Bounce diving and DCS risk

[bluebanded goby]

I'd be very interested in hearing the reaction of MD's here to a couple of different explanations I've heard about the DCS risk posed by bounce diving.

A message on another bulletin board quoted a message from yet another source that was attributed to George Irvine, the well-known proponent of the DIR philosophy. If you want to read the quoted message, you can see it at:

http://diver.net/bbs/messages/12918.shtml

The main point that I'm interested in is this assertion:

"(B)ouncing to 20 feet or whatever to pick up a bottle and immediately returning to the surface is the like giving yourself a home-made PFO: the bubbles in the venous side compress enough to get past the lungs and then will reexpand on the arterial side and lodge in the worst places, the spine and brain blood supplies."

By contrast, I noticed that Lawrence Martin MD has this to say about bounce diving in his book "Scuba Diving Explained," the text of which is visible online at:

http://www.mtsinai.org/pulmonary/books/scuba/sectiong.htm

"Bounce and sawtooth diving probably increase DCS risk because of silent bubble formation at depth. On ascent any silent bubbles that form begin to release nitrogen harmlessly. However, if the diver quickly returns to the deepest depth, more nitrogen will enter the tissues; _that_ nitrogen, on re-ascent, will then flow into the existing bubbles, which expand further. Thus, the risk of DCS increases by diving immediately after formation of any silent bubbles; bounce or sawtooth diving is one way to do this."

My question is this: Do we really know which of these scenarios is more correct? If so, which one? It seems to me that if re-descending to depth caused any currently outgassing bubbles to shrink and sneak through the lungs into the arteries, then a lot of divers would be experiencing a lot more problems on multiple dives. Thanks in advance for any reactions.

 

[Dr Deco]

Dear bluebanded goby:

1. ARTERIALIZATION

It is not clear which of these to possibilities is correct with the research currently performed. It is known that humans or animals in a normal posture in a chamber will not arterialize gas bubbles from the venous side during pressurization. This is not to say that it cannot occur when a diver with venous bubbles is in the water. It has not been tested as of yet. I mention this because of the studies being performed at NASA in support of decompression procedures to use in space. All research is performed on subjects that are seated (to reduce micronuclei formation from walking in gravity), but they are in an erect posture (generally). Some recent research work by other groups regarding PFO has shown, however, that recumbent individuals pass bubbles from the venous to the arterial side (generally through a PFO) more easily than erect one. This is with a Valsalva maneuver, or coughing, or some similar activity, and involves what is known as “rebound flow” (= large influx of blood to the heart when pressure in the chest is suddenly released). Regrettably this posture seems important and all work so far has been performed on erect people - - the wrong posture, apparently. In the end, it is unclear.

Normally gas bubbles do not arterialize with repressurization (in erect people, again). This has been a contribution Doppler bubble detectors. The statement that pressure per se reduces the size of bubbles and they pass through the lungs is not really supported by a several decades of research.

2. IN SITU (stationary, tissue) BUBBLES

There is at least one study that indicates that animals, when redived within a short time frame, will have increased DCS problems. There is a “window” where bubble growth is already begun and not enough time has elapsed for tissue bubble reduction. A redive here will add dissolved nitrogen to the tissues which will diffuse into the small bubbles. These can then really expand on the next ascent. Certainly, individuals who exercise too vigorously (lifting heavy scuba tanks) between dives are candidates for problems in the later dives because they are forming micronuclei with all of the musculoskeletal activity.

The question is avery good one and, it is still not settled as of yet.

Dr Deco

PS Read "Freediving after SCUBA" in the early DR DECO Forums.

 

[bluebanded goby]

Dr. Deco,

Thanks for the reply. It and the earlier thread on freediving were helpful.

I was interested in your statement:

"Certainly, individuals who exercise too vigorously (lifting heavy scuba tanks) between dives are candidates for problems in the later dives because they are forming micronuclei with all of the musculoskeletal activity."

It seemed to me that I'd seen an article on Scubadoc's site which (if memory serves correctly, this having been a few days ago) implied that a reasonably fit diver who exercises a dive may be helping the offgassing process by speeding up blood circulation. I'm interested in this question because I normally bicycle about five miles each morning and do a little work with free weights. Should I be observing a "no exercise" period before and after dives?

 

[Dr Deco]

Dear Blubanded Goby:

The operative word here is VIGOROUS. My friend here is demonstrating.>

Among these would be climbing ladders, lifting scuba tanks, playing volleyball or swimming. While these activities certainly promote blood flow increases and faster tissue offgassing, it comes at the possible expense of tissue micronuclei formation. These “seeds” can grow on the surface when one still retains some dissolved nitrogen. :nono:

The opposite of this vigorous exercise is mild exercise such as walking or sitting down and moving your arms and legs for a half minute, for example, every two or three minutes. The activity that one does not want to perform is sleeping. :tree: This reduces the blood flow in the tissues to their lowest level, and the offgassing proceeds at the slowest rate. One reenters the water with a greater than anticipated residual nitrogen load.

A rule of thumb might be that one should avoid vigorous activity for a four-hour period before and after the day's diving.

Dr Deco.

 

[Atomox]

Dr. Deco

An important qualifier regarding this statement was omitted.
The original statement by G. Irvine was directed at WKPP divers who had just completed extended Deco diving and than re-entered the water to retrieve stages or deco bottles. This is directed at divers who have a high residual gas/tissue loading and then do a quick bounce dive. Not a smart practise for any type of diving. The recommendation was to allow at least 4 hours after decompressing before doing any "cleanup" work (ie retrieve gear or bottles at depth).

Brad

 

[bluebanded goby]

Dr. Deco,

Thanks much for the information. Can you suggest a rule of thumb for how long one should stay awake following a dive? Gee, I've got a night dive planned tomorrow night ...

 

[Dr Deco]

Dear Goby:

[sp][1.] Regarding sleeping and offgassing, that is meant for individuals planning to redive. If that is the final dive of the day, then going to sleep is fine.

Sorry for the confusion.

[sp]2.Thanks also to ATOMOX for the clarification of the WKPP divers. They are obviously not marginally loaded with gas.

Dr Deco