new theory about risk in freediving (dcs/airtrapping)

[trux]

Thanks again. Nice to have you on board. Can you tell us which Oceanic / Aeris models exactly do have freediving modes including the DCS calculation in those modes?

 

[jome]

Thanks for the excellent input. Even if the simulator is limited, we have thus far had no guideline whatsoever.

I wonder, would it be totally inappropriate to hope you draft a simple table as a guideline?

For example assuming descent/ascent rate of 60m / min, and bottom time of 10s (typical cwt dive with a little margin) and the common "rule of thumb" of having a recovery time of 2x previous dive time.

List depths in 5 meter increments and on the other axis how many repetitions it would take to be likely to suffer dcs

I think that would cover about 90% of competitive type freediving training. Because such diving is usually very constant and predictable in profile. If you've seen dive proviles of competitive dives, they are for the most part an almost exactly symmetrical "V". Maybe the table could have 2-3 colums for differend speeds below and above 1 m/s.

Such a table would be really valuable for the freediving community

Of course, even more useful would be a similar guideline for repetitive recreational diving, but then you have so many variables, it would basically have to be computed real time to be of any use...(depth, bottom time, ascent/descent rates etc vary greatly with every dive).

So something like:
spd 0.9 m/s 1.0 m/s 1.1 m/s

10m 6 reps 7 reps 8 reps
15m 5 reps 6 reps 7 reps
20m 4 reps 5 reps 6 reps
25m 3 reps 4 reps 5 reps
30m 2 reps 3 reps 4 reps

The table would be read "worst case" type, like in scuba. Meaning if you dove to 30m at 0.9 m/s, the even if the successive dives are shallower, you have a total of 2 dives "allowed" for the session (of course these numbers are completely made up).

Naturally, no liabilities of any kind, but as a completely unofficial "off the record" recommendation, such would be very useful! If nothing else to give people a feel of the ballpark we should be operating in. For making it official there would certainly be commercial and legal aspects to be taken into account.

 

[diveoceanos]

Thanks for the excellent input. Even if the simulator is limited, we have thus far had no guideline whatsoever.

I wonder, would it be totally inappropriate to hope you draft a simple table as a guideline?

For example assuming descent/ascent rate of 60m / min, and bottom time of 10s (typical cwt dive with a little margin) and the common "rule of thumb" of having a recovery time of 2x previous dive time.

List depths in 5 meter increments and on the other axis how many repetitions it would take to be likely to suffer dcs

I think that would cover about 90% of competitive type freediving training. Because such diving is usually very constant and predictable in profile. If you've seen dive proviles of competitive dives, they are for the most part an almost exactly symmetrical "V". Maybe the table could have 2-3 colums for differend speeds below and above 1 m/s.

Such a table would be really valuable for the freediving community

Of course, even more useful would be a similar guideline for repetitive recreational diving, but then you have so many variables, it would basically have to be computed real time to be of any use...(depth, bottom time, ascent/descent rates etc vary greatly with every dive).

So something like:
spd 0.9 m/s 1.0 m/s 1.1 m/s

10m 6 reps 7 reps 8 reps
15m 5 reps 6 reps 7 reps
20m 4 reps 5 reps 6 reps
25m 3 reps 4 reps 5 reps
30m 2 reps 3 reps 4 reps

The table would be read "worst case" type, like in scuba. Meaning if you dove to 30m at 0.9 m/s, the even if the successive dives are shallower, you have a total of 2 dives "allowed" for the session (of course these numbers are completely made up).

Naturally, no liabilities of any kind, but as a completely unofficial "off the record" recommendation, such would be very useful! If nothing else to give people a feel of the ballpark we should be operating in. For making it official there would certainly be commercial and legal aspects to be taken into account.

Having read your post I am attaching this file for information purposes. This work is only an excercise and as such shall not be used in any way as a means to prevent or predict DCS.

View attachment AUTOPLAN free diving.pdf

 

[diveoceanos]

Thanks again. Nice to have you on board. Can you tell us which Oceanic / Aeris models exactly do have freediving modes including the DCS calculation in those modes?

Aeris models with integraded algorithm for free diving Nitrogen calculation are EPIC, MANTA and ELITE T3.

Oceanic models are ATOM-2, GEO, and VT-3 PRO.

 

[jome]

That is so very, very cool!

Thank you

 

[diveoceanos]

Something is better than nothing. This "modo" drives the decision to post these tables in this forum.

This table covers depths of 10 to 40 meters and bottom times between 1 and 5 minutes.

Allways use your judgment and knowledge that you acquired on your proper training.

Allthough, I have put some effort to make the tables as conservative as possible, there are considerable areas of uncertainty, hence do not use this table to plan your dives as they can not guarantee DCS free diving!!!

View attachment AUTOPLAN spearfishing.pdf

Dive safely!

 

[azapa]

Something is better than nothing. This "modo" drives the decision to post these tables in this forum.

This table covers depths of 10 to 40 meters and bottom times between 1 and 5 minutes.

Allways use your judgment and knowledge that you acquired on your proper training.

Allthough, I have put some effort to make the tables as conservative as possible, there are considerable areas of uncertainty, hence do not use this table to plan your dives as they can not guarantee DCS free diving!!!

View attachment 13828

Dive safely!

Thanks so much for this input, and as you request, I fully understand its experimental and no guarantee nature.
Thanks also for considering the spearos.
Here a question: under what basis did you select your recovery time multipliers? Is it arbitrary? Also, i assume you refer to real "lying flat on the bottom" or aspeto, as bottom time, not dive time (as measured by the suuntos etc). I, and every one I know, uses 2x dive time as a surface interval. Would you say that the x bottom time calculation to be safer?
Thanks so much.

 

[diveoceanos]

Thanks so much for this input, and as you request, I fully understand its experimental and no guarantee nature.
Thanks also for considering the spearos.
Here a question: under what basis did you select your recovery time multipliers? Is it arbitrary? Also, i assume you refer to real "lying flat on the bottom" or aspeto, as bottom time, not dive time (as measured by the suuntos etc). I, and every one I know, uses 2x dive time as a surface interval. Would you say that the x bottom time calculation to be safer?
Thanks so much.

The recovery times could have been selected randomly but that would affect the maximum number of allowed repetitions. Therefore I had to choose a simple rule. The rule is the one you described and is apparently being used by the majority of spearos.

Bottom time is the actual time spent at maximum depth. If you add the descent and ascent time you will find the total dive time. Multiply this X2 and you have the recovery time. In the table for spearos the ascent / descent rate is 0,7 m/s. Rounding up to the next minute has also been used. Generally the longer the recovery time is the higher is the safety.

Some thoughts to decrease the possibility of DCS are:

• If you have exceeded for example 1 minute bottom time by some seconds use the next greater time column (i.e. 2 minutes)
• If you have exceeded for example 25m slightly use the row for 30 meters aso.
• You may cosnider all of your dives as dives to the depth of your deepest dive in this day.
• You may also choose to increase the recovery time as you wish to be safer.
• If you feel cold get out of the water and seek warmth. Do not dive again for 24 hrs.
• If you feel thirsty at any given point during the dives, get out of the water and do not dive for the next 24 hrs.
• If you have excerted yourself too much beyond your comfort limits get out of the water and do not dive for the next 24 hrs.
• For very deep dives have oxygen handy (in the boat or car) and use it for 15-20 minutes or more. Demand valves are preferable as they provide higher percentage of oxygen and they can safe you some gas as well.

These simple rules can minimise the risk of DCS.

I didn't mean to repeat what other people wrote allready! Just to emphasize though, remember to apply all safety rules you have learned.

Dive safely!

 

[alexunder]

Hi,
This is a very interesting and comprehensive thread!

Can anybody recommend an official free-diving decompression table ?

Thanks
Sasha

 

[trux]

Hi,
This is a very interesting and comprehensive thread!

Can anybody recommend an official free-diving decompression table ?

Well, what exactly is official? I think AIDA recommends some minimal recovery times between dives as a function of the last dive depth, but I am not sure if it is available online. Some freediving tables are also linked here: decompression table @ APNEA.cz

You will find more about DCS here on DB too - just enter "DCS table" or just "DCS" into the search box. For example the following thread is also quite interesting: http://forums.deeperblue.com/safety/75779-can-freediving-cause-dcs.html

 

[efattah]

Unfortunately I can't agree at all with the tables posted in this thread. For one thing, the tables show a reduced risk o DCS if the descent is done at 1.3m/s and the ascent at 1.3m/s, compared to 0.7m/s or descent/ascent. In my experience of many 'bends', slow descent and slow ascent has a way less chance of DCS compared to fast descent and fast ascent.

 

[alexunder]

I know that AIDA has their tables,
where can i find them ?

 

[diveoceanos]

Unfortunately I can't agree at all with the tables posted in this thread. For one thing, the tables show a reduced risk o DCS if the descent is done at 1.3m/s and the ascent at 1.3m/s, compared to 0.7m/s or descent/ascent. In my experience of many 'bends', slow descent and slow ascent has a way less chance of DCS compared to fast descent and fast ascent.

As you might probably noticed under the published tables (which are a preliminary version of the extended set, i.e. there is a set of tables), there is a note about your observation! There are indeed serious drawbacks as for the validity of these tables for several reasons, not only for the reason you are providing in your above post. As the author of these tables I just wanted to make that clear to everyone. The tables are based on Buhlmann M-Values and ΔΜs for a 16 compartment model and they are mathematical models. Nothing more and nothing less. Below there is a text which was written in the original paper that presents the tables. Please read for more clarifications:


"This same algorithm although being already in use with relative safety for the planning of technical dives with open circuit SCUBA, its application in freediving remains to be proved with actual dive data. There are no human studies known in this field and thus any recommendation remains to be confirmed for validity in BHD.

In SCUBA diving it is widely accepted that nitrogen bubbles do form in virtually every dive. When those bubbles are formed their diameter is very small and do not exceed the critical size, therefore their presence is not associated with any clinical complications. The lung it self acts as a filter to those so called “micro-bubbles” or “silent bubbles”, in a way that the blood is being cleaned before it reaches the left atrium. Practically, bubbles with pathological size are never reaching systematic circulation on a healthy individual who dives within the no-decompression limits. In BHD the ascent rate is rather fast and definitely quite faster than the ascent rate in SCUBA diving. In SCUBA diving the abrupt changes in depth are absolutely contra-indicated and discouraged. They have also been linked to higher incidence of DCS. However the rapid depth change is a part of BHD definition. This might lead to doubts regarding the ability of the lung to act as a filtering apparatus. Perfusion rate of the lungs is limited and occasionally when the ascent rate is quite fast some bubbles might expand in size to exceed critical limits before they reach the pulmonary circulation. In BHD the perfusion of the lungs to ascent rate ratio is significantly lower than the one in SCUBA diving. The consequences of this fact with regards of the ability to deal with micro-bubbles are largely unknown in BHD and remain to be investigated.

In modern approaches of decompression theory deep safety stops are discussed. Some studies did show a beneficial outcome of the deep safety stops as it has been measured with Doppler bubble detection at the end of the dive. A lot of SCUBA divers are now considering the introduction of deep stops as an extra safety factor and they employ various approaches that all lead to the addition of deep stops. These are RGBM, VPM, Gradient Factors concepts introduced in Haldanean models. In BHD deep stops are not an option because the diver has minimum availability of time. Safety stops are normally applied by elite divers on record breaking attempts but they are largely very short with a duration not exceeding one minute.

Doppler technology for the detection of bubbles experience had shown that not all measurements can be explained with the classic mathematical models of decompression theory. Therefore one could easily suspect that this theory has its own limitations. Therefore, the suggested algorithm can not be treated as an exact science! This is one extra reason for more conservatism.

Carbon Dioxide built-up during SCUBA dives have been proved to make the diver more prone to DCS. By definition carbon dioxide is accumulated in the body fluids of a freediver during a BHD. It is not known if the diaphragmatic and expiratory muscle contractions that happen quite often during the struggle phase of a BHD do cause any right to left shunt and thus increase the likelihood for DCS in individuals with patent foramen ovale (PFO). This mechanism has been discussed in analogy in SCUBA diving, where certain conditions like coughing or sneezing, increase the intra-thoracic pressure and could possibly cause right to left shunt in individuals with PFO.

For the above reasons we decided to introduce two conservative factors in an attempt to increase the safety of our proposed method by in no way we are claiming that this method will guarantee DCS-free diving. These conservatism factors are:

1. The surface M values (M0) have been calculated as the surface would be at an altitude of 300 meters.

2. The surface M values have been reduced by a further 20%. The final M values are around 78% of the M values used in classic decompression theory for SCUBA diving."

 

[smithie]

Its pretty sad to read about animals being needlessly pointlessly tortured to death to verify these assumptions to support human risk taking...just a thought....

if you are convinced that you've suffered from dcs (I am not and rather tend to go for the air embolism), you should read the following two articles (if you haven't already done so):

Aerobic endurance training reduces bubble formation and increases survival in rats exposed to hyperbaric pressure -- Wislff and Brubakk 537 (2): 607 -- The Journal of Physiology Online

Aerobic exercise before diving reduces venous gas bubble formation in humans -- Dujic et al. 555 (3): 637 -- The Journal of Physiology Online

to give a quick summary:
gas bubbles which are the cause of dcs can only form from so called gas nuclei which are gas-filled bubble precursors (~1µm) attached to blood vessel endothelium. the amazing thing is that the number of these nucleation sites may be influenced by exercise. In the first article above the most noticeable protective effect of exercise against dcs was seen after a single bout of exercise 20h before decompression (in rats).
Same thing in humans after a single bout of exercise 24h before decompression (second article).

so, maybe it is worth a try next time you go free diving. you exercise as described in the second article 24 hour before going down. if you still get dcs symptoms, well that might indicate that dcs may not be the culprit after all and that one has to focus more on the air embolism due to lung squeeze theory, which btw. I find very intriguing. I mean if I hear these blood coughing stories it seems plausible to me that alveolar gas may enter the blood stream even through the smallest ruptures of the alveolar-capillary membrane especially in a hyperbaric condition. Although most of the blood coughing originates from upper respiratory tract vessels I wonder if this may not be an indicator for a possible rupture of the much more fragile alveolar-capillary membrane (0.6 µm). Also, it would be very interesting to see if there is a correlation between blood coughing and the occurrence of dcs symptoms which would also argue for micro air embolisms due to lung squeeze.

Considering frc-diving, lung squeeze should occur much earlier then in “normal” free diving because sub- residual volume volumes are reached much earlier. However, packing might cause micro ruptures even before going down which, in combination with pressure equalization techniques (valsalva), should worsen the risk for air embolism even more.

bube[/quote]

 

[trux]

Thanks for your input, bube, and welcome to DeeperBlue.

...
Considering frc-diving, lung squeeze should occur much earlier then in “normal” free diving because sub- residual volume volumes are reached much earlier. However, packing might cause micro ruptures even before going down which, in combination with pressure equalization techniques (valsalva), should worsen the risk for air embolism even more.

The theory discussed earlier in this thread, actually suspects "squeeze" or better told air embolism on the ascent, just before surfacing. When you pack your lungs before the dive, the air inside is pressured above the ambient pressure. As soon as you start descending both, inner and outer pressure are more or less equal. At big depth, blood shift serves for reducing the lung volume, hence keeping the inner lung pressure in equilibrium with the outside pressure. Now when you ascent, the air in lungs start expanding. Although some oxygen gets consumed, the air volume reduction is smaller than the lung volume reduction due to the blood shift. In consequence, just below the surface, the inner lung volume reaches its maximum that is even higher than it was just after the packing at the beginning of the dive (unless you exhale little bit in the last 2-3 meters).

However, there are certainly different cases. Some people may get "squeezed" already at immersion, when they do their duck-dive and compress so their diaphragm increasing the already critical lung pressure above safe limit. Others get regularly squeezed in depth due to the negative pressures, and again in other cases the squeeze is caused by contractions in depth (voluntary ones at Valsalva, or involuntary hypercapnic contractions). And, of course, air trapping can happen already at plain dry packing, so the barotrauma may happen at some individuals already before the immersion. All these cases were studies, confirmed and described in medical literature. And usually it is quite difficult to diagnose the real reason afterward.

Now, DCS is a different story, but although you are certainly right that some cases of DCS might have been mistaken air-trapping incidents, at many others it is not the case. DCS, though, is more deeply discussed in other threads.