Index of potency of general anaesthesia -

Minimum alveolar concentration is defined as the alveolar concentration of an inhaled anesthetic agent that prevents movement in 50% of patients in response to a standardized surgical stimulus. The MAC of a volatile substance is inversely propoional to its lipid solubility (oil: gas coefficient), in most cases. This is the Meyer-Oveon hypothesis put forward in 1899-1901 by Hans Horst Meyer and Charles Ernest Oveon. MAC is inversely related to potency, i.e. high MAC equals low potency. MAC values of adult Halothane: 0.75 Sevoflurane: 2.0 Isoflurane: 1.2 Desflurane: 6.0 Enflurane: 1.7 Xenon: 70 N2O: 104 Factors affecting: MAC increases with hypehermia, hypernatremia, and redheads. MAC decreases with anemia, hypercarbia, hypoxia, hypothermia, hypotension (MAP < 40 mmHg), and pregnancy. Medications, illicit drugs, and prior substance use history have also been found to affect MAC. For example, acute use of amphetamines, cocaine, ephedrine, and chronic use of alcohol increase MAC. Whereas, administration of propofol, etomidate, barbiturates, benzodiazepines, ketamine, opiates, local anesthetics, lithium, verapamil, and alpha 2-agonists (dexmedetomidine, clonidine) decrease MAC. Acute alcohol intoxication and chronic amphetamine use have also been found to decrease MAC. Ref: Miller's anesthesia 8th edition Ref: Morgan & Mikhail's clinical anesthesiology 6e