Eagle's nest accident report

[boulderjohn]

So are you guys saying that in that case, it’s better to breathe the Nitrox gas for a few mins and move up only after?

My reading comprehension is evidently not up to the task here. I have read the section describing the dive several times, and I still don't understand it. It sounds like they descended to the chimney and dropped off their O2 tanks, switched to an unidentified nitrox mix for the next phase, and went down to 130 feet, where they did a switch to back gas. (It has been years since I dived there, and I have not done it often--I remember the debris cone at 150 feet, but I could be wrong.)

That sequence seems strange to me. I would have been breathing the nitrox from the start, dropped the O2 bottle at the chimney, and switched to back gas there. I don't see a benefit to two switches.

Either way, the dive would have been only a few minutes long when she had her problem and switched tanks. The back gas must have been truly nearly pure helium. If she had had only 4% O2 at that point, she should have been fine. There should have been no concern about DCS on the ascent at that point. If they were at 130 feet, they were still within NDLs.

The body has nearly no warning sign of hypoxia, so if she made the switch herself, she must have been in serious trouble at that point (and lucky--it would be more likely to just go unconscious). It would have taken more than a few breaths to oxygenate her deprived tissues. If they began an immediate ascent, it would not be surprising for her to pass out by 100 feet, if not before. That had nothing to do with the ascent, though. It would have been the same if she had stayed at depth.

There would be no harm in terms of breathing the nitrox at depth if she had stayed, but if she was hypoxic enough to pass out, then she needed to get to the surface quickly. Getting an unconscious, unbreathing diver to the surface is critical.

 

[Nick_Radov]

Generally you would not, but in this instance, if you are already way low on your bodily oxygen levels, then reducing the ambient pressure even further by ascending

That is essentially the same physiological mechanism behind shallow water blackout in free divers. They metabolize most of the oxygen in their body while deep. Then they ascend, and the decreased ambient pressure in shallow water causes the PPO2 in their brain to suddenly drop below the threshold necessary to maintain consciousness.

 

[boulderjohn]

This article explains the mechanism of shallow water blackout in depth. It explains that the primary reason for shallow water blackout is simply a lack of oxygen. The reason ascent can be a factor is that the alveoli that were compressed at depth expand, limiting blood (and oxygen) supply to tissues. This would not be a factor with scuba.

There is a factor in that pressure at depth limits blood flow to the periphery, and when ambient pressure is decreased, that allows more blood flow to the periphery and thus takes blood away from other areas, but I cannot see that it would be a significant factor in this case.

 

[Kay Dee]

.........It would have been the same if she had stayed at depth.

There would be no harm in terms of breathing the nitrox at depth if she had stayed, but if she was hypoxic enough to pass out, then she needed to get to the surface quickly. Getting an unconscious, unbreathing diver to the surface is critical.

I beg to differ John, as she didnt pass out till she was ascending. So, although we will never know in this instance, I don't think 'you' can say she would have passed out a her max depth, if breathing her nitrox. The reduction in pressure on ascending could have been the straw that broke the camels back so to to speak, whereas breathing the nitrox at its MOD may have been just enough to strike the balance for a 'recovery'. Whatever, its bail out time, but reducing the ambient pressure is as I and @Nick_Radov states, setting up the scene for something almost akin to 'shallow water blackout'.

Don't get me wrong, I am not suggesting anyone do what I am implying, but given the laws of physics that's what I would (try) to do.

 

[boulderjohn]

I beg to differ John, as she didnt pass out till she was ascending.

According to the story, she passed out after ascending 30 feet--maybe 15 seconds after starting the ascent. There would not have been enough blood flow in that time to make a difference. If she was on the verge of passing out, that would not have made any difference. As I wrote before, it is a miracle she did not pass out first.

 

[Kay Dee]

According to the story, she passed out after ascending 30 feet--maybe 15 seconds after starting the ascent. If she was on the verge of passing out, that would not have made any difference. As I wrote before, it is a miracle she did not pass out first.

So say you. Like I said I beg to differ so we will just have to agree to disagree.

 

[dmaziuk]

According to the story, she passed out after ascending 30 feet--maybe 15 seconds after starting the ascent. There would not have been enough blood flow in that time to make a difference. If she was on the verge of passing out, that would not have made any difference. As I wrote before, it is a miracle she did not pass out first.

30 feet is almost an atmosphere. To me the whole mechanism of the sudden drop of ppO2 already in the blood -- as opposed to drop in the amount of O2 in the blood i.e. plain hypoxia -- does not sound very intuitive, but if we accept that SWBO works that way, 1 atm drop in ambient pressure could be enough to trigger it.

 

[Vicko]

A diver breathing 32% nitrox or whatever she had in that 130ft tank is not passing out from shallow water blackout at 2 bars of pressure.
Possible that the tank contained something with less o2 than expected.

 

[dmaziuk]

The description is not super-clear.

I think it means she went hypoxic breathing from her primary and had time to realize something was wrong and switch to nitrox -- but not enough time to re-oxygenate before they started the ascent. It doesn't mean not ascending would have made a difference, but adding a ppO2 drop on top couldn't have helped either.

 

[BlueTrin]

30 feet is almost an atmosphere. To me the whole mechanism of the sudden drop of ppO2 already in the blood -- as opposed to drop in the amount of O2 in the blood i.e. plain hypoxia -- does not sound very intuitive, but if we accept that SWBO works that way, 1 atm drop in ambient pressure could be enough to trigger it.

What does SWBO mean and do we know if PPO2 in blood is what matters?