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”.
In reference to the more known cause of right to left shunts, namely a PFO, there are arguments whether or not:
- A diver ought to get tested for a PFO
- A diver with an PFO should get it closed
One such argument is that the overall incidence of DCS is small (2.76/10,000 recreational dives) meanwhile, for divers with a PFO, the incidence is almost double that (5.35/10,000).Note1
Although 25% of the population has a PFO, almost all of us have IPAVA. Those have known to exist in humans for over 60 years, but their physiological and pathophysiological (i.e. disease) roles are only now emerging. In regards to us divers, these IPAVA may play an important role in the development of Type II DCS.
In order to better understand what all this means, I will summarize some previously published papers on
1. What IPAVA are
2. Conditions that open and close them
3. How their opening and thus potential shunting can be prevented
There is no surgery for IPAVA, but for us divers, we may have more simple options available.
How do Bubbles get into the arterial (left) side?Note2
During off-gassing, bubbles form on the venous (right) side and if large enough, get trapped in the lungs where they are being eliminated during expiration. However, this ability of the lungs to “filter” bubbles can be bypassed:
Microbubbles are too small to get trapped and get transported into the arterial side where they continue to grow during an ascent.
A PFO or a septal defect in the heart (whole in the heart) allows blood to bypass the lungs altogether.
Intrapulmonary shunts are open during off-gassing providing another pathway for blood to bypass the lungs.
IPAVA – What are they
In scientific terms: Intrapulmonary arteriovenous anastomoses prvodie a passage for blood flow to bypass a capillary bedNote6
For the layperson: In the lungs you have de-oxygenated blood coming from the right (via venules), passing over the gas filled lung fields (alveoli) and exiting oxygenated on the left (via arterioles). This “meeting” of deoxygenated to oxygenated blood via tiny vessels are capillaries.
This is a typical aka ‘normal’ scenario that occurs in our lungs. There are circumstances where a bypass is created and the blood flows directly from right to left, avoiding the capillary bed.
These intrapulmonary shunts have been dismissed as not playing an important role for most people, but for us divers, they could provide a pathway for bubbles to bypass the lung’s filtering mechanism and get into the arterial side.
IPAVA – When do they openNote 9
IPAVA are usually closed at rest. However, exercise, hypoxia, and catecholamines (e.g. adrenaline) may cause them to open. For most people, the timing of IPAVA opening is not critical. For us divers, however, it is. I think we can pretty much agree that we don’t want any right to left shunts occurring when we are bubbling. And most of us bubble to some degree, after the dive. It is less likely that we are hypoxic after a dive, but we might swim hard toward the boat, struggle up the ladder, or exert ourselves otherwise before we can rest on the boat. Stress releases catecholamines, too but I don’t know of a study that has related stress response after surfacing in divers.
How much does one have to ‘exercise’ for IPAVA to open?
In one studyNote 3 a significant shunt (bubble score of 2) was reported during attaining of a VO2max of 62-78% (exercising at 62-78% of maximum capacity). Depending on your fitness level, that corresponds to mild-moderate exercise for some or heavy in othersNote 4. Incidentally, there appears to be no gender difference in the occurrence of IPAVANote 5.
Other studiesNotes 7,8 identified that some divers are more susceptible to having IPAVA open than others. Thus, even basic activities such as an easy surface swim to the boat could be enough to open a shunt.
Can they be closed once open?
The general consensus is that IPAVA can be closed when one breaths oxygen. It was demonstrated Note 3 that IPAVA can be prevented from opening while exercising and simultaneously breathing oxygen. Another studyNote 7 showed an absence of bubbles after breathing oxygen for only 55 seconds but that bubbles would resume once breathing room air again.
How can we use this information?
It appears that 95% of the population has IPAVA that can open and allow bubbles (if present) to move to the left side.
IPAVA can open even during mild to moderate physical activity.
Some individuals are more susceptible to IPAVA opening than others.
Opening of IPAVA can be reduced or prevented by breathing oxygen.
So perhaps after the dive, flushing our loop with oxygen and staying on it until we sit on the boat for a couple of minutes is a good idea.
Peter Germonpre – Incidence of DCS in divers with RLS – A prospective study. From: 2015 DAN PFO and Fitness to Dive Consensus Workshop Proceedings, June 17, 2015 Montreal, Canada
Papadopoulou V, Tank MX, Balestra C, Eckersley RJ, Karapantsios TD – Circulatory bubble dynamics: From physical to biological aspects 2014 Advances in Colloid and Interface Science; 206: 239-249
Lovering T, Stickland MK, Amann M, Murphy JC, O’Brien MJ, Hokanson JS, Eldridge MW – Hyperoxia prevents exercise-induced intrapulmonary arteriovenous shunt in healthy humans J Physiol 2008; 586: 4559-4565
“for example, easy running is 65% to 75% of VO2 max; and your threshold run effort level is about 75-90% of VO2 max;” Source: Running for Fitness – Training Zones http://www.runningforfitness.org/book/chapter-10-putting-it-together/training-zones
Kennedy JM, Foster GE, Koehle MS, Potts JE, Sandor GG, Potts MT, Houghton KM, Henderson WR, Sheel AW – Exercise-induced intrapulmonary arteriovenous shunt in healthy women Respir Physiol Neurobiol 2012; 181: 8-13
Lovering AT, Duke JW, Elliott JE – Intrapulmonary arteriovenous anastomoses in humans – response to exercise and the environment J Physiol 2015; 593: 507-520
Madden D, Lozo M, Dujic Z, Ljubkovic M – Exercise after SCUBA diving increases the incidence of arterial gas embolism J Appl Physiol 2013; 115: 716-722
Madden D, Ljubkovic M, Dujic Z – Intrapulmonary shunt and SCUBA Diving: Another Risk Factor? J Cardiovasc Ultrasound and Allied Techn. 2015; 32: S205-S210
Eldridge MW – Inducible Intrapulmonary Arteriovenous Shunt Pathways: Are they important in DCS? From: 2015 DAN PFO and Fitness to Dive Consensus Workshop Proceedings, June 17, 2015 Montreal, Canada