Quantitative measurements of anesthetic potency are essential to study the pharmacology of anesthetic action. The minimum alveolar concentration (MAC) is defined as the alveolar partial pressure of a gas at which 50% of humans do not respond to a surgical incision.

1. The use of MAC as a measure of anesthetic potency has the advantages that it is an extremely reproducible measurement that is remarkably constant over a wide range of species, and the use of the end-tidal gas concentration provides an index of the “free” concentration of drug required to produce anesthesia because the end-tidal gas concentration is in equilibrium with the free concentration in plasma.
2. The MAC concept has several important limitations, particularly when trying to relate MAC values to anesthetic potency observed in vitro (Table 5-2: Limitations of the Minimum Alveolar Concentration (Mac) Concept).
3. Monitors that measure some correlate of anesthetic depth have been introduced into clinical practice.
   1. The most popular of these monitors converts spontaneous electroencephalogram waveforms into a single value that correlates with anesthetic depth for some general anesthetics.
   2. To date, these monitors have not been shown to be more effective at preventing awareness during anesthesia than simply maintaining an adequate end-tidal anesthetic concentration.

Mechanisms of Anesthesia and Consciousness

1. What is Anesthesia?
2. How is Anesthesia Measured?
3. What is the Chemical Nature of Anesthetic Target Sites?
4. How Do Anesthetics Interfere with the Electrophysiologic Function of the Nervous System?
5. Anesthetic Actions on Ion Channels
6. How are the Molecular Effects of Anesthetics Linked to Anesthesia in the Intact Organism?
7. Where in the Central Nervous System Do Anesthetics Work?
8. Conclusions