Anoxic Asphyxia?

[Akimbo]

Commercial and military saturation divers are well aware of the dangers of exposure to anoxic or extreme hypoxic gas mixtures.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1293382/pdf/jrsocmed00121-0051.pdf

The weird part that I have never found an explanation for is why do people black out almost instantly, often before completing a full breath? I also don't know what the Oxygen threshold (or range) is for "instant light-out" instead of the typical less dramatic evolution of hypoxia. Does the same thing happen on a 10% mix?

Surely oxygen levels in the blood could not be reduced that fast. It is hard to conceive of how this bit of evolution could occur given the rarity that ancient humans would be exposed to this condition.

Background:
This type of accident usually results from divers testing BIBS (emergency/treatment) masks or hats on the surface before the deep mix has been completely purged from the supply lines. "Deep mix" is in the 1% oxygen range at 1000' and 2% at 600'. Six percent is the highest I have used in sat. The diver typically falls away from the mask or hat and wakes up on their own -- usually with a bleeding wound or knot on their head. A few deaths have occurred because the console operator made a mistake and supplied the wrong gas. I have experienced this myself and only remember drawing part of a breath.

A less common and potentially more dangerous version of this accident occurs when there is a leak of deep mix or pure Helium that accumulates in a closed space, like the bell on the surface. The diver and supervisor "should" be monitoring O2 levels before entering... a problem of complacency.

 

[TSandM]

Actually, oxygen levels can fall very fast, depending on the starting point. At sea level, breathing air, you have a ppO2 of .21, which is only a little above the threshold for losing consciousness. You have a cardiac output of about 5L/m, and a good portion of that goes to the brain. Start breathing a mix with a ppO2 of essentially zero, and it doesn't take long for the brain cells to equilibrate with the essentially anoxic blood.

Things are different if you start from a higher setpoint. If we preoxygenate patients with 100% O2, we can have 4 minutes or more before they desaturate to the point where they would be unconscious.

 

[Akimbo]

Actually, oxygen levels can fall very fast, depending on the starting point...

Not that fast. I am not exaggerating when I wrote that "lights are out" before you complete an inhalation. Literally 1½-3 seconds with very low physical exertion... like standing up and holding an oral-nasal to your face. I have witnessed it twice and it happened to me once.

None of us remembered any unusual sensation before blacking out and woke up with no idea how long we were unconscious. I don't know, but am "guessing" that implies a pretty low level of consciousness even though we were out less than 10 seconds.

 

[Greenjuice]

Did you ever play around with helium balloons and funny voices? I don't know what those mixes are, but I don't recall any 'lights out' events. I'm thinking it would take more than one tidal exchange to be hypoxic enough to pass out, but I'm not an expert.

 

[Akimbo]

Although a good analogy, it isn't quite the same. The great majority of the time people don't black out from playing Donald Duck with Helium balloons because they are sucking in some air along with the Helium... but not always:

Compressed Gas Association

The CGA article can be a little confusing because they mix anoxia and barotrauma injuries to some extent. Barotrauma is from people trying to inhale directly off a first stage regulator on a Helium cylinder used to inflate balloons, which is far more pressure than human lungs can tolerate.

All the "Balloon Grade Helium" sold comes with a warning about anoxia. I have heard of some suppliers that add 16-20% Oxygen to balloon grade to prevent this problem and reduce cost.

Airgas - HE BL300 - Balloon Grade Helium, Size 300 Cylinder, CGA580 (Availability of this product will vary due to market and supply conditions.)

Hypoxia "starts" with a PPO2 around 0.16 ATA, or 16% at the surface, and has a host of warning symptoms that relatively slowly leads to blackout and death.