Can we infer DCI before symptoms develop?

[FPDocMatt]

If you aren't bent, then no sense in going through all the motions as if you were. ... Bottom line is that if you're asymptomatic, then you beat the odds (or the model you're using) that day.

Then why do they say that you can get the bends hours or even days later?

For example, you get back on the boat, feel no symptoms, then wake during the night with life-threatening bends. And you think, "Well, I read that guy's post, and he said I didn't need to do anything." Then you die, and your family reads this post on Scuba Board...

 

[DevonDiver]

noob question:

If you did something that you are fairly certain is likely to lead to DCS....... why not just get situated at the surface for a few mins.. and then dive down a little ways to keep the off gassing from occurring at a dangerous rate? or is it one of those things that once the process starts.. your kind of screwed?

Fair question, but it opens up a very complicated debate.

There's basically two aspects of discussion here:

Firstly, whether you can 'get back down' and complete missed stops to prevent DCI.

Secondly, whether you can go back down and re-compress to remove DCI.

Missed Deco - If you miss a stop, or believe you exceeded an ascent rate, then there is a finite window of opportunity for you to re-descend and re-initiate an appropriate off-gassing regimen. In very simplistic terms, this 'finite window of opportunity' is dictated by whether bubbles have begun to form or not. Typically, you're talking 60 seconds or so. Technical divers learn strict procedures for doing this. Recreational divers, who aren't in a mandatory deco state quite simply don't have a critical need to learn these procedures. Most dive computers will apply a (safety) stop if you exceed safe ascent rates - typically, they allow you about 1 min to re-descend and complete the stop. They also apply mandatory stops to clear deco, if you exceed an NDL. If you don't do that within the time limit, they'll lock out (if deco) or penalize you on subsequent dives (if fast ascent).

As a general rule of thumb, if you find yourself on the surface and are concerned that you ascended to quickly, or missed a safety stop that you'd otherwise have desired to complete - then you can re-descend and complete a 3min, or longer, stop - provided you make that decision immediately.

Emergency Recompression - If bubbles have begun to form (symptomatic DCS) then re-descending will be an attempt to 'treat' the DCS. This is an entirely different prospect and is highly unlikely to be beneficial without immediate and extensive logistical and medical support. The main issue is that DCS symptoms are likely to make it very unsafe, even life threatening, to remain underwater.

Secondly, the re-compression is unlikely to be effective at 'safety stop' depths - you need to go deeper (i.e. 18m/60ft to reduce the bubble growth through ambient pressure). You'd also need to spend considerable time at that depth, before slowly decompressing during a very slow, staged ascent. During that time...guess what.... you're still absorbing nitrogen... and making the problem worse. The solution for that is to breath pure O2... but guess what... pure O2 will cause convulsions at those depths....and that'll mean you drown. Also, the DCS victim needs medication... immuno-suppressants, IV hydration etc. They can't be administered easily underwater.

Thirdly, whilst underwater doing this improvised recompression, the victim is getting no closer to 'proper' medical and hyperbaric treatment.

Overall, emergency recompression will have more drawbacks than benefits... probably more drawbacks than just providing first aid, hydration and pure O2 on the dive boat (pending/during emergency evacuation to a chamber).

O2 Administration - As this thread already illustrates, there are different schools of thought on the administration of 'preventative' O2. It can mask, but not necessarily cure, DCS symptoms - preventing timely and appropriate evacuation and treatment. On the other hand, it can help prevent bubble formation in some DCS cases, thus negating an emergency at all.

My personal opinion is that I am generally willing to risk symptom masking in favor of possible prevention. Obviously, that depends on the exact situation; where I am, what dives I've done etc etc. Here in the Philippines, I don't have the option of coast-guard heli-evacuation, so it's better for me if symptoms don't present until I am back on dry-land and nearer a chamber. Masking them whilst still on the boat isn't so critical. That said, if I was in a remote location and other divers were entering the water for subsequent dives - I'd want symptoms to present immediately, so that diving can be immediately aborted and the boat can speed back to civilization ASAP. If I was back at home, diving in the UK, then I'd want the symptoms to present naturally - because I can be on a chopper and heading to a chamber in 30 minutes.

 

[FPDocMatt]

There's basically two aspects of discussion here:

Firstly, whether you can 'get back down' and complete missed stops to prevent DCI.

Secondly, whether you can go back down and re-compress to remove DCI.

Missed Deco - I

Emergency Recompression -

O2 Administration -

Very nice answer to the question, thanks for the information!

 

[Bubbletrubble]

As a general rule of thumb, if you find yourself on the surface and are concerned that you ascended to quickly, or missed a safety stop that you'd otherwise have desired to complete - then you can re-descend and complete a 3min, or longer, stop - provided you make that decision immediately.

I wouldn't call that a "general rule of thumb." The course of action you just outlined for a recreational diver missing a safety stop or ascending too quickly is absolutely debatable. I would argue that the asymptomatic recreational diver surfacing under those conditions would be safer if he/she just remained on the surface and monitored for DCI symptoms. With the aforementioned caveats, O2 administration could be considered.

 

[DevonDiver]

I wouldn't call that a "general rule of thumb." The course of action you just outlined for a recreational diver missing a safety stop or ascending too quickly is absolutely debatable.

Yep. That's why I said....

Fair question, but it opens up a very complicated debate.

I would argue that the asymptomatic recreational diver surfacing under those conditions would be safer if he/she just remained on the surface and monitored for DCI symptoms. With the aforementioned caveats, O2 administration could be considered.

That's what their computer is going to want/allow them to do anyway. I think the premise is that bubble formation takes a little time - so that's your window for getting back down to a depth (3-5m) where the physics pixies can help whisk the gas away before it coalesces into symptomatic bubbles. If we assume we all have 'micro-bubbles' on ascent, then staying at pressure to further off-gas prevents those bubble seeds from expanding. If we re-descend within a sufficient time-frame to prevent bubble growth, then the benefits are as per doing a safety stop without accidentally surfacing first. A recreational diver, in most cases, isn't sufficiently saturated to make this a bad option.

As a general principle, why sit and wait to see if it happens, rather than attempt to prevent it?

 

[Bubbletrubble]

That's what their computer is going to want/allow them to do anyway...

What a computer allows and the best course of action aren't necessarily one and the same thing.

I think the premise is that bubble formation takes a little time - so that's your window for getting back down to a depth (3-5m) where the physics pixies can help whisk the gas away before it coalesces into symptomatic bubbles. If we assume we all have 'micro-bubbles' on ascent, then staying at pressure to further off-gas prevents those bubble seeds from expanding. If we re-descend within a sufficient time-frame to prevent bubble growth, then the benefits are as per doing a safety stop without accidentally surfacing first. A recreational diver, in most cases, isn't sufficiently saturated to make this a bad option.

That's the theory. Then there are the practical issues of re-descending to do a missed safety stop. Does the diver have enough gas to remain at stop depth long enough to make the additional risk of re-descending worthwhile? How does the diver's buddy figure into all of this? Is he in the same situation? Will he accompany the diver to do the missed safety stop? Is he equipped to accompany the diver?

There's also the issue of why the diver experienced the runaway ascent or why the safety stop was missed. Was it an unintentional loss of buoyancy control? If so, how confident is the diver that he can re-descend and hold the stop? Was the diver low on air or out of air? If so, why did he have the gas supply issue and how quickly can the diver surface, swap tanks, and then re-descend? Was there a gear malfunction?

Technically, a safety stop is optional. Is the risk of bubble-pumping worth the marginal benefit of re-descending to do the missed safety stop?

As a general principle, why sit and wait to see if it happens, rather than attempt to prevent it?

As a general principle, why choose a course of action with uncertain benefit and the possibility of having an underwater incident when first aid care and close supervision is so readily available at the surface?

I'm not a tech diver, but I'm aware that tech divers are concerned with the quality of off-gassing on every dive. How "clean" does your deco look if you surface and then re-descend to do a missed safety stop?

Would you prescribe the same sort of action for a tech diver who just conducted an aggressive decompression dive but missed a 3 minute safety stop at 15 fsw (assuming that only regular air is available for the stop)? If not, why not? I realize that a tech diver would normally have other gases available to him which would theoretically accelerate off-gassing.

 

[lowviz]

Keeping it in "Basic Scuba Discussions", let's say I've been at 90' for about 25 minutes then decide to head up properly -but cork. Hit the surface breathing like a freight train through my reg, take 10 seconds to self-assess while hanging onto the downline, feel fine, What next???

 

[Mr Carcharodon]

As a general principle, why sit and wait to see if it happens, rather than attempt to prevent it?

Because you can get worse lose consciousness and die, as has happened to a number of people who have tried it. Or at least that is DAN's rational.

In water recompression should not be a first choice if transport to a medical facility is possible in a reasonable period of time.

 

[DevonDiver]

What a computer allows and the best course of action aren't necessarily one and the same thing.

My assumption on what computers will, and will not, allow is that it is largely based upon effective study, reinforced with a degree of conservatism.... and highly constrained by potential liability.

By allowing divers to re-descend, in fact - even by promoting that response, by locking out otherwise, and stating that in the manual - there'd be quite a large liability issue if it were based on a very faulty premise.

That's the theory. Then there are the practical issues...

Absolutely... and there are a tonne of them. Water conditions, gas supply, etc

Does the diver have enough gas to remain at stop depth long enough to make the additional risk of re-descending worthwhile?

For 3 minutes stop at 5m... I'd assume that most divers would, unless they initially missed their stop or ascended fast due to low/no gas.

Could they grab an alternative tank and re-descend? Maybe.

How does the diver's buddy figure into all of this? Is he in the same situation? Will he accompany the diver to do the missed safety stop? Is he equipped to accompany the diver?

If he was equipped for the initial dive, then I assume he's equipped to redescend to 5m for 3 mins. If he was low/out of air... then the diver concerned would have to make a judgement call that balanced the risk of DCS against the risk of going back to 5m alone for a few minutes. I won't pretend that's risk free, but it shouldn't be beyond the capabilities of a certified diver in most instances.

There's also the issue of why the diver experienced the runaway ascent or why the safety stop was missed. Was it an unintentional loss of buoyancy control? If so, how confident is the diver that he can re-descend and hold the stop?

If that was the limiting factor, on an otherwise prudent procedure, then the diver shouldn't be re-descending at all..... at least, not without significant remedial training at Open Water level.

Agreed, some novice divers do lose their buoyancy control occasionally - especially on ascent and especially in the shallows. Whoops... I made a mistake. That said, there's a significant difference between making a one-off mistake.... and not having the ability to do a safety stop. If a diver can't perform a safety stop, they shouldn't be diving - it is after all, a required skill for mastery on the OW course.

Was the diver low on air or out of air? If so, why did he have the gas supply issue and how quickly can the diver surface, swap tanks, and then re-descend? Was there a gear malfunction?

Absolutely, those are very valid variables - and something that should be obvious to the diver at the time. Many prudent dive centers put a 'drop tank' below the boat for such incidents (complete safety stop/emergency deco, even if air is low after deep dives).

Technically, a safety stop is optional. Is the risk of bubble-pumping worth the marginal benefit of re-descending to do the missed safety stop?

What we're really talking about here is fast ascents and/or deep dives. If a dive is not otherwise noteworthy, then missing a safety stop is not a big issue....neither is re-descending to complete it.

However, if the diver is concerned enough to determine that a safety stop would be beneficial - then they have the option to quickly re-descend and conduct one. 3 minutes later, they can be back on the boat, near the O2 and first-aid, as you've described. It's just another protective measure.

As a general principle, why choose a course of action with uncertain benefit when first aid care and close supervision is so readily available at the surface?

It's a case of being pro-active or being re-active. There is a certain benefit to it. If a diver is concerned about their dive, they should complete an (extended) safety stop - accidentally surfacing shouldn't change that. It has to be balanced against other factors though - and shouldn't present more definable risk than it otherwise mitigates.

I'm not a tech diver, but I'm aware that tech divers are concerned with the quality of off-gassing on every dive. How "clean" does your deco look if you surface and then re-descend to do a missed safety stop?

Who cares about clean?...it's an emergency procedure... if it happens, it has happened.... you can sit and bubble, or you can attempt to save yourself. As we've already agreed; it is situationally dependent though (as it is for a recreational diver). From the horse's mouth....

Missed decompression stop

1. If you miss a decompression stop, how you handle it depends on the situation.


2. If you can, immediately (less than a minute) redescend and complete the stop and add one minute, then decompress according to schedule.


3. If you cannot redescend (such as if you have a gas supply problem and must switch to a deco cylinder you can’t use at the deeper depth), stay at the next
stop for the combined time of both stops.


4. If you cannot redescend or it takes longer than a minute to do so, extend your 6 metre/20 foot stop and/or the final stop by 1.5 times the decompression normally
required. (Note: some dive computers calculate missed deco stop procedures automatically – see the manufacturer’s instructions.)


5. If you’re using a dive computer, it may lock up if you skip a stop and cannot redescend to complete it. You may need to decompress according to back up
tables.

 

[DevonDiver]

Because you can get worse lose consciousness and die, as has happened to a number of people who have tried it. Or at least that is DAN's rational.

In water recompression should not be a first choice if transport to a medical facility is possible in a reasonable period of time.

I clearly defined the difference between in-water recompression and DCS risk mitigation through re-descending to complete an optional stop.

Whilst I defined in-water recompression for the purposes of clarity, let's not get too entwined in that debate in this​ forum