CLINICAL EVALUATION OF THE GENERAL ANESTHETIC Equation FORDESFLURANE

Abstract

S341 INTRODUCTION: The general anesthetic equation (GAEq), based on the square root of time (SqRT) uptake model of Lowe and Ernst, describes what the required vaporizer settings are to obtain a constant end-expired concentration for any fresh gas flow (FGF). [1] Because the GAEq has not been evaluated clinically for desflurane, we studied the delivered desflurane concentration into a circle system required to provide a constant end-expired desflurane concentration (Etdes) for 7 different FGF. METHODS: After obtaining institutional approval and informed consent, 48 ASA I-III patients received general endotracheal anesthesia with desflurane in O2. Patients were assigned to 1 of 6 groups depending on the FGF used: 0.3 L/min (with an initial 4 L/min FGF during 30 s to hasten circuit wash-in), 0.5, 1, 2, 4, or 8 L/min. After IV induction, an Etdes of 0.65 MAC (4.5%) was reached as quickly as possible by using the maximum vaporizer dial setting (18%). Subsequently, Etdes was maintained at 4.5 % by adjusting the vaporizer. Vaporizer settings and Et (des) were recorded every min for 1 h. Observations were compared with the theoretical SqRT model for closed circuit anesthesia (CCA). [1] In addition, the cumulative amount of desflurane vaporized within each group (desflurane consumption) was calculated (FGF * vaporizer settings). RESULTS: Etdes reached 4.5% after 30 s (by protocol), 4 min 0 s, 1 min 30 s, 42 s, 24 s, and 18 s in the 0.3, 0.5, 1, 2, 4, or 8 L/min groups, respectively. Vaporizer settings with the different FGF and predictions by the theoretical GAEq for CCA are shown in Fig. 1. Desflurane consumption is presented in Fig. 2. DISCUSSION: Vaporizer settings at 0.3 L/min were very different from those predicted by Lowe and Ernst for CCA, with the rate of desflurane uptake decreasing less than predicted. Practically, a FGF of 0.5 L/min appears to provide a good combination of ease of use (number of vaporizer adjustments needed as reflected by the decay curve of the vaporizer settings) and desflurane consumption. After 15 minutes, there is no need for frequent vaporizer adjustments, regardless of FGF. Lowering FGF below 0.5 L/min does not further reduce desflurane consumption and costs substantially.