• Christian Thoben, Nils Dennhardt, Terence Krauß, Robert Sümpelmann, Stefan Zimmermann, Henrik Rüffert, and Sebastian Heiderich.
• From the Department of Sensors and Measurement Technology, Institute of Electrical Engineering and Measurement Technology, Leibniz University (CT, SZ), Clinic of Anaesthesiology and Intensive Care Medicine, Hannover Medical School, Hannover (ND, TK, RS, SH) and Clinic of Anaesthesiology and Intensive Care Medicine, Helios Klinik Schkeuditz, Leipzig, Germany (HR).
• Eur J Anaesthesiol. 2019 Nov 1; 36 (11): 851-856.

BackgroundTrigger-free anaesthesia is required for patients who are susceptible to malignant hyperthermia. Therefore, all trace of volatile anaesthetics should be removed from anaesthetic machines before induction of anaesthesia. Because the washout procedure is time consuming, activated charcoal filters have been introduced, but never tested under minimal flow conditions.ObjectiveTo investigate performance of activated charcoal filters during long duration (24 h) simulated ventilation.DesignA bench study.SettingA Primus anaesthesia machine (Dräger) was contaminated with either 4% sevoflurane or 8% desflurane by ventilating a test lung for 90 min. The machine was briefly flushed according to manufacturer instructions, activated charcoal filters were inserted and a test lung was ventilated in a 24 h test. Trace gas concentrations were measured using a closed gas loop high-resolution ion mobility spectrometer with gas chromatographic preseparation. During the experiment reduced fresh gas flows were tested. At the end of each experiment the activated charcoal filters were removed and the machine was set to standby for 10 min to test for residual contamination within the circuit. The activated charcoal filters were reconnected into the circuit to test their ability to continue removing volatile anaesthetics (functional test) from the gas. Control experiments were conducted without activated charcoal filters.Main Outcome MeasuresAbsolute concentrations of desflurane and sevoflurane.ResultsThe concentration of volatile anaesthetics dropped to less than 5 ppm (parts per million) following insertion of activated charcoal filters. In the desflurane experiments at least 1 l min FGF was needed to keep the concentration below an acceptable level (<5 ppm): 0.5 l min fresh gas flow was required in sevoflurane experiments. While activated charcoal filters in the sevoflurane tests passed the functional test after 24 h, activated charcoal filters in the desflurane tests failed.ConclusionActivated charcoal filters meet the requirements for trigger-free low flow (1 l min) ventilation over 24 h. Minimal flow (0.5 l min) ventilation may be possible for sevoflurane contaminated machines.

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