Agent consumption with the Zeus® in the automated closed circuit anesthesia mode with O₂/air mixtures

Background: Earlier software versions of the Zeus® (Lübeck, Dräger, Germany) failed to provide true closed circuit anesthesia (CCA) conditions. We examined whether the latest software (SW 4.03 MK 04672-00) achieves this goal. Methods. In 8 ASA I-III patients, the CCA mode of the Zeus® was used to maintain the inspired O₂(F_IO₂) and end-expired sevoflurane % (F_Asevo) at 50 and 1.8%, respectively. The fresh gas flow (FGF) of O₂and air and the sevoflurane injection rate (=Vinj_sevo, mL liquid sevo/h) were videotaped from the control screen and entered offline into a spreadsheet. Cumulative sevoflurane usage during early wash-in (=0-1 min, CD_sevo0-1), late wash-in (=1-5 min, CD_sevo1-5), and maintenance (=5-60 min, CD_sevo5-60) was calculated, and Vinj_sevobetween 1 and 60 min was compared with published uptake data. Results: F_Asevoreached 1.8% within 101 (23) sec. CD_sevo0-1 was between 1.24 (0.03) and 3.01(0.25) mL (a range is provided because no absolute Vinj_sevovalues were displayed once Vinj_sevowas > 100 mL/h, which occurred between 15 ± 2 and 46 ± 6 sec). CD_sevo1-5 was 0.81 (0.37) mL, and CD_sevo5-60 was 4.63 (0.94) mL. The Vinj_sevopattern between 1 and 60 min matched previously published uptake data. Brief high FGF periods were used to maintain the target F_IO₂, and to refill the reservoir bag after external pressure had been applied to the abdomen; subsequent "spikes" wasted 0.08-0.19 mL and 0.14-0.49 mL sevoflurane (1-3% and 3-9% of total agent usage between 1 and 60 min, respectively). Conclusion: Under the conditions specified, the Zeus® approaches CCA conditions so closely that further reductions in agent usage would have minimal economic significance.