Safety stop immediately after surfacing

[SkimFisher]

DAN: In-Water Recompression

 

[bleeb]

The downside is the AGE that can result from bubble pumping. This takes recompression after a dive where you have bubbled venous side and had those bubbles filtered out in the capillaries of the lungs. Repressurization can cause those bubbles to move through the capillary to the arterial side with AGE be the resulting condition.

Thal, I can't see how recompression would be a hazard in a healthy diver (having personally done this thousands of times). Do you have a reference on this?

I don't have a reference, here's the chain of reasoning, which seems sound. The problem is not so much the recompression, but decompression and recompression followed by a second decompression without first allowing enough time to adequately off-gas. Bubbles form/absorb more nitrogen during the first decompression, but the lungs do their job and prevent most of them from moving from the venous side to the arterial side where they can do damage. During the recompression, the bubbles are squashed down, so they can pass through the lung capillary filters. If someone subsequently decompresses again before those bubbles have a chance to dissipate, they re-expand through good old Boyle's Law, except now they're in the arterial side, with shrinking vein diameters and the potential to cause blockages.

It's less of an issue if you're doing a full-length deco stop after recompression. But I believe someone once commented that most of the time, you're talking tens of minutes, well beyond what most recreational NDL divers would consider a safety stop. Some food for thought: IIRC Doppler studies have found that for 'typical' NDL dives, detectable bubbles peak around 40-60 minutes into the SI, which would reinforce the suggestion that we're not talking a typical 3-5 minutes stop to avoid bubble pumping.

 

[Thalassamania]

Thal, I can't see how recompression would be a hazard in a healthy diver (having personally done this thousands of times). Do you have a reference on this?

Sorry ... I was unclear.

Recompression, as in going back in to the water for a short time (bounce dive, or trying to make up a 3 min safety stop) not Sur-D with a full schedule.

 

[DCBC]

I don't have a reference, here's the chain of reasoning, which seems sound. The problem is not so much the recompression, but decompression and recompression followed by a second decompression without first allowing enough time to adequately off-gas. Bubbles form/absorb more nitrogen during the first decompression, but the lungs do their job and prevent most of them from moving from the venous side to the arterial side where they can do damage. During the recompression, the bubbles are squashed down, so they can pass through the lung capillary filters. If someone subsequently decompresses again before those bubbles have a chance to dissipate, they re-expand through good old Boyle's Law, except now they're in the arterial side, with shrinking vein diameters and the potential to cause blockages.

It would stand to reason that the bubble size would not increase after the second recompression/decompression any more than it was after the first one. This assumes that the surface interval was less than 10 mins.

We did doppler testing at DCIEM several years back and did not find this to be an issue. I would however acknowledge that there is sure to be more current research than that which I was involved in, so I'd appreciate any references on current studies.

 

[Thalassamania]

Here's Lynne's comment on bubble pumping in an earlier thread:

Well, the issue with "bubble pumping" is allowing the bubbles to pass through the pulmonary filter, where they can reexpand on the arterial side. If you stay under pressure long enough, those bubbles will slowly offgas and resolve, and will not reexpand. How long you have to stay, and what gas you need to breathe to do that safely is probably not known. But going back down for three minutes on backgas and resurfacing is probably as close to a recipe for arterial bubbles as you can come up with.

 

[DCBC]

Here's Lynne's comment on bubble pumping in an earlier thread:

Thanks Thal, I understand her statement, but this runs contrary to my experience / research. That's why I was seeking supporting documentation.