Diving and Hyperbaric Medicine Volume 46 No. 2 June 2016 92 The duration of two carbon dioxide absorbents in a closed-circuit rebreather diving system David Harvey, Neal W Pollock, Nicholas Gant, Jason Hart, Peter Mesley and Simon J Mitchell Abstract (Harvey D, Pollock NW, Gant N, Hart J, Mesley P, Mitchell SJ. The duration of two carbon dioxide absorbents in a closed- circuit rebreather diving system. Diving and Hyperbaric Medicine. 2016 June;46(2):92-97.) Introduction: Diving rebreathers use canisters containing sodalime preparations to remove carbon dioxide (CO 2 ) 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 CO 2 -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 CO 2 introduced to the expiratory limb at 2 L∙min -1 . The P I CO 2 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 P I CO 2 reached 1 kPa (‘breakthrough’). Peak inspiratory and expiratory pressures during tidal ventilation of the circuit were also recorded. Results: The mean operating duration to CO 2 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 CO 2 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. Key words Technical diving; equipment; rebreathing; exercise; risk management; safety Introduction Effective removal of carbon dioxide (CO 2 ) is fundamental to the function of rebreathing systems, such as those widely used in anaesthesia and in semi- or fully closed- circuit rebreathers (CCRs) used in technical diving. This is most commonly achieved by passing exhaled gas through granular ‘sodalime’; a mix of sodium hydroxide and calcium hydroxide which reacts with CO 2 to produce calcium carbonate (CaCO 3 ) and water (H 2 O). This is a consumptive reaction and a given mass of sodalime therefore has a finite absorptive capacity. If this capacity is exceeded during a dive, exhaled CO 2 will ‘break through’ the scrubber canister and be rebreathed by the diver. 1 CO 2 rebreathing is hazardous because it may result in hypercapnia which, in turn, can cause debilitating symptoms and increase the risk of cerebral oxygen toxicity. 1 CCRs are usually tested for use with specific sodalime preparations. However, divers may use alternative preparations for reasons that include cost, availability, and/or for perceived advantages in endurance or work of breathing. For example, the Inspiration Rebreather™ (AP Diving, Helston, Cornwall, UK) is designed and tested to use Sofnolime 797™ (Molecular Products, Essex, UK), but divers often report using Spherasorb™ (Intersurgical, Berkshire, UK), a product commonly used in anaesthetic circle circuits in operating rooms. 2 There is controversy regarding the performance of these different sodalime preparations. For instance, an unpublished clinical study (which is nevertheless used in promotion of Spherasorb) concluded that Spherasorb has a 30% longer useful duration than Sofnolime 797, 3 and yet a recent diving fatality during use of an Inspiration rebreather was speculatively attributed to breakthrough with the use of Spherasorb. 2 There is a conspicuous absence of available data from independent sources describing the relative CO 2 absorbing performance of these two sodalime preparations, particularly in the context of their use in CCRs. We undertook a laboratory study in which the primary outcome was comparison of their respective durations to significant CO 2 breakthrough when used in a CCR under conditions simulating moderate but sustainable underwater work. The null hypothesis was that there would be no difference in duration of use to reach a breakthrough PCO 2 of 1 kPa (7.5 mmHg). A secondary outcome was comparison of the difference between peak inspiratory and expiratory pressures generated when moving a tidal volume of 1.5 L around the rebreather loop during use of the two absorbents. Methods Although primarily a bench test study, development of the protocol required human participation and the study was approved by the University of Auckland Human Participation Ethics Committee (Reference 015280).