Exhaustion by design

Abstract

We would like to highlight a problem with the design of the carbon dioxide (CO2) absorber of a Maquet FLOW-i® Anaesthesia Delivery System machine (Maquet Cardiovascular, LLC, Wayne, NJ, USA) that results in elevated inspired CO2 levels. The FLOW-i anaesthesia machine is fitted with a disposable proprietary CO2 absorber canister containing enabling rapid change of an exhausted absorber. Sofnolime™ granules are coated with an indicator that changes from white to purple at exhaustion. During a recent case under general anaesthesia with mechanical ventilation through a circle circuit with a fresh gas flow of 2 l.min−1, the baseline inspired CO2 level was found to be 0.53 kPa, even though only approximately one-third of the Sofnolime granules were visibly exhausted (Fig. 2a). The absorber function was not bypassed and the inspiratory and expiratory valves were working correctly. Attempts to decrease the inspired CO2 levels by gentle knocking of the absorber canister to settle the granules were unsucessful. As the inspired CO2 level continued to rise, we changed the absorber canister and the inspired CO2 fell back to zero. Examination of the CO2 absorber canister revealed uneven exhaustion of the absorber granules preferentially towards the machine-facing aspect of the canister (Fig. 2b), where both the inflow and exit pipe channels are situated (Fig. 2c). We think that there may be preferential airflow between these, leading to the clinical problems and granule exhaustion pattern we found. We invite Maquet to comment on whether they are considering redesign of the inflow/exit channel positions relative to each other, or the incorporation of baffles, to overcome this problem.