Inspection of a CCR after an accident…

One of the findings at Rebreather Forum 3.0 was for CCR manufacturers and other community members to publish a worksheet to help accident investigators collect meaningful data from rebreathers involved in diver deaths. A sombre topic for sure but the need to have some “standards” and some sort of unit-specific checklists is apparent given the wide gaps in information gathering to date.

Martin Parker — the managing director of Ambient Pressure Diving, the manufacturer of Vision Rebreathers (the Inspiration, Evolution and Evolution+) — recently posted a worksheet. It is available in PDF form from the link below.

Click Here

The description of some procedures are graphic and not suitable reading perhaps for the squeamish; however, I believe this is a good start… Kudos to Martin.

AN OPEN LETTER TO NEW DIVERS ABOUT STAYING ALIVE

Hi: and congratulations on your new open-water certification. Diving is extremely cool and I hope you get as much out of the experience as you can: talk about opportunities… wow!

You probably do not recall everything you read while doing the academic work to earn your certification, so I would like to take this chance to remind you of something important: An overhead is no place for an open-water diver: period. There should be no exceptions to this.

I’m writing to tell you this because there is a chance that sometime soon, someone – perhaps a buddy or a more experienced diver, maybe even an instructor – will try to tell you differently. Please tell them they are wrong. You can quote me if you like, and you can use stronger language too, but much more importantly, you can find the same advice in training manuals from EVERY agency and any reputable dive professional. Please take the time to check this out.

Caves, caverns, and the inside of wrecks, are NOT places to find yourself without specialized training. And I do not mean advanced open-water, or rescue diver or even a divemaster or instructor training but specialized training in those specific environments. If the person, even an instructor, trying to get you into a cavern or cave, calls you a wimp or says that everything will be OK as long as you “stay close and follow me,” please walk away. It could save your life and in any event, will send the right message.

If they are an instructor, suggest they reread their standards and procedures manual – it really does not matter which agency they teach for because they ALL forbid this sort of behavior and this type of dive. Fact is, suggesting that you let them “guide” you into an overhead environment could get them reprimanded and their teaching status suspended.

A few days ago, a buddy of mine pulled an open water diver out of a cave in North Florida. Against all odds in this sort of scenario, she was alive… shaken, but alive.

Her explanation was that she went into the cave just to take a quick peek at what was in there because someone (her Dad) had convinced her that as long as she did not go in too far, she’d be fine.

In a perfect and just world, she will be able to sit down sometime soon with her idiot father to have a chat. She almost died. She would have died if my friend had not been there to find and save her. (He is, by the way, a very experienced cave instructor and explorer.)

OK, so here are some things to think about. Untrained divers kill themselves in caves with sad regularity. This has been happening for years and unfortunately continues to happen even though it is preventable. People who should know better and who have read the “rules” choose to ignore common sense and believe they are immune from the laws of physics… and Murphy.

The poor sap who ends up dead may have been told that caverns and caves are safe, and believed it. They may also have been a super master specialty open-water instructor and have a collection of badges as long as their arm, but all that means bugger-all inside a cave when they realize they are lost, are sucking seeds and stems from their almost depleted single cylinder, and start to claw at the limestone ceiling and walls wailing for salvation.

Caves can be beautiful but that beauty can become extremely ugly in a couple of heartbeats. Here are some things that have happened to untrained divers who almost died in a cavern or cave but somehow managed to find their way out.

“We only intended to swim in a little way, but there were lots of passages and we got turned around…”
“The water was really clear but my buddy crashed into the bottom and I lost sight of him and the exit. I think he is still in there…”
“We had a light between us but it went out. It was really dark and I kept swimming into the walls…
“I swum in a little way and then my octo started to freeflow…”
“We followed a line and it just stopped and then I got tangled in it…”
“I panicked when I turned and could not see the exit.”

There are no grey areas when it comes to this overhead stuff. Going in there without the right kit and training is seriously tempting fate, and there are so many other ways to enjoy yourself with scuba. Please, please, do not go into an overhead until you get training in overhead diving and get yourself some serious kit and gain the experience to use it properly.

Thanks. Now go dive… in open water.

Steve Lewis
TDI instructor trainer #6

Air Breaks… what are they, and do people take them for the wrong reason?

I find the concept of taking air breaks to manage oxygen toxicity while decompressing comparable to using a paper towel to mop up an incoming tide at the beach. Or put another way, air breaks in this context are about as useful as ashtrays on a motorcycle.

Allow me to explain. I believe oxygen toxicity is one of the biggest risks to recreational divers, especially technical divers, but air-breaks as commonly described and executed, are no substitute for proper CNS planning… and are useless as a CNS management tool in any event.

The first time I remember hearing the term air-breaks was in a conversation with a hyperbaric doctor over a bottle of wine and a grilled fish supper some years back. The context was a discussion about the practice of getting hyperbaric chamber patients on air after 20-minute spells breathing pure oxygen at a “dry depth” of 18 metres (60 feet). Of course, this therapy – part of the procedures called for in the US Navy Diving Manual – delivers an oxygen partial pressure of 2.8 bar, well in excess of the 1.6 bar recommended as a maximum for recreational divers… technical or otherwise. I have no clue how or who decided that this term was the right one to use to describe the practice of switching to a low-oxygen content gas after breathing oxygen during staged decompression stops in the water. Nor can I fathom what it can possibly have to do with managing central nervous system (or pulmonary toxicity, gods forbid) while recreational diving.

Oxygen toxicity is a condition resulting from the harmful effects of breathing oxygen at elevated partial pressures. The most serious form of oxygen toxicity has the potential to affect a diver’s central nervous system and is a result of breathing very high-partial pressures (more than one bar or atmosphere) for a relatively short period of time (less than a few minutes at extreme levels). This type of toxicity may result in a clonic-tonic seizure; which in the water usually means death by embolism or drowning. Historically, this central nervous system condition was called the Paul Bert effect. The less problematic whole-body or pulmonary condition – a function of breathing lower partial pressures (less than one bar) over much longer periods – goes under the name the Lorrain Smith Effect, after the researchers who pioneered its discovery and description in the late 19th century.

I have heard and read that divers manage both Paul Bert and even Lorrain Smith effects by taking a short “air-break” during moderately long decompressions. The typical scenario is this: A diver conducts a deep or deepish dive which earns her a lengthy series of staged decompression stops on her way back to the surface. She finishes her dive by breathing pure oxygen at 6 metres on up. In this scenario, the decompression schedule requires the diver to breathe oxygen for around 20 minutes. There are a pile of variations on this theme, but the common thread is a fair amount of time breathing a gas that is delivering around 1.6 bar of oxygen… by the way, the NOAA limit for exposure to 1.6 bar of oxygen for a diver is 45 minutes, so this type of exposure does load a diver with the potential for a CNS incident… there is no argument there.

The “air-break” myth goes something like this. At some point during her spell breathing pure oxygen – sometimes at the end and sometime mid-stream – the diver will “RESET” her CNS “clock” by switching from breathing oxygen to breathing bottom mix, air, a less oxygen-rich nitrox (typically the mix she was breathing during her ascent to her final stops). Let’s illustrate the air-break protocol with a dive profile calling for a final decompression stop for 21 minutes at six metres or 20 feet. In this example, the diver might use oxygen for ten minutes, and then switch to say an EAN50 for five minutes, and finally switch back to oxygen for eleven minutes to finish up their deco. Typically, as in this example, the time spent on an “air-break” is not credited against the decompression obligation.

What I have yet to hear fully explained is how a five-minute break from breathing pure O2 resets a diver’s CNS loading during this procedure. Actually, you may also read postings from divers who rely on the same technique to manage Lorrain Smith effect, which shows an even greater misinterpretation of the mechanism behind the syndrome*.

OK, let’s take a step back and turn on the logic filter. According to NOAA – the folks who literally set the standards for nitrox use in the recreational dive community – a period of 20 minutes breathing oxygen at 6 metres – a practice that delivers a partial pressure or oxygen depth of around 1.6 bar/ata – has a corresponding time limit of 45 minutes. When we calculate the CNS loading for a dive, we are taught to account for the CNS loading for ALL phases of the dive. That’s to say, every minute spent breathing elevated levels of oxygen. Let’s ignore whatever came before during our example dive, and let us just focus on what happens at six metres or 20 feet. In a nutshell: The diver has to account for 20 minutes on pure oxygen. The NOAA tables don’t give a rat’s behind whether those 20 minutes are accumulated in one lump or two… or three or four. Twenty minutes is 20 minutes and uses up about 44-45 percent of the total allowable time regardless! The five minutes breathing another gas – in our example we can say she used EAN50 delivering an oxygen partial pressure of about 0.8 bar – simply adds a little to the total CNS loading, albeit a very tiny about (less than one percent). There is nothing in the NOAA dive manual or any of Hamilton’s published work that tells us anything different.

Now, to set the record straight, faced with the situation outlined above and breathing pure oxygen for that long, the chances are that I would take an air-break and recommend taking one to my team-mates; however, it has NOTHING to do with CNS but rather to help optimize off-gassing.

Oxygen is a vasoconstrictor – it causes some blood vessels to shut or partially shut – which may have some effect on general perfusion levels. This does not seem like a great plan for those of us trying to eliminate dissolved inert gas.
The bottom line is this: Let’s agree to take a break from pure O2 during our deco, but let’s not confuse the issue by suggesting that doing so magically helps manage CNS toxicity. Better yet, let’s opt to employ a better option and a slightly more helpful gas. But more about that later.

* Prolonged breathing of gas with an Fio2 (Fractional Inspired Oxygen) greater than 60 kPa (0.6 bar/ata)can lead to pulmonary toxicity and eventually irreversible pulmonary fibrosis, but this takes many hours or days and does not constitute an issue for the rank and file technical diver. Most likely, the “burning” sensation and pulmonary toxicity like symptoms mentioned by technical divers breathing oxygen and oxygen-rich gas during recreational decompression is a function of breathing cold, dry air (the dew-point of oxygen in the cylinders in my fill station is marked as -40! That’s dry.) This air has the ability to dry the mucus membranes lining our lungs and bringing on something called dry-air asthma. A less far-fetched probable outcome than pulmonary toxicity.