Temperature Dependence of the Potency of Volatile General Anesthetics

BACKGROUND: When performing experiments at room temperature with volatile general anesthetics and in vitro mammalian preparations (such as isolated neurons), the question arises as to which concentrations of anesthetics are "clinically relevant." Different choices can lead to different interpretations of the anesthetic sensitivities of putative target sites. METHODS: Published data on the temperature dependence of minimum alveolar concentration were analyzed. RESULTS: Although gas-phase potencies of volatile anesthetics increase markedly with decreasing temperature, the corresponding aqueous-phase potencies are relatively constant. Changes in minimum alveolar concentration with temperature can be accounted for, on physical grounds, in terms of the temperature dependencies of anesthetics binding to their central nervous system target sites. CONCLUSION: When performing room-temperature in vitro experiments on simple mammalian preparations with a volatile anesthetic, the aqueous-phase (but not the gas- phase) minimum alveolar concentration calculated at normal body temperature is, to a first approximation, the appropriate choice for a clinically relevant anesthetic concentration. Recommended aqueous-phase minimum alveolar concentration values (in MM) for desflurane, enflurane, halothane, isoflurane, and sevoflurane have have been calculated.