Anesthesia Pharm Exam 1 Practice

The main idea is that lipophilicity drives how widely a drug distributes beyond the plasma. Sufentanil is extremely lipid-soluble, so it readily partitions into fat tissue. In obese patients, there is more adipose tissue for the drug to distribute into, which increases the amount stored in tissues relative to the plasma. This broader distribution makes the apparent volume of distribution larger, since Vd is the ratio of the amount of drug in the body to the plasma concentration. Because more drug is sequestered in fat, the plasma concentration drops for a given dose, pushing the Vd higher and often prolonging redistribution and effect.

Low lipid solubility would keep more drug in the bloodstream and limit tissue distribution, not enlarge Vd. Lipid solubility does affect Vd, and while high protein binding can reduce the freely distributing portion, it does not explain the large Vd seen with a lipophilic drug in obesity.