The effect of fresh gas flow rate and type of anesthesia machine on time to reach target sevoflurane concentration

Background: Anesthesia machines have been developed by the application of new technology for rapid and easier control of anesthetic concentration. In this study, we used a test lung to investigate whether the time taken to reach the target sevoflurane concentration varies with the rate of fresh gas flow (FGF) and type of anesthesia machine (AM). Methods: We measured the times taken to reach the target sevoflurane concentration (2 minimum alveolar concentration = 4%) at variable rates of FGF (0.5, 1, or 3 L/min) and different types of AM (Primus (R), Perseus (R), and Zeus (R) [Zeus (R) -F; Zeus (R) fresh gas mode, Zeus (R) -A; Zeus (R) auto-mode]). Concomitant ventilation was supplied using 100% O-2. The AMs were connected to a test lung. A sevoflurane vaporizer setting of 6% was used in Primus (R), Perseus (R), and Zeus (R) -F; a target end-tidal setting of 4% was used in Zeus (R) -A (from a vaporizer setting of 0%). The time taken to reach the target concentration was measured in every group. Results: When the same AM was used (Primus (R), Perseus (R), or Zeus (R) -F), the times to target concentration shortened as the FGF rate increased (P<0.05). Conversely, when the same FGF rate was used, but with different AMs, the time to target concentration was shortest in Perseus (R), followed by Primus (R), and finally by Zeus (R) -F (P<0.05). With regards to both modes of Zeus (R), at FGF rates of 0.5 and 1 L/min, the time to target concentration was shorter in Zeus (R) -A than in Zeus (R) -F; however, the time was longer in Zeus (R) -A than in Zeus (R) -F at FGF rate of 3 L/min (P<0.05). Conclusion: Shorter times taken to reach the target concentration were associated with high FGF rates, smaller internal volume of the AM, proximity of the fresh gas inlets to patients, absence of a decoupling system, and use of blower-driven ventilators in AM.