MAC values are determined experimentally in healthy volunteers breathing just the anaesthetic agent in oxygen. Values quoted for MACs of different anaesthetics do not take into account the presence of any other agents.

In practice we often combine a volatile agent with nitrous oxide (N₂O) which itself has anaesthetic properties. MAC values are additive. Using half a MAC of volatile agent and half a MAC of N₂O results in a combined MAC of 1.

Although not done in practice, it would be theoretically possible to combine half a MAC of isoflurane with half a MAC of sevoflurane to produce the same level of anaesthesia as 1 MAC of either agent alone.

The apparent MAC of a volatile agent can also be influenced by a number of other factors (Table 1), such as co-administration of drugs with sedative or hypnotic effects and the metabolic state of the patient. If raised, the patient requires a higher MAC. If lowered, the patient requires a lower MAC