Speed of onset of action of local anaesthetics is indicated by -

Ans. is 'b' i.e., Blood : gas paition coefficient Minimal alveolar concentration It is the lowest concentration of the anaesthetic in pulmonary alveoli needed to produce immobility in response to a painful stimulus (surgical incision) in 50% individuals. It is the measure of potency of inhalation GAs. Blood : gas paition cofficient It is the measure of solubility of agent in blood. It determines the speed of onset and recovery. Higher the blood : gas paition cofficient, lesser the speed of induction and recovary --> more blood soluble agents have slower induction and recovery. Oil : gas paition cofficient It is the measure of lipid solubility of the agent, and therefore solubility in the fat - rich tissues of the CNS. This equates with the potency of individual agents. There is a direct relationship between the minimum alveolar concentration (MAC) value of inhaled anaesthetic agents and lipid solubility in terms of the oil/gas paition cofficient.