D. Yount: The parameters were not adjusted for repetitive control. In a way, that is an interesting extreme case.
R. Moon. Bruce, could I get you to elaborate on what I think I have heard? What I heard you say, I think, is that within relatively small delta P's that all of the models that you have tested produce under any circumstances deep versus shallow first, or shallow first and then deep about the same predicted decompression requirements. Under larger delta P's, what is the difference between deep versus shallow, versus shallow versus deep? In other words, I understand that the models may behave differently, but leaving that aside, is there any suggestion that those two circumstances may produce different decompression requirements?
B. Wienke: I have the feeling that the Haldane models will cluster together no matter what I dp for delta P, and the phase models will also cluster together. But the Haldane models and the phase models, as I increase the delta P, are going to diverge in the decompression formats that they suggest at the end of the repetitive dives, whether it is four, five, six or eight dives. I have actually run this. I just haven't collated it in a coherent fashion that I want to throw it up on the board. What you are going to see is Haldane models going one way for these repetitive dives, and you will see phase models going the other way. But the phase models are typically going to be what everybody here has been saying, deeper stops and shorter decompression times overall, because they are focusing on different dynamics as far as staging divers man Haldane is. So I think you will see essentially the same kinds of phenomena, but I think you are going to see that the classes of models will diverge as delta P gets larger.
B. Wienke: On these exercises that we do at the laboratory, we have a lot of leeway into how we schedule diving. Our concern is not to validate schedules and things like that. We may have to look at a potential nuclear device or a biological canister, and we have to assess, first of all, what is in there. So we have to take some measurements and we also bring some instruments down to see what is coming out of it. Then we call on the expertise of other people who know about this type of terrorist activities. Our experiences have been that whether we are diving nitrox or trimix or whether it is a deco dive or a number of repetitive insertions with a bunch of us, we generally don't exceed delta P's of about 50 or 60 feet of seawater, because the data we show for the 60 dives was an incidence rate of 4 out of 60, about 6 or 7 percent. As delta P went to 80 and 120, we had DCI incidence rates that were high enough that we couldn't live with. Generally, we don't do that, but we may have to. The delta P on the order of 60 feet of seawater is probably something that we have found to be important.
When I do modeling work or when I concoct schedules, I try to keep that in mind. You will see
model divergences once the increments, the reverse dive profile increments, are 60 or 70 feet. This is
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October 1999.
built into the Suunto implementation for recreational diving and actually uses some of the data from that table that I showed you.
W. Gerth: It is important in considering this divergence of model predictions or prescriptions for how to do decompression that we have to separate the issue of decompression efficiency versus the safety of a decompression. Different decompressions can be scheduled that are longer than necessary, but they are still safe, or shorter, and still safe. So it depends on how you allocate stops most efficiently.
ACCORDING TO A PHYSIOLOGICAL MODEL Valerie Flook University of Aberdeen and Unimed Scientific Limited 123 Ashgrove Road West Aberdeen, AB16 5FA SCOTLAND
This model is based entirely on data available in the physiological and physical literature. It has been used to study the effect of reversing the order of a 100 ft dive and a 60 ft dive. Two patterns of bottom time have been studied: 100 ft for 15 min and 60 ft for 30 min, for bottom times determined by the USN Tables NoD limits for repetitive dives. The conclusion is that reverse diving does not routinely carry a risk of more decompression bubbles - in some situations the reverse order results in less bubbles.
Introduction
The model that I use to study decompression is simple in concept but to be used to its full potential requires considerable knowledge of physiology. The basis is a simple model of gas dynamics well proven in terms of uptake and distribution of inert gases (Mapleson, 1963). This approach treats the body as if it were a number of compartments each containing tissues that have identical properties of gas dynamics. Onto this is grafted, for each compartment, the Van Liew-Burkard model of bubble dynamics (Van liew and Burkard, 1993). The minimum number of compartments required to give satisfactory simulation of gas dynamics is 8. However the model is designed in such a way that it is easy to divide any of the 8 to allow any tissue (or indeed any small section of tissue) to be treated as a separate compartment to study the effects of changes in conditions. For example, an extra compartment can be added to take account of areas of body surface subjected to heating through use of a hot water suit.
Taking a step further in complexity, given patience and persistence to amass and evaluate the relevant data, it can be used to study the effect of differential skin temperatures such as might arise in a less than perfect suit.
In addition to looking at what happens within the compartments, the model can be used to predict what happens in the venous blood draining the compartment and by using appropriate weighting, what happens in pulmonary artery blood. This is very useful for comparison with precordial bubble counts.
All parameter values required by the model are drawn from the physiological literature. This effectively sets limitations in that, given the complexity of the human body and the lack of detailed experimental data about relevant aspects of its function, the model should always be used wisely. That is accepting the fact that we cannot reproduce reality, accepting that results from it can refer only to what will happen on average for groups of divers undergoing identical experiences. These limitations can be offset by, for example, estimating the confidence limits based on known ranges of normal physiological values and perhaps, most importantly, by always presenting and interpreting results relative to what is known. For example, if a particular exposure is known to give few bubbles, then does the predicted performance for an unknown procedure suggest a few more or lots more bubbles? It is easy to believe that the numbers that come out from a model tell the truth, especially if you have confidence in every number which has gone into the calculation. It takes an effort to remember that each human body has a very wide range of variability built in and that no person is identical to any other person.
In terms of decompression procedures that this model can simulate, the scope is limited only by the data and the time available to build the profiles. To date it has been used to study: no-decompression dives (both planned and accidental), surface supplied air dives, Sur-D decompressions, heliox saturation decompressions, excursions during heliox saturation dives, nitrox and mixed gas exposures, treatment tables, submarine escape exposures and, at the other extreme, compressed air exposures in caisson workers that approximately correspond to saturation air dives with severely accelerated decompressions.
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October 1999.
Confidence in the model's ability to predict what might happen in the average person has increased over the four years of use as the opportunity has been taken to compare predictions with actual outcomes. Hie first comparisons were made with average bubble counts, recorded by trans-oesophageal ultrasonic scanning, in animals exposed to experimental hyperbaric profiles. Several of the commonly used operational profiles contributed to that study together with some more experimental procedures.
Agreement was good enough to make it possible to convert from model prediction to bubble counts for anaesthetised pigs. It has also been used to rank outcome, in terms of predicted pulmonary artery bubbles, for experimental exposures in humans, with good agreement with the experimental data once the exact physiological conditions in the experiments were allowed for. It predicted bubbles in compressed air workers for profiles believed to be bubble free; later measurement of bubbles using both Doppler and ultrasonic scanning confirmed the model's prediction. The gas dynamics part of the model has been tested in a number of situations by comparison with inert gas content of blood with very close agreement. This has been done for both the uptake and washout phase in mixed venous blood for a range of decompression profiles, in arterial blood of humans during the rapid compression of a submarine escape procedure, in animals, venous and arterial blood, during the rapid decompression of submarine escape both during air breathing and after a switch to oxygen. More recently the model was used to assist in the design of human trials of Sur-D decompressions aimed at reducing variability between subjects. This trial was successful in that objective and median pre-cordial bubble scores were within one grade of predicted values on the Kisman-Masurel scale. Unfortunately, most of this work is published, at the moment, only as confidential reports to the clients, but all details can easily be supplied in a non-confidential format to interested parties. Ultimately, this will all be written up and put into the public domain.
Results
For the purposes of this work, I have chosen to look at what happens in individual tissue types rather than at pulmonary artery gas that, though ideal for comparing with Doppler or scanning bubble assessment, is after all only a whole-body average. The four compartments that I want to focus on are:
• time constant 1.9 minutes, which represents viscera and in particular could be taken to represent the grey matter of the brain;
• time constant 5.3 minutes representative of brain white matter;
• time constant 51 minutes for resting muscle and skin under normal thermal conditions; and
• time constant 211 minutes to represent fatty marrow and fat.
Table 1 gives the predicted volume of gas carried as bubbles per ml of tissue, or of venous blood draining that tissue, for Series 2 100ft 15 minutes and 60ft 30 minute exposures. Both the 100-60 and 60- 100 exposures are given. The final column gives the ratio of the volume of gas carried as bubbles for the 60-100 exposure compared to the 100-60. The first conclusion from these numbers is that in the average diver all tissues are likely to form bubbles. The second conclusion is that, for the slower compartments, less gas is carried in bubbles when the surface interval is longer. The surface interval makes no difference to the gas in bubbles in the fast compartments, and this is because even the shortest surface interval, 1 hour, is long enough to ensure that each of the two exposures is a new event for these tissues. Because of this, the surface interval has no effect on the consequences of reversing the order of the dives. In fact for the fast tissues, the second dive is always the one that determines the formation of bubbles in these tissues; doing the deepest dive second will always give more bubbles at the end.
The ratios for the slower compartments also show that the deepest dive second will give more gas in
bubbles, but, for example, for resting muscle the extra gas is only on the order of 20%. Again, the benefit
of the increased surface interval is apparent. For the slowest tissue, the fat, the order in which the dives
are done is more important. Almost twice as much gas is carried as bubbles if the deepest dive comes
second. For these tissues the apparent effect of increasing the surface interval is to make the
consequences of the reverse dive worse. In fact, what happens is that the beneficial effect of the increased
surface interval is greater for the 100ft - 60ft than for the 60ft - 100ft dives. This is so because 15 minutes
at 100ft does not give a sufficient gas load in the slow tissue for bubbles to form on the first
decompression, and gas washout during the surface interval takes place at the appropriate rate for the
tissue time constant. The 30-minute dive to 60ft does give enough gas for bubbles to form so when the
Flook: According to a Physiological Model
60ft dive is the first dive, there are bubbles in the tissue throughout the surface interval and removal of inert gas is considerably slower. Therefore, the second dive is started with a very high gas load.
Table 1. 100ft for 15 ntins, 60ft for 30 mins. Gas carried in babbles (ml/ml * 104)
Surface interval 1 hour Time constant (mins)
1.9 5.3 51 211
100-60 2.05 14.3 105.0
68.8
60-100 4.3 30.4 129.9 133.6
Ratio 2.08 2.13 1.24 1.94 Surface interval 2 hours
1.9 5.3 51 211
2.05 14.3 97.1 62.6
4.3 30.4 117.4 127.1
2.08 2.13 1.21 2.03 Surface interval 3 hours
1.9 5 3 51 211
2.05 14.3 89.4 56.6
4.3 30.4 105.2 120.7
2.08 2.13 1.18 2.13
Figure 1 is presented to demonstrate the extent to which the formation of bubbles influences removal of gas from the body. This is for muscle at rest. The dotted line at the top of the figure shows the predicted bubble radius starting from the beginning of the initial decompression from 100ft. The solid line on the lower graph shows the arterial inert gas partial pressure and the shape corresponds to the pressure profile. The bubbles form at the end of the decompression and after reaching a maximum size decrease very slowly throughout the surface interval and are reduced very little by the start of the next compression. The line with the long dashes on the lower graph shows the tissue and venous blood inert gas partial pressure dropping as the bubbles form, reaching an equilibrium just above arterial partial pressure once bubbles have reached the maximum size. The tissue partial pressure then remains unchanged throughout the time that the bubbles decrease in size, and a dynamic equilibrium has been reached.
On the second compression the bubbles are squashed; the inert gas in them is expelled back into the
tissue or blood, and this gives an immediate rise in inert gas partial pressure that increases further during
the second dive, almost to saturation for that depth. The line with the shorter dashes shows what would
happen if no bubbles formed and inert gas removal from the body was governed by the same factor that
determines gas uptake. When no bubbles form, inert gas load at the end of the second dive is much less
than when bubbles do form. Removal of gas from the body is proportional to the tissue (or venous
blood) to arterial inert gas partial pressure gradient. On the figure this is displayed as the area bound by
the solid line representing arterial partial pressure and the dashed line representing tissue (or venous)
partial pressure. This area is obviously very much greater when there are no bubbles (short dashes) than
when there are bubbles (long dashes).
Lang and Lehner (Eds.): Reverse Dive Profiles Workshop, Smithsonian Institution, October 1999.
r 0.035 3J - 0.030 S - 0.025 &
- 0.020 = - 0.015 w - 0.010 7?
- 0.005 | 300
<D 250 5 200 -
£ 150 -
Q.
100 -
o> 50 -
r o
fc
— i —
25
Arterial
1—
50
Tissue w i t h b u b b l e s
without bubbles 0 25 50 75 100 125
Time from first decompression (minutes)
Figure 1. Bubble growth, arterial inert gas partial pressure and tissue inert gas partial pressure for decompression with bubbles and decompression without bubbles
Table 2 shows the same information as Table 1 but this time for 100ft and 60ft dives, each to the no decompression limits. Again, for the faster tissues, the amount of gas carried as bubbles is determined entirely by the second dive, but of course the duration of the second dive depends on the duration of the surface interval.
Table 2. 100ft NDL, 60ft NDL. Gas carried in bubbles (ml/ml * 104)
Surface interval 1 hour Time constant (mins)
1.9 S3 51 211
100-60 2.18 14.4 119.7 171.1
60-100 Ratio
Surface interval 2 hours 1.9
5 3 51 211
2.05 14.3 122.6 173.1
4.02 13.9 132.3 312.5
1.96 0.97 1.08' 1.81 Surface interval 3 hours
1.9 53 51 211
2.05 14.3 119.8 173.7
4.59 25.1 129.4 316.2
2.24 1.75 1.08"
1.82 Exercising muscle
11/51 11/51
250.9 173.7
230.6 316.2
0.92 1.82
Floolc According to a Physiological Model
I used the USN dive tables to determine the duration of the second dive. These require you to reduce the duration of the second dive according to the estimated amount of gas remaining in the body at the end of the surface interval. That estimate takes no account of the way in which bubbles, once formed, slow down the removal of gas. Thus, the residual gas is underestimated, and the example shown in Figure 1 demonstrates the magnitude of the underestimation for one situation. If the volume of residual gas as used in the USN tables were "correct," as judged by a model that takes account of the effect of bubbles, the predicted gas carried in bubbles for each compartment would be the same for any repetitive dive combination. Table 2 shows this not to be the case. Where the value for the ratio exceeds 1, the estimate of residual gas following the 60ft dive as the first dive, is a greater under-estimate than that when the 100ft dive was taken as the first dive.
Following a surface interval of 2 hours for the 5.3 minute tissue, reversing the order of the dives appears to be beneficial.
Of course no diver dives in order to lie on the seabed and gaze at the stars. The dives usually involve some physical activity and this influences gas uptake and removal in many parts of the body. I have taken a very quick look at how physical activity might affect the outcome for the muscle. I have assumed that throughout the dives and the decompression there is a level of physical activity equivalent to heart rate of 100 to 120 beats per minute. In normal conditions, this would be taken as low level activity for somebody of average fitness. The appropriate increase in muscle blood flow has the effect of reducing the muscle time constant to about 11 minutes. This means that the 100ft NDL almost saturates the muscle and the 60ft NDL does saturate the muscle. During the surface interval muscle blood flow returns to normal resting levels. The penultimate row on Table 2 shows the outcome. Reversing the order of the dives is certainly beneficial. The reason of course is that the duration of the second dive is too short to allow more than a small increase in gas load even with the short time constant of the exercising muscle.
A different picture emerges when we do the same for the 3-hour surface interval. The final row in Table 2 shows why in this case reversing the order is not a good idea. Why does this differ from what happens for the 2-hour surface interval? The increased duration of the second dive (an extra 4 minutes) more than offsets the extra hour of the surface interval. Again, this is a consequence of the apparent under-estimation of the residual gas.
Discussion
In addition to the complexity of what happens in the different tissues of the body and the infinite range of dive time/surface interval combinations, there is the question of what is worse and by how much? Table 1 shows that for the short time-constant tissues making the 100ft dive, the second one gives approximately twice as many bubbles after the pair of dives than when the 60ft dive is second. Is the reverse order really twice as bad, twice as damaging? Figure 2 shows the pattern of bubble growth and decay predicted to occur in brain white matter for both combinations of 100ft for 15 minutes and 60ft for 30 minutes, with a surface interval of 1 hour. When the deeper dive is made second, the maximum bubble size after the dives is 0.0206 cm. When the deep dive is made first, the maximum bubble size after the first dive of the pair is 0.0203 cm. There is an equivalent similarity after the 60ft dive whether it comes first or second. There is a difference in the time taken for the bubbles to decay, in that when the less deep dive is second, bubbles are predicted to last for 82 minutes after the final decompression; when the deeper dive is second, bubbles last for 128 minutes. Is the reverse order of the dives really twice as bad, twice as dangerous, as doing the deep dive first?
Conclusions
The work presented here has been limited by the terms of reference of the workshop, but even within these limits we begin to see that there might not be a simple answer to the question: Is it better to do the deeper or the shallower dive first? What seems to be clear is that if you are going to make a second dive, it is probably better to work from tables that give some allowance for residual gas even if that allowance is an under-estimate.
The use of a physiological model draws attention to the complexity of the problem and illustrates the need for more clear and precise thinking about what we mean when we try to assess the risk of