The duration of two carbon dioxide absorbents in a closed-circuit rebreather diving system.

INTRODUCTION: 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.