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

Abstract

AGC® (Automatic Gas Control) is the FLOW-i's automated low flow tool (Maquet, Solna, Sweden) that target controls the inspired O2 (FIO2) and end-expired desflurane concentration (FAdes) while (by design) exponentially decreasing fresh gas flow (FGF) during wash-in to a maintenance default FGF of 300mL 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 FIO2 was set at 35%, while the FAdes 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 FIO2 and -3-1% for FAdes; median absolute performance error was 1-2% for FIO2 and 0-3% for FAdes. MAC increased faster in N2O groups, but total MAC decreased 0.1-0.25 MAC below that in the O2/air groups after 60min. 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 1h, the wash-in speeds tested are unlikely to affect desflurane usage. N2O usage decreases Vdes proportionally with its reduction in FAtdes.