🔹The placenta forms a barrier to the transfer of drugs between the mother and the fetus, but increasing lipid-solubility, decreasing maternal protein binding, decreasing molecular weight, increased materno-fetal concentration gradient and placental blood flow etc will increase the placental transfer of drugs
🔹The relative distribution of the drug across the placenta is represented by Feto-Maternal (F/ M) concentration ratio
🔹Pethidine and diamorphine are both metabolised in the fetus to less lipid-soluble products like norpethidine and morphine respectively, which remain on the fetal side of the placenta. The elimination half-lives of these drugs are also longer in the fetus because of immature hepatic metabolism. This again prolongs its existence in the fetal side.
🔹Lipid solubility of drugs like thiopentone sodium are high; so they cross the placenta easily, and can accumulate as the pH is lower in the fetus
🔹Diazepam is metabolised to less lipid-soluble products. So it can have an F/ M ratio of 2 even one hour after maternal administration.
🔹Local anaesthetic agents are weak bases which are largely UN-IONISED at physiological pH, and cross the placenta readily. But fetal 'trapping' occurs in severe acidosis, when the molecules become IONIZED in the fetal side.
#pharmacology , #DrugMetabolism ,#PlacentalTransfer , #anesthesia
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