Minimum Alveolar Concentration (MAC): This is the concentration of an inhaled anesthetic in the alveoli at which 50% of patients do not move in response to a surgical stimulus.
It is a measure of potency (MAC is inversely proportional to potency).
Factors that ↑ MAC (less potent): Hyperthermia, chronic alcohol use, and sympathomimetics.
Factors that ↓ MAC (more potent): Hypothermia, pregnancy, older age, and use of other anesthetics (e.g., opioids, benzodiazepines).
Blood/Gas Partition Coefficient: This determines the speed of induction and recovery.
It describes the solubility of the anesthetic in the blood.
The clinical effect of an inhaled anesthetic is not determined by the total amount of the drug in the body, but by its partial pressure in the brain. Anesthesia occurs when the partial pressure of the agent in the brain reaches a therapeutic level.
Low coefficient = less soluble in blood = faster equilibrium between inhaled and brain partial pressures = rapid onset and recovery. (e.g., Nitrous Oxide, Desflurane, Sevoflurane).
Because it doesn’t dissolve well, the “free,” unbound gas molecules rapidly build up pressure in the blood.
High coefficient = more soluble in blood = more anesthetic must be absorbed into the blood before the partial pressure in the brain rises = slow onset and recovery.
Oil/Gas Partition Coefficient: This reflects the lipid solubility of the anesthetic.
It is directly proportional to potency (and inversely proportional to MAC). A higher oil/gas coefficient means the agent is more potent.
Adverse effects
Postoperative nausea and vomiting
Risk of malignant hyperthermia (except nitrous oxide)
Decrease vascular resistance in brain and lead to an increase in cerebral blood flow → increased ICP