General Anaesthetics (MAC Value, Blood : Gas Partition Co-efficient & Oil:Gas Partition Co-efficient

Minimum Alveolar Concentration (MAC):  It is the minimum concentration of an inhalational agent required in the alveoli to produce unresponsiveness to the skin incision in 50% of the patients. It is the measure of potency of an agent.  Greater is the MAC, lesser is the potency. – Nitrous oxide is a gas with maximum MAC and thus least potency. Its MAC is 104% i.e. even with pure (100%) nitric oxide alone, we cannot get complete anaesthesia. This is thus, not a complete anaesthetic agent.  Methoxyflurane is the most potent agent (having least MAC). Blood: Gas partition coefficient –It is determined by solubility of an agent in the blood. It determines the speed of onset and recovery of an anaesthetic drug.  Greater is blood gas partition coefficient, lesser is the speed of onset and recovery and vice versa.  Desflurane is the fastest acting agent as it has minimum blood gas partition coefficient. – Methoxyflurane is the slowest acting agent (maximum blood gas partition coefficient). Oil: Gas partition coefficient- solubility in fat. If solubility of anaesthtics in lipid will be high means potency will also be high. Anaesthetics with high oil:gas partition coefficient denotes stronger action.