nitrogen nightmare?????

[Jason B]

As far as I know PADI has no standards regarding snorkeling for crayfish after a scuba dive, if you can find any, I suggest you post them,

That's not what I was getting at. What I was getting at is that PADI standards are designed to teach the same safety minded diving regardless of the country the course is taught in. I'm sure the New Zealand PADI OW manual doesn't promote sucking a tank dry so I don't think that we should promote it either. Say what you want, I refuse to approve.

Jason

 

[PacNWdiver]

Cray, snorkling after diving is like shaking an opened soda. It can make the bubbling worse. It is not how long you stay down that could cause problems, but the multiple rapid descents and ascents that could give you DCS, so be really careful. Why not get your crayfish before you run out of air..

I don't get this from a physiological stand point. Please splain it to me.

You finish a dive and you have a certain amount of inert gas in your tissue. Maybe you have some micro bubbles already in your blood and tissue. But, hopefully, you don't have any overt signs of DCS. How is free diving going to change any of this? You are not taking on any more nitrogen because you are breathing gas at one ATA on the surface (well maybe just a tiny bit from the air in your lungs but not enough to matter). Now, as I said, removing yourself from 1 ATA for a significant amount of time (say 20 or 30 - 30 second free dives) over the course of your surface interval might be enought to knock off the calculations for repetative dives. That could be a problem. But from only a single dive and free diving afterword? I'm not convinced that alone puts you at any greater risk of DCS.

 

[DandyDon]

I don't get this from a physiological stand point. Please splain it to me.

You finish a dive and you have a certain amount of inert gas in your tissue. Maybe you have some micro bubbles already in your blood and tissue. But, hopefully, you don't have any overt signs of DCS. How is free diving going to change any of this? You are not taking on any more nitrogen because you are breathing gas at one ATA on the surface (well maybe just a tiny bit from the air in your lungs but not enough to matter). Now, as I said, removing yourself from 1 ATA for a significant amount of time (say 20 or 30 - 30 second free dives) over the course of your surface interval might be enought to knock off the calculations for repetative dives. That could be a problem. But from only a single dive and free diving afterword? I'm not convinced that alone puts you at any greater risk of DCS.

This is the way I was taught...

Imagine a hot beer or coke, supersaturated with dissolved carbon dioxide. Now shake it up and open it. That's what you're trying to do to your body, not really knowing how much dissolve nitrogen you have. Except with no place to spew like the beverage, the bubbles block your system, and we find you bent in pain.

 

[Xman]

Yep, I did, but did you see what the course director from his country said?? Remember that I am a good kiwi boy at heart Don and I do remember what I used to do as a young felluh back home. The question was a good honest and innocent one, and I thought he got pounced on rather than counseled.

Now, do I think deep apnea freediving with a high nitrogen load is healthy, absolutely not, far from it, but also there is probably little harm in splashing around in the shallows if the nitrogen load is low.

The best suggestions so far are

Get a bigger tank and do it on scuba.

Dont suck your tank dry, and you wont have to free dive for them.

Xman, by far the most dangerous animal in New Zealand waters is the scallop, but as you say, anyone that has ever latched onto a great cray hole has at one time been suprised how quickly their guage has gone from plenty to OMG.

So what are they doing in Godzone to teach better gas management?

Ok, firstly, I would like to clarify that I am a Divemaster, not an instructor. I have been meaning to change my settings so my Instructor forum membership doesn't show. After reading this response I finally got around to doing it. Don't want to mislead anyone. Perhaps "Dive Professional" would be a better title since it is open to all dive professionals rather than just instructor.

As far as scallops being the greatest danger, I beg to differ.
Whilst the attitude described by cancun mark is prevalent amongst New Zealand hunter/gatherer divers, I don't think that is a good thing, and I think any sensible advise that is given, can't do any harm, and may get through to a few of them.
Too many NZ divers die easily preventable deaths by diving solo (or effectively diving solo) without the right training/redundancy/experience, by poor gas management, and by using inadequate equipment. Please note that I am not against Solo diving. I just think that it requires excellent awareness, and preferably redundancy.
All to many dive without any formal training, without octis or sometimes without SPGs (yes, I know this is how everyone used to dive, but I believe it is irresponsible now that we have the option), and are totally focussed on catching crayfish (rather than maintaining general awareness).
Whilst none of this is directed at the original poster, I believe the attitude that sucking your tank dry to get those last crays is part of the generally dangerous attitude of a lot of New Zealand Divers.

 

[pete340]

I don't get this from a physiological stand point. Please splain it to me.

Ordinarily, small bubbles pass into your capillaries, move from there to larger veins, and eventually reach your lungs, where they get filtered out of your blood. When you dive again, bubbles get crushed smaller, and aren't as easy to filter out. Some might get past your lungs and end up in your arteries. When you surface they expand. They continue to circulate into smaller and smaller arteries until they get trapped, blocking your circulation. That's called an arterial gas embolism, or AGE. It's the most dangerous form of DCS. The blocked artery could be one that supplies blood to your brain or your heart.

 

[Mako Mark]

I don't get this from a physiological stand point. Please splain it to me..

Just open a coke bottle and watch the bubbles form and the liquid start to gas off, this is similar to what is happening in your body after a dive, the bubbles are small and pose no real problem.

Then put your thumb over the top and shake the bottle, and the coke will spray all over the place. No more gas has been added, but the agitation has caused a problem. THe repeated rapid compression and decompression of the body by doing snorkle dives can have the same effects, thus increasing the risk of DCS.

As you say though, the fractional amount of nitrogen added from the one held breath is so negligible not to be a factor here, although there are cases of spear fishermen, pearl divers and Okinawan women divers that have gained enough nitrogen to get bent.

Ok, firstly, I would like to clarify that I am a Divemaster, not an instructor. .

no clarification needed Xman, thanks for speaking up, and I think all of us agree, there is no reason to suck a tank dry, especially for an extra Crayfish. Anyone that cant bag their limit of crays in the first 20 minutes of the dive sucks the big kumara anyway..

, small bubbles pass into your capillaries, move from there to larger veins, and eventually reach your lungs, where they get filtered out of your blood. .

Good explanation Pete, the whole concept of the lungs acting as a filter for microbubbles is fairly new, but very important, it explains why people with PFO have a higher risk of DCS.

 

[PacNWdiver]

Then put your thumb over the top and shake the bottle, and the coke will spray all over the place. No more gas has been added, but the agitation has caused a problem. THe repeated rapid compression and decompression of the body by doing snorkle dives can have the same effects, thus increasing the risk of DCS.

Agitation? Dosen't seem like an acurate analogy. Say you opened the bottle, then filled it up with Coke so there was NO airspace, at all (just like the cells in, say... the human body?!). Then shake it up. Would it spray then?

Anyway, here's the reply I got from DAN on the issue:

Thanks for the inquiry.

As you indicated, the offgassing process is slowed down by free diving
on the surface interval. And that may change the amount of Nitrogen in
the tissues going into the next dive. You are right as well as far as
the free diving not increasing the risk of DCS from the first dive.
There is no evidence that recreational free diving on a surface
interval
increases the risk of DCS without additional Nitrogen exposure from
later dives. The body has had the opportunity on the ascent of the
first dive to form bubbles. Then as you are on the surface, the
Nitrogen is decreasing and therefore makes the possibility of
supersaturation on a free dive very unlikely.

I hope that this is helpful.

Laurie Gowen, NREMT-B, DMT
DAN Medical Services
Department of Anesthesiology
Duke University Medical Center
919-684-2948 ext 235


So, like I said. Mult dives, could be a problem. Free diving after a single dive with no more planned scuba? Follow the rules for having a beer!

 

[Mako Mark]

Agitation? Dosen't seem like an acurate analogy. Say you opened the bottle, then filled it up with Coke so there was NO airspace, at all (just like the cells in, say... the human body?!). Then shake it up. Would it spray then?
!

Bubble dynamics area very complex issue and therefore hard to explain and understand with simple analogies, while I agree with the DAN conclusion that free diving would probably not add enough dissolved gas to noticeably change your decompression status, I would be concerned with rapid compression and decompression of micro bubbles due to rapid descents and ascents

As ScubaDadMiami mentioned earlier, it could reduce the efficiency of the lungs as a filter for micro bubbles, allowing them to form in the arterial rather than just the venous system. This IMO would be a risk factor that should be considered.

 

[ScubaDadMiami]

You are right as well as far as the free diving not increasing the risk of DCS from the first dive.

I must say that I am shocked that this would be a response from an authority like DAN. I have been a member for years up to the level of instructor trainer. Yet, I have a hard time seeing how DAN, who is obviously aware of modern decompression theory and research, can come out with such a response. This may have been the belief years ago, but I find it hard to believe that anyone would still feel this way today in the light of modern decompression theory.

Let's look at the explanation provided:

As you indicated, the offgassing process is slowed down by free diving on the surface interval. And that may change the amount of Nitrogen in
the tissues going into the next dive.

No problem here. This is true as far as dissolved gas in the tissues goes. In fact, this comment is nothing more than a restatement of the basic impact of residual nitrogen in tissues after making a dive.

However, this facet of the equation does not include the examination of bubble mechanics and free phase bubbles existing as a result of the previous dives. It ignores 50 percent of what is going on which, in this case, is the most dangerous part of the risk involved. This is where I (and I suspect a lot of other people) have a big problem.

Without retyping all that has been said above and the links, I strongly suspect that submerging will take any bubbles that exist, crunch them down and allow them to travel to parts of the body that can cause serious injury or death. Surfacing allows them to enlarge, getting trapped in those areas, depriving vital body parts, like the brain and spinal cord, of oxygen, causing these injuries.

There is no evidence that recreational free diving on a surface
interval increases the risk of DCS without additional Nitrogen exposure from
later dives. The body has had the opportunity on the ascent of the
first dive to form bubbles. Then as you are on the surface, the
Nitrogen is decreasing and therefore makes the possibility of
supersaturation on a free dive very unlikely.

This is the shocking part. While the statement admits that there are bubbles existing, it also states that free diving has no effect on these existing bubbles in precipitating DCS.

Yes, I would agree that, if you are well within the No Decompression limits of a recreational dive, you are probably not increasing your risk much by doing some light free diving. I have done this a million times . . . that is, back before I knew better. These days, no way would I do so unless my prior scuba dives had a hugh extra margin of safety added before commencing the free diving.

However, clearly, if you are even close to the no deco limits, free diving after scuba is playing with fire especially if you go deep and repeatedly.

That's my story, and I'm sticking to it.

 

[dc4bs]

Hmm...

I don't remember when or where, but I remember reading about a case of the bends resulting entirely from freediving.

As I recal, it was during submariner training for free ascent escape from a disabled sub. The submariners would go through an airlock and into the base of a tall tube of water and had to swim to the surface on a single breath.

There were safety divers on scuba in the tube in case of problems but there was also a freediving safety officer. After freediving down the tube many, many times one training day, he had symtoms of DCS and was run through a chamber session wich cleared them up.

I'll have to see if I can find the reference and post it later...