> 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.
Please tell me you are not really a doctor.
[0] https://www.sciencedirect.com/topics/medicine-and-dentistry/...
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 ;)