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by Dr. Andrew Fock
We surveys CCR divers from around the world. Here are the results.
I will admit to being quite floored when Michael Menduno asked me to offer an opinion on mouthpiece restraining straps (MPRS), as I considered this subject fairly done and dusted. But then I guess I should not have been totally surprised given that when I provide medical support on Pete Mesley’s Truk trips I still see few people using them. The recent death of a colleague of mine who drowned after becoming unconscious underwater without one brought this matter back for discussion once again.
An analysis of CCR related deaths shows that the vast majority of them are due to either hyperoxia, hypoxia, or CO2 retention.(1) This situation does not seem to have changed over the years, although fortunately the number of fatalities seems to be low. In each case, the primary cause is not immediately fatal; however, the consequence of each one is usually unconsciousness with the loss of the CCR mouthpiece from the mouth. In the aquatic environment, water not being a respirable medium, the result is usually fatal.
The obvious solution would therefore seem to be to use a full face mask (FFM), and indeed, if applied properly this would prevent drowning in the event of unconsciousness as has been attested to by many commercial and military divers. Unfortunately, nothing comes for free, and in the tech diving world, where we are observation divers rather than working divers, FFMs usually come with a reduced field of view and are difficult to bailout from should there be an issue with the CCR (in itself an extremely rare event). They also make gas switching more difficult, although I would hope that that old chestnut has also gone the way of the dinosaur!
So, if FFMs are not so good, then is there an alternative that can allow easy bailout but still save a life in the event of the diver becoming unconscious?
The only large study looking at this was conducted by the French navy and was published in “Military Medicine” in 2011 by Gempp et al.(2) This study looked at accidents on Rebreathers between 1979 and 2009. The number of accidents reported was 153, of which gas toxicities were thought to be responsible in 68% of cases, and loss of consciousness was reported in 58 cases. Despite this, there were only three fatal cases, all of which were caught in wrecks. All divers used a MPRS with a lip guard. This is in stark contrast to the civilian CCR deaths, where unconsciousness while underwater is almost universally fatal. It is also poignant to note that military divers always dive in pairs so that it is possible for the unimpaired diver to recover his unconscious buddy.
So, is this study only relevant to military combat divers?
While all the divers in the study were military, not all were using oxygen rebreathers and, in any case, the point is that in 58 cases where a diver became unconscious underwater, none of the divers who were not trapped drowned. To me this is a pretty compelling case for the use of a MPRS with a lip guard.
A range of arguments are often put forward as reasons not to use MPRS. One is that if the diver convulses then you cannot take them off O2. This is just silly. First, during a convulsion the mouthpiece almost always comes out of the mouth if there is no MPRS. From the multiple case reports I’ve read, there are very few cases where the buddy was able to place an alternate air source successfully in the diver’s mouth and reinitiate breathing. In most cases, the victim was sent to the surface where they died of drowning and severe DCI. Had a MPRS with a lip guard been in place, the CCR could have been flushed with diluent, and the diver kept at depth without the buddy having to worry about maintaining the airway.
The next argument against MPRS is that they make emergency bailout or gas switching more difficult. The most common version of the MPRS with built in lip guard is the Draeger type. This comes with a rubber head strap. It is undeniable that this head strap can be difficult to adjust for comfort, and when properly fitted, can also be difficult to doff (especially without dislodging the face mask). In contrast, the MPRS built by AP Diving is very easy to adjust, easy to don and doff, and due to the very elastic silicone straps, is very easy to remove in an emergency. However, as it lacks the lip guard, it is also less than ideal. The ideal in my opinion, is a combination of both, utilizing the lip guard of the Draeger system with the strap of the APD system. The added advantage of this combined system is dramatically increased comfort and reduced jaw fatigue during long decompressions.
I’ve been using this system for almost a decade now and would not dive any other way. There seems to be no downside to a system like this, and it might just save a life, so why would you not use one?
1. Fock AW. Analysis of recreational closed-circuit rebreather deaths 1998-2010. Diving Hyperb Med. 2013;43(2):78-85.
2. Gempp E, Louge P, Blatteau JE, Hugon M. Descriptive epidemiology of 153 diving injuries with rebreathers among French military divers from 1979 to 2009. Military medicine. 2011;176(4):446-50.
Dr Andrew Fock MB; BS, FANZCA, Dip AdvDHM(ANZCA), Dip DHM(SPUMS), is Head of Hyperbaric Services at the Alfred Hospital, Melbourne Australia. Andrew’s primary specialist training is as an anesthetist, completing his fellowship in cardiac anesthesia. While still maintaining a private practice in anesthesia, Andrew took up a VMO position at the Hyperbaric Unit in 2004, subsequently gaining his diploma in Diving and Hyperbaric Medicine with his thesis in Decompression theory and Deep decompression stops.
He is widely published in the peer-reviewed diving literature and is a regularly- invited speaker to both scientific hyperbaric meetings and diving industry conferences around the world. He was a board member of the Divers Alert Network Asia-Pacific and is a senior medical advisor to the DAN Australasian diver mortality study. He is an active technical mixed gas diver and has led a number of diving expeditions to shipwrecks around the world to depths in excess of 130 msw. He also holds a commission in the RANR, providing expertise in the areas of Diving, Hyperbaric and Submarine medicine to the ADF. He is an Adjunct Senior Researcher for the School of Public Health and Preventative Medicine Monash University.
Evolution of Dive Planning
Unbeknown to many tekkies, sophisticated dive computers like Shearwater’s offer divers real-time dive planning tools that enable them to adjust their dive plan on the fly. This summary of erudite tech educator Mark Powell’s latest blog outlines some of the specifics. Be sure to check out his blog.
Header image courtesy of Trisha Stovel
Find the full story in the Shearwater Blog
Mark Powell discusses decompression planning for technical dives with diving computers and PC planning tools. More time is spent planning technical dives than recreational dives. This is due to increased risks, greater depths, high gas usage at depth, increased decompression obligations, increased oxygen toxicity loading, and a host of other reasons. He describes how in the early days of technical diving, there were no PC planning tools or dive computers suitable for technical dive planning. Now computers are much more available and reliable. Also, the costs have reduced so much that many people have backup computers. The flexibility offered by the computer is in contrast to the rigid nature of tables. Unfortunately, when your backup is based on written tables, you can’t make full use of this flexibility. When you have a backup computer, suddenly this flexibility comes into its own and this is where significant changes to planning styles started to be adopted.
In reality, a dive computer gives a much more flexible tool for managing the dive. However, many divers keep the tables mindset even when using a very reliable and flexible planning tool. It is important to understand the features incorporated in your dive computer as they can provide additional information that can be used to manage the situation. On the Shearwater computer range, the NDL is shown on the display and counts down the time available until it reaches 0. Once the diver goes into deco, this field can be configured to show several other pieces of information. Any one of these can be selected to be shown in the NDL space once the NDL reaches zero. Alternatively, all of the following options can be viewed together by stepping through the display options: TTS, @+5, Δ +5, CEIL, GF99 setting, and SurfGF display. This article is intended to show that, far from removing the need to plan a dive, the sophisticated dive computers available today can help to improve the planning process. They can be used to provide a more realistic and more flexible planning tool. They can also be used to adapt the plan when the situation changes. This is only possible if the diver understands the tools they have at their disposal and practices using them.
After reading and digesting the information contained in this article, I would encourage you to make sure you know where to find the various display options on your computer. On your next dive look at the SurfGF value during the dive and watch the relationship between it and the NDL value. During the NDL ascent, look at the GF99 and SurfGF values. Then on a decompression dive, compare the CEIL and Stop Depth values as well as comparing the CEIL, GF99, and SurfGF values. It is essential that you understand all of the information in this article and practice it before using it to plan your dive or modify your dive plan. Like any tool, you must practice before using them for real. However, a bit of investment in time and practice will give you the ability to manage your ascent in a much more intelligent way than blindly following your computer or a fixed set of deco tables.
Read the full article on Shearwater’s Blog.
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