KEY POINTS

1. The definition of "depth of anesthesia" has constantly evolved since the first demonstration of clinical anesthesia in the 1840s. The changing definitions have revolved around the available drugs used to provide anesthesia and the body of knowledge on their effects in humans.
2. Anesthesia is not a single pharmacologic process. It is a complex interaction of multiple stimuli, diverse responses, and the drug-induced probability of nonresponsiveness to the stimuli.
3. Anesthesia can be defined by its hypnotic (unconsciousness) and analgesic (pain relief) components. The hypnotic component can be induced by intravenous and inhaled anesthetics, whereas the analgesic component can be induced by opioids and local anesthetics. Some drugs, such as ether, nitrous oxide, and ketamine, provide both hypnotic and analgesic components to some degree.
4. Hypnotics, when given alone, will allow significant hemodynamic response to intense noxious stimuli. Opioids, when given alone, do not guarantee consistent unconsciousness or lack of movement response to intense noxious stimuli. A combination of the two can result in predictable unconsciousness and lack of hemodynamic response to intense noxious stimuli.
5. The interaction of the hypnotic and analgesic components can be characterized by a three-dimensional surface with hypnotic concentration on the y axis, analgesic concentration on the x axis, and probability of nonresponse on the z axis.
6. Characterization of the three-dimensional surface requires precise stimuli to be applied and specific responses to be measured at defined effect-site concentrations of the hypnotic and analgesic.
7. The specific stimuli-response pairs used to define anesthetic depth range from easily suppressed responses to mild stimulation, such as the verbal response to a verbal command, to difficult-to-suppress responses to intense stimuli, such as the hemodynamic response to intubation.
8. The interaction of hypnotics and analgesics is generally synergistic.
9. Current clinical anesthesia involves the physician carefully observing the clinical response to defined stimuli and then adjusting the hypnotic or analgesic dosage, or both, by using their synergistic interaction to achieve the clinical goals of hemodynamic control, lack of awareness, and rapid, safe induction and emergence.
10. The hypnotic effects of the intravenous and inhaled anesthetics can be measured with the bispectral index, an empirically derived EEG measure that has extensive clinical calibration.
11. The bispectral index is an empirically derived measure of hypnotic drug effect, including hypnotic-induced sedation, amnesia, loss of consciousness, and reduced cerebral metabolic rate.
12. A consequence of inadequate anesthetic depth is intraoperative awareness. The incidence of awareness in healthy patients is approximately 0.2% and can increase to 1.0% to 1.5% in higher-risk patient populations. Emerging evidence is indicating that intraoperative monitoring of the hypnotic component of an anesthetic can significantly decrease the risk of awareness.