Minimum Alveolar Concentration

In 1965, Eger et al. introduced the concept of minimum alveolar concentration (MAC), and it has since been used as the standard measure of potency for volatile anesthetic agents. Previous attempts to create a measure to determine the adequacy of anesthetic dosing, such as the Guedel stages of anesthesia or Woodbridge concept of “nothria” were all qualitative measures that varied depending on the inhaled anesthetics. The concept of MAC negated the variability by measuring a single quantitative endpoint: immobility. It is defined as the concentration of inhaled anesthetic within the alveoli at which 50% of people do not move in response to a surgical stimulus. MAC uses the measurement of end-tidal anesthetic as a measure of the level of anesthetic within the alveoli and, in turn, at the level of the central nervous system. It is a useful and reproducible metric in both animals and humans across all volatile anesthetics, making it the standard for comparison of volatile anesthetic potency.