median minimal alveolar concentration (MAC)

The median minimal alveolar concentration (MAC) value (the MAC of inhaled anesthetic required for immobility in 50% of patients) remains the gold standard for dosing inhaled anesthetics. However, MAC cannot be used to define or predict brain states in anesthetized patients. Animal experiments have demonstrated that there is no clear association between anesthetic-induced EEG patterns and immobility, and that inhaled anesthetics produce immobility primarily by acting in the spinal cord, rather than the brain.

Regardless, end-tidal anesthetic concentration is widely used as a way to monitor the level of consciousness induced by inhaled anesthetics and guide anesthetic dosing.

1. This use of MAC has been supported by the B-Unaware Trial that reported no difference in the incidence of intraoperative awareness with an anesthetic protocol that maintained the BIS between 40 and 60 and a protocol that maintained the end-tidal anesthetic between 0.7 and 1.3 MAC.

Unlike the BIS, PSI and Narcotrend, which provide EEG-based measures of brain activity, the end-tidal anesthetic criterion is related very indirectly to brain activity through the concentration of anesthetic expired in the lungs.

A key drawback is that it cannot be used with a total intravenous anesthetic.