To the Editor:—
I read the article by Hendrickx et al. 1with great interest. In their study, the authors found that during minimal-flow anesthesia (MFA; 0.5 l/min), the vaporizer dial setting required to maintain the end-tidal sevoflurane concentration constant at 1.3% is lower when sevoflurane is delivered in an oxygen–nitrous oxide mixture than in oxygen alone because less gas and vapor are wasted through the pop-off valve with the oxygen–nitrous oxide mixture. During low-flow anesthesia (LFA; 1 l/min), however, vaporizer dial settings are similar with oxygen–nitrous oxide or oxygen, presumably because the proportion of excess gas leaving the pop-off valve relative to the amount taken up by the patient increases. However, I carefully examined the authors’ experimental design and found that the vaporizer setting required to maintain the end-tidal sevoflurane concentration constant at 1.3% is lower when sevoflurane is delivered in an oxygen–nitrous oxide mixture than in oxygen alone during MFA, simply because of the differences in the initial flow setup and not because less gas and vapor are wasted through the pop-off valve. We must recognize that the very large space of the anesthesia circuit (4.3 l) and the patient's functional residual capacity (2.5–3 l) existed before the patient's alveolar membrane; therefore, the use of MFA with oxygen alone certainly requires a much higher vaporizer dial setting to maintain the end-tidal sevoflurane concentration constant at 1.3%. On the other hand, MFA with oxygen–nitrous oxide mixture and a high fresh gas flow of 6 l/min was used for 10 min; therefore, the circuit and functional residual capacity were prefilled to keep the end-tidal sevoflurane concentration constant at 1.3%. Certainly lower sevoflurane vaporizer dial settings are required afterwards, even at MFA.
