Describe the elements of placental drug transfer

Describe the elements of placental drug transfer.

Placental drug transfer occurs by diffusion. Fick’s equa-tion describes the factors governing the transfer of drugs across the placenta.

Q_d = K_d × A × [P_d(m) − P_d(f )]/b

where:

Q_d = quantity of drug transferred per unit time

K_d = diffusion constant for the drug

A = surface area of the placenta

P_d(m) = mean drug concentration of maternal blood in the intervillous space 

P_d(f) = mean drug concentration of fetal blood in the intervillous space

b = thickness of the placenta.

Factors over which the anesthesiologist has control are limited to the specific drug administered and the amount used. Other factors, such as the surface area and thickness of the placenta, are clearly not under our control. In order to minimize the amount of drug reaching the placenta, the quantity of maternally administered drug needs to be reduced.

Diffusion constants, which vary from one drug to another, are determined by four main properties: molecu-lar weight, lipid solubility, protein binding, and electrical charge. Placental transfer of drug is facilitated by a molec-ular weight of less than 500, high lipid solubility, minimal maternal protein binding, and a low degree of ionization. Thus, fentanyl, a non-ionized, highly lipid-soluble mole-cule with a low molecular weight, crosses the placenta easily. In contrast, succinylcholine, a highly ionized mole-cule, does not cross the placenta readily.