Desflurane usage during anesthesia with and without N₂O using FLOW-i Automatic Gas Control with three different...

AGC^® (Automatic Gas Control) is the FLOW-i's automated low flow tool (Maquet, Solna, Sweden) that target controls the inspired O₂ (F_IO₂) and end-expired desflurane concentration (F_Ades) while (by design) exponentially decreasing fresh gas flow (FGF) during wash-in to a maintenance default FGF of 300 mL min^-1. It also offers a choice of wash-in speeds for the inhaled agents. We examined AGC performance and hypothesized that the use of lower wash-in speeds and N₂O both reduce desflurane usage (Vdes). After obtaining IRB approval and patient consent, 78 ASA I-II patients undergoing abdominal surgery were randomly assigned to 1 of 6 groups (n = 13 each), depending on carrier gas (O₂/air or O₂/N₂O) and wash-in speed (AGC speed 2, 4, or 6) of desflurane, resulting in groups air/2, air/4, air/6, N₂O/2, N₂O/4, and N₂O/6. The target for F_IO₂ was set at 35%, while the F_Ades target was selected so that the AGC displayed 1.3 MAC (corrected for the additive affect of N₂O if used). AGC was activated upon starting mechanical ventilation. Varvel's criteria were used to describe performance of achieving the targets. Patient demographics, end-expired N₂O concentration, MAC, FGF, and Vdes were compared using ANOVA. Data are presented as mean ± standard deviation, except for Varvel's criteria (median ± quartiles). Patient demographics did not differ among the groups. Median performance error was -2-0% for F_IO₂ and -3-1% for F_Ades; median absolute performance error was 1-2% for F_IO₂ and 0-3% for F_Ades. MAC increased faster in N₂O groups, but total MAC decreased 0.1-0.25 MAC below that in the O₂/air groups after 60 min. The effect of wash-in speed on Vdes faded over time. N₂O decreased Vdes by 62%. AGC performance for O₂ and desflurane targeting is excellent. After 1 h, the wash-in speeds tested are unlikely to affect desflurane usage. N₂O usage decreases Vdes proportionally with its reduction in F_Atdes.