- General anesthesia reversibly alters consciousness, without shutting down the brain globally. Depending on the anesthetic agent and dose, it may produce different consciousness states including:-
→ a complete absence of subjective experience (unconsciousness),
→ a conscious experience without perception of the environment (disconnected consciousness, like during dreaming), or
→ episodes of oriented consciousness with awareness of the environment (connected consciousness).
- Each consciousness state may potentially be followed by explicit or implicit memories after the procedure. In this respect, anesthesia can be considered as a proxy to explore consciousness.
- During recent years, progress in the exploration of brain function has allowed a better understanding of the neural correlates of consciousness, and of their alterations during anesthesia. Several changes in:-
→ functional and effective between-region brain connectivity,
→ consciousness network topology, and
→ spatio-temporal dynamics of between-region interactions
have been evidenced during anesthesia.
- Despite a set of effects that are common to many anesthetic agents, it is still uneasy to draw a comprehensive picture of the precise cascades during general anesthesia.
- Several questions remain unsolved, including:-
- the exact identification of the neural substrate of consciousness and its components,
- the detection of specific consciousness states in unresponsive patients and their associated memory processes,
- the processing of sensory information during anesthesia,
- the pharmacodynamic interactions between anesthetic agents,
- the direction-dependent hysteresis phenomenon during the transitions between consciousness states,
- the mechanisms of cognitive alterations that follow an anesthetic procedure,
- the identification of an eventual unitary mechanism of anesthesia-induced alteration of consciousness,
- the relationship between network effects and the biochemical or sleep-wake cycle targets of anesthetic agents, as well as
- the vast between-studies variations in dose and administration mode, leading to difficulties in between-studies comparisons.
- In this narrative review, we draw the picture of the current state of knowledge in anesthesia-induced unconsciousness, from insights gathered on propofol, halogenated vapors, ketamine, dexmedetomidine, benzodiazepines and xenon.
- We also describe how anesthesia can help understanding consciousness, we develop the above-mentioned unresolved questions, and propose tracks for future research.
Text Colour Conventions (see disclaimer)
- Blue: Text by me; © Theo Todman, 2022
- Mauve: Text by correspondent(s) or other author(s); © the author(s)