Disturbing 'advice' from DAN in Alert Diver

[TSandM]

I think the biggest worry with bubble-crushing is on deeper dives, where the bubbles can be made small enough to pass the pulmonary filter, and then expand on ascent in the arterial circulation. Going back down 15 feet to do a safety stop is unlikely to do this to any great degree. I would agree, however, that it is not necessary.

 

[Blackwood]

I think the biggest worry with bubble-crushing is on deeper dives, where the bubbles can be made small enough to pass the pulmonary filter, and then expand on ascent in the arterial circulation. Going back down 15 feet to do a safety stop is unlikely to do this to any great degree. I would agree, however, that it is not necessary.

Granted, big bubbles will likely remain too big to pass the filter after a shallow descent. But smaller bubbles (those I would imagine to be likelier from a short dive with a questionable ascent) present that posibility.

 

[b1gcountry]

I don't think the dive computer logging features really play into this argument one way or the other. Whether or not a computer counts a dive as one or two is probably thought of as a convenience feature more than something they spent any amount of time worrying about from a safety perspective.

Tom

 

[Dan Weidert]

A Safety Stop is generally short, on the order of 3-5 minutes. Re-descending after a dive during which bubbling has occurred (on an no-stop single gas profile, they're most likely to appear at the highest gradient, i.e. the surface, and they will certainly be at their largest there) introduces the potential for those bubbles to be compressed and passed from the venous side over to the arterial side in just enough time to re-ascend, turning a benign bubble into a potential type II neurological hit.

Blackwood

Your assertions here confuse me.

First let me state my assumption that inert gas bubbles do not form preferentially in the arterial, venous, or capillary system. (Someone familiar with doppler studies might be able to confirm/refute that assumption.)

Once blood has passed through capillaries (in the body) and into the veins, the vessels get larger and larger as the the blood progresses to the heart. It enters the right side of the heart and pumps into the lungs through smaller and smaller vessels until it once again passes through capillaries. Then the vessels get larger and larger as the oxygenated blood returns to the left side of the heart where it is pumped to the body in arteries that get smaller and smaller until it, once again, passes through capillaries (in the body).

So the capillaries in the lungs would catch enlarging bubbles from the venous system before they could enter the arterial system. But that would not matter because there would be the same bubbles in the arterial system already (based on my assumption).

However, I do wonder if once micro bubbles have formed, what is the theoretical benefit is returning to safety stop depth? It seems like the benefit of the safety stop is that you off gas before you make your greatest pressure gradient change (the last 15 feet of ascent) and therefore minimize micro bubble formation. Do I misunderstand how the safety stop functions?

Dan

 

[Rick Inman]

First let me state my assumption that inert gas bubbles do not form preferentially in the arterial, venous, or capillary system,etc...

Dan,

I can understand why you are interested in what might happen to the gas in your blood if you should lose control of your buoyancy and end up unintentionally on the surface.

Just kidding!!

 

[TSandM]

I believe that in normal people, bubbles are not detected in the arterial circulation. There is no gas exchange with the tissues from the arteries, not until you get to the capillary level. Gas is reabsorbed from the tissues in the capillaries and bubbles pass into the venous circulation. Arterial gas bubbles are dangerous and are the mechanism of injury in gas embolism.

 

[lowviz]

I believe that in normal people, bubbles are not detected in the arterial circulation. There is no gas exchange with the tissues from the arteries, not until you get to the capillary level. Gas is reabsorbed from the tissues in the capillaries and bubbles pass into the venous circulation. Arterial gas bubbles are dangerous and are the mechanism of injury in gas embolism.

-and if you wouldn't mind explaining just how efficient the lungs are at scavenging bubbles, this would be a really nice paragraph.

 

[TSandM]

The lungs are an effective filter, because they have miles of capillaries. Bubbles are trapped there, and held where they are exposed to inspired gas tensions, and therefore tend to offgas and collapse. Bubbles probably do create embolism-type issues in the lungs, but as long as the amount of them is kept small and the size is kept fairly small, the lungs are capable of doing the filtration.

Even small bubbles on the arterial side are dangerous, because the first takeoffs from the aorta go to the brain, and the brain doesn't tolerate an interruption to circulation well, even to very small areas.

 

[cwkline]

A Safety Stop is generally short, on the order of 3-5 minutes. Re-descending after a dive during which bubbling has occurred (on an no-stop single gas profile, they're most likely to appear at the highest gradient, i.e. the surface, and they will certainly be at their largest there) introduces the potential for those bubbles to be compressed and passed from the venous side over to the arterial side in just enough time to re-ascend, turning a benign bubble into a potential type II neurological hit.
I need to stop posting when I'm really tired.

 

[dumpsterDiver]

I think the biggest worry with bubble-crushing is on deeper dives, where the bubbles can be made small enough to pass the pulmonary filter, and then expand on ascent in the arterial circulation. Going back down 15 feet to do a safety stop is unlikely to do this to any great degree. I would agree, however, that it is not necessary.

I thought Geo Irvine specifically warned about this issue and referenced divers retreiving deco bottles getting bent. I always assumed the deco bottles were "shallow".

Do I misunderstand his admonishments or has the thinking changed in the DIR world?