Monitoring anesthetic depth

Monitoring “depth of anesthesia” is an elusive goal, in part because the term “depth of anesthesia” is poorly defined. Anesthesia is a combination of effects, that include the prevention of pain perception (analgesia or antinociception), conscious perception (unconsciousness), and recall (amnesia). Most available monitors claim to reflect the hypnotic component of anesthesia (level of consciousness/unconsciousness), but analysis of the level of consciousness still lacks precision. The term “level of consciousness or unconsciousness” may refer to a dichotomous – i.e., “all or none” – phenomenon rather than to a gradual process. This may contradict the clinical picture that some patients seem to be in a “deeper” state of unconsciousness compared to others. The clinical assessment of this “depth” is based on reactions to a stimulus, i.e., assessing whether unconsciousness can be entirely or partially reversed. Yet, this clinical assessment may not truly reflect the level of unconsciousness, but a phenomenon composed of analgesia and hypnosis. Even in the scientific literature, the terms “consciousness,” “awareness,” “recall,” and “memory” are often used interchangeably. The respective phenomena are closely related to each other, but should be separated. Amnesia and sedation or unconsciousness are separate phenomena that are both evoked by anesthetics. Wakefulness or consciousness may occur without preserved memory function. As a consequence of its widespread use and of differences in the underlying definitions, the term “awareness” may lead to confusion.