Deng C, Pollock NW, Gant N, Hannam JA, Dooley A, Mesley P, Mitchell SJ. The five-minute prebreathe in evaluating carbon dioxide absorption in a closed-circuit rebreather: a randomized single-blind study.
Diving Hyperb Med 2015;
45:16-24. [PMID:
25964034]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2014] [Accepted: 01/14/2015] [Indexed: 06/04/2023]
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 (PICO₂) 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 PICO₂ 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.
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