Effect of N2O on sevoflurane vaporizer settings during minimal- and low-flow anesthesia

Abstract

Background: Uptake of a second gas of a delivered gas mixture decreases the amount of carrier gas and potent inhaled anesthetic leaving the circle system through the pop-off valve. The authors hypothesized that the vaporizer settings required to maintain constant end-expired sevoflurane concentration (Etsevo) during minimal-flow anesthesia (MFA, fresh gas flow of 0.5 1/min) or low-flow anesthesia (LFA, fresh gas flow of 1 1/min) would be lower when sevoflurane is used in oxygennitrous oxide than in oxygen. Methods: Fifty-six patients receiving general anesthesia were randomly assigned to one of four groups (n = 14 each), depending on the carrier gas and fresh gas flow used: group Ox.5 1 (oxygen, MFA), group NOx.5 1 (oxygen-nitrous oxide, MFA after 10 min high fresh gas flow), group Ox1 1 (oxygen, LFA), and group NOx1 1 (oxygen-nitrous oxide, LFA after 10 min high fresh gas flow). The vaporizer dial settings required to maintain Etsevo at 1.3% were compared between groups. Results: Vaporizer settings were higher in group Ox.5 1 than in groups NOx.5 1, Ox1 1, and NOx 1; vaporizer settings were higher in group NOx.5 1 than in group NOx1 I between 23 and 47 min, and vaporizer settings did not differ between groups Ox1 1 and NOx1 1. Conclusions: When using oxygen-nitrous oxide as the carrier gas, less gas and vapor are wasted through the pop-off valve than when 100% oxygen is used. During MFA with an oxygennitrous oxide mixture, when almost all of the delivered oxygen and nitrous oxide is taken up by the patient, the vaporizer dial setting required to maintain a constant Etsevo is lower than when 100% oxygen is used. With higher fresh gas flows (LFA), this effect of nitrous oxide becomes insignificant, presumably because the proportion of excess gas leaving the pop-off valve relative to the amount taken up by the patient increases. However, other unexplored factors affecting gas kinetics in a circle system may contribute to our observations.