Ans. (A) High blood: gas partition coefficient(Ref: Katzung 11th/e p427)The depth of anesthesia depends on the partial pressure of anesthetic in CNS. The transfer of anesthetic into the brain starts only after the blood is fully saturated (or, in other words partial pressure of the anesthetic in blood equals the partial pressure in the inspired air). The speed of transfer of anesthetic to the brain determines its onset of action (rapid vs slow induction of anesthesia) and is dependent on the solubility of anesthetic in the blood. Solubility of an anesthetic is directly related to its blood/gas partition coefficient: highly soluble anesthetics have high blood/gas partition coefficient.If the agent is poorly soluble the amount of gas needed to saturate the blood is small and saturation occurs fairly quickly. Nitrous oxide is an example of poorly soluble gas with a blood/gas partition coefficient of 47. On the graph above the curve of partial pressure of NO in blood rises rapidly. In the highest point on the curve the partial pressure on NO in blood equals that in the inspired air, and the transfer to brain occurs.The second curve (drug A) portrays the process of blood saturation for a highly soluble gas. The higher the solubility the more gas can be taken up by blood before it is saturated. Note that the curve of the partial pressure of drug A in blood rises slower than that for NO. When the blood is fully saturated with NO the partial pressure of drug A in blood is approximately 25% of that in inspired air. For drug A, it takes a longer time to fully saturate the blood and to start transfer in tissues. Drug A, therefore is characterized with high blood/gas partition coefficient and slower onset of action.EffectPoorly soluble gas NOHighly soluble gas (Halothane)Amount needed to saturate the bloodSmallLargeRise in tension of gas in bloodRapidSlowEquilibrium with the brainRapidSlowOnset of actionRapidSlow