Shallow- and Deep-Water Blackouts

[simskiscuba]

Extremely educational thread. Thanks guys!

 

[Hank49]

While I agree with DD about not diving alone, the training will help people avoid blackout by helping them learn their personal limits under controlled conditions.

Problem is, I'm not thinking about, or paying attention to personal limits when a 40 lb cubera is approaching at 60 feet.
"Come on.....a little closer....keep coming....." and that "lyin' bastard" has already spoken...but you don't feel the need to breath as you're waiting behind that rock. ya man...
The only safe way is a buddy on the surface watching you.

 

[feitr]

Wow, this diagram shows quite clearly that hypercapnia is your friend. When you get the strong urge to breathe, that's the time to surface.

I presume that holding one's breath for longer and longer periods is not so much learning how to ignore the urge to breathe, but by extending the time before you become hypercapneic and/or hypoxic...?

That diagram looks to me like a caricature to just illustrate the process. I've seen several graphs of statics with pulse oximeters (measure % of hemoglobin with attached oxygen) and the urge to breathe often comes before the O2 levels even start dipping (in a trained freediver this will be 3-4 mins). Will depend on training, personal differences (some people don't even get contractions, albeit rare), degree of hyperventilation and so on, but that graph isn't based on any experimental data it's just an attempt to demonstrate a concept.

Hypercapnia causes the urge to breathe so you can't separate the two - the more you become tolerant of high CO2 levels the longer it takes before contractions start and the more comfortable you become during contractions (staying relaxed) the longer you'll be able to push through them. You can do O2 tables to tolerate low O2 (same period of rest, longer statics each time or you can do exhale statics) and CO2 tables to tolerate high CO2 (same length static followed by progressively shorter rests) but until you have a high CO2 tolerance that is always going to be the limiting factor.

 

[Thalassamania]

Problem is, I'm not thinking about, or paying attention to personal limits when a 40 lb cubera is approaching at 60 feet.
"Come on.....a little closer....keep coming....." and that "lyin' bastard" has already spoken...but you don't feel the need to breath as you're waiting behind that rock. ya man...
The only safe way is a buddy on the surface watching you.

I agree, but then we're not all as blood thirsty as some.

 

[FPDocMatt]

Sounds like what's needed is an underwater pulse oximeter. I wonder if such a thing exists. Then you'd know when your oxygen is approaching a dangerous level.

 

[Thalassamania]

Would not work, you're fine at depth, the problem is when the ambient pressure drops and thus causes the ppO2 to drop likewise. Using such a device would mean: start up, beep! beep! queep.

 

[FPDocMatt]

Would not work, you're fine at depth, the problem is when the ambient pressure drops and thus causes the ppO2 to drop likewise. Using such a device would mean: start up, beep! beep! queep.

Good point.

But couldn't you translate the pulse oximetry at the depth you're at to the equivalent hemoglobin saturation at the surface? So you'd say, "At the surface I am hypoxic at a pulse ox of A, and that means that at my current depth I need to surface when my pulse ox reaches A+X", where X is the correction for the depth you're at.

Furthermore, this could be incorporated into a free diving computer with pulse ox attachment, which would do the calculation for you. Sort of like a scuba air-integrated dive computer. This would be a free-diving air-integrated dive computer. It's just the air would be in your lungs rather than your scuba tank.

 

[TSandM]

Matt, if you think about the oxyhemoglobin dissociation curve, you'll see the fallacy. At depth, you're going to be in the flat area on the far right of the curve, but as you ascend, you'll hit the sigmoid portion. Because there is so little difference between .21ATA and 1.6 ATA in terms of Hb saturation and oxygen carrying capacity, you're going to see very little difference in the time to the steep portion of the curve.

 

[FPDocMatt]

Matt, if you think about the oxyhemoglobin dissociation curve, you'll see the fallacy. At depth, you're going to be in the flat area on the far right of the curve, but as you ascend, you'll hit the sigmoid portion. Because there is so little difference between .21ATA and 1.6 ATA in terms of Hb saturation and oxygen carrying capacity, you're going to see very little difference in the time to the steep portion of the curve.

So what are you saying, there's no way to tell when you should ascend? No matter when you ascend, you're going to be hypoxic by the time you reach the surface? This doesn't sound right to me.

 

[Hank49]

Why wouldn't you be hypoxic? If you lay on your couch and hold your breath until you feel the urge to breath, you're very likely hypoxic to some degree.

(based on my understanding of the term "hypoxic")