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Decompression Theory

Multi model approach

Support information for Ratio Deco Decompression Procedures

There’s a difference between a decompression model and a decompression strategy, one of the best proposition for it is what JP Imbert called a “multi model approach to decompression safety”, other interesting idea is “Ratio Deco.” Both are valid, very useful and base on the same principles; but be aware, the applications are differents.

First, will start on the easy one, critical volume model, here the decompression strategy consists in managing and controlling bubble size in one specific tissue to avoid type 1 DCS during ascent. Critical volume is the maximum volume of released inert gases that the body can deal with in a limited period of time. In order to avoid DCS, this total volume should never exceed the critical volume at any time.

Tables like Comex or Navy are good on preventing DCS and by-product of build in procedures also prevent type 2 DCS- mainly by slow ascent procedures to the first stop or restrictions on repetitive diving, etc.

So, as we studied up until now about gas phase models , compartment models, only cover pain- Type 1 DCS. So, the second idea is the use of complementary decompression procedure.

The third idea is that lungs act as a bubble filter, when you decompress, bubbles are formed and collected in the capillaries and evaporated in the lungs.

If bubbles pass the lung- filter, it will be dumped at the wrong place, directly to critical central organs, such as the central nervous system or the heart.

To understand more about this refer to The Arterial Bubble Model.

Arterial Bubble Model provide an explanation for the onset of type 2 DCS. JP Imbert explained that this idea can be traced to Haldane’s publication 1908 in the page 352: “If small bubbles are carried through the lung capillaries and pass, for instance, to a slowly desaturating part of the spinal cord, they will increase in size and my produce serious blockage of the circulation or direct mechanical damage.” Later in the with the incoming Doppler technology, the arterial bubbles were used to discuss the cerebral perfusion deficit in divers who had symptoms mainly referable to the spinal cord and the possible role of a PFO in divers susceptibility to type 2 DCS.

To complicate the matter, the lung filter capacity varies on the person’s age, physical fitness, smoking, fatigue, etc. Plus as we discuss in earlier modules, CO2 retention could onset neurological symptoms. Remember that CO2 retention could be triggered by Stress, anxiety, hyperventilation, had work, cold, low-flow regulator, etc.

So JP imbert advised: “ What we can take from this, is to avoid repetitive ascent- descent variations, or in shallow water, repetitive ascent to the surface (yo-yo diving). On open sea air divers should avoid 3m stop and accumulate their spot time at 6. Boyle’s law is quite effective and the lung likely to be involved in the process of filtering bubbles. Moreover, in a 3m stops a Valsalva maneuver –in decompression– is a nasty thing. It induces a significant lung overpressure and may produce arterial bubble as “water is squeezed out of a sponge.”

This concept is useful for a qualitative analysis, but not as an operational tool, JP continue to explain: who could put a mathematical figure on the efficiency of your lungs (in an individual basis).

In this regards the only thing you can do is add procedure that adjust your dive profiles when such factor are present.

Our next idea is to explain why the first few minutes of the ascent to our first stop is critical and have so much influence on the quality for the rest of the decompression.

So microbubbles are introduced, this bubbles do not quite follow the physic of ordinary bubbles (refer to bubbles mechanics post, especially the concept or dirty molecules) and are bubbles sufficiently small that can pass the lung filter.

As you already readed, if are not controlled properly, the seeds will grow up into bubbles later in decompression.
So Tiny Bubbles Assumption comes to play.

Tom Hennessy,  the designer of the BSAC ‘88 tables, defined the origin of microbubbles as being cavitation at the tips of the heart valves.

The survival of this microbubbles would depend on the amount of the gas dissolved in the blood. If this microbubbles last long enough, they could reach a tissue where they could receive more gas and grow. JP Imbert explain that these seeds could pass several cycles before they reach the size of a bubble in the arterial bed.

The ascent protocol is hard to defined, a mid stop procedure was implemented by the Royal Navy on the use of air tables. A fast/slow ascent protocol is an approach used by Van Liew on his microbubble model. When such dives are analyzed in the light of MB model it appears that divers can off-gas fast tissues such as brain by a rapid initial ascent of small distance. This way microbubbles won’t encounter supersaturate gas to feed the growth.

Then with a slower ascent and mid-range stops (starting at 40 meters), control the survival distance of microbubbles and reduce shower cycling of microbubbles through the body and prepare for a clean decompression at shallower stops.

I suggest review the thermodynamic model of hills.

The Comex deep dive studies at the Hyperbaric Center of Marseille in the mid-1970:

Divers were pressurized in 15 minutes to 180 meters and run High Pressure Nervous Syndrome tests for 2 hours. At a time was difficult to find a mathematical model that cover the decompression and then simple start to drawing the ascent profile in a logarithm scales versus depth on paper, and then designed a very successful decompression table for 180 meters and 120 minutes dive with a 48 hours of decompression requirement.

So  JP Imbert became interested in implementing the same semi-log scale procedure for  deep cave diving with trimix, adding slower ascent rate and deeper stops were classic model fails to deliver.

So he explain that what he learned from that experiences was:

  • Decompression profiles appears roughly as lines in the log/P plot.
  • The slope of the line is related to the % of O2, meaning that the higher the O2 content, the faster the ascent.
  • A change in deco mix accelerates ascent.
  • The longer the bottom time, the slower the ascent. 2 dives with the same depth and mixes but with different bottom times display parallel lines.
  • If the bottom time increases, the deco’s tend to adopt the same stop times close to the surface. This correspond to the last tissues controlling the ascent, i.e. the saturation end of decompression.

(For an example of this procedure review the 170 m for 20 min dive of Pascal Bernabe.)

JP Imbert continue explaining that all comes down to this: use tissue compartment models to avoid type 1 DCS, use procedures to avoid Type 2 DCS and deep to middle stops to avoid vestibular hits in deep fast decompression.

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