test subjects with 4 reported cases of DCS (Thalmann, 1986). The decompression was specified by a (non- probabilistic) decompression algorithm that has since been modified.
ft
100
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1 i i i i i i .1 i
•
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i
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i i
i !
i !
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Figure 2. "EDU885S-U".
Figure 3 shows a Navy experimental dive from the early 1980's. Twenty subjects completed this
"reverse" multilevel dive while breathing from a 0.7 ata constant PO2 rig. None suffered DCS (Thalmann, 1984). The tables obtained from this trial are still in Navy use for scheduling both "regular" and "reverse"
profiles.
EDU184, Profile 14
I
o.£ 40
120 180 240 Time (min)
Figure 3. "EDU184-14".
The profile illustrated in Figure 4 is a "reverse" repetitive sequence from a 1957 Navy validation trial of a repetitive diving calculation method (des Granges, 1957). The two divers on this profile spent an hour at 140 few followed by a surface interval of just over an hour, and then completed a dive to 220 fsw with a 20 min bottom time. While both divers suffered DCS, a total of 140 other man-dives on a wide variety of different profiles prescribed by the same method were completed with only 2 additional cases of DCS. The trial was consequently declared a success. The Navy no longer authorizes routine air dives to 220 fsw, but it does authorize use of the repetitive scheme validated in this trial to plan dives in both
"regular" and "reverse" profiles.
The final profile in Figure 5 is not formally "reverse", but is included to demonstrate the severity of repetitive air dives that can be successfully managed with a probabilistic model. This test subject went to 1015 fsw three times over a 7 hour period, staying for the maximum no-decompression time allowed by a
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October 1999.
probabilistic model. Another 19 subjects made very similar dives, all with only a single marginal DCS symptom noted (Thalmann et ah, 1999).
EDU657
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j- 165
« no
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•
-
90 180 2 7 0 360
Time
Figure 4. "EDU657-42".
102
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-
-
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Figures. "PARA-36".
Discussion
A central question for diving "reverse" profiles is: Do such profiles incur DCS risk by different mechanisms from those governing DCS risk in other types of diving? We believe the answer is NO. We find no evidence that there is a "qualitatively" different operative mechanism. Instead, in concert with most of the Workshop speakers, we approach all diving questions with the same quantitative analysis.
The analysis we prefer is undertaken with a minimum number of questionable assumptions and with the strongest and most direct connection possible to human experimental data. Since the mid-1980s, a formal, statistically valid methodology has been available to combine observed DCS outcome information from a large number and variety of different profiles under a single probabilistic model, simultaneously
Weathersby and Gerth: Reverse Dive Profiles in the NMRI Database
(Gerth and Vann, 1996; 1997; Parker et al, 1992; 1998; Survanshi et al., 1997; Temple et al., 1999; Thalmann, 1984; 1986; Thalmann et al, 1997; 1999; Weathersby et al., 1992). The method ensures that each model provides its closest possible agreement with the observed data. The models are readily used to estimate the probabilities of DCS in different profiles and compute decompression schedules to keep estimated DCS probability within user-specified limits.
Three carefully constructed and tested probabilistic models for DCS incidence and time of occurrence in many forms of N2-C>2 diving have been published (USN93: Parker et al, 1992; Survanshi et al, 1997;
Thalmann et al., 1997; Duke BVM(3): Gerth and Vann, 1996; 1997; JAP98-2: Parker et al., 1998). All use hazard/risk functions of varying complexity with parameters estimated from over 3000 human dives.
The "reverse" data discussed for Figures 1, 2, 3, and 5 were all included in the calibration data for these models. The performance of the models is documented in the literature cited.
Model predictions of DCS outcomes for the "reverse" profiles outlined above are compared with observed outcomes in Table 2. For each set of profiles (identified by Figure number), columns have: total number of divers, observed number of DCS and marginal cases, and the 95% binomial confidence limits for the observed number of DCS cases. The next columns have the upper and lower 95% confidence limits for the total number of DCS cases predicted by each of the three models. All the models provide estimates of DCS incidence 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 profile and that reverse profiles are not "spedal."
This model success indicates that comprehensive risk management of "all" dives - reverse or not - is feasible (see Weathersby paper in these Proceedings).
Table 2. Probabilistic Model Predictions for Tested Reverse Profiles
Profile Figurel Figure2 Figure 3 Figure 4 Figure5
Divers 54 38 20 2 20
DCS 0 4 0 2 0
Observed Marginal 2 0 0 0 1
95%CLon DCS cases 0 - 3 1-10 0 - 3 0 - 2 0-3
Predicted, USN93D 1.6-25 1.6-2.3 0.7-1.1 0.4-0.5 1.0-1.7
low-high 95% CL BVM(3) 1.2-4.9 1.6-2.1 0.7-0.9 0.3-0.4 0.9-1.6
JAP98-2 1.4-2.4 1.6-2.4 0.8 - \2 0.4-0.6 0.9-1.6
Literature Cited
Des Granges, M. 1957. Repetitive diving decompression tables. Navy Experimental Diving Unit Report 6- 57, Washington, D.C.
Gerth, W.A. and R.D. Vann. 1997. Probabilistic gas and bubble dynamics models of decompression sickness occurrence in air and nitrogen-oxygen diving. Undersea Hyperbaric Med. 24:275-292.
Gerth, W.A. and R.D. Vann. 1996. Development of Iso-DCS Risk Air and Nitrox Decompression Tables Using Statistical Bubble Dynamics Models. Final Report, NOAA Award No. NA46RU0505.
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October 1999.
Parker, E.C., S.S. Survanshi, P.K. Weathersby, and E.D. Thalmann. 1992. Statistically based decompression tables. VIII. Linear-exponential kinetics. Technical Report of the Naval Medical Research Institute, NMRI92-73, Bethesda, Maryland.
Parker, E.C., S.S. Survanshi, P.B. Massell, and P.K. Weathersby. 1998. Probabilistic models of the role of oxygen in human decompression sickness. J Appl Physiol, 84:1096-1102.
Survanshi, S.S., E.C. Parker, E.D. Thalmann, and P.K. Weathersby. 1997. Statistically based decompression tables XII. Repetitive Decompression Tables for Air and Constant 0.7 ATA POz in N2
using a Probabilistic Model (Volume I). Technical Report of the Naval Medical Research Institute, NMRI 97-36, Bethesda, Maryland.
Temple, D.J., R. Ball, P.K. Weathersby, E.C. Parker, S.S. Survanshi. 1999. The dive profiles and manifestations of decompression sickness cases after air and nitrogen-oxygen dives. Vol. I: Data set summaries, manifestation descriptions and key files, Vol II: Complete profiles and graphic representations for DCS events. Naval Medical Research Center Technical Report 99-03. Bethesda, Maryland.
Thalmann, E.D. 1984. Phase n testing of decompression algorithms for use in the U.S. Navy underwater decompression computer. Navy Experimental Diving Unit Report 1-84. Panama City, Florida.
Thalmann, E.D. 1986. Air-N2O2 decompression computer algorithm development. Navy Experimental Diving Unit Report 8-85. Panama City, Florida.
Thalmann, E.D., E.C. Parker, S.S. Survanshi, and P.K. Weathersby. 1997. Improved probabilistic decompression model risk predictions using linear-exponential kinetics. Undersea Hyperbaric Med.
24:255-274.
Thalmann, E.D., P.C. Kelleher, S.S. Survanshi, E.C. Parker, P.K. Weathersby. 1999. Statistically based decompression tables. XI: Manned validation trial of the LE probabilistic model for air and nitrogen- oxygen diving. Joint Technical Report NMRC 99-01 and NEDU1-99. Bethesda, Maryland. 81 pp.
Weathersby, P.K., S.S. Survanshi, R.Y. Nishi, and E.D. Thalmann. 1992. Statistically based decompression tables. VII. Selection and treatment of primary air and N J O J data. Joint Technical Report NMRI 92- 85 and NSMRL No. 1182. Bethesda, Maryland.
THEORETICAL, EXPERIMENTAL AND OPERATIONAL ASPECTS OF REVERSE DIVE PROFILES Michael L. Gernhardt Astronaut Office, Code CB NASA Johnston Space Center Houston, TEXAS 77058 U.S.A.
There have been no definitive laboratory studies to directly compare the decompression stresses associated with forward and reverse dive profiles. For this reason, assessment of the risks associated with reverse dive profiles will not be "hard science," but the result of thoughtful analysis of a xxiriety of information sources including limited laboratory trials, well-documented operational records, field experience and theoretical predictions. This paper addresses the decompression stresses associated with reverse dive profiles, based on analysis with the Bubble Dynamics Model (Gernhardt, 1991). It also addresses a number of related special topics including: multi-gas, multi-depth, reverse dive profiles; the hang-off technique used in commercial diving; comparison of "Revet-Up" vs. "Repet-Down" dive profiles; and, the influence of exercise on decompression stress, as a function of dive profile.
Bubble Dynamics Model
The conceptual formulation of the bubble dynamics model assumes that DCS is not a localized threshold phenomenon that always occurs when some critical value of decompression stress is exceeded.
The observations that decompression can result in a spectrum of different symptoms that can occur at different sites in the body with different degrees of severity suggests that DCS is a generalized systemic phenomenon of graded degrees (Lambertsen, 1989). Specific symptoms of DCS would be local expressions of generalized DCS. There are also likely to be other forms of asymptomatic DCS that can occur at multiple sites and go unrecognized because the degree of gas phase separation and expansion is not severe enough to elicit detectable symptoms at a particular anatomical site (Lambertsen, 1989).
Since the specific tissue types and sites that result in the spectrum of DCS symptoms are not known with certainty, then the physical, physiological and biochemical parameters of those tissue sites cannot be precisely defined.
Given these limitations, it is not practical to model decompression stresses at specific anatomical sites.
It is also not sensible to assume that there is one worst case theoretical tissue site that would apply to all types of decompression and at all points in a decompression profile. Instead, it is more reasonable to model decompression stress as a generalized systemic phenomenon resulting from gas phase separation, with bubble growth of multiple degrees and at multiple sites.
This is accomplished by modeling the growth of a single theoretical bubble, resulting from an assumed nucleus in each of a spectrum of tissue compartments that collectively provide an adequate description of the whole body's inert gas exchange. Prediction of gas phase growth and resolution is accomplished within an integrated system of tissue gas exchange, bubble dynamics and oxygen effect (Lambertsen et al., 1991). The highest level of decompression stress (determined by the largest theoretical bubble) occurring in the spectrum of tissue compartments can then be used as a "worst case" general description of the levels of decompression stress resulting from the much more complicated and interrelated physical, physiological and biochemical phenomena that produce DCS symptoms.
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October1999.
Assumptions of the Model The specific assumptions of the bubble dynamics model are:
1. Gas bubbles are the initial cause of DCS symptoms;
2. Gas nuclei are assumed to exist or form normally in tissues during decompression;
3. Gas bubbles grow prior to symptoms of DCS;
4. The inert gas exchange between a "well-stirred" tissue and a free gas bubble is limited by diffusion through a diffusion barrier;
5. The inert gas exchanges between the lungs and the tissues can be described with a multi- compartment, exponential inert gas exchange model;
6. The volume of gas in an extravascular bubble is much smaller than the volume of gas dissolved in the tissues, and initial pre-DCS bubble growth does not appreciably lower tissue inert gas tensions; and,
7. There is a worst case tissue for defining maximum decompression stress, but the specific tissue type depends on the dive profile and type of exposure.
The detailed rationale and supporting data for these assumptions and the mathematical derivation of the model are beyond the scope of this paper (Gernhardt, 1991; Lambertsen, 1989). A graphic illustration of the bubble dynamics model is shown in Figure 1.
Figure 1. Bubble Dynamics Model.
Retrospective Validation of the Model
The bubble dynamics model was evaluated by comparing the decompression stress predictions of the model to DCS observations in a variety of laboratory decompression trials (Gernhardt, 1991; Lambertsen, et cd., 1991) using the logistic regression method (Lee, 1980). The statistical analysis involved analyzing 6457 decompression exposures, which resulted in 430 cases of DCS. The decompression data (provided by the International Diving, Hyperbaric Therapy and Aerospace Data Center) included a wide range of decompression techniques (Table 1).
Data sets were combined based on the likelihood ratio test (Lee, 1980). The results of the statistical analysis are shown below in Table 2.
The DCS incidence data associated with different degrees of theoretical bubble growth were plotted as a histogram. The x-axis denotes the bubble growth index (the maximum bubble radius in any tissue
Gernhardt:- Theoretical, Experimental and Operational Aspects of Reverse Dive Profiles
compartment divided by the initial radius) and the y-axis denotes the associated DCS incidence. The number of man dives associated with each interval is shown at the top of each bar.
Table 1. Summary of Laboratory Decompression Data Decompression Procedure
No Stop Ascent Submarine Escape
Air Decompression In-Water N2 - O2 Decompression 02 Decompression "In-Water"
Surface Decompression With Oxygen With Air
Total Numbers
Man Trials 674 299 2,687
488 301 1,733
275 6,457
DCS 52
4 133 33
8 156 44 430
Table 2. Summary of statistical analysis of nitrogen-based decompression data (Lambertsen et ah, 1991.).
Data Set: In-Water Decompression on Air Index
Null Set Bubble Growth Index Relative Supersatu ration Exposure Index
Log- Likelihood -529 -498 -524 -505
Test for Improvement
62.8
10.8 47..9
p-valuc
.000 .001 .000
Test for Goodness or Fit x!
4.8 19.4
30.5 p-value/
df
0.77/8 0.08/12 0.00/9
NITROGEN BASED DIVING IN-WATER DECOMPRESSION WITH AIR
o 6O SO
*O 3O 2O
1O O
N — NUMBER OF DIVES
9 1O 11 12 BUBBLE GROWTH INDEX
Figure 2. Bubble Growth Index vs. DCS incidence for combined nitrogen-based decompression data (Lambertsen et al, 1991; Gemhardt 1991).
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October 1999.
The results of the statistical analysis showed that the bubble dynamics model provided a statistically significant prediction of the occurrence of DCS (p < .05) and an adequate fit of the DCS incidence data (p
> .05).
Prospective Validation of the Bubble Dynamics Model
In 1992, animal studies using a pig model were performed to compare the gas phase generated with the Bubble Dynamics Tables (generated by the Institute for Environmental Medicine (IFEM)) to equivalent USN Sur-D-02 Tables. The results indicated significantly less gas phase in the IFEM tables (Brubakk, 1993).
The first human trials of decompression tables based on the Bubble Dynamics model were performed at the National Hyperbaric Center in Aberdeen, Scotland, sponsored and directed by the British Department of Energy (Robertson and Simpson, 1997). A summary of these trials is shown below in Table 3.
Table 3. National Hyperbaric Center Laboratory Trials.
Profile (FSWmin)...
110/40 100/50 8QT70 120/40 150/40 150/60
Total
n
10 16 12 30 6 3
77
.! ...DCS
1 1 0
; 0 5 1
8
i %mptoms
Shoulder pain, spontaneous resolution Itching
: Rash Rash/Visual
*7/8-SWnRash
These trials were compromised by procedural anomalies that were not representative of operational circumstances. Limitations in the depressurization rates of the wet pot required the diver subjects to exit the wet pot at depth, remove their hot water suits and perform the ascent and water stop portions of the dive profile in a cold air environment.
The majority of the symptoms reported (> 87%) included skin rash. Since skin rash is not a common symptom for these types of profiles, it is likely that the sudden change in thermal environment (from hot water to cold air) during the stressful ascent and water stop phases resulted in skin circulation changes that interfered with nitrogen elimination. Because of these procedural anomalies, an additional set of laboratory trials was performed at the Sub-Sea International Hyperbaric Center in New Orleans. The results of these trials are shown below in Table 4.
Table 4. Subsea International Laboratory Trials.
Profile V.GE* | FSWrrin n DCS .j_.@ade%4Lj
9080 6 0 ; 0
120*40 6 0 ; 1 - G r a d e 4 ;
"'". ~<" i 13CK40 3 : 0 i 0 j
150*40 9 0 : 3=Qade3 \
Total 24 0 4(16%) j
Gernhardt: Theoretical, Experimental and Operational Aspects of Reverse Dive Profiles
These results, 24 dives with zero DCS incidents, were more in line with the theoretical predictions.
Based on the successful laboratory trials in New Orleans, operational sea trials were conducted. The sea trials involved accurate time-depth recorders and post-dive Doppler measurements. The results are summarized below:
74 working dives with exposures u p to 140 fsw for 90 minutes - 0%DCS
- 6.7% Grade IE and IV VGE
The tables were then released for operational use. Summaries of the operational results are shown below in Table 5.
Table 5. Summary of Operational Results.
Phase
III Ill 199S-5
IV
Decompression Procedure
•No
Decompression- Air SUR-DO2*
with N2-O2*
Air SUR-D-O2 Multi-Depth
Offshore i Dives
i 20,000 i 4,000**
; 500*
2,500**
DCS Incidents
0 !
9 I
i
1
DCS
%
0%
0 0 /
.2%
-. - _.
.04%
The initial operational implementation of the tables (Phase III) resulted in over 20,000 no- decompression dives with zero DCS incidents, and 4500 decompression dives with nine cases of DCS (.2%). The majority of the DCS incidents were on dives with bottom times beyond the recommended operational limits. The final operational table system, which incorporated in-water decompression, surface decompression on oxygen and multi-level diving (Phase IV) were then implemented with slightly reduced bottom times, still within the "Z" to extreme exposure range of equivalent USN tables. The integrated Phase IV tables also included multi-depth diving with the capability to perform forward or reverse dive profiles. These tables resulted in one DCS incident in 2,500 dives.
Analysis of Reverse Dive Profiles with the Bubble Dynamics Model
The purpose of the previous discussion and data was to establish the long history and validation of the Bubble Dynamics Model. It is important to state that the following are theoretical predictions of the model, which was validated primarily on conventional forward dive profiles. No model can provide a true description of the complicated physical, physiological, statistical and biochemical processes involved in DCS.
Figure 3 illustrates the decompression stresses occurring on forward and reverse dive profiles for dives to 100 fsw and 60 fsw. The graph plots Bubble Growth Index (BGD against time. The IFEM tables based on the bubble dynamics model controlled decompression stresses to a BGI level of three. For this comparison, there are no differences in decompression stresses between forward and reverse dive profiles.
Figure 4 illustrates no significant differences in decompression stresses for forward or reverse dive profiles for these exposures with a 2-hour surface interval, compared to a one-hour surface interval.
Figure 5 illustrates the decompression stresses associated with forward and reverse dive profiles with dives to the USN no-decompression limits at 100 fsw and 60 fsw, with a 2-hour surface interval. There are small differences in the decompression stresses. However, these differences are related to differences in the no-decompression limits at 100 fsw and 60 fsw, and not to the dive profile sequence.
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October 1999.
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