Disposition of the adrenergic blocker metoprolol in the late-pregnant woman, the amniotic fluid, the cord blood and the neonate.

Pharmacokinetic studies on the concentration of the beta 1-blocker metoprolol have been performed in maternal plasma, amniotic fluid, breast milk and the plasma of the newborn. The concentration of metoprolol in maternal plasma exceeded that in the amniotic fluid initially , but not later on. The quotient between the metoprolol concentration in the maternal venous blood and the mixed cord blood at the time of parturition is at about 1. In the maternal plasma the metoprolol concentration decreases rapidly after the latest dose, probably because of an increased clearance rate. Blood levels of metoprolol during the first postnatal hours increase almost fourfold and are generally followed by a decrease over the next 15 hours. Active metabolites of metoprolol (alfa-OH-metoprolol and 0-demethyl-metoprolol) are found in the urine of the newborn. The breast milk concentration of metoprolol is three times as high as in the maternal plasma but the suckling newborn will only show very low or unmeasurable plasma levels between consecutive breast feeding periods, in the majority of cases.