Having surfaced after a routine , incident free 30m wreck dive , confused and bewildered , the diver suddenly became aware that he was now back underwater , alone and rapidly sinking . With a strong tide sweeping him off the site , he realised he was in mortal danger , and with his ear drums feeling like they were about to implode , he instinctively reached for his BC inflation device and squeezed .
The rapid inrush of gas began arresting his uncontrolled descent so he next switched his rebreather Bailout Valve ( BOV ) to open circuit . The diver eventually arrived back at the surface and was recovered by the safety boat crew , who having witnessed the diver go unconscious at the surface , thought he was lost beyond help . Back on board , the magnitude of what had just happened began to dawn upon the diver ; he had just survived a chain of events that in every case he had read or heard about , the outcome had been death by drowning . However , against the odds , today he would be returning home to see his wife and two young children .
This story of survival is not fiction – it is a recently documented case of a rebreather diver losing unconsciousness and sinking beyond the assistance of those around him . While the sequence of events that led to Loss of Consciousness ( LoC ) are important from an accident analysis and lessons identified perspective , what is arguably of equal importance and what this article focuses upon , is the circumstances that led to the diver ’ s unlikely survival .
The hazard of inappropriate breathing gas
When we dive a rebreather , we are breathing an artificially maintained and highly dynamic breathing mixture . As a consequence , it is widely acknowledged that when compared to open circuit scuba , the probability of exposure to an inappropriate breathing gas is greatly increased when using rebreathers . A serious incident or fatal accident is therefore more likely when rebreather diving , with one study estimating that the fatality potential is possibly four to ten times greater when using a rebreather , compared to diving a manifolded twin set .
Inappropriate breathing gas scenarios most frequently associated with rebreather use are : ( 1 ) hypoxia ; ( 2 ) hypercapnia , resulting from increased levels of inspired Carbon Dioxide ( CO2 ) or hypoventilation leading to retained CO2 ; ( 3 ) hyperoxia . Frequently referred to as the rebreather ‘ 3H hazards ’, all can and do lead to loss of consciousness with little or no warning .
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