VAPORIZER SETTING VARIABILITY INCREASES WITH LOWER FRESH GAS FLOWS

Abstract

S344 INTRODUCTION: In a circle system, rebreathing increases when fresh gas flow (FGF) is lowered. [1] Because individual anesthetic uptake is unpredictable and differs to a large degree, the range of vaporizer settings [Vap] between patients needed to maintain a constant end-expired anesthetic concentration [Et] is likely to be wider with lower FGF. [2] We tested this hypothesis for sevoflurane (sevo). METHODS: With IRB approval and informed consent, 48 ASA I-III patients received general endotracheal anesthesia with sevo in O2. Patients were assigned to 1 of 7 groups depending on the FGF used: 0.2 L/min with or without the use of an initial 4 L/min FGF during 30 s to hasten circuit wash-in (0.2a and 0.2b), 0.5, 1, 2, 4, or 8 L/min. After IV induction, [Etsevo] was maintained at 0.65 MAC (1.3%). [Vap] was recorded every min for 1 h. The variability (coefficient of variation) of [Vap] at 15, 30, 45, and 60 min was calculated. RESULTS: Variability of [Vap] was <8% with FGF of 4-8 L/min, and was significantly higher (14-24%) with lower FGF (<or=to0.5 L/min) (Table 1). All patients were hemodynamically stable.Table 1: Variability of [Vap] (%) over time (min) with different FGF (L/min).DISCUSSION: Our results confirm the hypothesis that [Vap] diverge wider when FGF is lower. Marked interpatient variability of uptake at a constant [Et] probably explains this finding. During closed-circuit anesthesia, isoflurane uptake differs up to 50% between patients. [2] However, even though variability of [Vap] is higher with lower FGF, the number of vaporizer adjustments in the individual patient that is needed to maintain the desired [Et] once the vessel-rich group (VRG) group has been saturated (approximately 10-15 min) is minimal because the rate of uptake decreases little during the ensuing 45-90 min.