Scuba Nitrox Incident for Analysis

[OTF]

Exactly! Where did you find that picture?
And what is its English name?
Or is this another of those Italian-only diving techniques which never landed in other countries?

We call it a "Duck dive" although there may be other words for it. Standard practice in freedving.

 

[tursiops]

PFOs can spontaneously occur.

You've said this twice now, but have no source other than an N=1 anecdote that is not compelling.

 

[Angelo Farina]

We call it a "Duck dive" although there may be other words for it. Standard practice in freediving.

I did always think that "duck diving" means "capovolta in raccolta", which differs as you bend your knees and then you extend them suddenly providing additional push...
But this is done only freediving, it is cumbersome with full scuba equipment...

 

[tursiops]

We call it a "Duck dive" although there may be other words for it. Standard practice in freedving.

I've always heard this described as a jackknife surface dive.

 

[Angelo Farina]

I've always heard this described as a jackknife surface dive.

Ah, great, this is it the proper wording. "jackknife surface dive"
Found the description here:
Military Swimming Training from 1944 | The Art of Manliness

And here the picture taken from above:

In the freedive version (duck dive) there is no "arm opposition" movement, the freediver is often carrying a weapon and a torch in his hands, so he simply extend the arms towards the bottom and follows with torso, and later raising the legs out of water, but to a much lower elevation, hence getting less thrust.
here the duck dive:

 

[boulderjohn]

How can someone who's fit and healthy suddenly succumb to a PFO in a gentle dive where they've made far more aggressive dives in the past?

A PFO means you are somewhat more likely to get DCS on a dive. It does not mean you WILL get DCS on a dive. I am sure there are people with PFOs who have done thousands of dives without any DCS symptoms. Repairing a PFO for diving or continuing to dive with a PFO are controversial issues. Many doctors say the risks associated with a PFO repair are not worth it, and because the PFO only makes the divers marginally more likely to get DCS, the diver would be better off continuing to dive with the PFO. Some even question whether a PFO is enough of a problem to be a real worry.

Nearly all PFOs are congenital. Everyone starts life with a PFO. In the womb that opening has an important function because you are not breathing through the lungs. After birth, it begins to close over, and in most adults, it closes over completely. In others, it remains open to some degree, and for most people, it is not a big deal. It is, however, often associated with some chronic maladies, such as migraine headaches.

Some people will get what is in essence a PFO as a result of catheter surgeries in which a hole is made there so a catheter can pass from one side of the heart to the other in order to perform work on the other side of the heart. The hole it leaves is effectively a PFO. It takes about 6 months for it to heal in most cases, although some larger holes may take longer.

SUMMARY: Everyone starts life with a PFO. In most cases, it heals, but in some cases, it remains throughout life. It probably makes the diver somewhat more susceptible to DCS, but the degree to which this happens is questionable. If a diver who has done hundreds of dives without getting DCS gets bent on a dive and has a test that reveals a PFO, that does not mean the PFO magically appeared just before that dive. That PFO was there all along. It also does not mean the PFO was a factor in the diver getting bent--having it could be just a coincidence.

 

[wnissen]

Did you mean CO and not CO2?

I did not. It's less common, but still possible. CO, as I understand it, will not cause the breathing instinct like CO2 will.

Having listened to @jagfish 's analysis, I agree that he's probably right that they were right on the cusp of narcosis and hypercapnia kicked in due to shallow breathing. Ultimately it was a CO2 issue, but more likely self-inflicted rather than bad gas. Though since there is no systematic studies of SCUBA gas contaminants, we won't really know.

 

[kammel78]

I did not. It's less common, but still possible. CO, as I understand it, will not cause the breathing instinct like CO2 will.

Having listened to @jagfish 's analysis, I agree that he's probably right that they were right on the cusp of narcosis and hypercapnia kicked in due to shallow breathing. Ultimately it was a CO2 issue, but more likely self-inflicted rather than bad gas. Though since there is no systematic studies of SCUBA gas contaminants, we won't really know.

How does one get CO2 contamination?

 

[wnissen]

How does one get CO2 contamination?

E.g.: Compressed Air Systems: Causes of CO2 Contamination - Compressed Air Testing - Trace Analytics

 

[John C. Ratliff]

He did not start with "an oxygen debt" (hypoxia), he started with a lot of CO2 in his blood, causing him a "short breath" (Hypercapnia). It is not the same thing, albeit you seem to confuse the two concepts. But a visual look separate them easily:
- hypoxia your skin is pale, you are weak, and you are not over-breathing
- hypercapnia you skin is red or even blue, and you breath very quickly.
In the video, his face is quite pinky!
Carrying a weight over a slope causes a lot of CO2, but does not burn too much oxygen...
All what happened later just made the CO2 retention to worsen...
But going deep provided him with A LOT of oxygen, which did not improve the situation, of course, as lack of oxygen had never been the problem.

Angelo,

He never went deep. Everything happened either before entry, or on the surface.b. Thanks for the correction on hypoxia/hypercapnia. But I think there is such a thing as “oxygen debt,” based upon my past experience. I’ll look it up to be sure though.

SeaRat