- Waste Anesthetic Gases: Focus on a Major Problem
- ASA Monitor 9 2019, Vol.83, 26-28.
- Uday Jain, B.S.E.E., M.D., Ph.D., FASA; Dolores Njoku, M.D.; Chris R. Giordano, M.D., FASA
1-1-8 one-step sevoflurane wash-in scheme for low-flow anesthesia: simple, rapid, and predictable induction
BMC Anesthesiology volume 20, Article number: 23 (2020)
Keypoints: Due to Sevoflurance’s high cost, Low-Flow Anesthesia (LFA; fresh gas flow (FGF) ≤ 1 L·min− 1) is used to reduce the amount needed, thus significantly lowering costs.
For example, one case that is one hour long could cost:
- FGF 6 L/min = $40/hour
- FGF 3 L/min (standard flow rate) = $20/hour
- FGF 1.5 L/min = $10/hour, then switch to Isoflurane = $2/hour
Low-flow anesthesia (LFA; fresh gas flow (FGF) ≤ 1 L·min− 1) is gaining in popularity because it has a relatively lower cost, causes less environmental burden, and medically because it increases the humidity and temperature of inspired gas, leading to improved mucociliary function of the patient . Since the use of low FGF leads to a long time constant, a wash-in phase using a high FGF and high vaporizer concentration of volatile anesthetic (FV) is warranted in order to rapidly achieve the required concentration of inhalation anesthetic in the breathing system. Sevoflurane—when used with strong base-free CO2 absorbent—is suitable for use in LFA because it has low blood-gas solubility.
Sevoflurane is a popular and widely used volatile anesthetic because it does not irritate the airway, hence it can be used as an induction agent, especially in children. Moreover, its low blood and fat solubility leads to rapid onset, easy depth of anesthesia adjustment, and early recovery . Due to its high cost, however, LFA is used to reduce the amount needed . The more important reasons to implement sevoflurane LFA are benefits to environment and mucociliary function of the patient . Previously, the recommended lowest FGF to be used with sevoflurane was 1 L·min− 1 for exposures up to 1 h and 2 L·min− 1 for exposures > 1 h because of compound A concern . With the introduction of strong base-free CO2 absorbents (e.g., Amsorb Plus and Litholyme), the issue with compound A from sevoflurane has been resolved and sevoflurane can be safely used in LFA . LFA, however, needs a wash-in phase to rapidly build up FAS to the required target concentration. The wash-in can be achieved by (a) increasing FGF to reduce the time constant ; (b) increasing FVS to induce a concentration effect ; or (c) integrating both methods.
A few studies have addressed the wash-in technique for sevoflurane LFA. Lindqvist et al. reported a 2-step wash-in technique to achieve a FAS of 1.2%; starting with FGF 1 L·min− 1 and FVS 8% for 1 min, then reducing FGF to 1, 0.7, 0.5, and 0.3 L·min− 1. They found that the respective time to achieve the target FAS was 1.8, 1.5, 2.5, and 3.6 min
The 1-1-8 wash-in scheme for sevoflurane LFA has many advantages, including simplicity, coverage, swiftness, safety, economy, and that it can be used with both N2O and Air. A respective FAS of 1, 1.5, 2, 2.5, 3, and 3.5% when used with N2O and Air can be expected at 1, 1.5, 2, 3, 3.5, and 4.5 min and 1, 1.5, 2, 3, 4, and 5 min.
In patients requiring general anesthesia with endotracheal intubation and controlled ventilation, the 1-1-8 wash-in scheme for sevoflurane LFA yields a respective FAS of 1, 1.5, 2, 2.5, 3, and 3.5% at 1, 1.5, 2, 3, 3.5, and 4.5 min when used with N2O and at 1, 1.5, 2, 3, 4, and 5 min when used with Air. This technique uses a one-step setting for O2:N2O or O2:Air 1:1 L·min− 1 with sevoflurane 8%. There were statistically but not clinically significant changes in heart rate and blood pressure during the wash-in process.