microbubble variability

[Amar]

Hi Dr Deco,

I was reading some of the threads in this forum and was interested in the varriability in susceptibility to DCS between individuals. I was wondering what research had gone into this and how varried the levels of susceptibilities are. I also wondered what proportion of individuls are kept safe by dive table limits and computer limits and are there people who would regularly get DCS is diving to the limit? can most people increase the no deco time limit on there computer by another 50% say, not do any deco and get away with it? Not that I would ever do that as it's asking for trouble but was just curious because until now, I thought if you miss deco then you are bound to get bent.

Amar

 

[gcbryan]

I had a similar question and as I recall one answer is that there aren't any studies where they are continued to the point where 50% of the people get bent.

I think a 5% rate on the one of the navy tables would include an area out to 45 minutes or so on air at 100 fsw so 25 minutes past the NDL and you still only have 5 % getting bent.

This is off the top of my head so I'm sure you will get a better answer soon.

 

[Dr Deco]

Hello Amar :

You have a question that has been asked in years past. I do, indeed, know of data on per cent DCS and gas loads. I will put something together, but I am busy this morning [Thursday].

Dr Deco :doctor:

 

[Mr Carcharodon]

The other thing that would be interesting to know is the severity of the outcome. It seems all DCS gets lumped together from niggles to death. A 1 percent chance of minor symptoms might be OK especially with ready access to oxygen and a chamber.

 

[Dr Deco]

Hello Amar:

Risk and US Navy Tables

The first attachment [DCS and Bottom Time] shows a calculated risk for DCS. As calculated [I believe] by DAN. Differing bottom depths are shown with the DCS risk and the bottom time. What is very clear is that the risk is not low and then suddenly jumps. This would be a discreet bends/no-bends limit. Such does not exist. The risk rises continuously as the nitrogen “dose” or load increases.

You do not exceed some [mythical] “table limit” and suddenly acquire the bends. Many divers believe this to be true. [I would certainly suspect this from the questions asked over the past ten years.]

Increasing Bottom Time and DCS Risk

You can see that, yes, you can increase the limits on your tables or computer and not greatly increase the DCS risk. I would not recommend this, however, since other factors than nitrogen load go into the DCS risk for any given dive. You would be loading the dice against you and in favor of more DCS risk.

Bottom Time and Per Cent of Individuals with DCS

In the other figure [Hypobaric depress] we see data from a Russian study with test subjects in a hypobaric [altitude] chamber. Subjects went to increasing altitude [essentially increased load] until DCS was encountered. Those subjects were removed from the test series and a larger ascent was made with the remainder. Those with DCS were removed and the process repeated to greater heights until all eventually had evidence of DCS.

Altitude is better for such an experiment as spinal DCS rarely shows up in altitude work. In a hyperbaric situation, spinal DCS would be a possible DCS end-point.

Diving to the Table Limits

The limits are set far away from the bends point for all of the divers tested. No one should get DCS with the table limit. You could add a lot of time and probably “get away with it.” I am certainly not suggesting this, nor were you planning to try. It was solely for scientific curiosity. Remember - curiosity only!

Dr Deco :doctor:

 

[Amar]

Thanks for that answer Dr Deco, It was very informative.
I noticed that comparing table limits with the risks on the graph showed a higher risk than I imagined, for example, the padi RDP suggests a NDL of 25 minutes at 90 feet and according to the graph this has a 1.5% risk. Is this because the risks on the graph are for faster assent rates than those advised by the RDP? or does diving to the limits or the RDP carry that much risk?

Thanks again for your reply

 

[Dr Deco]

No, the PADI RDP does not carry such a risk. The graph is calculated from data different from that used by PADI. Rest assured.