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- Title
The five-minute prebreathe in evaluating carbon dioxide absorption in a closed-circuit rebreather: a randomized single-blind study.
- Authors
Deng, Carolyn; Pollock, Neal W.; Gant, Nicholas; Hannam, Jacqueline A.; Dooley, Adam; Mesley, Peter; Mitchell, Simon J.
- Abstract
Introduction: Closed-circuit underwater rebreather apparatus (CCR) recycles expired gas through a carbon dioxide (CO2) 'scrubber'. Prior to diving, users perform a five-minute 'prebreathe' during which they self-check for symptoms of hypercapnia that might indicate a failure in the scrubber. There is doubt that this strategy is valid. Methods: Thirty divers were block-randomized to breathe for five minutes on a circuit in two of the following three conditions: normal scrubber, partly-failed scrubber, and absent scrubber. Subjects were blind to trial allocation and instructed to terminate the prebreathe on suspicion of hypercapnia. Results: Early termination was seen in 0/20, 2/20, and 15/20 of the normal, partly-failed, and absent absorber conditions, respectively. Subjects in the absent group experienced a steady, uncontrolled rise in inspired (P1CO2) and end-tidal CO2 (PETCO2). Seven subjects exhibited little or no increase in minute volume yet reported dyspnoea at termination, suggesting a biochemically-mediated stimulus to terminate. This was consistent with results in the partly-failed condition (which resulted in a plateaued mean P1CO2 near 20 mmHg), where a small increase in ventilation typically compensated for the inspired CO2 increase. Consequently, mean PETCO2 did not change and in the absence of a hypercapnic biochemical stimulus, subjects were very insensitive to this condition. Conclusions: While prebreathes are useful to evaluate other primary functions, the five-minute prebreathe is insensitive for CO2 scrubber faults in a rebreather. Partly-failed conditions are dangerous because most will not be detected at the surface, even though they may become very important at depth.
- Subjects
RESPIRATION; BREATHING apparatus; CARBON dioxide; RANDOMIZED controlled trials; VENTILATION
- Publication
Diving & Hyperbaric Medicine: Journal of the South Pacific Underwater Medicine Society, 2015, Vol 45, Issue 1, p16
- ISSN
1833-3516
- Publication type
Article