Background: The 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.
Methods: Twelve 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.
Results: Sevoflurane 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.
Conclusions: An 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 registration: Clinical Trials NCT01699802.
Keywords: airway; anaesthetic techniques; carbon dioxide; dead space; inhalation; rebreathing.
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