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. 2022 Dec 20;52(4):237-244.
doi: 10.28920/dhm52.4.237-244.

Hypoxia signatures in closed-circuit rebreather divers

Daniel Popa  1   2 , Craig Kutz  3 , Morgan Carlile  3 , Kaighley Brett  4 , Esteban A Moya  5 , Frank Powell  5 , Peter Witucki  3 , Richard Sadler  6 , Charlotte Sadler  3
Affiliations

Hypoxia signatures in closed-circuit rebreather divers

Daniel Popa et al. Diving Hyperb Med. .

Abstract

Introduction: Faults or errors during use of closed-circuit rebreathers (CCRs) can cause hypoxia. Military aviators face a similar risk of hypoxia and undergo awareness training to determine their 'hypoxia signature', a personalised, reproducible set of symptoms. We aimed to establish a hypoxia signature among divers, and to investigate their ability to detect hypoxia and self-rescue while cognitively overloaded.

Methods: Eight CCR divers and 12 scuba divers underwent an initial unblinded hypoxia exposure followed by three trials; a second hypoxic trial and two normoxic trials in randomised order. Hypoxia was induced by breathing on a CCR with no oxygen supply. Subjects pedalled on a cycle ergometer while playing a neurocognitive computer game to simulate real world task loading. Subjects identified hypoxia symptoms by pointing to a board listing common hypoxia symptoms, and were instructed to perform a 'bailout' procedure to mimic self-rescue if they perceived hypoxia. Divers were prompted to bailout if peripheral oxygen saturation fell to 75%, or after six minutes during normoxic trials. Subsequently we interviewed subjects to determine their ability to distinguish hypoxia from normoxia.

Results: Ninety-five percent of subjects (19/20) showed agreement between unblinded and blinded hypoxia symptoms. Subjects correctly identified the gas mixture in 85% of the trials. During unblinded hypoxia, only 25% (5/20) of subjects performed unprompted bailout. Fifty-five percent of subjects (11/20) correctly performed the bailout but only when prompted, while 15% (3/20) were unable to bailout despite prompting. During blinded hypoxia 45% of subjects (9/20) performed the bailout unprompted while 15% (3/20) remained unable to bailout despite prompting.

Conclusions: Although our data support a normobaric hypoxia signature among both CCR and scuba divers under experimental conditions, most subjects were unable to recognise hypoxia in real time and perform a self-rescue unprompted, although this improved in the second hypoxia trial. These results do not support hypoxia exposure training for CCR divers.

Keywords: Physiology; Rescue; Safety; Technical diving.

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Conflict of interest statement

Conflict of interest and funding

No conflicts of interest were declared. The Diver’s Alert Network (DAN)/R.W. Bill Hamilton Memorial Dive Medicine Research Grant funded this study. PADI Foundation Grant 47849 provided additional support.

Figures

Figure 1
Figure 1
Study protocol; CCR – closed circuit rebreather; EtCO2 – end tidal carbon dioxide; EtO2 – end tidal oxygen; SpO2 – peripheral oxygen saturation
Figure 2
Figure 2
Reported hypoxia symptoms in descending order of frequency of occurrence at left. Percentage of blinded trials with recurrence of the symptoms reported during the unblinded hypoxia trial at right
Figure 3
Figure 3
Blinded gas identification among scuba and closed circuit rebreather (CCR) divers after completion of each experimental trial
Figure 4
Figure 4
Performance of the bailout procedure with and without prompting during both hypoxia trials. Prompting occurred only when oxygen saturation reached 75%; CCR – closed circuit rebreather
See this image and copyright information in PMC

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