Deep-shallow-deep profile affects off-gassing?

[jmcd16]

I am reading through my dive con manual and I found a point I need more clarification on.

It says that you should only dive deep and then get progressively shallower throughout a dive -- and never return to deeper water because the residual nitrogen in your system affects your means of off-gassing. It doesn't go into detail about why -- stating that it's too complex and "just think of it like a sponge."

The part that I can't wrap my head around is -- you've almost always got residual nitrogen when you're on a dive trip, right? I mean, that's the whole point of keeping track of multiple dives with designation groups or computers -- and it's the reason for the no fly rules. So how is a deep-shallow-deep profile in a single dive affected my off-gassing ability differently then getting back in the water in a few hours? There must be more to it?

Thanks,
Jen

 

[Bubbletrubble]

That's a great question. There really isn't a simple answer. Yes, there's "more to it."

All mathematical decompression algorithms in current use account for the partial pressure difference of an inert gas (nitrogen) between a diver's tissues and her inspired gas. That's essentially what underlies the concept of off-gassing and on-gassing. If the tissues contain a higher partial pressure of nitrogen than the inspired gas, then off-gassing occurs. (For simplicity, I've omitted the "middle man" role of the circulatory system.) This concept of off/on-gassing is based on simple diffusion. Not being able to clear the nitrogen gas quickly enough from the tissues is probably an important factor predisposing a diver to DCI. This can happen due to spending too much time at depth, ascending too quickly, or a combination of the two.

We still know very little about the pathophysiology of DCI. Considering only the physics of bubble growth and assuming perfect reversibility of bubble growth dynamics, a deep-shallow-deep profile would seem to be OK. Empirically (real-life data), we know that's not the case. One hypothesis regarding DCS pathophysiology is that inflammation is involved. If this is the case, the disease process is certainly not "reversible." It likely involves a complex cascade of cytokines and activation of immune system cells. This is a possible reason for a longer surface interval not being able to "compensate" for a suboptimal profile (deep-shallow-deep).

There's also the notion that going up and down in the water column increases what's called "bubble pumping." If the diver has anatomy that would allow the passage of venous gas emboli to the arterial circulation, e.g., patent foramen ovale, then this might increase the probability of arterial gas embolism (AGE).

FYI, in recent years, some research has shown that, for recreational dives, reverse profiles (compared to forward profiles) do not necessarily increase DCS risk.

Confused yet? Well, join the club! Your question should spark an interesting discussion. I look forward to following this thread.

 

[gcbryan]

From a practical point of view rather than thinking about it as a "must do" think of it as a good idea to do. If you spend the first part of your dive "deep' and then spend the rest of your dive gradually getting "shallow" then most of your dive has been a series of deco stops in effect.

If you don't do the deeper portion of your dive profile until much later in the dive and let's say you get close to the NDL then you will have had much less time offgassing while underwater still under some pressure to keep the bubble size under control.

If you stay within the NDL's your chances of not taking a hit are good anyway but they're probably even better if you dive a profile with a gradual ascent that's more likely with the deeper portion being done first.

It's true that if you're going to have a surface interval and get back in that you are going to accumulate more nitrogen either way but it's more about slowing the process down while you're under additional pressure to keep the bubble formation/size down.

It's better to spend a little extra time under the water than to spend that same time on the surface from the perspective of bubble formation. It's faster on the surface but it's better from a bubble formation point of view to not get to the surface so quickly.

That would be my take on it anyway.

 

[TSandM]

There is not a whole lot of good evidence for those recommendations. The Smithsonian conference (proceedings available on the Rubicon site) concluded that there was really no reason to condemn reverse profiles with depth differences of 40 feet or less. This would imply that greater differences were known to be bad, but that isn't actually the case.

Cave profiles often involve the kind of sequence you describe. What you have to think about is that, as you move up to the shallow portion of the profile, you do need to deco out whatever you have already done.

I would be, if you surveyed people here, there are a lot of us who have, for example, surfaced in the middle of a dive to take bearings. There are definite theoretical arguments against such practices, but again, I don't think there's much if any real data to validate them.

 

[MauiScubaSteve]

In resort areas where morning and afternoon boat trips are the norm, the afternoon trip usually has a first dive deeper than the second dive of the first trip. Here in Hawaii that is compressed to morning and mid-day trips, as we start earlier and do not take a lunch break typically. Either way the profiles are similar; 80-130' dive, 45-60 min SI, 30-60' dive, 60-90 min SI, 60-100' dive, 45-60 min SI, 30-60' dive.

There are dive leaders who probably do this more than a couple times a week.

 

[gcbryan]

Popping up to the surface early in the dive and then going back down doesn't bother me nearly as much as doing it later in the dive. I dislike doing it at all however.

Reverse dives as in shallow one first and deeper one next don't bother me as much with a decent surface interval as doing it that way within the same dive.

I just visualize it in my mind as all the bad things in diving happen on the way up and I try to slow that process down as much as possible. I don't know how instructors in resorts do it constantly going up and down after separated divers.

 

[DandyDon]

In resort areas where morning and afternoon boat trips are the norm, the afternoon trip usually has a first dive deeper than the second dive of the first trip. Here in Hawaii that is compressed to morning and mid-day trips, as we start earlier and do not take a lunch break typically. Either way the profiles are similar; 80-130' dive, 45-60 min SI, 30-60' dive, 60-90 min SI, 60-100' dive, 45-60 min SI, 30-60' dive.

There are dive leaders who probably do this more than a couple times a week.

And that's when I like to mix in a Nitrox tank on dive 3, if not more than one. Common in Cozumel.

 

[Mike Boswell]

"What is quite remarkable about the issue of reverse profiles is that nobody seems to know why this guideline is in place and where it came from.... The earliest documented recommendation to perform the deepest dive first can be traced back to a manual published by PADI in 1972. In each subsequent PADI manual the rule is repeated in increasingly strong terms up to 1999 where it is clearly stated that reverse profile dives have a high incidence of decompression illness."

"In 1999 a workshop was held on reverse diving profiles...The workshop was hosted by the American Academy of Underwater Sciences (AAUS) and was attended by most of the leading decompression researchers and theorists... It became clear that there is no basis in diving experience to draw the conclusion that reverse profiles are inherently more dangerous than forward profiles."

"Conclusion:
We find no reason for the diving communities to prohibit reverse dive profiles for no-decompression dive profiles less than 40msw/130fsw and depth differentials less than 12 msw/40fsw."*

* Mark Powell, Deco For Divers P. 82-84, 2009

Italics are mine.

 

[Dr Deco]

Hello readers:

To the best of my knowledge, the concept of deep-to-shallow diving arose initially from the issue of gas management. This allowed a diver to use breathing gas in the most economical way such that bottom time was maximized. When this changed to a plan to minimize decompression risk is not known to me.

As other posters have commented, a study commission found that the DCS problem was nonexistent if the dives were relatively close in depth, i.e., forty feet or less. It required time for shallow-to-deep dives to be collected and analyzed so that a scientific decision could be made.

The concept of deep to shallow still holds for economy of gas usage.

Dr Deco :doctor:

 

[jmcd16]

Thanks guys! I appreciate the discussion. I always hate when organizations give you rules and/or recommendations without reasoning.

And it's good to know that there is no real evidence that surfacing mid-dive to get bearings or retrieve a confused/lost diver isn't going to kill me straight off. Deep-shallow-deep isn't something I've made a habit of -- but depending on the dive, I've definitely popped up to check on the whereabouts of the boat and then jumped back down.