Comparing charcoal and zeolite reflection filters for volatile anaesthetics: A laboratory evaluation.

Background: A modified heat-moisture exchanger that incorporates a reflecting filter for use with partial rebreathing of exhaled volatile anaesthetics has been commercially available since the 1990 s. The main advantages of the device are efficient delivery of inhaled sedation to intensive care patients and reduced anaesthetic consumption during anaesthesia. However, elevated arterial CO2 values have been observed with an anaesthetic conserving device compared with a conventional heat and moisture exchanger, despite compensation for larger apparatus dead space.

Objective: The objective of this study is to thoroughly explore the properties of two reflecting materials (charcoal and zeolites).

Design: A controlled, prospective, observational laboratory study.

Setting: Lund University Hospital, Sweden, from December 2011 to December 2012.

Participants: None.

Interventions: Three filters, with identical volumes, were compared using different volatile anaesthetics at different conditions of temperature and moisture. The filtering materials were charcoal or zeolite. Glass spheres were used as an inert control.

Main Outcome Measures: Consumption of volatile anaesthetics using different reflecting materials in filters at different conditions regarding temperature and moisture. CO2 reflection by the filtering materials: glass spheres, charcoal or zeolite.

Results: Isoflurane consumption in an open system was 60.8 g h(-1). The isoflurane consumption in dry, warm air was 39.8 g h(-1) with glass spheres. Changing to charcoal and zeolite had a profound effect on isoflurane consumption, 11.8 and 10.7 g h(-1), respectively. Heating and humidifying the air as well as the addition of N2O created only minor changes in consumption. The percentage of isoflurane conserved by the charcoal filter was independent of the isoflurane concentration (0.5 to 4.5%). Reflection of sevoflurane, desflurane and halothane by the charcoal filter was similar to reflection of isoflurane. Both charcoal and zeolite filters had CO2 reflecting properties and end-tidal CO2 increased by 3 to 3.7% compared with glass spheres. This increase was attenuated to 1 to 1.4% when the air was heated and humidified, and isoflurane was added.

Conclusion: Charcoal and zeolite possess gas-reflecting properties, which can be used to conserve volatile anaesthetics. They also reflect CO2. The degree of CO2 reflection was reduced by heating and humidifying the air.