1-1-8 one-step sevoflurane-nitrous oxide wash-in scheme for low-flow anesthesia: simple, rapid, and predictable induction

Abstract Background Sevoflurane is suitable for low-flow anesthesia (LFA). LFA needs a wash-in phase. The reported sevoflurane wash-in schemes lack simplicity, target coverage, and applicability. We proposed a one-step 1-1-8 wash-in scheme for sevoflurane-nitrous oxide (N2O) LFA. The objective of our study was to identify times to achieve every alveolar concentration of sevoflurane (FAS) from 1% to 3.5%. Methods We recruited 102 adults requiring general anesthesia with endotracheal intubation and controlled ventilation. After induction and intubation, a wash-in was started using a fresh gas flow of oxygen (O2):N2O at 1:1 L·min-1 plus sevoflurane 8%. The ventilation was controlled to maintain end-tidal carbon dioxide (CO2) of 30-35 mmHg. Results The rising patterns of FAS and inspired concentration of sevoflurane (FIS) are similar and parallel. The FAS/FIS ratio increased from 0.46 to 0.72 within 260 sec. The respective times to achieve FAS of 1%, 1.5%, 2%, 2.5%, 3% and 3.5% were 1, 1.5, 2, 3, 3.5, and 4.5 min. The heart rate and blood pressure significantly increased initially then gradually decreased as FAS increased. Conclusions The 1-1-8 wash-in scheme for sevoflurane has many advantages, including simplicity, coverage, swiftness, safety, and economy. A respective FAS of 1%, 1.5%, 2%, 2.5%, 3%, and 3.5% can be expected at 1, 1.5, 2, 3, 3.5, and 4.5 min. This scheme may be applied for LFA in the situation where anesthetic gas analyzer is not available.