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In unpremedicated subjects MAC (sometimes referred to as 1 MAC) is the alveolar concentration of a volatile anaesthetic in oxygen which will prevent movement in response to a standard surgical stimulus in one half of the population. MAC is, thus, the ED50 (the effective dose in 50% of patients). MAC may be higher or lower in the other 50% of the population. MAC may, therefore, be considered to equal the median of a normal distribution of alveolar concentrations. It is important to consider where in the distribution an individual patient lies. The issue can only be resolved by clinical observation and monitoring. During anaesthesia (assuming normal respiratory function) the alveolar concentration of a volatile agent, or its MAC, is close to the concentration of the agent measured at the end of exhalation, the end tidal concentration.
MAC is the standard way to compare the potencies of volatile agents, and is also useful because it is additive. Thus, if the patient inhales 0.5 MAC of one agent and inhales 0.5 MAC of another agent, the MAC value is 0.5 + 0.5 = 1.0. MAC does not always relate to the "depth' of anaesthesia. Various reasons apply and these are listed in Figure IN.3.
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