Agent consumption with the Zeus® in the automated closed circuit anesthesia mode with O(2)/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 O(2) (F(I)O(2)) and end-expired sevoflurane % (F(Asevo)) at 50 and 1.8%, respectively. The fresh gas flow (FGF) of O(2) and 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(sevo)0-1), late wash-in (=1-5 min, CD(sevo)1-5), and maintenance (=5-60 min, CD(sevo)5-60) was calculated, and Vinj(sevo) between 1 and 60 min was compared with published uptake data. RESULTS: F(Asevo) reached 1.8% within 101 (23) sec. CD(sevo)0-1 was between 1.24 (0.03) and 3.01(0.25) mL (a range is provided because no absolute Vinj(sevo) values were displayed once Vinj(sevo) was > 100 mL/h, which occurred between 15 ± 2 and 46 ± 6 sec). CD(sevo)1-5 was 0.81 (0.37) mL, and CD(sevo)5-60 was 4.63 (0.94) mL. The Vinj(sevo) pattern between 1 and 60 min matched previously published uptake data. Brief high FGF periods were used to maintain the target F(I)O(2), 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.