minimum alveolar concentration minimum alveolar concentration minimum alveolar concentration or mac is the concentration, often expressed as a percentage by volume, of a vapour in the alveoli of the lungs that is needed to prevent movement (motor response) in 50% of subjects in response to surgical (pain) stimulus. mac is used to compare the strengths, or potency, of anaesthetic vapours. the concept of mac was first introduced in 1965. mac actually is a median value, not a minimum as term implies. the original paper proposed mac as the minimal alveolar concentration, which was shortly thereafter revised to minimum alveolar concentration. a lower mac value represents a more potent volatile anesthetic. other uses of mac include mac-bar (1.7–2.0 mac), which is the concentration required to block autonomic reflexes to nociceptive stimuli, and mac-awake (0.3–0.5 mac), the concentration required to block voluntary reflexes and control perceptive awareness. formal definition the mac is the concentration of the vapour (measured as a percentage at 1 atmosphere, i.e. the partial pressure) that prevents patient movement in response to a supramaximal stimulus (traditionally a set depth and width of skin incisions) in 50% of subjects. this measurement is done at steady state (assuming a constant alveolar concentration for 15 minutes), under the assumption that this allows for an equilibration between the gasses in the alveoli, the blood and the brain. mac is accepted as a valid measure of potency of inhalational general anaesthetics because it remains fairly constant for a given species even under varying conditions. meyer-overton hypothesis