Shallow Water Blackout

[SCUBAMedic]

Sorry about this it must have been covered a hundred times. But I have been unable to explain SWB to someone who has it stuck in their head that it is all about CO2 and that O2 only has a minor part to play.

Simple question "what role does carbon dioxide play in SWB" and how does it interact with ppO2 levels.

I am looking for the deeper medical side of it please.

 

[Walter]

CO2 triggers the breathing reflex. If you purge CO2 prior to a breath hold dive, it takes longer before CO2 reaches a level which will trigger the need to breathe.

While you are underwater, you are using O2. As the CO2 level increases, you start to feel an urge to breathe, so you begin your ascent. As you ascend, the ambient pressure decreases lowering the PO2. If the PO2 drops too low you will black out.

 

[The Iceni]

SCUBAMedic once bubbled...
what role does carbon dioxide play in SWB" and how does it interact with ppO2 levels.

Hi SCUBAMedic,

I can understand you own and your friend's confusion. It is best to think of shallow water blackout as simply unconciousness developing during ascent from any cause and not as a term implying a specific cause.

This goes way back to the war years when the British Navy "frogmen" were starting to use oxygen rebreathers - no bubbles for the enemy to see (limited to 30 feet of course). The scrubbers used were not very efficient, at least at first, and many divers suffered from carbon dioxide poisoning, which was sufficent to cause unconciousnes even in the presence of sufficient oxygen. This become known as shallow water blackout although it is caused by carbon dioxide poisoning.

Since then technology has greatly improved, as you know, and CO2 poisoning is now quite rare. However hypoxia will also cause unconciousness. In (military) divers, who still use oxygen rebreathers, this is a risk particularly during ascent to the shallows when the ppO2 is greatly reduced. This is confusingly also known as "shallow water blackout" but should more properly and specifically be known as dilution hypoxia. (ppO2 << 0.1 bar)

Freedivers can suffer a form of shallow water blackout.

One of the medical websites has a good description. If I can find it I will edit this post and insert a link.

Hope this helps. :doctor:

 

[BillP]

Howdy SCUBAMedic:

Scubadoc has a good article on freediving and shallow water blackout at:

http://www.scuba-doc.com/latenthypoxia.html

Mt. Sinai's website has an article from "Scuba Diving Explained" that discusses both CO2 buildup and shallow water blackout in freedivers at:

http://www.mtsinai.org/pulmonary/books/scuba/gaspress.htm

HTH,

Bill

 

[The Iceni]

Hi Bill,

Those are excellent links!

This is the one I was referring to

http://www.mtsinai.org/pulmonary/books/scuba/gaspress.htm

Unfortunately there remains some confusion.

In Dr Campbell's article at

http://www.scuba-doc.com/Brdprpgases.html

he states

The term shallow-water blackout is sometimes applied, but it is best reserved for its original meaning: unconsciousness from CO2 buildup in rebreathing types of scuba. (Hypoxia is also a potential problem in rebreathing units if O2 is displaced by excess N2).

Dr Martin states

Shallow-water blackout is a sudden unconsciousness from lack of oxygen during a breath-hold dive. (The term was originally applied, in the 1940s, to blackout from CO2 buildup seen with re-breathers; over the years the term's definition has been changed.)

This seems to confirm that "shallow water blackout" can have many causes and is therefore, possibly, a term that is best avoided.

 

[Padipro]

Any thoughts on whether this might have been what killed Audrey Mestre?

Scott

 

[The Iceni]

Padipro once bubbled...
Any thoughts on whether this might have been what killed Audrey Mestre?

Scott

Sadly, scott I fear this is quite likely.

One of the reports reads

An initial autopsy conducted in Santo Domingo found drowning to be the cause of death, but a final report could take weeks to be released, he said.

"We believe something hit the sled,'' Serra said earlier. "When she came out of the water she was foaming from the mouth and bleeding.''

Regardless of whether something hit the sled or not, the finding of blood-stained fluid in the airways is typical of salt water drowning.

At these depths the outcome is either another world record broken or another fatality. I do not think there can be an intermediate outcome.

The free diver takes a fixed, and very limited, amount of oxygen with them when they dive. They consciously resist respiratory drive until either

1. They surface and take a breath of air.
2. They fall unconscious due to lack of oxygen, lose this conscious effort and reflexly take a breath underwater, inhaling salt water.
3. Although still conscious they can no longer withstand carbon dioxide-mediated reflex respiratory drive and inhale salt water.
   [/list=1] As salt water is three times the osmotic pressure of body fluids it produces a kind of chemical burn in lung tissue. The alveolar walls become leaky and serous fluid migrates into the airways.
   
   This effectively prevents gaseous exchange so if 100% oxygen is given immediately on surfacing, even if ventilated by means of an endotracheal airway, little effective oxygenation of the blood can take place.
   
   The brain is already hypoxic. . . . :boom:
   
   I have done many dangerous sports in my time but extreme free-diving pushes the body well beyond what I personally consider to be sensible.:doctor:
   
   I suggest you take a look at Dr Campbell's article under ;
   
   SHALLOW-WATER BLACKOUT (Latent hypoxia)

 

[SCUBAMedic]

Thanks guys, you helped support my stand and gave us a whole lot more. What an excellent resource