The minimal alveolar concentration of an inhalational anaesthetic is a measure of its:

Minimal alveolar concentration (MAC) :- is the lowest concentration of the anaesthetic in pulmonary alveoli needed to produce immobility in response to a painful stimulus (surgical incision) in 50% individuals. It is accepted as a valid measure of potency of inhalational GAs, because it remains fairly constant for most young adults. The MAC of all inhalational anaesthetics declines progressively as age advances beyond 50 years. The MAC of a number of GAs shows excellent correlation with their oil/gas paition coefficient. However, this only reflects capacity of the anaesthetic to enter into CNS and attain sufficient concentration in the neuronal membrane, but not the mechanism by which anaesthesia is produced.