IV of page 9 of NMRC 99-2.
TYPE OF DIVES HI. REPETITIVE &
MULTILEVEL (AIR)
IV. REPETITIVE*
MULTILEVEL (NON-AIR)
A.
B.
C.
D.
E.
F.
A.
B.
C.
D.
E.
DATA SET