Desflurane consumption during automated closed-circuit delivery is higher than when a conventional anesthesia machine is used with a simple vaporizer-O(2)-N(2)O fresh gas flow sequence

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), identical at 4 min (P > 0.05), and slightly higher after 4 min (P < 0.05). Desflurane consumption was higher in the Zeus group at all times, a difference that persisted after correcting for the small difference in F(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.