DCS & free-diving

[Soggy]

Nessie once bubbled...
No 4 l
Because there is a difference between the volume of your lungs (which is something about 7 liters) and the volume you can breathe in ( sorry, do not know the English terms).

Yes, very true. Approximately 1/3 of the gas in our lungs cannot be expelled...As a trumpet player who has done a fair amount of study about breathing, I sometimes forget that there is a difference between the actually volume and the usable volume of the lungs.

 

[rmediver2002]

Now we are getting into some pretty theoretical areas as far as getting bent on one breath or not but some facts can be stated as truth:

The partial pressure of the Nitrogen in your lungs will increase on decent, our bodies do not form rigid walled containers, this allows a volume change on decent causing an increase in the density of the air contained within and a corresponding rise in partial pressure of the gasses involved...

The rapid acsent would most likely be the origin of the problem as any gas that is absorbed during the free dive may not have time to off-gas through respiration as it would with a controlled ascent. If the ascent is fast enough the bubbles could come out of solution within the tissues of the body, this could cause symptoms of DCS.

Jeff Lane

 

[Soggy]

rmediver2002 once bubbled...
The partial pressure of the Nitrogen in your lungs will increase on decent, our bodies do not form rigid walled containers, this allows a volume change on decent causing an increase in the density of the air contained within and a corresponding rise in partial pressure of the gasses involved...

True, but the ribs do form a chamber which would prevent a fair amount of squeeze, correct? Otherwise at 500 ft, that freediver would look more like a stick than a person.

 

[rmediver2002]

Using the middle ear as an example:

The majority of the middle ear wall is semi rigid, the area most affected by the volume change is the tempatic membrane --- The same holds true for the lungs, although you are correct about the semi-rigidity of the chest wall the diaphram is by nature very flexible...

The entire container or chamber must be considered in the example as far as amount of volume change in an air space...

Jeff Lane

 

[Soggy]

rmediver2002 once bubbled...
Using the middle ear as an example:

Perhaps I misunderstood, but the relationship between the tympanic membrane/middle ear is different than the lungs and ribs. The ribs form a mildly flexible, but mostly rigid structure around the outside of the lungs. It seems to me that the ribs would have to be compressed substantially to have much affect on the volume of the lung inside. For that much change to occur, our ribs would have broken many times over.

 

[rmediver2002]

The point of the last example was that the ribs are not the only feature surrounding the respiratory system, the diaphram is the lower boundry of the lungs.

The diaphram as well as the ribs will reach a point where they can no longer off-set the compression taking place on the thoractic cavity.

A decent is continued the volume change does not stop, it continues until a level where something (in the case of the lungs blood) either blood or tissue will have to fill the void.

I would agree with your opinion in one respect that if our bodies were not able to replace this pressure differential with blood or tissue then something would have to give. Either a tear in the diaphram or cracking of the ribs as you mentioned...

An additional item to consider is the upper respiratory tracts role in this as the mouth, tracea, and bronchi will all be undergoing the same decrease in volume but offering little in the way of flexibility to off-set the effects...

 

[Soggy]

Thanks for the clarification...that makes much more sense. I was (incorrectly) thinking that since the abdomen is basically liquid, it would not compress...but since the lungs are filled with gas, the abdomen will push against the lungs, which will compress, thus the partial pressure will increase more or less linearly with depth.

 

[pescador775]

Brown, I analyzed this subject of lung content, etc about a year ago during discussion with some Israeli college student. There is more than enough nitrogen in the lungs to cause bends if the diver could stay down and deep long enough. Normally, the body carries about one liter of disolved nitrogen. Doubling this amount would constitute saturation at 33 feet, the no deco limit. The diver's lungs may contain up to four liters of nitrogen, more than enough to cause trouble.

I have seen bends develop after a tank dive which was preceded by several hours of deep free diving. Never the opposite. The freediver who contemplates a follow up SCUBA dive must compute repetitive dive status. Freediving after SCUBA is safe as far as my experience. Sawtooth and rectified diffusion do not apply to freedivers, or if it does, the effect would likely be quite small to vanishing. IMO, the frequency of pressure changes is just too slow to create the conditions for this phenomenon and that is the mathematical model for freediving deco hazards.

 

[Soggy]

But could a single dive possibly cause DCS? What kind of depth would we be talking about. I know there is no line where DCS will occur, but we have ideas.

I just read the Rodale's article about a woman who freedived to 525 ft, breaking the current world record. I think she's crazy...but it's cool.

 

[pescador775]

My understanding is that woman drowned. Memory fuzzy, maybe someone can confirm.

You need to separate your question into its components, actual and theoretical. Theoretically, there is enough nitrogen in the lungs to cause bends. Actually, this cannot happen on a single breath dive because the diver will run out of oxygen before he gets bent.

Reported cases of bends while freediving are documented but rare. Cumulative N2 from repetitive dives.

The subject of scuba diving, either ante or post free diving is controversial. Mostly, the argument is theoretical. However, in my 45 years of diving, I have witnessed two cases of bends that could only be explained in light of the fact that the divers proceeded on scuba dives directly after freediving to 50-80 feet for several hours. I have never seen or heard of the opposite case.