The performance of 'temperature stick' carbon dioxide absorbent monitors in diving rebreathers.

Introduction: Diving rebreathers use canisters containing soda lime to remove carbon dioxide (CO2) from expired gas. Soda lime has a finite ability to absorb CO2. Temperature sticks monitor the exothermic reaction between CO2 and soda lime to predict remaining absorptive capacity. The accuracy of these predictions was investigated in two rebreathers that utilise temperature sticks. Methods: Inspiration and rEvo rebreathers filled with new soda lime were immersed in water at 19 degrees C and operated on mechanical circuits whose ventilation and CO2-addition parameters simulated dives involving either moderate exercise (6 MET) throughout (mod-ex), or 90 minutes of 6 MET exercise followed by 2 MET exercise (low-ex) until breakthrough (inspired PCO2 [PiCO2] = 1 kPa). Simulated dives were conducted at surface pressure (sea-level) (low-ex: Inspiration, n = 5; rEvo, n = 5; mod-ex: Inspiration, n = 7, rEvo, n = 5) and at 3-6 metres' sea water (msw) depth (mod-ex protocol only: Inspiration, n = 8; rEvo, n = 5). Results: Operated at surface pressure, both rebreathers warned appropriately in four o five low-ex tests but failed to do so in the 12 mod-ex tests. At 3-6 msw depth, warnings preceded breakthrough in 11 of 13 mod-ex tests. The rEvo warned conservatively in all five tests (approximately 60 minutes prior). Inspiration warnings immediately preceded breakthrough in six of eight tests, but were marginally late in one test and 13 minutes late in another. Conclusion: When operated at even shallow depth, temperature sticks provided timely warning of significant CO2 breakthrough in the scenarios examined. They are much less accurate during simulated exercise at surface pressure.