Desflurane consumption during automated closed-circuit delivery is higher than when a conventional anesthesia machine is used with a simple vaporizer-O2-N2O fresh gas flow sequence

Abstract Background The Zeus ® (Dräger, Lübeck, Germany), an automated closed-circuit anesthesia machine, uses high fresh gas flows (FGF) to wash-in the circuit and the lungs, and intermittently flushes the system to remove unwanted N 2 . We hypothesized this could increase desflurane consumption to such an extent that agent consumption might become higher than with a conventional anesthesia machine (Anesthesia Delivery Unit [ADU ® ], GE, Helsinki, Finland) used with a previously derived desflurane-O 2 -N 2 O administration schedule that allows early FGF reduction. Methods Thirty-four ASA PS I or II patients undergoing plastic, urologic, or gynecologic surgery received desflurane in O 2 /N 2 O. In the ADU group (n = 24), an initial 3 min high FGF of O 2 and N 2 O (2 and 4 L.min -1 , respectively) was used, followed by 0.3 L.min -1 O 2 + 0.4 L.min -1 N 2 O. The desflurane vaporizer setting (F D ) was 6.5% for the first 15 min, and 5.5% during the next 25 min. In the Zeus group (n = 10), the Zeus ® was used in automated closed circuit anesthesia mode with a selected end-expired (F A ) desflurane target of 4.6%, and O 2 /N 2 O as the carrier gases with a target inspired O 2 % of 30%. Desflurane F A and consumption during the first 40 min were compared using repeated measures one-way ANOVA. Results Age and weight did not differ between the groups (P > 0.05), but patients in the Zeus group were taller (P = 0.04). In the Zeus group, the desflurane F A was lower during the first 3 min (P 0.05), and slightly higher after 4 min (P A between the two groups. Conclusion Agent consumption with an automated closed-circuit anesthesia machine is higher than with a conventional anesthesia machine when the latter is used with a specific vaporizer-FGF sequence. Agent consumption during automated delivery might be further reduced by optimizing the algorithm(s) that manages the initial FGF or by tolerating some N 2 in the circuit to minimize the need for intermittent flushing.