BLOOD/GAS SOLUBILITY AND ALVEOLAR RATE OF RISE OF POTENT INHALED ANESTHETICS DURING OPEN-LOOP FEEDBACK ADMINISTRATION USING A CLOSED-CIRCUIT ANESTHESIA LIQUID INJECTION TECHNIQUE

Abstract

S342 INTRODUCTION: When using a high fresh gas flow (FGF) and a constant inspired concentration, the end-expired concentration [Et] of potent inhaled anesthetics rises faster with agents that have a lower blood/gas solubility (lambdaB/G): fast with desflurane (des), slower with sevoflurane (sevo), and slowest with isoflurane (iso) (lambdaB/G = 0.42, 0.60, and 1.38, respectively). [1] We determined whether this is still true when FGF is very low and an attempt is made to reach the [Et] as quickly as possible by using an overpressure technique. METHODS: With IRB approval and informed consent, 54 ASA I-III patients undergoing elective surgery under general endotracheal anesthesia with controlled mechanical ventilation were assigned to 1 of 3 groups, depending on the anesthetic studied: des (n=10), sevo (n=34), and iso (n=10). After IV induction and endotracheal intubation, liquid anesthetic was injected into the inspiratory limb of a closed circle system using a syringe pump. An [Et] of 1.3 MAC of des, sevo, or iso (8.5, 2.6, or 1.5%, respectively) was obtained as quickly as possible (overpressure). The initial bolus of liquid anesthetic was administered based on the known amount needed to prime the anesthesia circuit and functional residual capacity. Subsequently, the infusion rate of the syringe pump was adjusted to obtain and maintain the [Et] at 1.3 MAC. The [Et] was recorded every minute (Datex-Engstrom AS/3 multigas analyzer). The rate of rise of the three agents, expressed as a fraction of MAC, was compared. RESULTS: The [Et] over time for each agent is presented in Fig. 1. The rate of rise of [Et] did not differ between the three groups. DISCUSSION: When overpressure is used during an open-loop feedback closed-circuit anesthesia liquid injection technique, the rate of rise of [Et] of des, sevo, and iso is identical, and thus becomes independent of lambdaB/G. Commonly used pharmacokinetic principles of uptake of potent inhaled anesthetics do not apply when an overpressure open- or closed-loop (e.g., Physioflex, Drager) feedback closed-circuit anesthesia liquid injection technique is used.