A practical description of the anesthetic state is a collection of “component” changes in behavior or perception (components of the anesthetic state include unconsciousness, amnesia, analgesia, immobility, and attenuation of autonomic responses to noxious stimulation). Regardless of which definition of anesthesia is used, rapid and reversible drug-induced changes in behavior or perception are essential to anesthesia. As such, anesthesia can only be defined and measured in an intact organism.
- It has long been assumed that anesthesia is a state that is achieved when an anesthetic agent reaches a specific concentration at its effect site in the brain and that if tolerance to the anesthetic develops, increasing concentrations of anesthetic might be required to maintain a constant level of anesthesia during prolonged anesthetic administration.
- The recent finding that it takes a higher anesthetic brain concentration to induce anesthesia than to maintain anesthesia (emergence occurs at a significantly lower concentration than induction) contradicts these assumptions. This phenomenon is referred to as neural inertia and suggests that the mechanisms of anesthetic induction and emergence may be different.