Emergency Ascent practices

[Sailfish]

Hope this does not come off too horribly lame and I realize that I am getting ahead of myself since I have not even gotten certified, but I had a thought and maybe you guys can help.

Say you're at 90-120 ft and run into trouble. Gauges were wrong or cut a hose or what have you. At any rate you're out of air. You don't have enough air in your lungs to make the proper ascent in the proper interval of time. Gotta get to the surface and fast! Once on the surface, how long have you got before you realize the bends? How much time do you have to get to a decompression chamber? Is it possible to just put on a new set of gear or tank, go back to the depth you were at when the problem occured and make a proper or extra slow ascent with enough air now at your disposal to do so?

Way ahead of myself but just curious.

Thanks.

 

[Dr Deco]

Dear Sailfish:

If you have been at the depth for a period within the table guidelines, then you probably do not have much to worry about unless you held your breath on the way up. [From that depth, you are in serious trouble on the surface.]

If you missed decompression that would amount to a few minutes, there is probably only a very small chance of getting DCS. As the missed decompression becomes greater in duration (as with the divers in The last Dive ), the problem becomes more acute. :boom:

There is usually a DCS-free interval of about 5 minutes where you can be safely repressurized. This is not a procedure that is performed without some caution. In caisson work, it is termed “decanting” and in diving, it is “surface decompression.” There is no rule of thumb.

This is really a difficult question, since it is not possible to assay an individual without a Doppler bubble detector. With this, you could get some idea of his decompression status. Suffice it to say that you would have a few minutes to gather together some more gas cylinders, make peace with God, and write your last will and testament (should it be needed).

Historically people have gotten away with it. In the 1800s, when decompression was poorly understood, compressed air workers missed tens of minutes by today’s standards. Most were OK, but some died. :reaper:

That is why we do not do this any more.

Dr Deco :doctor:

 

[The Iceni]

Dear Sailfish,

I am sure you will be taught all about secondary air sources and buddy diving.

In the UK all sports divers have at least two ways of avoiding an unplanned rapid ascent due to equipment failure.

The first is the almost universal use of a pony cylinder or an isolatable twin set, which both provide a supply of breathable air to get you to the surface at the normal rate, even if you suffer a catastrophic first stage failure. (There are others, for example an Octopus rig (a spare second stage) allows you to breath from your main cylinder following a main second stage failure. Unfortunatlely these are useless for an uncorrectable second stage free-flow, which are not uncommon in our colder waters.) Nearly all so-called technical divers use isolatable twins.

The second is the standard practice of buddy diving. :1st: If all your kit becomes totally US you simply take you buddy's octopus and ascend with him. Failing that, there is buddy breathing, of course (however no longer a part of the BSAC syllabus).

If I may expand on what Dr Deco said above;-

1) The main risk of a rapid ascent is pulmonary barotrauma ( Burst Lung ) but it also makes the risk of DCI more likely.

2) In water re-compression has little to recomend it. After all who do you expect to act as your attendant?

(By the way, I like all those extra smilie icons, Dr D!)

 

[bluce]

Are there any statistics kept on catastrophic equipment failures, particularly first stage failures? For example, number of first stage failures per number of dives in the world last year, that sort of thing. Thanks ... Bob

 

[devjr]

Ok, you survived the trip up and you want to know what to do next. The whole matter may or may not be urgent. I've seen a couple of emergency ascents where nothing bad happened. I've read about less fortunate situations but which always involved deco dives. One should probably take certain precautionary steps if the situation, including repetitive dive status, is close to or above the decompression limit. As Dr. Deco has averred, if no more than a few minutes have passed since surfacing you can recapitulate by donning another set and submerging for a deco(safety) stop. That's one option of several.

The question you posed seems constraining since it is really a subset of the old question of what to do about a bends hit. In other words, try looking at it as seeking a solution to one of several variants of the same problem. There are equally many options, and as alluded to by Dr Thomas-- some may not be practical on that day, at that time. They may too complex, dangerous, require too much support or are too time consuming. Ergo, certain procedures such as in water recompression on air or NITROX are farther down on the list, way down for the average diver.

For the rest of us, carrying an oxygen kit on board as the main treatment modality seems like a good idea. However, the usual kits, the ones that are normally carried on ambulances and the like might not be the best choice for a boat. You need something that can be used for in water recompression or for treating a diver on board the boat. For these purposes, I would suggest that a 72 cf steel SCUBA tank, cleaned and charged with O2, be used. A SCUBA tank is capable of holding enough oxygen for sustained treatment and will accept standard backpacs and various regulators. In this regard, there should be two regulators in the kit, an O2 ready SCUBA reg and a modified oxygen flow reg(16 lpm), plus tubing and mask. With older O2 continuous flow regs(E Bay) the oxygen nipple and 1/4" pipe can be removed and replaced by a SCUBA yoke(NE SCUBA supply). Thus, after mods and cleaning either the SCUBA reg or continuous flow oxygen reg can be attached to the tank, and alternately used with oxygen on deck or underwater as required.

Some training and orientation are needed for using O2 and this is not the place to work on that. I can make a few general remarks. If you suspect but are not sure that something bad is about to happen, and it is too late to regroup for a safety stop, surface breathing of oxygen seems prudent. If you believe you have bends, and are still physically able, rig up for an oxygen dive not to exceed 33 feet/30 minutes. Continue on board with the mask while heading home.

In water O2 recompression is sometimes called the "Hawaian method". Sources for study or discussion of this and O2 treatment in general are available on the web and in print.

 

[VTWarrenG]

Several points:

A) Regulator failure is a non-issue. High-pressure hose rupture is a slow leak (the orifice on the first stage is very small). Low-pressure hose rupture, second-stage freeflow, HP seat failure, and O-ring failure are all moderate leaks. In any of these situations, unless you were already nearly out of gas to begin with (a no-no), you'll have plenty of air to get to the surface.

B) The only real concern with "express elevator" ascents is breath-holding. If you're doing deco, of course, the rules are significantly different.

C) If you missed a decompression obligation, you might feel a hit before you even break the surface -- or you might not feel it until the next day while driving home from work. DCI is a very complex, very poorly understood collection of phenomena that never really obey any known rules. If you miss any substantial decompression obligation (the definition of "substantial" depends heavily upon time, depth, and mix), you'd be a smart man to get the recompression chamber ready, even if you're asymptomatic.

D) There is no "time limit" to getting into the chamber. It's often several hours once it's all said and done. Like many other things in life, recompression is a "better late than never" prospect.

E) It's generally accepted that three minutes of surface exposure while under a decompression obligation is about all that one can safely withstand. If you have to surface during deco (for instance, because you ran out of deco gas), you need to get back under pressure ASAP. You'd want to return to a stop deeper than the one you aborted, and you'd be wise to extend your deco as long as possible, using all of your available gas. (Side note: deep stops can sometimes actually load you with more nitrogen than you started the ascent with, especially if you were on a helium-based bottom gas, and are using a nitrox deco gas... in such situations, you wouldn't want to extend those deep stops, or you would want to stay on bottom mix -- but this is becoming quite complicated, so I'll just say that missing a deep stop is very, very touchy.)

F) Don't ever, ever, ever deco with air. Bad, bad, bad.

G) In-water shallow recompression (like the 20 fsw oxygen dive) is generally a bad idea. Most bubbles form in the bloodstream on the venous side. When you recompress just a bit, like to 20 fsw, the bubbles get small enough to squeeze through the capillary beds in the lungs, and into the arterial side. Once there, they're very hard, if not impossible, to eliminate. The rule of thumb: if you blew a deep deco stop, get more gas, go back down, and do a long, careful deco. If you blew a shallow stop, get on the boat, get on oxygen, and call the Coast Gaurd.

H) Any of these botched-deco situations are examples of horrendously poor planning, and very bad diving. They shouldn't ever happen, and certainly never need to happen.

I) Don't ever do any kind of deco or mixed-gas diving without the proper training, equipment, and support.

- Warren

 

[devjr]

Actually, the permanent nerve damage resulting from type 2 bends begins within 20 minutes; so, there is a time limit. A lot of divers have been saved by in water recompression on O2 or by the alternate "Australian" method of air and O2.

 

[VTWarrenG]

devjr,

I'd love to discuss some examples of such saves, and specifically why it was in-water 02 recompression that saved them.

Once you have type II bends, 0.6 ATA more pressure is not going to cause a meaningful decrease in bubble size. Bubbles big enough to cause type II will still be far too large to fit through capillaries at 20 ft, and those tissues will be still be oxygen-starved. Unfortunately, though, the bubbles that ARE small enough will now be in the arterial side, and will stay there and cause much more damage for a much longer time. The lungs trap bubbles on the venous side, and are very effective at eliminating them (in fact, they're much more efficient at eliminating bubbles than dissolved gas). Once you've bubbled, it's virtually always preferable to stay bubbled, so that the lungs can do their job.

Bounce diving to 20 feet after a steep deco is a very, very bad idea. In-water recompression after a hit is a very, very bad idea. It's an old concept that had promise, but has been disproven, like labotomy.

Comments welcome.

- Warren

 

[Se7en]

Firstly: Don't deco dive, don't dive deeper than your training, don't dive on air, don't try IWR, blah blah blah.

Now - I belive there are at least three different published formal methods of IWR.

US NAVY is o2 to 10 metres for a time period (60 minutes), then a series of stops to the surface.

'Australian' method is again a time period at 10 metres (30 minutes, or until symptoms stop) and then a very slow ascent (in the region of 1 foot per minute) all on o2.

Hawaiian method is as per Australian method, except it's preceded by a bounce dive to the depth at which symptoms disappear. Bounce dive on air, then deco out as per Australian method on o2.

Richard Pyle, of deep stop fame/notoriety, did an extensive, footnoted literature review of known IWR cases. From memory the success rate was something like 80% with no problems on surfacing, and high 90% with no significant problems. Plus a few who were never seen again...

There were also a couple of case studies where a group of divers all bent, those who completed IWR survived, those who tried for a chamber didn't make it. So there would seem to be a time limit for recompresion. Indeed, I'd go so far as to suggest that time is critical.

Interestingly, I think most of the reported cases used air for IWR, rather than o2...

So - I'd suggest that IF you have a large supply of o2 (enough for 90 minutes at least at 10m), AND a support diver to aid, AND you know what you are doing, AND you can deal with the inwater issues (such as thermal exposure etc), AND a chamber is a significant time away (6 hours plus), THEN IWR may be beneficial.

But if you have done all the planning to organise the above conditions, you probably should plan to dive conservatively enough so it's not an issue...

I also belive that the venous / arterial bubble issue was a theory that was yet to be demonstrated - but it appears likely given that bubble formation is likely to be linked to microbubble formation in body tissues, and the only way these bubbles can move from the tissues is in the direction of blood flow. Therefore bubbles would be present in the venous system, to the heart, then to the lungs. Hence the issue with a PFO - which allows un oxegenated (pre lung) blood to mix with oxygenated (post lung) blood - and hence supplies a means for the bubbles to be sent out with arterial blood.

Mike

 

[devjr]

Did I get the Aussies and Hawaians switched at birth? Yikes, formal apology, and please don't sue. Thanks for the input.