Why You Want to Stay on Your Loop AFTER the Dive

[dreamdive]

Why You Want to Stay on Your Loop AFTER the Dive

December 28, 2016
I recently summarized a 2013 article about IPAVA (Intrapulmonary Arterio-Venous Anastomoses) shunting bubbles from the right side to the left thus risking type II DCS. Further review on this topic revealed more articles and discussions within the science community. My goal is to summarize and relate their findings for you.

There is compelling evidence that EVERY diver is susceptible to having a right to left shunt WITHOUT also having a PFO (Persistent Foramen Ovale) and that opening of these IPAVA may explain why some of us experience “undeserved hits”.

The full article is on the non-for-profit TRERO website. Here is the link:

The Rebreather Exploration and Research Organization

 

[Jack Hammer]

Interesting

 

[Duke Dive Medicine]

Claudia, great summary of intrapulmonary shunts.

Though breathing of surface O2 for a couple of minutes would change the inert gas pressure gradient and therefore the amount of inert gas that's off-gassed, the effect of hyperoxia on intrapulmonary shunts would cease very soon after going off-loop on the surface and, depending on dive profile, may or may not reduce post-dive VGE significantly. I think one also has to account for the fact that the number of detectable venous gas emboli increases with time after surfacing (and also with repetitive dive days).

Influence of repeated daily diving on decompression stress. - PubMed - NCBI

One could likely derive more benefit from a more conservative decompression profile, or by increasing the length of the safety stop or last decompression stop.

Effect of in-water recompression with oxygen to 6 msw versus normobaric oxygen breathing on bubble formation in divers. - PubMed - NCBI

Best regards,
DDM

 

[dreamdive]

Claudia, great summary of intrapulmonary shunts.

Though breathing of surface O2 for a couple of minutes would change the inert gas pressure gradient and therefore the amount of inert gas that's off-gassed, the effect of hyperoxia on intrapulmonary shunts would cease very soon after going off-loop on the surface and, depending on dive profile, may or may not reduce post-dive VGE significantly. I think one also has to account for the fact that the number of detectable venous gas emboli increases with time after surfacing (and also with repetitive dive days).

Influence of repeated daily diving on decompression stress. - PubMed - NCBI

One could likely derive more benefit from a more conservative decompression profile, or by increasing the length of the safety stop or last decompression stop.

Effect of in-water recompression with oxygen to 6 msw versus normobaric oxygen breathing on bubble formation in divers. - PubMed - NCBI

Best regards,
DDM

Thank you Duke for your response.

Please note that the suggestion is to continue to breathe oxygen until one has rested for a few minutes. The studies are quite clear that IPAVA open in some people even during mild exercise. Breathing oxygen in the context of IPAVA opening prevention is different than managing post-dive VGE as you would with dive profiles. It is also a good reminder to 'take it easy' after the dive since, as you correctly pointed out, post-dive VGE increase with time after surfacing. While there are many strategies to minimize post-dive VGE, adding a strategy to minimize a potential right to left shunt while bubbles are present should also be considered.

Claudia

 

[Duke Dive Medicine]

Thank you Duke for your response.

Please note that the suggestion is to continue to breathe oxygen until one has rested for a few minutes. The studies are quite clear that IPAVA open in some people even during mild exercise. Breathing oxygen in the context of IPAVA opening prevention is different than managing post-dive VGE as you would with dive profiles. It is also a good reminder to 'take it easy' after the dive since, as you correctly pointed out, post-dive VGE increase with time after surfacing. While there are many strategies to minimize post-dive VGE, adding a strategy to minimize a potential right to left shunt while bubbles are present should also be considered.

Claudia

Claudia, thanks for your response. I guess what I'm saying is that the vasoconstrictive effect of the oxygen goes away very soon after the oxygen is removed, which in this context is sooner than many VGE appear. If the goal is to minimize arterialization of VGE through an intrapulmonary shunt, your recommendation to rest after diving is certainly prudent, but breathing 100% surface O2 for a couple of minutes post-dive would likely be of little benefit.

Best regards,
DDM

 

[dreamdive]

Claudia, thanks for your response. I guess what I'm saying is that the vasoconstrictive effect of the oxygen goes away very soon after the oxygen is removed, which in this context is sooner than many VGE appear. If the goal is to minimize arterialization of VGE through an intrapulmonary shunt, your recommendation to rest after diving is certainly prudent, but breathing 100% surface O2 for a couple of minutes post-dive would likely be of little benefit.

Best regards,
DDM

OK Duke, perhaps we are misunderstanding each other, my apologies.

I think the point Lovering etal. and Madden etal. are making is that IPAVA open during exercise and oxygen closes them, and that the idea is, that while you have oxygen available and are exercising towards the boat, climbing up a ladder (which may be harder for some than for others, which may be easy in nice but tougher in rough seas), etc. you might be able to prevent a R-L shunt from an open IPAVA.

Your IPAVA should remain closed although you are breathing 21% oxygen later, provided you don't start "exercising" again. Thus, bubbles that are occurring after you have closed the IPAVA, do not reach the left side through it.

Thank you in advance! Always looking forward to hearing from you guys/gals
C

 

[Duke Dive Medicine]

Ah I get it, apologies for the misunderstanding. Staying on the loop while swimming and climbing the ladder is a great idea in any event, and if you can bump it to 100% O2 while doing so that certainly couldn't hurt.

Happy New Year!

Best regards,
DDM

 

[crookshanky]

Great read Claudia. Before your article I was only familiar with shunting via a PFO.

When I did my deco training, my instructor recommended, once on the surface and still in the water, to remain stationary and continue breathing deco gas for 5 minutes and then remain stationary while breathing atmosphere for 5 minutes before attempting to go to shore/climb onto the boat.

I have also had debates with other tec divers in regads to the best deco gas to carry. Some argue that EAN50 is advantageous as a diver can get onto it deeper to slow ongassing and begin offgassing sooner. I am now under the opinion that a diver should carry a main deco gas (ie. EAN50) as well as a cylinder of oxygen (perhaps just 40 cf) and remain on the oxygen until the diver is at rest (on shore/boat) to reduce the risk of an IPAVA shunt. I am curious as to what everyone's opinions are on this deco procedure.

 

[Duke Dive Medicine]

Derek, I think what you're describing in the first paragraph is similar to what Claudia is recommending above. I don't think it can hurt, but a lot of venous gas emboli form after that time so I'm not sure how much benefit a diver would derive from it. From an inert gas loading standpoint, the extra 10 minutes on O2 may be better spent at the last water stop and on ascent to the surface. Staying on the loop or on a breathing gas source is a good idea until you're back on the boat and if you have 100% O2 to breathe on the surface while swimming it probably won't be of any harm.

Best regards,
DDM