Free Diving and DCS

[The Iceni]

Frank O once bubbled...
. . . This fellow apparently was going up and down and up and down, to depths quite a bit over 100 ft -- but was only freediving, with no scuba involved. I wouldn't be surprised if, all told, his bottom time exceeded his surface recoveries. If memory serves correctly, the outcome of the incident was that he was in a wheelchair for several months. So, although rare, it does happen!

Hi Frank,

Yes indeed. I suppose I should never use the word never!

Another concept that may have played a part in such cases is asymmetry (or applied RGBM?).

If it takes longer to offgas a particular tissue than it does to ongas, repetative exposures will crank up the net amount of gas dissolved is those tissues and increase the size and number of bubbles due to micronuclei formation, which will in turn will increase asymmetrical gas transfer.

Your link made interesting reading, Pipedome!

 

[Ash]

Hi Dr. Thomas

I have recently been involved in a discussion about freediving and DCS in another forum.

One of the posters there mentions a dive doctor in South Africa who treated a spearfisherman for a case of DCS. This doctor made the following recommendation:

"An objective guideline would be to ensure that the total dive time does not exceed the no-decompression limits for compressed-air diving, even though it is known that repeated ascents while diving with compressed air increases the risk of developing the bends. Adequate hydration - the intake of 250 to 300 milliliters of fluid every hour - should be mandatory. Overall better record keeping of dive profiles by surface support personnel will lead to a better understanding of the problem and its scope"

I found this concept quite intriguing because it's fairly easy for a good freediver or spearo to exceed the air NDL in the course of a dive day if you add up their total time underwater.

In fact, there is a diver being discussed on that thread who does 20 breath hold dives of at least 2 minutes total time to 38-40m in a day. If you work out his theoretical dive profile, it is well outside the NDL.

He wants to increase that amount to 25-30 dives a day over 2-3 days.

I suppose the real question comes down to surface intervals. How long would his surface intervals need to be in order to keep him from getting DCS?

You said that as long as the surface intervals are longer than the dive time he would be unlikely to develop DCS over a multiple day period?

I know there is no hard data on this type of situation but I was curious as to your thoughts in this type of extreme case.

Thanks and best regards

Ash

 

[Dr Deco]

Hi Ash:

Free Diving

This is a question that we see frequently. There are at least two difficult questions here. These are nuclei formation and gas exchange.

Nuclei Formation

Free divers are an active bounce underwater if they are going to a deeper depth. This involves hard finning to reach the depth. This will possibly create micronuclei from the musculoskeletal activity as will a swimming ascent. This activity could be more than is accounted for in normal table development, and it is dependent on the skill of the diver.

Gas Exchange

The greatest uptake (from the lungs in a single breath) will occur with the diving being as active as possible while at depth. This is probably true with some freedivers. Since they will most likely rest while on the surface, the diver would go into a quite mode while on the boat. This will cause the circulatory system to shift to very low blood flows. Clearly this is bad for nitrogen elimination. To account for the rapid nitrogen uptake and slow elimination, I might suggest that at least five times the surface interval be maintained in comparison to the bottom time for any single dive.

One should recognize that none of this has been rigorously tested (controlled study) to my knowledge.

Dr Deco :doctor:

 

[Karl_in_Calif]

Dr Paul Thomas once bubbled...
I suggest that, as a rule of thumb, it would appear to be safe to free dive before scuba but post dive free-diving ought to be avoided at least until it is safe to fly.

This is the current generally accepted view.

When you surface from scuba diving, the nitrogen dissolved in your blood, nerves, and fat will be off-gassing for up to 120 hours, to return to pressure equilibrium at the surface. The blood offgasses quickly, but the nerves and fat will continue to seep nitrogen into the blood for days. However the first 20 hours seem to be the most critical time, during wich equilibrium with the surface pressure is being re-established.

The most dangerous thing you could do after a scuba dive is a subsequent reverse profile deep bounce dive. This upsets the off-gassing mechanism that is taking place in your venous blood supply, and possibly recompresses the gasses in the highly saturated venous blood system such that it by-passes the lungs and goes on into the arterial blood supply. Upon re-surfacing after the bounce dive, saturated gas in the arterial blood supply could form a blockage and cause a stroke.

Many of us know of divers who have suffered strokes after reverse profile bounce dives. Often that happens when a diver goes back down to free an anchor, and then ascends too fast without a safety stop.

Freediving after scuba diving entails the same hazards of reverse profile bounce dives on scuba. And with freediving, you do not have time for a safety stop of 3 to 5 minutes.

Although I do not know of anyone who has suffered a stroke from freediving after scuba, the problem seems like it could feasibly occur. Who wants to run the risk of finding out for themselves? Your talking about a wheelchair and adult diapers for the rest of your life.

 

[pescador775]

I need to remind some of you folks, especially the theoreticians, that I have actually HAD the bends. The incident occurred in the Bahamas. I was treated at Miami Mercy during Oct 31-Nov 6, 1999. There was paralysis and partial disability which took some time to get over. The bends resulted from a SCUBA dive to 100 feet. It was a no decompression dive to Navy tables. Total air consumption was 72 cu ft. Slow ascent but no 'safety' stop. I've made 100's of similar dives with no apparent problem. However, this SCUBA dive followed 4 hours of free diving to depths of 50-60 feet, also some shallower dives to 25 feet. Entering the SCUBA dive I was dehydrated, chilled and a bit tired. Later, I estimated that my initial group on entering the water was "C" which I neglected to take into account and which naturally did not show up on my backup computer. I have had absolutely no problem with doing shallow free dives (30 feet) after SCUBA diving. I rarely do the opposite, SCUBA dive after free diving but the particular opportunity seemed too good to pass up. I should have taken into account the free diving when estimating my final decompression status.

 

[pescador775]

In case somebody doesn't get it, Doc Deco has stated on numerous occasions that dehydration is a secondary risk factor as are fatigue and cold. These risk factors constitute a crucial tipping point for marginal SCUBA dives which follow free diving. These secondary risk factors directly arise from free diving and contribute to risk for any subsequent SCUBA dives. Deep breathing by the free diver expels humidity; fatigue and cold cause urination which further reduces hydration. Cold as a risk factor...., well, hopefully you get it. Everyone is calculating nitrogen effects based on theories originally developed for analysis of volcanos and such. Admittedly, this concept does have merit at high frequency exposures such as whales blasted by high power sonar but in my opinion does not apply to most human diving situations and has nothing to do with free diving at moderate depths. To understand the relationship between free diving and SCUBA and bends it is necessary to look at the big picture. Forget the calculus and look at the secondary risk factors accepted by virtually every diving authority. These factors lead to the conclusion that nitrogen absorption and secondary risk factors such as dehydration must be taken into account when a SCUBA dive is planned following on a significant period of moderate to deep free diving.

 

[The Iceni]

Ash once bubbled...
. . . I found this concept quite intriguing because it's fairly easy for a good freediver or spearo to exceed the air NDL in the course of a dive day if you add up their total time underwater.

In fact, there is a diver being discussed on that thread who does 20 breath hold dives of at least 2 minutes total time to 38-40m in a day. If you work out his theoretical dive profile, it is well outside the NDL.

He wants to increase that amount to 25-30 dives a day over 2-3 days.

I suppose the real question comes down to surface intervals. How long would his surface intervals need to be in order to keep him from getting DCS?

Hi Ash,

Good Lord, that is taking it to the extreme and I doubt any diving doctor or biophysicist would dare attempt an answer. No one really knows.

By the way I read in my newspaper of a report from Nature Magazine where post mortem examinations on 14 whales stranded during a naval exercise off the Canaries last year found that 10 of them had “gas bubbles in their blood, and holes in their internal organs”. The exercise involved the use of mid frequency sonar by the ships and was terminated when the beached beaked whales were discovered.

Dr Paul Jepson, the leader of a joint project between the Zoological Society of London and The University of Las Palmas is reported to have stated, “This challenges the widely held notion that whales and dolphins cannot suffer from decompression sickness.”

I believe the words “from natural causes” should have been added to the end of this last sentence as this investigation very strongly suggests that sonar was the cause of the onset of decompression illness (or other pathology?) in these animals.

One possibility is that the animals were startled by the unusual noise and so ascended too rapidly but I tend to believe the sonar was powerful enough to agitate the tissue fluid and cause the formation of a large number of those little nasties, micronuclei. If their hypothgesis is true - and DCI the cause of the problem - clearly certain tissues, in these cetaceans at least, contain inert gas of sufficient quantity in the liquid phase for any generated micronuclei to grow and cause DCI.

Obviously these researchers appear to be calling for tighter controls on the use of sonar by submarines which might have biased their perspective. However, I most certainly would not want to be diving anywhere near one!

The sort of diving described by Ash will obviously increase fatigue, dehydration and inert gas loading, particularly if gas transfer is assymetrical, as Dr Deco highlights, regardless of the ratio of bottom time to surface interval.

I was not aware that any snorkellers (free divers) were so aggressive.

 

[Karl_in_Calif]

Since most off-gassing tends to occur within 20 hours, if your surface interval after your last scuba dive is 20 hours, then it would seem that there is very little risk to subsequent freediving.

Total offgassing requires several days, however. Although this should not affect freediving on alternative days.

 

[pescador775]

One possibility is that the animals were startled by the unusual noise and so
ascended too rapidly but I tend to believe the sonar was powerful enough to agitate
the tissue fluid and cause the formation of a large number of those little nasties,
micronuclei. If their hypothgesis is true - and DCI the cause of the problem - clearly

Current theory has it that micronuclei are a preexisting or latent condition, not the result of high frequency waves or pulses. Rather, the high frequency pulses result in rarifaction of fluids, the minute bubbles of which attach and grow around the preexisting micronuclei.