speed of descent

[sky50960]

Well just red this one...


http://divermag.com/archives/nov2000/divedoctor_nov00.html


refered above.

I do totally agree.........

 

[Dr Deco]

Dear readers:

This question continues in the column, and I am simply expressing a different point of view than has been the traditional one. That is, again, the table limits to not define a bends/no-bends point. The risk does not increase rapidly past this point. The major difference, I believe, in deserved vs undeserved hits lies in the surface activity of the diver.

Why do I believe this? Because of the work that I have witnessed while performing studies in hypobaric or altitude decompression. Here the DCS rates are very high, and the gas bubble generation is large. It is very easy to pick out those factors that are responsible for DCS. When you do a study with deep dive table, the DCS rate is very low. For example, we had no DCS in the 1200 dives made for the DSAT table testing. From this, it would be difficult to detect the risk factors. When you perform profiles with a 20% DCS incidence and 50% of the subjects have Doppler bubble, these factors begin to stand out.

Because NASA is very interested in DCS and what causes it in their EVA astronauts, we have devoted considerable time to studies, both in our laboratories and in analysis of the data in the literature. It is very clear from all of this that while gas loads are very important (they are the major determinant in a wide range of loadings), the next biggest factor is activity level. Since all astronauts on EVA are performing physical activity, we ALWAYS factor this into the test program. There is no question that activity (even whether you are standing or sitting) is a major player in DCS incidence. For this reason, we have the test subjects perform activities that will mimic the work astronauts will perform in space. {For example, they do not stand.}

Table test programs do not have subjects perform strenuous activities after the depress. Decompression is not over when you reach the surface. If you perform Valsalva-like maneuvers, strain when lifting, play volleyball, or conversely, go to sleep, you will affect your status with regard to the generation of tissue nuclei or vary the rate of off gassing. :boom:

While the speed of descent will change the tissue gas loads, I believe that it can be shown that this is minor compared to the activities just outlined. We continue to stress gas loads since that is the easiest factor to model. A quote from my Decompression Physiology call for more than a decade is, “BECAUSE DIVE TABLES ARE PRINTED WITH DISCRETE NUMBERS, THEY GIVE THE IMPRESSION OF A PRECISION THAT DOES NOT ACTUALLY EXIST IN THE BIOPHYSICAL REALM.” I am not trying to imply that all is a fiction. I am trying to impress you that there is more to decompression!!! :nono:

Dr Deco

:doctor:For those of you who are interested, this is a reminder of the [red]Decompression Physiology[/red] class I have at the USC Catalina Island station. For more information, connect on:
http://wrigley.usc.edu/hyperbaric/advdeco.htm

 

[sky50960]

" I believe, in deserved vs undeserved hits lies in the surface activity of the diver. "

Whould this be true even whith a PFO.
I mean that there would not be an increased risck of DCS if PFO is prersent, but only if performing valsalva.
Would there be a higher risck of DCS when PFO+ only and only regarding surface activity.
And would there not be an incresed risck of DCS in PFO if not performing surface activity ( important valsalva ).

Hope you did understand me.

Thanks.

I am sorry, can't come to your deco class on Catalina : live in Belgium.

Fa

NB : Would it be possible to get any deco class notes later on?

Thanks

 

[Dr Deco]

Dear sky:

Not everything is clear with the PFO situation and diving.

It appears that there are differing “grades” of PFOs depending upon the “hemodynamic significance” (= how much it affects the blood flow) of the lesion (= defect or injury). This can be measured by injection of saline contrast agent and viewing the heart with an echo ultrasound device.

Some researchers have found better sensitivity with the ultrasound probe measuring the blood flow in the arteries of the brain (transcranial Doppler, TCD) and checking for the saline contrast bubbles. When this is done, several things are noted. The first is that about one quarter of the people has a PFO that opens when a Valsalva maneuver is performed (“augmented PFO”). About one tenth of the adult population has a PFO that is open even without the Valsalva (“resting PFO”).

Now, of these people, the hemodynamic significance will vary. This has been checked in individuals with recurrent “crytogenic stroke” (= a stroke cause by a blood clot from the venous side passing into the arterial circulation). What was found was a range of patency that went from minimal to very big. The classification was with regard to bubbles and was “few”, “several”, “shower” and “curtain.” The last category is not good with respect to stroke.

If a diver were to have the last two categories, dives that produced bubbles would be a disaster, most likely. Generally divers do not

• - produce bubbles and,
  - have a big PFO and,
  - perform a Valsalva maneuver.

Straining maneuvers when holding you breath are just Valsalva maneuvers with a different name. Similarly, this is true for coughing or sneezing, or any forceful exhalation. People have had cryptogenic strokes when straining on the toilet, lifting boxes, pulling up weeds, etc.

This should be avoided following a dive. I mean of course, lifting objects on the dive boat, not pulling up weeds on the boat. :nono:

Dr Deco

PS Possibly some day the class will be held in Europe.

 

[sky50960]

You hit it once again.
Well hope to see you in Belgium or somewhere else in Europe
Thanks dear Doctor

Fa