Blow a Safety Stop? Redescend ?

[The Chairman]

Well, the issue with "bubble pumping" is allowing the bubbles to pass through the pulmonary filter,

Not to be obtuse here Lynne, but what are you referring to here? N₂ crossing alveolar-capillary barrier into the blood stream, or already absorbed N₂ passing through the alveolar capillaries from the venous side to the arterial side?

If the former, then how does the N2 go against the tension? If the latter, then the capillaries would have to be mighty small and the bubbles mighty big for there to make much of a difference.

 

[Thalassamania]

Neither, we are talking about AIR BUBBLES that have been filtered out by the venous side of the pulmonary capillary bed. You bubble there first because of the lower blood pressure; it is not a problem because except in the hypatic circulation the veins get larger and larger as you more toward the heart, until they hit the pulmonary capillary bed where things get small again and they block up with bubbles. No big damage done ... the bubbles outgas through the lungs in time, but there may be a feeling of fatigue ... this is one of the ways that some folks think EAN works with respect to divers having, "more energy."

What's the big problem with a patent ductus arteriosus? It is that bubble laden venous blood crosses the heart into the arterial circulation with a possible next stop at the bifurcation of the internal and external carotid artery - queep!!!! Exactly the same thing can happen with bubble pumping, just the path is different, compression of bubbles at the bed, movement through to the arterial circulation then reexpansion on ascent -- queep!!!!

 

[dpaustex]

Okay, for those of us that aren't doctors, can the answers be posted in non-techno/medical jargon? I have always found it best to talk to the technical level of the general audience, and explain the "whys" in lay terms and descriptions.

 

[H2Andy]

No-decompression limit is a misnomer.

yup

Rather, what the PADI (DSAT) table displays are no-STOP limits

that's just another misnomer too

pick which one you like and go with it

 

[Blackwood]

that's just another misnomer too

How so? It's a time limit at a given depth beyond which you can no longer make an ascent at the stated rate without stopping and still comply with the maximum tissue gradients in the model. Seems apt to me.

 

[TSandM]

To try to make it clearer: Most of us bubble after dives (this has been shown by Doppler studies). The bubbles form in the venous circulation. They get bigger as they absorb nitrogen, and they get smaller as they give that nitrogen off, or when they are compressed by higher pressure.

Venous bubbles come back to the right side of the heart, and get pumped into the lungs. The small vessels (capillaries) in the lung act like a fine filter, trapping the bubbles until they give off enough gas to get small enough to collapse. If there aren't too many bubbles, the process works well. If there are too many, you get the "chokes".

If you go back to depth after the bubbles have formed, they get smaller because of the increased ambient pressure. This may allow them to pass the pulmonary filter, because they are now small enough to get through the capillaries instead of getting stuck. Then they come back to the left side of the heart, and get pumped into the arterial circulation. The first branches of the arteries go to the brain and spinal cord.

If you only spend a small amount of time at depth, then when you ascend again, these bubbles can expand back to the size they were when they were trapped in the lungs. This allows them to get trapped in the capillaries elsewhere -- but unlike the lungs, the brain and spinal cord aren't tolerant of having small blood vessels blocked.

If you take a chamber treatment, they take you to depth and HOLD you there for a long time. They also give you 100% oxygen to breathe, to encourage nitrogen to come out of the bubbles. A combination of sufficient decompression time and the right deco gas results in shrinking the bubbles, wherever they are, to the point where they don't cause problems (or even collapse and disappear). Three minutes on air or regular Nitrox is almost certainly not enough to accomplish this.

 

[The Chairman]

Thanks... that's lucid and it makes sense.

 

[HowardE]

WHY did the person miss the safety stop in the first place? What caused that? Uncontrolled rapid ascent?

 

[H2Andy]

How so? It's a time limit at a given depth beyond which you can no longer make an ascent at the stated rate without stopping and still comply with the maximum tissue gradients in the model. Seems apt to me.

because it's a model ... it can only approximate, not describe exactly, what is going on

on a given day, under a given set of circumstances, you can follow that model and get bent

it's all misnomers when it comes to this stuff ... just pick which terms you feel most comfortable with

 

[I Dive]

My take on this is, if a person blows a safety stop on a dive, what makes them think they will be able to hold a safety stop on a subsequent dive? From my perspective, "if at first you don't succeed, try again" doesn't apply here...