Why plan decompression with a Gf (lo)?

[Nirvana]

There's a bunch of published research regards deep stops, micro-emboli considerations etc; much of it referenced and extensively discussed here on Scubaboard.

GF high and low offers divers the capacity to tweak their ascent curve according to their individual preferences and needs.

Mark Powell's 'Deco for Divers' is probably the best starting point for a layperson understanding of gradient factors.

Decent tech instructors will expand on that with supplementary lectures in their classes.

These lectures should link the theory with the practical application; there's a myriad of personal preferences and specific dive parameter needs that dictate how GF may be used to create a desired outcome.

Thanks, but I have already read 'Deco for Divers', as well as other often recommended publications directed to divers.

Regarding 'Deco for Divers', its section on gradient factors is located in chapter 6, titled 'Deep Stops and Bubble Models'. This chapter begins with a historical account of the realisation that even asymptomatic dives produce bubbles and with the gain in popularity of Pyle Stops. The description of the purpose of gradient factors reads as follows:

'Gradient factors on the other hand provide an approach which modifies the specific calculations of the dissolved gas model. They are an attempt to include a method of reducing asymptomatic DCS within the framework of the Bühlmann model. The basic concept behind gradient factors is that if there is a danger of bubbling even if we stay within the M-Value, then to increase safety we can simply stay further away from the M-Value. Gradient Factors are just a way of controlling how close we are prepared to get to the M-Value.'
(Powell, 2012)

As I stated on the OP, the problem is that, according to what I have been able to understand from more recent developments (the NEDU study in particular, whose validity I really DON'T want to discuss on this thread), emphasising deep stops may not reduce bubbling substantially, and the consequent increase in supersaturation of slow and medium tissues that it imposes may make DCS more likely.

The question, then, is, what makes you believe that controlling the supersaturation of faster tissues on the initial phase of ascent is beneficial?

Reference:
Powell, M. (2012). Deco for Divers. Essex: Aquapress, pp. 137-139, 144-145.

 

[DevonDiver]

The description of GF that you quoted has no specific or implied relationship to deep stops, Pyle stops or replication of bubble modelling.

GF high and low allow individual tweaking of an otherwise unilateral decompression ascent curve.

Yes, that function can be used, if desired, to allow a dissolved gas model to replicate the curve otherwise given by a free-phase (bubble) or dual-phase model.

No, that's not the sole purpose or means of utilisation with GF.

Back when free-phase models were 'en-vogue' (prior to contemporary studies), some divers did indeed use GF to replicate free-phase model ascents using dissolved gas algorithms. But not all. Some, I perceive an increasing minority, still apply that practice.

As I said earlier, GF allows divers to customize their dissolved gas model ascent in recognition of myriad personal preferences, individual needs and dive specific parameters. It's purpose is not solely to allow one algorithm to mimic another.

 

[Kevrumbo]

The question, then, is, what makes you believe that controlling the supersaturation of faster tissues on the initial phase of ascent is beneficial?

More great @Dr Simon Mitchell snippets (sifting through the Deep Stops Increases DCS thread again):

...I am saying (based largely on the results of the NEDU study) that transient high / peak supersaturation in fast tissues does not seem to matter as much as we thought it might, and therefore that protecting fast tissues from supersaturation early in the ascent by using deep stops does not seem as effective as assumed by bubble models. This is especially so when it comes at the cost of increased supersaturation (both in terms of peak levels and duration) in slower tissues later in the ascent. The NEDU study is telling us that this is where the problems seem to come from.

@Kevrumbo asks:
. . .Explain from a physiological basis why allowing supersaturation of Fast Tissues in this instance is less risky to those tissues and results in a lower overall incidence of DCS in the shallow stops trials of the NEDU Study. . . ?

Hello Kev,

The bubble models and the deep stop approach were originally promoted on the basis that they were more successful at controlling bubble formation. The attempts to evaluate this notion in decompression dives in humans that I am aware of have shown that gas content models (or decompression procedures that have backed off deep stops to some extent) actually produce less bubbles when measured after surfacing. Neal Pollock presented some fascinating work they have been doing at the inner space event at a NOAA / AAUS rebreather diving forum . . . In any event, the more we investigate it, the more the "control bubbles by deep stopping" concept appears to need reconsideration. What this is suggesting is that the bubbles are coming from the slower tissues that absorb more inert gas during the deep stops. It also implies that the faster tissues that deep stops attempt to protect from supersaturation are less prone to bubble formation when they become supersaturated. You are seeking a physiological explanation for this, and while I can't be definitive, I would suggest that it makes sense that a tissue washing inert gas out quickly might be less prone to bubble formation and growth than a tissue with slower inert gas kinetics where the supersaturation persists for longer (there's that time integral again).

Simon M

Deep Stops Increases DCS

I am interested in knowing what are the theoretical and practical bases for planning decompression using gradient factors Low and High. That is, the reason behind choosing a Gf Lo and a different Gf Hi. Why ZHL-16C 40/70 and not ZHL-16C 70/70, for instance. . .

edited for clarity

". . .This brings us to the fact that we have seen the recent emergence of data that suggest the deep stop approach might not be the best one. In particular, these data suggest that the logic of protecting fast tissues from supersaturation early in the ascent at the expense of increased supersaturation in slow tissues later in the ascent may be flawed. Whether one likes the relevant studies or not, they are all we have at the present time to illuminate this issue.

So, was the bubble model fashion a bad idea, and where does this leave the majority who have been using some degree of "deep stopping" with apparently good outcomes? Do we abandon deep stops entirely eg go to 90/90 as you hypothesised, do nothing, or do something in between?

I guess the first thing to acknowledge is that the studies indicating a disadvantage for deep stops are not the diving equivalent of multiple large multicentre randomized trials all showing that the risks of a widely used drug are greater than its benefits. On the basis of that sort of data you would probably stop using the drug overnight. The diving world data concerning deep stops are not as definitive as that. As more data emerge the situation may become clearer (or more blurred)! So, at the present time, as an informed commentator, I would not go so far as to recommend that the entire world dives 90/90 or 80/80 from tomorrow onward even though I personally would not be surprised if the outcomes were as good as (or better than) what we are getting now (if we were able to measure them!). Partly, this hesitancy to advocate substantial wholesale change arises from the certainty that every diver who subsequently got bent would inevitably blame it on the change in their decompression practice! The deep stop trend evolved over a substantial period, and if the data continue to be supportive, it may have to "de-evolve" over a substantial period.

Equally, I do believe the data are strong enough (and bear in mind they are the only data) to consider a change in practice if you are a strong "emphasizer" of deep stops. In practical terms, "de-emphasizing" deep stops (or lessening any potential disadvantage) would mean using bubble models on very high conservatism settings, and with gradient factors, avoiding very low GF-lo values. I have been evolving my own use of GFs and am currently around 50/80 . . .sometimes as low as 70 for the high value when we are at places like Bikini and I am the only diving physician. Pre-NEDU study I was GF lo of 20. This is my personal perception of a sensible graduated response to the way the evidence is currently evolving. I may well go further in future (guided by the evidence).

Sorry about the long post. Hope it makes sense."

Simon M

Deep stops debate (split from ascent rate thread) - Page 13

 

[joe10540]

At the risk of sounding dense and totally off the wall what would be the problem with a higher GF early in the ascent with a lower GF for surfacing? Say 80/50 so that you get shallow quicker but surface with a theoretical lower level of supersaturation. Keeping your ascent below 80% of maximum and continuing to deco out to a lower level of saturation would more closely replicate the profile in the NEDU study which had extremely long final stops as well as maximize the time spent at shallower depths which could provide more time with less gas consumption.

Before anyone totally tears me to pieces I should point out that I am as of yet NOT deco certified and everything I have learned has been read from books and articles about the theory. Obviously practice is different than theoretical ideals and that may be why this isn’t done. Just my random thoughts in the middle of the night.

 

[KenGordon]

At the risk of sounding dense and totally off the wall what would be the problem with a higher GF early in the ascent with a lower GF for surfacing? Say 80/50 so that you get shallow quicker but surface with a theoretical lower level of supersaturation. Keeping your ascent below 80% of maximum and continuing to deco out to a lower level of saturation would more closely replicate the profile in the NEDU study which had extremely long final stops as well as maximize the time spent at shallower depths which could provide more time with less gas consumption.

Before anyone totally tears me to pieces I should point out that I am as of yet NOT deco certified and everything I have learned has been read from books and articles about the theory. Obviously practice is different than theoretical ideals and that may be why this isn’t done. Just my random thoughts in the middle of the night.

80/50 would be like doing an 80/80 ascent and then padding the shallow stop(s) before surfacing. I would have to run some numbers but it might be what a fairly aggressive deepish bounce dive just within NDL would look like.

 

[Kevrumbo]

At the risk of sounding dense and totally off the wall what would be the problem with a higher GF early in the ascent with a lower GF for surfacing? Say 80/50 so that you get shallow quicker but surface with a theoretical lower level of supersaturation. Keeping your ascent below 80% of maximum and continuing to deco out to a lower level of saturation would more closely replicate the profile in the NEDU study which had extremely long final stops as well as maximize the time spent at shallower depths which could provide more time with less gas consumption.

Before anyone totally tears me to pieces I should point out that I am as of yet NOT deco certified and everything I have learned has been read from books and articles about the theory. Obviously practice is different than theoretical ideals and that may be why this isn’t done. Just my random thoughts in the middle of the night.

There's a few other dynamic factors to consider on a decompression schedule with a surfacing GF hi of 50 -for instance carrying enough volume of deco gases like Nitrox50, as well as a relatively long shallow profile breathing Oxygen at 20fsw/6msw along with potentially maxing out the accompanying O2 CNS exposure way past recommended levels, and increasing the risk of Oxygen Toxicity Syndrome (see this article regarding this risk though, Advanced Knowledge Series: Carbon Dioxide Retention | Dive Magazine). Other conditions such as water temperature, weather and sea state for example may impact the option of extending an O2 profile to a GF hi of 50 or lower . . .

 

[joe10540]

The two dives below are identical dives to 160’ for 20min with deco gas of 100% at 20’. The first to GF80/50 and the second is 50/80. For the extra couple of minutes of time in the water there are fewer stops but wouldn’t you exit the water with a lower theoretical risk of DCS?

Obviously this reversed GF would become impractical as dives get longer and deeper but isn’t that part of planning? What is the most appropriate GF settings for a particular dive on a particular day. Obviously if you are drifting in open water and the weather is going to hell you want to get out ASAP, likewise if you’ve had a problem but if you are comfortable and have the gas to do a little more deco to get a lowered risk wouldn’t you want to do that?

 

[Diver0001]

Thanks for your reply.

Protecting the faster tissues certainly was one of the reasons for introducing deep stops. But do they, in fact, protect those tissues?

Yes. It is quite clear that deep stops protect fast tissues.

However.... The longer the dive, the more relevant slower tissues become. This should be conceptually obvious because slower tissues on-gas more slowly but also off-gas more slowly. As the dive becomes longer the critical compartments in the model move down the chain from fast to slow.

A little while ago a threw together a spreadsheet that implements the Buhlmann model and indeed in that sheet the longer your bottom time is, the more the controlling compartment moves from fast to slow. Using what I thought were the most extreme dive profiles I could personally make based on the gear I own the controlling compartments were 8-5, which have 1/2 times of 27-77 minutes. These are not fast compartments. In fact, on ANY interesting profile over the NDL's that I entered the controlling compartments for decompression were never the fast ones.

Research done by NEDU, while done for very different reasons, seems to highlight the same kind of thing. On a sufficiently aggressive dive the controlling compartments for decompression are never the fast ones. Heat maps of tissue loading upon surfacing clearly indicate that deep stops protect fast tissues but that the expense of this when calculated with existing bubble models is that the intermediate tissues are pushed to the limits (or over). NEDU tested a particular bubble model against buhlmann and the results were clear. Using that particular bubble model the efficiency of decompression with a deep ascent line was clearly less than when the diver went shallower faster.

Let me say that again because there is a LOT of confusion about this online. The EFFICIENCY of the bubble model they tested was less. This was proven by picking settings for the two models that resulted in a nearly identical dive LENGTH. In reality people don't use their models like this. What it would probably would have meant in a real-world dive is that the bubble model would have calculated a much longer ascent when used with settings that are more the norm. Nevertheless, a highly significant calibration issue was brought to light by NEDU that seems to affect all existing bubble models, not just the one they tested.

NEDU never tested if deep stops were GOOD or BAD. Their research was to test the relative efficiency of the decompression. The conclusion is quite clear that models that include deep stops are not going to be as efficient. People on the internet have allowed that conclusion to mutate in a judgement call, saying "deep stops = bad", or "bubble models = bad" but NEDU never said that, nor could that be directly concluded from their research. What CAN be concluded from that research is that deep stops don't help you get to the surface faster or cleaner and that bubble models would seem to be in need of calibration.

So where does this leave you as a technical diver? My personal conclusion (and many with me) is to stop using bubble models entirely for decompression diving until such time that re-calibration has happened. My personal conviction is that it WILL happen, but it hasn't yet.

Secondly, I'm setting GFlo higher than I did in the past in order to calculate shallower deep stops. That second thing is a bit of a fence sitting position but since decompression models are mathematical constructs I'm still intuitively reluctant to just race to the surface. I'm happy to do a little more hang time anyway because of my age so a few minutes of deep stops during a typical ascent isn't going to drastically change my decompression obligation so I'm happy to live with this decision.

As I said in my previous post, I don't think anyone really knows what settings are optimal right now and we're all feeling our way through this.

I believe that neurological DCS is not necessarily produced by bubbles generated locally, but more often than not by bubbles generated throughout the body that travel to the brain. It doesn't seem that making deeper stops (setting a lower Gf Lo) correlates directly with reducing the incidence of DCS in faster tissues.

Yeah, a bubble is a bubble. That's true.

R..

 

[Diver0001]

At the risk of sounding dense and totally off the wall what would be the problem with a higher GF early in the ascent with a lower GF for surfacing? Say 80/50 so that you get shallow quicker but surface with a theoretical lower level of supersaturation. Keeping your ascent below 80% of maximum and continuing to deco out to a lower level of saturation would more closely replicate the profile in the NEDU study which had extremely long final stops as well as maximize the time spent at shallower depths which could provide more time with less gas consumption.

Before anyone totally tears me to pieces I should point out that I am as of yet NOT deco certified and everything I have learned has been read from books and articles about the theory. Obviously practice is different than theoretical ideals and that may be why this isn’t done. Just my random thoughts in the middle of the night.

I don't think anyone is experimenting with GFlo higher than GFhi yet but the idea is not without theoretical merit for short bounce dives. Then again... on a short bounce dive the deep stops aren't going to cause you to rack up piles of extra deco either.... so .... yeah.... I'm not personally sure what you would gain by it.

On (very) long dives my feeling is that it could lead to a situation whereby the GFhi controls the ceiling throughout the dive. In that case it would be like setting the gradient factors to 80/80, for example.

R..

 

[Kevrumbo]

View attachment 453366 View attachment 453367 View attachment 453368 The two dives below are identical dives to 160’ for 20min with deco gas of 100% at 20’. The first to GF80/50 and the second is 50/80. For the extra couple of minutes of time in the water there are fewer stops but wouldn’t you exit the water with a lower theoretical risk of DCS?

Obviously this reversed GF would become impractical as dives get longer and deeper but isn’t that part of planning? What is the most appropriate GF settings for a particular dive on a particular day. Obviously if you are drifting in open water and the weather is going to hell you want to get out ASAP, likewise if you’ve had a problem but if you are comfortable and have the gas to do a little more deco to get a lowered risk wouldn’t you want to do that?

Yes, if you are comfortable and have the gas -and this happens to be the only dive profile of the day or trip excursion- it doesn't hurt to have some extra "insurance" with an extended O2 stop.

Where you have to start being cognizant of inert N2 loading and CNS/O2 Toxicity is if you do two (or more?!) of these mandatory deco dive profiles on Deep Air per day over consecutive days of a week long or greater dive expedition -especially using a gross deepstop GF profile of something like 30/85 or the original Ratio Deco algorithm with 75% max depth as a First DeepStop- for each and every dive over that consecutive day itinerary. . .

Lastly & btw, here's another reason why you may want to quickly and efficiently complete a deco schedule and get out of the water ASAP: