• C L Knaggs and G B Drummond.
• Clinical Neurosciences, Royal Infirmary, Edinburgh EH16 4SA, UK.
• Br J Anaesth. 2005 Aug 1;95(2):178-82.

BackgroundRebreathing will occur if a low gas flow and a Mapleson D circuit are used to induce anaesthesia with a volatile anaesthetic agent. This has the advantage that it allows ventilation to be sustained when consciousness is lost, and specific manoeuvres such as breath-holding or vital capacity breaths are not needed to facilitate induction of anaesthesia. However, if the fresh gas flow were too small, this would slow induction by limiting the rate of delivery of the anaesthetic agent. To assess the impact of fresh gas flow and rebreathing, we compared induction using sevoflurane 8% given by three different methods.MethodsWe randomly allocated 65 patients to receive induction of anaesthesia from either a Mapleson A breathing system with a fresh gas flow of 9 litre min(-1) (group A9), a Mapleson D system with a fresh flow of 6 litre min(-1) (group D6) or a Mapleson D system with a fresh flow of 3 litre min(-1) (group D3). We measured times for induction, end-tidal sevoflurane and end-tidal carbon dioxide.ResultsThe median (quartiles) induction times were 58 (45, 72), 50 (42, 65) and 64 (52, 92) s in the groups A9, D6 and D3 respectively. Induction of anaesthesia took longer (P<0.01) and was more variable in group D3. In this group, end-tidal sevoflurane concentration at the time of induction of anaesthesia was lower (P<0.05). In group A9, end-tidal carbon dioxide was less (P<0.05).ConclusionsIn adult patients allowed to breathe normally, prompt and consistent inhalation induction of anaesthesia with sevoflurane is obtained when fresh gas flow is limited to 6 litre min(-1) from a Mapleson D circuit, but smaller flows are impractical.

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