From individual to population: the minimum alveolar concentration curve

The concept of minimum alveolar concentration is central to the study of inhalational anesthetics. The minimum alveolar concentration curve is a population concentration-response curve that describes the relationship between anesthetic concentration and oblation of the movement response to painful stimuli. Two features of the minimum alveolar concentration curve, the anesthetic concentration that immobilizes 50% of patients and the steep slope of the curve, may contain useful mechanistic information. There is some dispute, however, about the interpretation of this information. This review examines hypotheses about the shape of the minimum alveolar concentration curve, and recent theoretical and experimental approaches to the question. The major determining factor for the slope of a quantal, population concentration-response curve is individual variability. The slope of the underlying in-vitro concentration-response curves contributes as well. At the molecular level, in-vitro curves are not steep. The integration of molecular signals that occurs in cells and neuronal circuits can result in steep in-vitro curves, due to multiple molecular targets, amplification and co-operativity. The shape of the minimum alveolar concentration curve has not provided any unambiguous clues about the sites or mechanisms of general anesthesia; however, the universality of anesthetic-induced immobility suggests some future research directions.