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Decompression Strategies


We were not sure what to call this page, but we decided upon strategies since the idea is to give helpful hints on reducing the risk of decompression sickness.  Many have been taught that diving within accepted time limits will prevent DCS while others have been taught that it is unpredictable and if it is your day to get a hit, then you will.  The truth is that a diver can make either good or bad decisions that will affect the risk of getting DCS.  It is not simply up to the decompression model with it either being blamed for a failure or given credit for a success.  The truth is that ALL models (including Departure) do not account for the divers ability to control variables ... or make them worse.

Micronuclei

The starting point is to understand more about what causes DCS.  Everyone understands that excessive bubbles will cause DCS.  But what a lot of divers don't realize is how these bubbles start ... which will help a diver to understand how to decrease their risk.  When a diver goes underwater, the diver's tissues start taking in inert gas due to the pressure.  So when the diver surfaces, the tissues have an excess of inert gas in them.  But, contrary to what divers believe, this is not the problem.  A diver could take up a very large amount of inert gas and never get DCS if it wasn't for one thing ... the presence of micronuclei ... or bubble seeds.  In other words, a diver could dive to very deep depths for very long times without ever needing to decompress if it wasn't for the micronuclei.  These micronuclei act as a source for bubbles to start occurring.  They may be viewed as very tiny bubbles themselves.  What happens while ascending is that the gas built up in the tissues from the dive are now in a supersaturated and high pressure state.  This gas wants to start leaving the tissue and escaping.  This gas will go wherever possible.  One route is for it to enter the blood and exit that way.  Another possible route is that the gas leaving randomly bumps into and enters a nuclei.  This nuclei will continue to grow as more gas enters it and/or the diver ascends towards the surface causing it to expand according to Boyle's law.  If this nuclei gets too big, a bubble results.  So why is this being mentioned?  In addition to the obvious point of slowly ascending/decompressing through the water column towards the surface, the control of nuclei is a topic that should be discussed.  It has been shown that activity increases resulting bubbling from a decompression.  Since micronuclei can't be seen, they are postulated due to the known increase in bubbling from activity.  Most activity generates micronuclei, but the activity performed by divers are especially bad for generating micronuclei such as hauling gear to and from the water or climbing up the ladder onto a boat.  The greater the activity, the more nucleation that will occur.  It doesn't matter when the nucleation occurs.  It can occur from pre-dive activity or post-dive activity.  As long as there is an excess of gas in the tissues from the dive, nucleation will generate more places for gas to enter while trying to leave the tissues ... and the more places gas has to enter, the greater the number of resulting bubbles ... and the greater the risk of decompression sickness.  So the moral of the story is not to believe that a model is responsible for your safety, but instead understand that your own activity can be too and be aware that pre and post dive activity (as well as that during the dive itself) can substantially increase the risk of getting decompression sickness.

The (bad) assumptions

A model is only as valid as the assumptions behind it.  Most models are based upon certain "knowns" such as accepted no-stop time limits for the different depths.  But these time limits/tables are only valid if the diver performs the dive in the same manner the tables were tested.  Most tables were tested with only mild to no exercise being performed before, during or after the dive ... so their time limits would give a lower incidence of bubbling than what the diving public would probably experience.  So always dive conservative since you are more than likely to have more micronuclei and thus more bubbling after a dive than the acceptable limits of the table when doing the same dive as a table test subject (who was probably sitting around with very little activity waiting his turn to get in the chamber for his test dive).

The conditioning

As much as many of us would hate to admit it, the condition of our bodies do make a difference.  Age has been shown to affect the degree of bubbling from a dive.  Older divers have more bubbles from a dive than younger divers.  The reason is not exactly known but may be accounted for by having more adipose tissue.  Adipose tissue can act as a gas storage area and could result in an overload and release of gas later.

The physical shape of a diver is also important.  It has also been shown that the greater the body uses oxygen, the less risk one has for DCS.  The usage of oxygen simply shows  the oxygen used by the tissues which is an indication of blood flow rates.  Obviously, the better the blood flow the better the off-gassing during decompression or after a dive.

Decompression Tactics

All divers need to decompress properly.  This includes recreational divers doing deep safety stops and ascending slowly.  The benefits of a slow depressurization can be seen by a simple experiment.  Take two soda bottles side by side and open one very slowly and the other one rapidly.  Notice that the one with the rapid decompression results in more bubbling.

In addition to the rate of decompression through the stops and making sure enough pressure is kept on the diver during decompression, divers can also speed up their off-gassing (as well as in-gassing if not careful at depth) by doing very super mild movements.  We hesitate to call it exercise because that makes it sound like work must be performed ... which is exactly what a diver does not want to do.  Performing work or exercise will generate micronuclei and make decompression worse.  Instead, very slow and ultra gentle movements are desired, such as bending the arms and legs slowly.  It is the change in muscle form that will open up capillaries and make blood flow occur and result in better off-gassing due to the increase in blood flow.  But again, anything more that amounts to activity can generate micronuclei and make things worse ... so always error on the side of caution by doing ultra gentle movements   The exercise that some divers think is helpful during decompression is a big no-no. 

Another procedure a diver can perform to reduce decompression risk is to obviously spend more time decompressing.  But the point of this article is to suggest and explain procedures that a diver may not have thought about.  Another of these procedures is for a diver to remain on their final decompression gas for a period of time after surfacing (in addition to avoiding anything other than mild activity).  This will assist with the elimination of gas whether bubbles are present or not.  But assuming a proper conservative decompression was done, bubbles should not yet really be present.  Bubbles take time to generate after a dive (but this may differ with mandatory decompression stop dives).  The reason for this is that it takes awhile for gas leaving the tissues to randomly bump into a micronuclei.  As the micronuclei grow and provide a larger surface area, the rate of gas bumping into the micronuclei increase.  If bubbles are avoided upon the initial surfacing, then there will be a window of time in which the diver can increase gas elimination by breathing a decompression mix at the surface before the bubbles grow - which in turn will reduce decompression risk.  But again, even if bubbles are present, this procedure will still assist in a diver eliminating gas.

Putting it together

Divers should be aware that they have control over nucleating events and they should minimize pre and post dive activity (including underwater activity).  Minimizing activity may help reduce decompression risk and performing activity will certainly increase it.  Of course dives should be planned in a conservative fashion since factors such as age and body fat can also increase decompression risk.  But putting this aside, some individuals are simply more susceptible to DCS than others.  Since most of these divers do not know who they are until it is possibly too late, dives should always be planned in a conservative fashion.  Very mild muscle movement can also be done to eliminate gas faster during decompression.  But if this is started too deep, there is a possibility that it will also result in more gas being taken in.  Divers can also remain on their final decompression gas while at the surface to increase the gas elimination rate before the gas can enter bubbles leading to bubble expansion.

 

       For additional information on controlling decompression risk in diving, further explanations and education can be found at Ascending From a Dive.

Brian R. Morris, www.diverssupport.com




 
775-323-3483 (phone)
departure@diverssupport.com