Placental transfer of etomidate in pregnant ewes after an intravenous bolus dose and continuous infusion.

Etomidate (ETO) is a short-acting intravenous (IV) anaesthetic characterised by cardiopulmonary stability and favourable pharmacokinetics. Although ETO has been used satisfactorily in obstetrical anaesthesia, little is known about placental transfer and the drug's pharmacokinetics in the fetus. Placental transfer in pregnant ewes has been evaluated following the administration of an IV bolus of 1mg/kg ETO; and after a 1-h infusion of 100 microg/kg min(-1) ETO preceded by an IV bolus of 1mg/kg. In ewes, ETO concentration and AUC were higher than those found in fetuses. After the ETO bolus dose, the fetus:ewe AUC ratio was 0.45+/-0.32, and the mean residence time (MRT) was 20+/-7 min for dams and 22+/-3 min for the fetuses. After ETO infusion, the AUC ratio was 0.37+/-0.08, and MRT was 46+/-12 min for ewes and 46+/-22 min for fetuses. Although ETO crosses the placenta very rapidly and reaches the fetus in high amounts, a certain placental barrier effect limits its transfer. There is no evidence of cumulative effects of the drug in the fetus as fetal ETO elimination was as rapid as in the dam.