Anesthesia Pharm Exam 1 Practice

Opioids cross the placenta due to what property?

They cross the placenta due to lipophilicity, and accumulate in the fetus as ion trapping in the acidic fetal environment. They cannot cross the placenta. Placental transfer is prevented by pregnancy hormones. They are actively pumped out by placental transporters.

Correct Answer:

They cross the placenta due to lipophilicity, and accumulate in the fetus as ion trapping in the acidic fetal environment.

Explanation:

Opioids cross the placenta mainly because of their lipid-loving, lipophilic nature, which allows them to diffuse readily across the placental membranes. Once in the fetal circulation, many opioids are weak bases, and the fetal environment is slightly more acidic than the maternal one. In this acidic setting, the drug tends to become protonated and charged, which limits its ability to cross membranes back out of the fetus. This results in ion trapping and accumulation of the drug in fetal tissues and fluids, increasing fetal exposure. Other options aren’t correct because placental transfer is not blocked by hormones, nor is it prevented by a lack of crossing; and while transporters exist, active pumping out isn’t the primary reason for entry or fetal accumulation.

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