Closed-circuit rebreathers (CCRs) offer longer dive times, lower consumption of (expensive) breathing gas and no emission of air bubbles, but also carry a higher risk - including hypoxia (lack of oxygen) if the device malfunctions - than conventional diving.
In aviation, crews undergo hypoxia signature training to recognize and respond to their individual oxygen deprivation symptoms - a practice that could also increase safety for CCR divers. The question is whether divers are better able to recognize hypoxia and initiate self-rescue measures after such training. This idea was tested in a study.
The study
Eight CCR divers and 12 scuba divers participated, who were first exposed to unblinded hypoxia to familiarize themselves with the occurrence of individual hypoxia symptoms, i.e. their own sensitivity to hypoxia. They then underwent three randomized trials: a second hypoxia exposure and two normoxia trials. Hypoxia was induced by having participants breathe on a CCR without oxygen while pedaling on a cycle ergometer and completing a computer game that simulated a real-life cognitive taskload.
Participants were instructed to watch for symptoms of hypoxia, referring to an information board that listed common warning signs. If they detected hypoxia based on their symptoms, they were instructed to perform a "bailout" maneuver that imitated a self-rescue, i.e. switch from CCR to a conventional breathing system. When a participant's peripheral oxygen saturation dropped to 75% (indicating severe oxygen deprivation), they were asked to perform the bailout. In post-trial interviews, participants' ability to distinguish hypoxia from normoxia was assessed.
Sobering results
95 % of the participants showed the same symptoms in unblinded and blinded hypoxia. However, in unblinded hypoxia, only 45% performed the bailout without prompting. 15% did not even manage to do so when prompted.
These are sobering results. Not even half of the participants managed to bail out in time, and some did not even do so when prompted!
The authors concluded that although the subjects had a "hypoxia signature", most were unable to recognize hypoxia in real time and perform a bailout maneuver without prompting. These results suggest that hypoxia awareness training does little to improve safety in CCR diving.
Hypoxia signature: wishful thinking
The problem is that the concept of a hypoxia signature is more wishful thinking than reality. A recently published study from the field of aviation medicine dispels the notion that aircraft crews have their own hypoxia signature and can recognize it in time. In fact, it found similar results as the CCR study: around half of the test subjects reported no oxygen deficiency symptoms when inhaling hypoxic gas.
Is that very surprising? I don't think so, because hypoxia leads to impaired mental cognition, which means that self-awareness is impaired precisely when it is most needed. This is clearly shown in the study. An oxygen saturation of 75% represents a severe oxygen deficiency. This is probably the reason why some test subjects were no longer able to perform the bailout, even when prompted to do so. So the cat is biting its own tail here.
This is not good news for CCR diving. It shows how dependent CCR diving is on a perfectly functioning technique.
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