The placental transfer of acetylsalicylic acid in near-term ewes.

The placental transfer of acetylsalicylic acid was studied in five near-term ewes and fetuses. The total clearance (Ctot) of acetylsalicylic acid from the fetal compartment was determined by bolus injection of 14C-acetylsalicylic acid into the fetal compartment. Ctot = Cp + Cf where Cp is the placental clearance and Cf is the fetal tissue clearance. 14C-acetylsalicylic acid was then infused into the ewe. Fetal (F) and maternal (M) steady-state concentrations of acetylsalicylic acid were measured. During steady-state conditions, the influx of acetylsalicylic acid into the fetal compartment will equal the efflux, or Cp X (M - F) = Cf X F. Combining these two equations yields the following description of placental clearance: Cp = Ctot x F/M. Using this equation, we calculated Cp = 0.57 +/- 0.12 ml/min . kg and Cf = 1.93 +/- 0.22 ml/min . kg. Maternal and fetal placental flows were also measured using radioactively labeled microspheres. From these flows, a 99% degree of diffusion limitation was determined for the placental transfer of acetylsalicylic acid.