Dr. Simon Mitchell of New Zealand, recently conducted a study that will no doubt rock diver’s understanding of their pre-breathe sequence. The study has not yet been published, but Mitchell felt it was important enough to present a preview at Eurotek last weekend in Birmingham, England. He also posted to CCR Explorers today. He says:
I presented two studies at Eurotek.
The first aimed to evaluate the efficacy of the 5 minute prebreathe in evaluation of CO2 scrubber function prior to diving a rebreather. This has been a controversial subject and we sought to gather some definitive data. Since this study is being written up for publication in a medical journal I am limited to the amount of detail I can provide here because “pre-publication” on the internet could compromise acceptance by a journal. However, I am happy to provide some basic details and the essential findings of the study… since having presented it at a conference the results will probably be discussed on line anyway.
We randomised diver subjects (the majority of whom were rebreather divers) to undertake 5 minute prebreathes on an Evo Plus rebreather. They were instructed to conduct the prebreathe in the same way that is recommended in training. Thus, they sat at rest breathing on the unit with the nose blocked. They were instructed to terminate the prebreathe if they considered they had any symptoms of CO2 toxicity. 60 prebreathes were conducted, comprised of 20 in each of 3 conditions: normal scrubber, completely absent scrubber, partial failure of the scrubber. The “partial failure” was achieved by leaving the O ring and spacer out of the inspo scrubber assembly. The subjects were blinded to the condition, and we masked any changes in resistance to breathing associated with the different scrubber conditions. We measured a variety of physiological parameters during each prebreathe including pulse rate, breathing rate, tidal volume (the volume of each breath), minute volume (the volume of gas moved in and out of the lungs each minute), the inspired PCO2, and the end tidal PCO2.
The primary outcome was comparison of the proportion of subjects who terminated the prebreathe in each condition.
No subjects terminated when a normal scrubber was in place (as you would expect).
25% of subjects did not terminate the prebreathe when there was no scrubber present despite dramatic changes in the physiological parameters.
90% of subjects did not terminate the prebreathe in the partial failure condition despite significant CO2 break through and some changes in the physiological parameters.
The changes in physiological parameters were fascinating and helped considerably with interpretation of the above results, but I will not discuss those at this stage because this will form much of the discussion in the paper. Once the study is published we will be able to exploit the significant educational potential of those results.
The obvious conclusion is that even in a study where there would have been a high expectation of scrubber problems among subjects, the 5 minute prebreathe had only mediocre sensitivity for detecting complete absence of a scrubber, and extremely poor sensitivity for detection of a significant partial failure. We therefore believe that it is not a valid intervention. I hasten to add that this is NOT to say that there should be no prebreathe. The prebreathe also gives the user the opportunity to ensure that other systems (like the oxygen controller) are working correctly. But it does not need to be 5 minutes long in the belief that this allows detection of problems with the carbon dioxide scrubber.
I will write on the second study in a separate post.
Hope this makes sense.
Simon Mitchell
