Minimum alveolar concentration: ongoing relevance and clinical utility

Since its introduction in 1965, minimum alveolar concentration (MAC) has served as the standard measure of potency for volatile anaesthetic agents. It is defined as the minimum alveolar concentration of inhaled anaesthetic at which 50% of people do not move in response to a noxious stimulus. Within the last 20 years, it has been discovered that volatile anaesthetics inhibit mobility largely through action on the spinal cord, whereas the amnesic and hypnotic effects are mediated by the brain. Studies suggest that the concentration of volatile anaesthetic needed to prevent explicit memory from developing, and to produce unconsciousness, is usually substantially lower than the concentration required to prevent movement in response to surgery. This review highlights the contributions and limitations of MAC and its derivatives as metrics of anaesthetic potency with respect to particular behavioural outcomes. Recent evidence is presented suggesting that a protocol that alerts anaesthetists whenever MAC falls to < 0.5 or 0.7 has the potential to decrease intra-operative awareness with explicit recall, possibly to a similar extent as does a protocol based on processed electroencephalography-driven alerting.