Hello Readers:
"Unexpected DCS" is the better term. In the piece that follows, I will discuss bubble formation/growth and DCS in subjects without a large PFO. A large hole in the heart will make everything written here just that much worse.
Bubbles Do Not Equal DCS 
Several decades ago, in the 1960s, when bubble detection systems were first being investigated, it was universally believed that
when bubbles appeared in the diver’s body, DCS would shortly follow. Under the Haldane concept, bubbles could appear most anywhere and placement of a decompression bubble detector was considered a matter of convenience. Since bubbles could appear anywhere [blood or tissues], blood was thought a very good place to check. Blood entering the heart was seen as the collection spot for gas bubbles from all of the body and a good location for an “early warning” system.
With more use, it became clear within a short time [early 1970s] that bubbles could be
found in many dive situations AND were also present in individuals without DCS. Thus was gradually born the idea of
subclinical DCS. It is clear that many dives probably have what would be described as
“subclinical DCS.” This being true, it is clear that
some DCS-free dives could also be very close to clinical-DCS dives, i.e., where pain is actually felt.
We see that the
boundary between DCS and no DCS can be fuzzy. The same dive that produces no bad response in one diver could provoke a problem in his dive bubby. “Undeserved DCS” is fuzzy in the same manner. That is, a diver is experiencing a decompression that elicits DCS that day. [I am speaking here of normal recreational diving. – no rapid ascents, etc.] Thus “unexpected DCS” is really a part of the spectrum of outcomes. Tables will naturally reduce this outcome to a very small percentage.
Calculating Decompression Tables 
As all know, these are formulated from actual dive outcomes and do not really have anything to do with real, physiological blood flows. I know that is surprising but it is true. The “models” purport to have real physiology in them, but this is really just a pious wish. Actual, measured blood flows [perfusion] do not correspond to the values given in decompression table calculations. Long haltimes require unrealistic perfusions.
Dive tables do recognize that blood flow is different in tissues and use halftimes to reflect this. Actual blood flow would not yield the requisite halftimes require to make the algorithm produce decompression tables that reflect the dive data. The algorithms must definitely produce real deco times independently of the words used to describe the process.
Blood Flow
The actual
blood flows in the body do change as requirements for oxygen and nutrients change. Active and seated dives will have different perfusions in the tissues. Heavy work will produce large tissue nitrogen uptakes and flows while quietly seated conditions will yield very slow nitrogen elimination rates. There are not any ways to change blood flows in a model [during depress] even though the body does. This can produce a mismatch in the gas loads and the table calculations. :confused6:
All of these activities would be described as normal and should not give cause for concern. But it might result in DCS – “undeserved” or otherwise. :depressed:
Nuclei in the Tissues
There is not enough supersaturation in the body to produce
de novo nucleation. That requires a change of hundreds of atmospheres of pressure. All real liquids do have microbubbles in them. It is these microbubbles that can be the nuclei or “seeds” in the tissues that grow during the ascent phase of the dive. While some dual-phase models do have nuclei present initially, the concentration of these “motes” does not change with activity during the dive. [The model says that some will be squeezed by pressure during descent, but they will not be augmented in concentration by physical activity on the surface.]
The Haldane tables were based upon the concept of limited supersaturation and the metastable limit [a stable state below which there was no bubble formation]. This idea embodied the principle that a liquid could hold a given quantity of dissolved nitrogen gas and that, if below the limit, the solution would
remain supersaturated forever. This concept was originally investigated in detail by the German chemist Wilhelm Ostwald [about 1910] who concluded that supersaturation was only possible
if there were no impurities in the solution at all.
Liquids contain minute gas bubbles that function as micronuclei. One way these are
generated is by body movement (kinetic activity). The process is termed hydrodynamic cavitation. When a certain saturation is exceeded, these will grow as determined by surface tension. [How surface tension changes in the bubble-forming tissues currently is not known.]
It is know that physical activity can greatly change the outcome of a decompression in altitude decompression [airplanes, astronauts, etc.]. I studied this
hypobaric situation for 15 years while I was at NASA but did not have any opportunities to apply it to deep diving [hyperbaric situations]. I suspect that it will hold true for diving, since decompression results are essentially the same for hyperbaric or hypobaric decompression situations. Pressure change is pressure change.
Chaotic Systems and Nuclei
Chaotic systems have as one hallmark that small
changes in initial conditions will result in vastly different outcomes. What does this mean for diving and decompression? The algorithms are based on data reflecting actual dive outcomes. The purported "model” is generally independent – it need not reflect reality. The whole system does require some conditions, however.
One is that the
nuclei concentration or “seeds” in the tissues be within expected limits. A diver with many, many nuclei and/or large “seeds” will experience
greater than expected decompression gas phase growth. Subclinical DCS can become clinical DCS. How can this tendency to increase nuclei number be increased? Physical activity is one way. [Activity might also promote some factor or factors for nuclei formation, some "biochemical mechanism." The evidence is not there yet to elucidate the mechanism.]
For nitrogen to exit the tissues, it is necessary that it be dissolved. If it finds its way into nascent bubbles, the driving force is gone and the nitrogen is
sequestered, trapped. Therefore, bubble formation early in the decompression could lead to DCS problems. Something as simple as climbing a ladder into the boat with all of your dive gear could cause bubble formation.
Bad luck you could call it.
Chaotic Systems and Blood Flow
Blood flow is also a concern. If the blood flow should cease for a short time in connective tissue, nitrogen elimination is markedly decreased. Flow might slow if the diver is chilled, sits in a cramped position, or simply because all capillaries are not open all of the time.
Open and closing of capillaries actually cycles! This is normal physiology, but one can see what this leads to. Large nuclei at a critical time when local perfusion is [unfortunately] reduced can lead to DCS. This is rare to be sure but is one way that “undeserved” hits could arise.
The best way to avoid getting a DCS hit is to
avoid strenuous physical activity during the gas
uptake phase (bottom) since this
increases tissue blood flow and to avoid similar activity when offgassing (topside) since this
increases nuclei formation or enlargement. [Some mild activity will help with blood flow and off gassing.] The whole process is a biophysical one and “underserved DCS” is a misnomer. This does not imply culpability and bad technique, just to indicate that things do have a reason on this planet. We are not at the whim of a capricious god.
Many responders in the past have indicated that there are, naturally, conditions that set the diver
outside normal limits for the population as a whole. Here we could include poor physical conditioning, overweight, advancing age, and a PFO.
All in the above paragraph, except PFO, are nonspecific as far as a mechanism is concerned. Blood flow and micronuclei are specific as far as a mechanism goes. If bubbles arterialize through a PFO, there is a certain degree of "bad luck" if they enter the brain or cord and then become
permanently lodged blocking blood flow.
Dr Deco :doctor:
