Second lesson to be learned and this I posted before long ago - a 5 minute pre-breathe is insufficient to detect CO2 problems - do 10 minutes minimum.
I am sure most readers here are aware of the recent study on the effectiveness of pre-breathing to detect the presence of CO2 completed by Dr. Simon Mitchell.
His initial presentation was made at EuroTEK this past summer, and we await the full paper. In the meanwhile, here are the major points in Simon's own words.
"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."
Certainly something to think seriously about.