title: The five-minute prebreathe in evaluating carbon dioxide absorption in a closed-circuit rebreather: a randomized single-blind study.
abstract: BACKGROUND
  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.
authors.affiliation: Department of Anaesthesiology, University of Auckland.
authors.forename: Carolyn
authors.initials: C
authors.lastname: Deng
authors.affiliation: Center for Hyperbaric Medicine and Environmental Physiology, Duke University Medical Center, Divers Alert Network, Durham, NC, USA.
authors.forename: Neal W
authors.initials: NW
authors.lastname: Pollock
authors.affiliation: Department of Sport and Exercise Science, University of Auckland.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Gant
authors.affiliation: Department of Anaesthesiology, University of Auckland.
authors.forename: Jacqueline A
authors.initials: JA
authors.lastname: Hannam
authors.forename: Adam
authors.initials: A
authors.lastname: Dooley
authors.affiliation: Divetec New Zealand, Auckland.
authors.forename: Peter
authors.initials: P
authors.lastname: Mesley
authors.affiliation: Department of Anaesthesia, Auckland City Hospital, Head of Department, Department of Anaesthesiology University of Auckland, Private Bag 92019, Auckland, New Zealand, Phone: +64-(0)9-923-2569, E-mail: sj.mitchell@auckland.ac.nz.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Mitchell
mesh_terms: D065966:Absorption, Physicochemical; D000328:Adult; D002245:Carbon Dioxide / Q000032:analysis / Q000737:chemistry*; D004242:Diving; D004868:Equipment Failure; D005260:Female; D015920:Gas Scavengers; D006801:Humans; D006935:Hypercapnia / Q000175:diagnosis / Q000209:etiology / Q000517:prevention & control*; D008297:Male; D012134:Respiratory Protective Devices; D016037:Single-Blind Method; D013997:Time Factors
chemical_list: D002245:Carbon Dioxide
keywords: Scuba diving; capnography; carbon dioxide; hypercapnia; physiology; rebreathers/closed circuit; rebreathing