> The minimum alveolar concentration (MAC) is the minimum concentration of an inhaled anesthetic at 1 atm of pressure that prevents skeletal muscle movement in response to a surgical incision in 50% of patients.
So first, you do not measure the depth of anesthesia, you measure the concentration of the anesthetic. Second, you judge this concentration by the prevention of muscle movement. Called paralysis.
Edit: In case you are wondering why this response doesn't really fit the parent comment, the parent saw fit to completely replace his comment without an indication that he did so. Originally there was a claim in the parent comment about "MAC being the primary indication of anesthetic depth being the textbook definition" or something to that effect. To which I responded. I guess I must have hit a nerve there ;)
"MAC being the primary indication of anesthetic depth being the textbook definition"
I am an attending anesthesiologist and this is true. MAC cannot be interpreted at face value, though. You've got other drugs on board (not accounted for in MAC), the patient might be frail or very old, etc. etc. All things changing MAC interpretation, which is why there are still anesthesia providers instead of robots ;-) We currently have no way of faithfully measuring the depth of anesthesia, and our understanding of consciousness/awareness is incomplete. Anesthesiologists have to rely mostly on know-how, even in 2025.
