• L W Sturesson, M Bodelsson, B Jonson, and G Malmkvist.
• Section of Anaesthesiology and Intensive Care, Lund University and Skane University Hospital, SE-221 85 Lund, Sweden louise.walther_sturesson@med.lu.se.
• Br J Anaesth. 2014 Sep 1;113(3):508-14.

BackgroundThe anaesthetic conserving device AnaConDa (ACD) reflects exhaled anaesthetic agents thereby facilitating the use of inhaled anaesthetic agents outside operating theatres. Expired CO₂ is, however, also reflected causing a dead space effect in excess of the ACD internal volume. CO₂ reflection from the ACD is attenuated by humidity. This study tests the hypothesis that sevoflurane further attenuates reflection of CO₂. An analysis of clinical implications of our findings was performed.MethodsTwelve postoperative patients received mechanical ventilation using a conventional heat and moisture exchanger (HME, internal volume 50 ml) and an ACD (100 ml), the latter with or without administration of sevoflurane. The ACD was also studied with a test lung at high sevoflurane concentrations. Reflection of CO₂ and dead space effects were evaluated with the single-breath test for CO2.ResultsSevoflurane reduced but did not abolish CO₂ reflection. In patients, the mean dead space effect with 0.8% sevoflurane was 88 ml larger using the ACD compared with the HME (P<0.001), of which 38 ml was due to CO₂ reflection. Our calculations show that with the use of the ACD, normocapnia cannot be achieved with tidal volume <6 ml kg(-1) even when respiratory rate is increased.ConclusionsAn ACD causes a dead space effect larger than its internal volume due to reflection of CO₂, which is attenuated but not abolished by sevoflurane administration. CO₂ reflection from the ACD limits its use with low tidal volume ventilation, such as with lung protection ventilation strategies.Clinical Trial RegistrationClinical Trials NCT01699802.© The Author [2014]. Published by Oxford University Press on behalf of the British Journal of Anaesthesia. All rights reserved. For Permissions, please email: journals.permissions@oup.com.

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