Diver activity during and after diving
Discussion of possible reasons for differences between algorithm decompression profiles and DCS.
During any ascent or exercise activity there is an increased risk of nuclei growing into micro bubbles – therefore an increased risk of active bubble numbers, requiring further conservatism in diving algorithm coefficients.
An increased risk of nuclei growing into micro bubbles and therefore an increased risk of active bubble numbers can occur when performing:
- Any ascent
- Any exercise
- Any increased stress
The main and first function that produces bubbles is ascent from the bottom depth. Depending on the dive, this could be to the surface or first stop.
After a dive is completed, we know a diver should rest and not exercise for some time to ensure DCS is not triggered after a successful decompression or dive. (decompression is continuing during the surface interval).
After a dive, we know that excessive muscle strain, such as that required to get out of the water, or lift tanks out of the water, has a higher chance of creating some DCS in the muscles or joints used for that activity.
For astronauts performing an EVA or space walk, we know that DCS can appear many hours into the EVA. This can not be explained using diving half tissue over pressure tolerances. Because the EVA is a strenuous activity it could be concluded that the preparation decompression required prior to an EVA is performed to an algorithm that is much more conservative than that used for diving. This could suggest that the extended stress and activity requires a much lower inert gas loading than would be predicted by algorithm coefficients used for diving. Therefore diving coefficients also would need to be more conservative for surface or decompression stages if exercise or other stressful bubble creating activities are to be performed instead of rest.
In other words, divers would also have to prepare for the post dive or decompression activity. This would be on top of that currently required for “rest assumed” decompression solutions.
Another implication of this extra conservatism if activity is to be performed follows the trends predicted in the VGM algorithm. The anecdotal data that makes algorithm coefficients more conservative as divers move from simple no-stop dives to long decompressions can also be justified based on continued activity after the initial ascent from depth during decompression rather than rest after the initial ascent.
Bubble numbers
Below is a comparison of active bubble numbers assumed by VPM compared to that justifiable by VGM.
The VPM model assumes the number of active bubbles stays constant after reaching the first decompression stop. The VGM model can be thought of as assuming the number of bubbles can continue to increase even after reaching the first decompression stop. Although this further increase is modest, this would be sufficient to add further conservatism in line with the continued activity.
Other conclusions and model requirements
Active bubble numbers is not the only way of explaining why further conservatism is required for more complicated decompressions. A more complete model would be able to take account of other bubble and physiological phenomena associated with decompression sickness.
So what could these other bubble phenomena be?
- Bubble size distributions
- Different bubble numbers associated with different tissue types
- Different bubble activation processes for different tissue types
- Boundary conditions at surface between one tissue type and another
VR Technology are currently sponsoring further mathematical modeling research to try and achieve a complete model of the human body from a decompression standpoint. If complete, this model would also be able to predict the types of adjustments needed to compensate for differences in human physiology associated with:
- Male / female
- Body fat
- Body fitness
A complete model would also be able to give better decompression estimates based on activity associated with every part of the dive or decompression. This would achieve a decompression suitable for space EVA. It could also achieve a better decompression if it was known that a diver required to do heavy exercise after a dive as might be required for military post dive scenario.
The applied research into better mathematical models seems to be an area that requires no cost prohibitive physiological research, but can still lead to an improved model with the existing anecdotal and academic research available.
Nick Bushell
All information is subject to change. Copyright Nick Bushell 2008/9
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