median minimal alveolar concentration (MAC)

The median minimal alveolar concentration (MAC) value (the MAC of inhaled anesthetic required for immobility, or more precisely lack of movement in response to a noxious stimuli, 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 in the brain.

Regardless, the 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 and BAG-RECALL trials that reported no difference or small increase, respectively, in the incidence of intraoperative awareness with an anesthetic protocol that maintained the BIS between 40 and 60 compared with a protocol that maintained the end-tidal anesthetic between 0.7 and 1.3 MAC.

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

Two concepts related to MAC are (1) MAC-awake, which is the MAC of inhaled anesthetic required for unconsciousness, which is approximately one-third of MAC; and (2) MAC-BAR (block of adrenergic response), which is the MAC of inhaled anesthetic required for the blunting of autonomic responses to nociception, and is approximately two times the MAC.

1. The use of opioids can reduce MAC, MAC-awake, and, in particular, MAC-BAR, by decreasing nociception-induced arousal.

A key drawback with MAC is that it cannot be used with total intravenous anesthesia.