Just how often does "undeserved" DCS occur?

[The Iceni]

Dr Deco said

Decompression sickness requires the presence of two factors. These are:
[1.] An amount of dissolved gas in excess of what can be held in solution, at equilibrium, at that pressure, and

[2.] The presence of minute gas bubbles that can grow when the supersaturation exceeds the Laplace pressure (= contracting pressure resulting from surface tension).

The use of gas mixes and decompression profiles on ascent reduce the amount of inert gas in solution but there will always be micronuclei present and excess dissolved gas in the blood and tissues at any given time during and immediately after the ascent phase of a dive.

As I see it ALL diving predisposes to factor [1] the nature of the dive and individual variation affect factor [2] but there is a very real statistical probability of DCI following any dive.

I suspect there may indeed be a third factor. In any population there is a normal distrubution in risk associated with any activity. Those with a PFO will be in the minority at the high-risk end of the bell curve for DCI, the majority with other yet-to-be-identified minor risk factors in the middle and the relatively risk free individuals at the other end.

A reduction in both factors [1] and [2] will reduce the statistical risk of DCI close to zero but in the presence of any excess gas in solution the statistaical risk will never actually be zero. The only way to completely eliminate the risk of DCI is to have no excess inert gas in solution and of course this can only be retained by not diving.

One contributor compared the risks to crossing the road. This is a good analogy. If you elect to cross the road you run the risk of being hit by a passing car or truck. No matter how careful you are the risk is there but, of course, by chosing a pedestrian crossing, looking both ways, listening and taking great care you minimise the risk of being an RTA fatality.

I do not have the figures to hand but do know that the actuarial risk associated with the US Navy tables is greater than the "more conservative" BSAC '88 tables. So the risk of unexpected DCI is greater using US Navy tables (This risk is accepted as it is based on opertional requirements and the knowledge that recompression facilities are immediately available).

Statistically you are as likely to suffer a DCI hit using established tables with conservative diving as you are at winning a prize on the National Lottery. It is a lottery. "If you are not in it you cannot win it" so even if you undertake the most conservative diving you run a small but very real risk of DCI. If you are unlucky this 1 in 20,000 chance (say) will occur within your first few dives. This is statistics!

Current decompression tables are not based on theory alone they are based on empirical data collected over the years and have been shown to work reasonably well.

While we have a much greater understanding of the origins of DCI it has yet to be shown whether the newer theories will give rise to safer diving practices. For example, we know that small PFOs are present in about 20% of the population but a screening programme to detect these shunts is currently prohibitively expensive.

Diving is not a risk-free activity and a DCI hit does not ALWAYS imply bad diving practices. At least we are now beginning to realise that and divers are more ready to accept treatment than in the past, when a DCI was always considered to be a failing in the diver's practices - something to be ashamed of and to hide.

 

[Dr Deco]

Dear Readers:

As has been said on more than one occasion in this FORUM, decompression sickness is the result of a combination of dissolved gas supersaturation and tissue micronuclei.

Dissolved nitrogen is what causes the micronuclei to grow, to enlarge to the point where excessive pressure builds in the (connective) tissues and pain ensues.

Micronuclei are what allow for a decompression gas phase per se . Without some method of overcoming the enormous inward pressures of surface tension in a very small bubble (on the order of millimicrons), no bubbles could ever form. This is true whether we are discussing effervescence or boiling.

Micronuclei by themselves will not result in DCS. This can be demonstrated by exercising until your generate so many microbubbles that your arms fall off. Will you get sore muscles? Yes. Will you get DCS? No.

There currently does not exist a way to measure dissolved nitrogen in your body, but, since the time of J. S. Haldane, there is a method to estimate it. That one factor, gas supersaturation, is only a part of the story. It is the factor that is in decompression computers, the wonderful child of the computer generation. They are wonderful guides or “roadmaps” but they are not absolute since they omit the important factor of micronuclei concentration and size/number distribution.

Divers acquire what they refer to as “undeserved hits” when they are within the “table limits.” There does not exist the physical reality of a “bends/no bends” zone in decompression. It is probabilistic and the dive DCS outcome depends on [a] supersaturation and nuclei concentration. One “hedges their bet” by eschewing those activities that will increase nuclei formation (e.g., surface activities of lifting, straining, running), and by attempting to reduce gas loads (e.g., shorter bottom times, less musculoskeletal activity at depth, mild exercise during the surface interval).

Curiously, nuclei control is without cost while supersaturation estimation cost hundreds. I believe that nuclei control, like drinking water, will be passed over until someone charges bucks for it. (Back to the drawing board for my BENDS BUSTER deco meter!)

Dr Deco
:doctor:

 

[Uncle Pug]

Originally posted by Dr Deco
I believe that nuclei control, like drinking water, will be passed over until someone charges bucks for it.

That is a very interesting thought.... :mean:

 

[The Iceni]

Sadly Dr Deco you are so right,

Medical practice is dominated by medicines, which make megabucks for the pharmaceutical industry but for the prevention and treatment of conditions such as diabetes and ischaemic heart disease, interventions such as adequate excercise and a balanced diet are much more effective and cost so little. But these are not "sexy"and a salad diet is hardly exciting.

While I am bombarded with reps peddling the latest insulin secretagogue or statin (to fight cholesterol) I don't see much promotional materal for the latter interventions.

AND my government makes a fortune on cigarette sales (about $3 a pack)

That's life!

Dr Deco. I think we may have our wires crosssed. I hope you don't think i was disagreeing with your statements. However, may I ask if you consider the factors [1] and [2] are the complete story? I am still not sure whether you believe there is a real, but small, statistical risk of DCI even if micronuclei formation is minimised. Personally, I think there must remain a small risk because not all micronuclei can ever be eliminated.

Human beings are not machines and do not behave like machines and to some extent an individual's pathophysiological behaviour is unpredictable, while statistics can be used to predict that behaviour for the benefit of the majority (in the realms of public health, say). This is why some die from meningitis while others simply catch a "cold", for example.

Do any of us know which group we are in?

 

[Dr Deco]

Dear Paul and Readers:

The truth is, prevention is always better than treatment, as we all really know, at heart. We also know that we are our worst enemies. We have very often an inactive lifestyle, filled with the wrong foods, eaten with too much salt, and washed down with too much beer.:nono:

Can you still get the bends if everything has been followed? Quite possibly since you are supersaturated. In addition to all of the [semi-]known factors such as blood flow and nuclei, there is always that imponderable condition often referred to as “chaos” or sensitive dependence on initial conditions.

We know that blood flow is not always predictable on a small scale, just as wind is not predictable on a small scale. [It is certainly possible to predict a big squall, but impossible to say if, within that storm, a very big gust will develop and knock down trees in your backyard.] Similarly, the flow of blood is variable within any microvolume.

It would certainly be a case of bad luck – but quite possible – that someone would have a region with no flow (and no nitrogen washout) at the same time that they perform some small muscle movement. The pair of events could function in concert to result in the growth of a gas phase in an unlikely circumstance. The result would be a very “undeserved” case of DCS.

All supersaturations will not result in DCS since not all microbubble nuclei diameters are present. In an airliner, for example, no one gets “the bends” since it requires micronuclei of about 10 microns in diameter (for the 8,000 foot cabin altitude). Such apparently do not exist in sufficient numbers (if any at all) to cause a problem.

It is true that some individuals appear to be sensitive to decompression more so than others. Currently, there is not any way to assay for this sensitivity. In addition, there is no proof that the resistance to DCS would be constant from day to day.

Dr Deco
:doctor: