DCS hit during final stop? Has it ever happened?

[MaxTorque]

I have just finished reading the excellent book "Deco for divers" by Mark Powell, and also reading a fair amouint of recent info on the controversy around deeps stops and trying to find an optimum ascent profile etc


On thing that really stuck with me is:

Draw a square (of any size) now draw a line through the middle, splitting it into two different but idential halves, of those two area's:

The one of the left is eqivalent to the change in gas volume by dscending from the surface to 10m depth.

The one of the right is equivalent to the change in gas volume by descending from 10m to an INFINITE depth!!

ie, the volume change in the first 10meters is the same as the total volume change from there (10M) to the bottom of the mariana trench !!



So, given that perhaps surprising knowlege, and the general trend that the larger the inert gas bubbles that result the more serious any symptoms of DCS are likely to be, is it possible to "get bent" ie to have observable physical symptoms of any kind during the final stop, lets say at between 6 and 3m depth?

(lets assume realistic maximum depth/time profiles for open circuit mixed gas tech diving, but not saturation diving, CCR or similar)

I'd be interested to hear anyones thoughts or links to any objective data on the subject?

 

[Whitrzac]

Jill H talks about getting a dcs hit while still doing deco in 'into the planet'
I've heard of it a few other times as well.

 

[boulderjohn]

It is possible for DCS symptoms to appear during a dive, and that would mean at any time during the dive.

 

[Zef]

I have just finished reading the excellent book "Deco for divers" by Mark Powell, and also reading a fair amouint of recent info on the controversy around deeps stops and trying to find an optimum ascent profile etc


On thing that really stuck with me is:

Draw a square (of any size) now draw a line through the middle, splitting it into two different but idential halves, of those two area's:

The one of the left is eqivalent to the change in gas volume by dscending from the surface to 10m depth.

The one of the right is equivalent to the change in gas volume by descending from 10m to an INFINITE depth!!

ie, the volume change in the first 10meters is the same as the total volume change from there (10M) to the bottom of the mariana trench !!



So, given that perhaps surprising knowlege, and the general trend that the larger the inert gas bubbles that result the more serious any symptoms of DCS are likely to be, is it possible to "get bent" ie to have observable physical symptoms of any kind during the final stop, lets say at between 6 and 3m depth?

(lets assume realistic maximum depth/time profiles for open circuit mixed gas tech diving, but not saturation diving, CCR or similar)

I'd be interested to hear anyones thoughts or links to any objective data on the subject?

Given the largest change in pressure happens between 10m depth and the surface, why would one think that signs and symptoms of DCS would/could not manifest during a final stop between 3m and 6m?

-Z

 

[MaxTorque]

Given the largest change in pressure happens between 10m depth and the surface, why would one think that signs and symptoms of DCS would/could not manifest during a final stop between 3m and 6m?

-Z

I wasn't suggesting that symptoms would not mainfest between a final stop and the surface, more asking what is the observable probability of them being manifest AT the deep stop.

Even with a "shallow" 3m stop, any bubble still has a 33% expansion left to go from that depth to the surface, because 3m is 77% of the relative volume compared to the surface (and 6m is 63% of the volume).

Now clearly, a bubble or bubbles being 33% smaller isn't a deffinite "no DCS" type afair, but it does sound somewhat significant given that the next 33% of compression of volume would take you down from 3m to 12.5m>

 

[boulderjohn]

I wasn't suggesting that symptoms would not mainfest between a final stop and the surface, more asking what is the observable probability of them being manifest AT the deep stop.

Even with a "shallow" 3m stop, any bubble still has a 33% expansion left to go from that depth to the surface, because 3m is 77% of the relative volume compared to the surface (and 6m is 63% of the volume).

Now clearly, a bubble or bubbles being 33% smaller isn't a deffinite "no DCS" type afair, but it does sound somewhat significant given that the next 33% of compression of volume would take you down from 3m to 12.5m>

I am probably confused, but it seems to me you are saying that DCS does not occur until a bubble reaches maximum size.

 

[MaxTorque]

no, not at all ;-)

Merely that the bigger the bubble is, the more likely DCS is to cause symptoms. So at some depth (pressure gradient) DCS is basically effectively impossible, and at zero depth (or worse altitude) is bascially got the most probability of occurring, and somewhere in between the probability follows some sort of curve or other, but one that is most likely to follow the Pressure/Volume laws, ie be inversely exponential to depth

 

[Zef]

From my understanding, deep stops are no longer recommended for no-deco diving due to slower tissue compartments continuing to absorb nitrogen even though faster tissue compartments are off-gassing.

That being said, I think, and may be wrong, that speed of ascent to the deep stop may be more critical in the manifestation of DCS at depth as small bubbles may form in the tissues due to rapid ascent that can cause impairment. I have no idea what the critical depth is that relates to nitrogen bubbles sizable enough to cause issues but I am familiar with recommended ascent speeds and read a study conducted a bunch of years ago that indicated it is better to ascend at a rate between 9 and 10 meters per minute to the safety stop rather than ascend at a slower rate.

Perhaps at deep stop depths (12m and below from limited knowledge) the pressure gradient is not enough to cause significant bubble formation, or the bubbles are small enough that the body can still clear them. Or perhaps task loading while under water keeps folks from realiizing they are experiencing symptoms that become more apparent when they are at the surface where a DCS issue that started at any depth will be worse due to further expansion.

It would be interesting to read some studies on this.

-Z

 

[rjack321]

Can confirm, you can get bent in the water. In my case I noticed symptoms after leaving the 9m stop. I went back down to 15m and repeated a whole bunch of time.

The probability of DCS manifesting at a (now discredited) deep stop is basically nil. These were never anywhere close to even 50% GF. ie by definition they are/were set at very low GF usually between -10 and 30% Yes back in the deep stop heyday people sometimes stopped or slowed their ascent before Buhlmann would have said you're even offgassing. Could someone have gotten bend way down there? Sure anything is possible. If you could find more than even one case study describing such an event I would be shocked.

ps 12m is not a deep stop by any common definition/barometer

 

[Akimbo]

Early HeO2 bounce and saturation divers were definitely bent well below their final stop. I have met several of them. Most were still walking because they were in chambers already so treatment was almost immediate. A few probably became symptomatic in the water but weren't officially diagnosed until they were back in the chamber. In hindsight, those pioneers did some pretty scary stuff but it got us to where we are today.