Confident CESA depth?

[CWK]

Dave Bevan,

My BCD is washed and disinfected after every dive trip. I keep the container for my alternative air supply fit for purpose. At any rate, it is preferable to have a chest infection than to suffocate or drown, is it not?

It is possible to use the BCD as a rebreather (inhale air from the BCD and exhale back into the BCD). This can be done if you are deep, say 40m, and are concerned that there may not be sufficient air in the BCD to get you to the surface. I would not suggest doing this for more than one or two breaths as it introduces co2 into the BCD.

If anyone is really concerned about OOA situations, think about how you can utilise the air in your BCD. If you can utilise it, OOA at 40m should not be terminal.

 

[Blasto]

Personally, in your current physical condition, what depth are you reasonably confident you can successfully perform a controlled emergency swimming ascent?

Not a planned training exercise with a free diving style breathe up or necessarily full lungs to start. I mean a mid ventilation cycle out of air where your next breath must be from the surface to survive and you can arrive there in good health.

Probably pretty deep. For this particular scenario, unplanned CESA initially out of air, the deepest I've done it was just around 60', but there was a lot of headroom. That specific situation was a buddy breathing failure; chasing the buddy was clearly less practical than ascending. At surfacing I felt at most 1/3 of the way from breathe-out to where I'd be forced to ascend while freediving.

Doing it from much deeper than 150 ft, I'd be worried about blackout in addition to the usual risk of bends. So the deepest I'd consider CESA a safe option without a RAS is probably 165 ft - not that I'd want to go there without a RAS in the first place. Anywhere deeper, I'd rather save the oxygen for my brain and go for a buoyant ascent.

OTOH, the lungs hold a lot more air even post-exhale at 165 ft than at 60 ft, so you get back to that full lungs situation sooner.

 

[dmaziuk]

At any rate, it is preferable to have a chest infection than to suffocate or drown, is it not?

If you have an atypical immune response and don't get to a good hospital fast, the difference may only be 48 hours or so.

Doing it from much deeper than 150 ft, I'd be worried about blackout in addition to the usual risk of bends. So the deepest I'd consider CESA a safe option without a RAS is probably 165 ft - not that I'd want to go there without a RAS in the first place. Anywhere deeper, I'd rather save the oxygen for my brain and go for a buoyant ascent.

That's a good point, though as I understand it it's not the amount of O2, it's the PPO2 drop that does it. So you have to either slow down so it doesn't drop so rapidly, or let it drop and wear a BCD that will float you face up after you do black out.

 

[Blasto]

My understanding is that it's deficiency in absolute O2 partial pressure that causes LOC, and even more precisely that deficiency integrated over (a short) time. IOW there's basically nothing that can be done to maintain consciousness below a particular blood ppO2 threshold, and the best mitigation is to keep it a bit higher.

 

[Akimbo]

My understanding is that it's deficiency in absolute O2 partial pressure that causes LOC, and even more precisely that deficiency integrated over (a short) time. IOW there's basically nothing that can be done to maintain consciousness below a particular blood ppO2 threshold, and the best mitigation is to keep it a bit higher.

I'm not sure I understand. Blood O2 and your faster tissues are highly-saturated*, even if you only spend 10 minutes at 60'. There's no way that you can pass out from hypoxia from recreational depths.

* meaning higher than breathing pure Oxygen at the surface for a short time but far from fully saturated

 

[dmaziuk]

Yes, on second thought I believe @Akimbo is right: there shouldn't be an ascent blackout after breathing compressed air during the dive. Hyperoxia should "cancel out" any PPO2 drop, and the deeper you start, the more hyperoxic you are.

 

[Blasto]

I suppose that might be right. Just not really sure how to account for oxygen dissolved in the tissues. To clarify, my concern is for fast bounce dives to 165+ ft (below normal recreational depths).

 

[northernone]

I suppose that might be right. Just not really sure how to account for oxygen dissolved in the tissues. To clarify, my concern is for fast bounce dives to 165+ ft (below normal recreational depths).

Here's a little anecdotal support:

I've experienced shallow water black out free diving 3 times (years ago now) but never from deep water CESA practice. Those I learned from talked of black out as a more significant risk if an entanglement or struggle at depth preceded the swimming ascent. How this corresponds to the level of oxygen saturation of the arterial blood necessary to maintain consciousness is beyond anecdotes.

Perhaps there is some military research I've not come across which would be relevant.

Regards,
Cameron

 

[Colliam7]

I know I can do it from 33'. I had to do one from 27' two years ago and, frankly, it was pretty much a piece of cake. But, I think that was because I practice it, regularly - every time I teach OW. I want to try it from 66', as I think I can probably do it without too much difficulty. But, I had to cast my vote for 33' because I have not really practiced it sufficiently from 66' to be confident.

 

[mcohen1021]

Reviving this thread (sorry)...

Can someone please tell me what is taught/required of a student depth wise in an out of air emergency ascent? What I mean is, the diver is coming up at the normal ascent rate, out of air, exhaling that last breath all the way up...