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FACT FICTION AND VOODOO - THE ART OF DECOMPRESSION DIVING


The scope of this article is to discuss the practical elements of mixed gas decompressions without delving too deeply into the science, partly so as not to generate confusion and partly because the science is still dynamic. For now at least, the philosophy of 'what works, works', is still relevant.

Over the years the art and science of decompression diving has evolved almost from folklore, through early experiments by pioneers such as Haldane, to eventually being taken up as virtual field trials by the mass recreational market. In more recent years sport divers have had access to 'exotic' gas mixtures such as Nitrox and Trimix which, especially in the case of Trimix, have been used predominantly by divers simply extrapolating Algorithms such as that developed by Professor Buhlmann for the deeper depths.

Research has come up with a variety of decompression 'models' designed to predict the decompression profile for a range of gases on any particular dive profile. The models themselves follow a range of theories, each trying to explain why divers suffer from DCI and how to avoid it. Although a lot is understood about the mechanics of decompression, there is still much we do not know. The bottom line is bubbles. As we descend under pressure our bodies absorb gas. Looking at one of the classic solution models, it is assumed our bodies are divided into a series of 'compartments' which absorb and release this gas at certain rates. If we stay submerged long enough the partial pressure of gas in any given compartment will eventually equal the partial pressure of the same gas in the mix we breathe. This is known as saturation.

As we ascend gas expands within the theoretical compartment. How far we can come up is defined by the amount of 'over pressure' any one compartment can withstand before gas (in theory) comes out of solution, forms bubbles and potentially causes injury. This phenomenon of over pressure is also known as 'supersaturation'. For instance, while a compartment at a certain depth may saturate with a partial pressure of 1 bar, it may be able to withstand 1.5 bar supersaturation (a reduction in depth pressure that causes the gas within the compartment to expand to create a tissue tension of 1.5 bar) before theoretical injury. A rise past this decompression ceiling (supersaturation over pressure) may cause gas to come out of solution and bubbles to occur. The length of time we spend at a decompression stop is controlled by how long it takes the compartment to 'off-gas' to a new safe level where a standard change in depth (often 3m) does not cause excessive over pressurisation.

Some computers allow for a sliding decompression ceiling rather than fixed 3m increments allowing the user to ascend in a smoother manner also known as 'flying the curve'. The end result is the same. In theory (following the mentioned model), if we conduct the decompression within over pressurisation limits our decompression will be 'safe'. Safe doesn't necessarily mean we are not getting bent, it just means we don't manifest sufficient symptoms to warrant treatment. A good indicator of decompression safety is how awake we feel after a dive. Post dive tiredness is a sign of sub-clinical (not presenting classic DCI symptoms) DCI.

However, the control of bubbles using saturation models theories is apparently not enough. There is evidence to suggest that our bodies naturally generate bubbles as a result of cavitation at the heart valves (1). The bubbles take the form of micronuclei which absorb gas during the dive. During an ascent, even though excessive overpressurisation (and bubble formation as a result of that) does not occur, the now expanded nuclei may form bubbles themselves. Bubbles of a certain size are the ideal way of removing gas from the body providing they become trapped within the lung's alveoli. If they are too small however (micro bubbles), they may pass through the lung and end up back on the arterial side of the circulatory system. Continuing ascents will then cause further expansion and excessive bubble formation in the CNS (Central nervous System) and other vital areas resulting in DCI.

So how do we control these micro bubbles and exactly what is the optimum way to decompress from a deep, possibly mixed gas dive? It would appear that a combination of both models is what's needed.

From a micro bubble standpoint it has been noted (2) that conducting short decompression stops below the predicted first 'real' decompression ceiling can reduce the symptoms of post dive fatigue, the assumption being that micro bubble growth is being controlled. Several physiologists have tried to predict these deep-water stops and their duration (3). To date there remains no commercially available algorithm which has been clinically trialled although many groups of extreme divers have their own empirical systems which 'seem' to work. The affect of these deep stops is to reduce the ascent rate significantly. It is worth noting at this stage that some classic solution models give bottom times which assume an instantaneous ascent could take place to the first decompression ceiling. Current thinking would seem to indicate that any form of rapid ascent is a bad thing and rates of 10m/min or less are more suitable even when combined with deep stops.

Taking the solution model first, what are the established rules for decompression from say a deep Trimix dive?
Helium is a 'faster' gas (absorbs quicker) than the other main component of Trimix - nitrogen. Most Trimix dives use large fractions of helium in the mix to combat narcosis. High helium contents have also been proven to reduce the probability of CO2 retention (4) and its associated problems (oxygen toxicity, pH balance changes in the blood etc). Due to its absorption rates, helium produces more decompression in short bottom times (less than 2 hours) than nitrogen. Hence when looking at any compartment load at the end of a Trimix dive bottom time, there will be more helium absorbed than nitrogen. As a result the first part of the decompression is designed to remove the helium. Also being noted as a 'friendlier' gas from a DCI stress standpoint makes the use of large fractions of helium a much preferred option over Heliair (often mixed as low helium fraction/high narcosis) and deep air diving.

Hence a deep water Trimix decompression is done in two ways. Firstly if during the decompression the breathing gas can be substituted for a mixture weak (or non-existent) in helium then suitable off gassing of helium will occur. The trade off is that if the helium-less gas is breathed at depth (normally air or Nitrox), then nitrogen may again be absorbed in some compartments. Dependant on the switch depth and profile chosen (narcosis aside) this may or may not lengthen the overall decompression. This is however of secondary concern when we look at the next problem providing a depth limit is established for this type of gas switch. Secondly the breathing mixture should maintain a suitably high partial pressure of oxygen. The normal range of PO2s should be kept within 1.6 to 1.0 bar.

Authors note: Down to 0.75 bar is seen to be acceptable in practice for extreme exposure dives in order to reduce gas handling logistics (carrying/mixing multiple gases). However it is advisable to build in additional safety factors when the PO2 is taken this low.

In other words, taking air as an example, air has a PO2 of 1.0 at approximately 37m, so if it were employed as a deep decompression gas, the switch to another oxygen rich gas should be made at around 37m. Keeping the PO2s high again elevates the off gassing gradient during the predominantly nitrogen decompression portion of the dive. Oxygen is the gas that supports life and is at the centre of a healthy metabolism. High PO2 levels help produce an efficient decompression profile providing they are balanced against oxygen toxicity.

In extreme deep diving an intermediate trimix may be employed to a number of advantages. Firstly it keeps the PO2 within the aforementioned acceptable levels. Secondly it keeps nitrogen narcosis at bay and thirdly it provides ever reducing levels of helium (and increased levels of nitrogen) which are needed to remove the high levels of helium saturated during the dive. The main question is where should this intermediate Trimix concept be used? There appear to be several theories.

Some groups advocate using Bottom mix Trimix up to the shallow stops (21m) and then switching to high levels of oxygen to complete the decompression, avoiding on gassing of nitrogen if lower FO2 (high FN2) gases were used at deeper stops. While in practice this seems to combat a problem of on gassing too much nitrogen it does expose the diver to potentially high Helium compartment over pressures, which themselves may cause symptoms of DCI. The question is by using Trimix in this fashion followed by high doses of oxygen in shallow water are we treating rather than preventing DCI?

To explain, as previously discussed, a good decompression is probably a balance of two theories. That of solution models and that of bubble mechanics. The gas switch concept (Trimix to 21m option) mentioned in the previous paragraph may actually generate decompression complications in deep water (even if micro bubble stops are employed). The resultant high FO2/shallow water deco section being used to treat the problem. Is there another way to do it with potentially less stress on the diver? Possibly. Taking on board the concepts of adequate PO2s, the inert gas switch and bubble mechanics, the following is offered.

Assuming the body has generated micro bubble nuclei, these absorb gas during the dive. To control their expansion deep water stops are employed. With the lack of an algorithm the accepted method of halving the depth between the bottom and the first stop and then completing a one to two minute stop seems applicable. This process is repeated if the depth between that micro bubble stop and the first real stop is greater than 10m. That gets the micro bubbles under control at this phase of the dive.

Now to Oxygen. If the depth where the PO2 in our Trimix equals 1 bar is not too deep from a nitrogen narcosis standpoint, then air or a Nitrox (if toxicity allows) is suitable. The subsequent increased level of nitrogen in the new breathing mix is of secondary importance (providing it is used no deeper than around 51m) when compared to the oxygen level remaining at 1 bar or greater. If the depth is too great from a narcosis standpoint then an intermediate Trimix high in oxygen (1.6 max) and lower in Helium can be employed. One note worth mentioning when using high PN2 mixes is solubility. Nitrogen is more soluble than helium and any preformed bubbles will quickly absorb nitrogen. This is a good reason for using intermediate trimixes (creating small shifts in PN2) coupled with micro bubble stops.

With the new gas, which is low in helium, the off gassing gradient is now established. While some compartments may 'on-load' nitrogen again, for the short duration of these deeper stops, this is of minor concern. The next phase combines a range of gas switches with ever increasing FO2s to maintain the oxygen levels as discussed (1.6 to approx 1.0) and control oxygen toxicity (more later). The increasing FO2s hence reduce the FN2s and continue a suitable off gassing profile.

So is that the end of it? Probably not! Assuming our deep stops limited bubble growth and our subsequent gas switch profile avoided gas coming out of solution, could micro bubbles again become a problem in shallow water? Probably! The critical depth range would appear to start at 21m (which is coincidentally the depth where the proponents of running intermediate trimixes switch to high FO2 Nitrox). In this area and up to 6m, practical experience (5) has shown a short extension of the decompression (10-20%) within this zone is beneficial, although this may to some extent be negated by good bubble controlling stops.

It is worth also mentioning that gas switches need time to physiologically take place, hence a short time extension (1-2 minutes) should be added at each gas switch point. This added to the final 'trick' of doing decompression at a minimum of 4m-6m helps reduce supersaturation and bubble growth problems in shallow water and keeps the overall oxygen load under control.
A final note on Bubbles:
In the air range it is an established practice to accept that multiple days of diving generate a residual load from an inert gas standpoint. With high helium Trimix diving this is not always the case as the highly saturated helium is often sufficiently dissipated prior to the dive's end, leaving only a small nitrogen (and rapidly reducing helium) residual on surfacing. So from a saturation model standpoint, it is often applicable to assume no saturation pre-load on subsequent days of diving, especially if the overall dive trip is relatively short. This is certainly applicable in the 'recreational' Trimix range (50-80m).Longer duration projects may need breaks in the diving to deal with inert gas pre-load.

What about the bubbles?
While our body naturally generates micronuclei, it appears that regular compressions actually break them down and providing they are not given time to re-form, the noted quantity on subsequent dives is found to reduce. Are multiple days of gas diving actually beneficial from a bubble reduction standpoint?
Problems with Oxygen
Oxygen would appear to be the answer to all our problems, the more the better. Hmmm... perhaps not. Oxygen is toxic, narcotic, can cause temporary bends and is a vasoconstrictor. What does this mean to us as divers? Oxygen is potentially twice as narcotic as Nitrogen. The upside being that at the partial pressures of oxygen within which we normally operate (0.21 to 1.6) oxygen narcosis is not a relevant issue. Nitrogen, which is breathed at far higher partial pressures, is the overiding concern. The vasoconstricting effects of oxygen are well documented.There is some evidence that the resultant reduction in perfusion (blood flow) may reduce off gassing by as much as 20%, this however only manifests as a problem at PO2s in excess of 2.0.
Finally how do we control the onset of an oxygen convulsion? Estimated times of breathing oxygen at elevated partial pressures have been documented for some time (6). The Percentage CNS method of calculating a CNS oxygen load has been used as a 'guide' for recreational divers for some time and is suitable for 'recreational' mixed gas diving. Longer, deeper exposures however need a different approach to oxygen control. Several tips and tricks apply.

  • As with all diving related problems, stay well hydrated.
  • The established chamber treatment method of taking an 'air break' for 5 minutes in every 25 breathing oxygen helps reduce the onset of symptoms. This is often applied when the theoretical CNS exposure exceeds 150% or in some cases more or simply when at the oxygen stop.
  • The majority of the elevated FO2 decompression should be conducted at a PO2 of no greater than 1.4 bar.
  • Remain at rest with limited exercise during the high FO2 decompression. This reduces CO2 levels, the resultant vasodilatation in itself being a possible precursor to oxygen convulsions due to increased blood flow and delivery of oxygen.
    Guidelines Summary
  • Use lots of helium (30 to 40m equivalent narcosis depths) in bottom mix.
  • Get off helium during decompression as soon as narcosis and oxygen toxicity allows.
  • Employ 'back switching' techniques to minimise narcotic shock at deep water switches.
  • Maintain oxygen levels during decompression (1.6 to absolute minimum 0.75 PO2).
  • Maximise bottom mix PO2s to 1.4 on extended exposure dives.
  • Keep CO2 levels down at all phases of the dive.
  • Do not use air past 51m as a decompression gas.
  • Employ micro bubble controlling stops.
  • Gas choice is always a compromise between handling logistics, deco time, gas management, oxygen toxicity, safety and narcosis. Either carry lots of gases or be prepared to hang around.

(1) Henessey (2) Pyle and others (3) Imbert (4) EDU (5) Britannic 97 and others (6) NOAA Oxygen Limits

Below: Typical schedules for a 120m and 70m dive. PO2 shifts detailed. Stop times are for reference only.

Depth
Time
B. mix
  
120m
25
11/60
  
Stop Depth
Time
Gas
PO2
4.5
97
100
1.45
6
15
100
1.6
9
31
40
0.76
12
19
40
0.88
15
13
40
1
18
9
40
1.12
21
7
40
1.24
24
5
40
1.36
27
4
40
1.48
30
4
40
1.6
33
3
21
0.903
36
2
21
0.966
39
2
21
1.029
42
2
21
1.092
45
1
21
1.155
48
1
21
1.218
51
1
21
1.281
54
2
11/60
0.704
Note marginal PO2 at this depth. Intermediate trimix could be used or air taken one stop deeper
57
1
11/60
0.737
60
1
11/60
0.77
67.5
1
11/60
0.8525
75
1
11/60
0.935
90
1
11/60
1.1
Depth
Time
B. mix
  
70m
25
18/40
  
Stop Depth
Time
Gas
PO2
4.5
97
100
1.45
6
15
100
1.6
9
31
40
0.76
12
19
40
0.88
15
13
40
1
18
9
40
1.12
21
7
40
1.24
24
5
40
1.36
27
4
40
1.48
34.5
1
18/40
0.801
42
1
18/40
0.936

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 



By Kevin Gurr.