IN THE MIX
This rEvo mouthpiece has a mouthpiece restraining strap attached
Once diving incident triggers have been initiated and inappropriate breathing gas encountered , what then is the typical chain of events following LoC ? Inevitably , jaw muscles relax ; the time for this varies and to a certain degree is dependent on the disabling injury encountered . For example , during a hypoxic or hypercapnic event , it may be near instantaneous .
Alternatively , during a central nervous system oxygen toxicity event , the upper and lower jaw may initially remain clamped together during convulsions . However , regardless of the reason for LoC , the diver ’ s body will eventually relax , the likely consequence being loss of airway protection as the rebreather mouthpiece is dropped . If this occurs underwater , without immediate intervention , the following outcomes are inevitable :
■ Fluid aspiration and asphyxiation
■ Venting of breathing loop
■ Whole or partial flooding of the breathing loop
■ Significant loss of buoyancy
■ Drowning . “ I approached the diver and noticed the mouthpiece was out ” is a common rebreather fatality eye witness statement . In a sport diving fatality study undertaken
by Divers Alert Network ( DAN ), the actual cause of death as certified by a Medical Examiner , was drowning in 94 % of rebreather cases . Since this study , the trend in rebreather divers drowning following LoC has continued as a consequence of an unprotected airway . In one documented case , the time from entering the water to LoC and water aspiration leading to drowning was just 150 seconds .
A look in the rear-view mirror
Is the increased likelihood of loss of consciousness associated with rebreather use something new to us ? The answer is no , not at all . Twenty-five years ago the sport diving industry had limited rebreather experience and so in 1996 , the industry organised Rebreather Forum Two ( RF2 ). The conference objective was to address the major challenges involved in bringing rebreather technology to the consumer marketplace . Drawing on the collective experience of numerous delegates from sport , military and occupational diving backgrounds , a risk consensus was developed in order to help shape future sport rebreather diving practice .
Two key consensuses were agreed :
■ Rebreathers are much more complex than open circuit , with insidious risk .
■ Loss of consciousness presents a significant hazard when using rebreathers , likely to result in death by drowning .
One technical diving pioneer stated at RF2 : “ The challenge is going to bring the technology to market without killing too many divers along the way ”.
What can you do to increase the probability of surviving LoC ?
The risk associated with inappropriate breathing gas has been recognised by militaries since The Second World War . Recognising its life-preserving potential , when not using a full-face mask , the use of a Mouthpiece Retaining Strap ( MRS ) has and remains near universal among the global military rebreather diving community . In one military rebreather accident study , drowning after LoC occurred in only 5.5 % of cases over a 30-year period . This very low percentage of drowning was attributed to the implementation of safety protocols that included the use of an MRS to preserve the airway .
What is a mouthpiece retaining strap ?
An MRS is an adjustable means of securely holding a rebreather Dive Surface Valve ( DSV ) or BOV in position in the event of a loss of consciousness . To optimise its design intent , a well-designed MRS incorporates a padded flange of suitable material that when correctly fitted , serves to reinforce the lip seal around the bite mouthpiece , while stabilising the DSV / BOV in position . When worn correctly , evidence indicates an appropriately designed strap will prevent the rebreather DSV / BOV from being dropped . This will increase the probability of preserving the airway , potentially preventing or limiting water aspiration , breathing loop flood , loss of buoyancy and the likelihood of drowning . Because of these potentially life preserving functions , an MRS is a mandatory design requirement for all rebreathers sold within the European Union and United Kingdom . The rebreather standard specifies a design requirement for a mouthpiece assembly , “ which shall minimise the ingress
Both images show the mouthpiece retaining strap used with AP rebreathers
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