MAC, the minimum alveolar concentration, is the steady-state agent concentration at which half of patients do not move in response to a surgical incision. It is the standard yardstick of anaesthetic potency, and a depth of around 1.2 to 1.3 MAC prevents movement in most patients.

How is MAC adjusted for age?

MAC peaks in infancy and declines steadily in adulthood. This tool uses the Mapleson correction, MAC at age 40 times 10 to the power of negative 0.00269 times age minus 40, which works out to roughly a 6 percent fall per decade above 40 and a rise below it.

How does nitrous oxide change the requirement?

Nitrous oxide is approximately additive with volatile agents. Its own MAC is about 104 percent, so the fraction you inspire contributes that fraction of a MAC. The tool subtracts the nitrous oxide credit and asks the volatile agent to supply only the remaining depth.

Why can I never reach 1 MAC of nitrous oxide alone?

Because nitrous oxide's MAC of 104 percent exceeds 100 percent, you cannot deliver a full MAC of it at one atmosphere without removing all the oxygen. That is why it is used as an adjunct to lower the volatile requirement rather than as a sole agent.

Does this include opioids or temperature?

No. Opioids, hypothermia, benzodiazepines, and other sedatives all lower MAC, sometimes substantially, but they are not modelled here. Treat the output as a starting estimate and titrate to the patient and the monitored end-tidal concentration.

MAC (Minimum Alveolar Concentration) Calculator

The minimum alveolar concentration, MAC, is how anaesthetists compare the potency of inhalation agents and judge depth of anaesthesia. Because anaesthetic requirement changes with age and is shared additively with nitrous oxide, the bedside target is not a single fixed number. This calculator does the adjustment and tells you the vol% to set.

How it works

The reference is MAC at age 40 in oxygen. Age scales it through the Mapleson relationship, which is a smooth exponential roughly equal to a 6 percent change per decade:

MAC(age) = MAC40 x 10^(-0.00269 x (age - 40))

Nitrous oxide is treated as additive. With its MAC near 104 percent, the inspired fraction contributes its own slice of MAC and the volatile agent only needs to make up the difference:

N2O MAC      = N2O_inspired% / 104
agent MAC    = target_total - N2O MAC
agent vol%   = agent MAC x MAC(age)

Example and notes

A 70-year-old needing 1 MAC of sevoflurane needs less than the textbook 2 percent because age has lowered the requirement to about 1.7 percent for 1 MAC. Add 50 percent nitrous oxide and that contributes roughly 0.48 MAC, so the sevoflurane only has to supply about 0.52 MAC, dropping the dial to under 1 percent. Opioids, hypothermia, and other sedatives reduce MAC further and are not included here; always titrate to the patient and the monitored end-tidal agent concentration.