Minimum alveolar concentration or MAC is a concept used to compare the strengths, or potency, of anaesthetic vapours; in simple terms, it is defined as the concentration of the vapour in the lungs that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus. Thus, it is actually a median value; the use of minimum would appear to be descended from the original paper in which the concept appeared, although the term there was minimal alveolar concentration.
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