Effects of nitrous oxide on MAC

The minimal alveolar concentration (MAC) is defined as the end-expiratory concentration of an inhalational anaesthetic that is required to suppress gross purposeful movement to a painful stimulus in 50% of subjects. The MAC of nitrous oxide is 1•04 atmospheres absolute (corresponding to 104 vol%). The MAC criterion has served since the early 1960s as a gold standard to compare the relative potencies of inhalational anaesthetics. Since more recent studies have demonstrated that determining the MAC measures an anaesthetic effect localized at the spinal cord and being independent from anaesthetic action at the brain the clinical relevance of MAC values are questioned. However, there is still no commonly accepted substitute to the MAC for comparing the relative potencies of inhalational and intravenous anaesthetics and related drugs.