Maintaining sevoflurane anesthesia during low-flow anesthesia using a single vaporizer setting change after overpressure induction

Abstract

Study Objective: A sevoflurane vaporizer dial setting of 1.9% was previously found to maintain the end-expired sevoflurane concentration (Et sevo) at 1.3% during maintenance of anesthesia for procedures up to one hour with an O 2 FGF of 1 L/min. We examined whether applying these parameters could simplify low-flow sevoflurane anesthesia after overpressure induction using two slightly different techniques. Design: Prospective clinical study. Setting: Large teaching hospital. Patients: Sixteen patients receiving general anesthesia for a variety of peripheral procedures. Interventions: Anesthesia was induced with overpressure with sevoflurane (8%) in an 8 L · min −1 O 2/N 2O mixture (30%/70%). After a laryngeal mask airway (LMA) was placed, fresh gas flow (FGF) was lowered to 1 L · min −1 using O 2 and N 2O (FiO 2 30%) with patients breathing spontaneously. In group I patients (n = 8), the vaporizer dial was set at 1.9% at the same time the FGF was lowered. In group II patients (n = 8), the vaporizer was turned off until Et sevo had decreased to 1.3%, after which the dial was set at 1.9%. The course of Et sevo in the two groups was examined. Measurements and Main Results: In group I, Et sevo after 3 min was 4.88 ± 1.12%. Et sevo decreased slowly after reduction of FGF to 1.83 ± 0.19%, 1.59 ± 0.18%, and 1.52 ± 0.19% at 10, 20, and 30 min, respectively. In group II, Et sevo after 3 min was 4.34 ± 0.84%, and decreased more rapidly after reduction of FGF to 1 L · min −1 than in group I. Et sevo was 1.40 ± 0.09%, 1.40 ± 0.11%, and 1.38 ± 0.13% at 10, 20, and 30 min, respectively. Conclusions: After high-flow overpressure induction with sevoflurane, a single change in vaporizer setting (to 1.9%) and FGF (to 1 L · min −1) suffices for the Et sevo to approach the predicted Et sevo (1.3%) within 10–15 min; thereafter the Et sevo remains nearly constant. As expected, the predicted Et sevo is attained slightly faster when the vaporizer is temporarily turned off. Clinically applying previously derived pharmacokinetic parameters simplifies low-flow sevoflurane anesthesia after overpressure induction.