awake
immobility
intubation
MAC-BAR
What the MAC levels mean
The concentration at which about half of patients open their eyes and respond to command. It marks the threshold between asleep and awake, which is why emergence happens well below the surgical level.
The defining point of MAC: the concentration that stops half of patients from moving in response to a skin incision. This is the usual reference depth for maintaining general anesthesia.
A slightly deeper level that blunts coughing and movement during laryngoscopy. It sits above the surgical value because manipulating the airway is more stimulating than a skin incision.
BAR stands for “blocks adrenergic response.” It is the deepest of these benchmarks, the concentration that blunts the sympathetic surge (rising heart rate and blood pressure) to a noxious surgical stimulus.
How it works and assumptions
MAC_age = MAC₄₀ × 10^(−0.00269 × (age − 40)), which works out to about a 6% drop per decade. Temperature follows a rule of thumb of about 5% per °C, so a colder patient needs less. N₂O is treated as additive: its share of the total is FiN₂O ÷ MAC_N₂O and the volatile agent makes up the rest. The patient modifiers (pregnancy ×0.7, chronic alcohol ×1.3, opioids ×0.7, red hair ×1.19) are simplified teaching values, and MAC-awake is estimated at 0.34 × MAC.One thing to notice: N₂O has a MAC of about 104%, so you can never reach a full 1 MAC with it alone. That would take more than 100% inspired gas and leave nothing for oxygen, which is why nitrous is always used as an add-on rather than a primary agent.