title: The duration of two carbon dioxide absorbents in a closed-circuit rebreather diving system.
abstract: BACKGROUND
  Diving rebreathers use canisters containing sodalime preparations to remove carbon dioxide (CO₂) from the expired gas. These preparations have a limited absorptive capacity and therefore may limit dive duration. The Inspiration™ rebreather is designed for use with Sofnolime 797™ but some divers use Spherasorb™ as an alternative. There are no published data comparing the CO2-absorbing efficacy of these sodalime preparations in an Inspiration rebreather.
  
  
  METHODS
  An Inspiration rebreather was operated in a benchtop circuit under conditions simulating work at 6 metabolic equivalents (MET). Ventilation was maintained at 45 L·min⁻¹ (tidal volume 1·5 L; respiratory rate 30 min⁻¹) with CO₂ introduced to the expiratory limb at 2·L·min⁻¹. The PiCO₂ was continuously monitored in the inspiratory limb. The rebreather canister was packed to full volume with either Sofnolime or Spherasorb and 10 trials were conducted (five using each absorbent), in which the circuit was continuously run until the PiCO₂ reached 1 kPa ('breakthrough'). Peak inspiratory and expiratory pressures during tidal ventilation of the circuit were also recorded.
  
  
  RESULTS
  The mean operating duration to CO₂ breakthrough was 138 ± 4 (SD) minutes for 2.38·kg Spherasorb and 202 ± minutes for 2.64·kg Sofnolime (P < 0.0001). The difference between peak inspiratory and expiratory pressures was 10% less during use of Spherasorb, suggesting lower work of breathing.
  
  
  CONCLUSIONS
  Under conditions simulating work at 6 MET during use of an Inspiration rebreather a canister packed with Spherasorb reached CO₂ breakthrough 32% earlier with 10% less mass than Sofnolime packed to similar volume. Divers cannot alternate between these two preparations and expect the same endurance.
authors.affiliation: Department of Anaesthesia, Auckland City Hospital, New Zealand.
authors.forename: David
authors.initials: D
authors.lastname: Harvey
authors.affiliation: Divers Alert Network, Durham, NC, USA.|Department of Anaesthesiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland New Zealand, sj.mitchell@auckland.ac.nz.
authors.forename: Neal W
authors.initials: NW
authors.lastname: Pollock
authors.affiliation: Exercise Neurometabolism Laboratory, Department of Exercise Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Gant
authors.affiliation: Exercise Neurometabolism Laboratory, Department of Exercise Sciences, University of Auckland, Auckland, New Zealand.
authors.forename: Jason
authors.initials: J
authors.lastname: Hart
authors.affiliation: Lust for Rust Diving, Auckland, New Zealand.
authors.forename: Peter
authors.initials: P
authors.lastname: Mesley
authors.affiliation: Department of Anaesthesia, Auckland City Hospital, New Zealand.|Department of Anaesthesiology, School of Medicine, University of Auckland, Private Bag 92019, Auckland, New Zealand, Phone: +64-(0)2-7414-1212, E-mail: sj.mitchell@auckland.ac.nz.
authors.forename: Simon J
authors.initials: SJ
authors.lastname: Mitchell
mesh_terms: D065966:Absorption, Physicochemical; D017610:Calcium Compounds / Q000737:chemistry*; D002126:Calcium Hydroxide / Q000737:chemistry*; D002245:Carbon Dioxide / Q000737:chemistry*; D004242:Diving; D001239:Inhalation; D010087:Oxides / Q000737:chemistry*; D015203:Reproducibility of Results; D012119:Respiration; D012134:Respiratory Protective Devices; D012972:Sodium Hydroxide / Q000737:chemistry*; D013997:Time Factors
chemical_list: D017610:Calcium Compounds; D010087:Oxides; C475558:spherasorb; D002245:Carbon Dioxide; C004569:soda lime; D012972:Sodium Hydroxide; D002126:Calcium Hydroxide
keywords: Technical diving; equipment; exercise; rebreathing; risk management; safety