Pharmacokinetics of terbutaline during pregnancy

Terbutaline in plasma was determined in three groups of women by gas chromatography-mass spectrometry. Eight women received a single i.v. dose of 0.25 mg terbutaline sulphate during pregnancy and 3-6 months after delivery. Mean plasma clearance was 29% higher during pregnancy than after delivery. There was a subsequent decrease in mean terminal half-life from 5.3 to 3.7 h and in mean residence time from 5.3 to 3.4 h. There was no change in volume of distribution. A second group of pregnant women in premature labour (n = 8) received oral terbutaline 5 mg t.d.s. The dosing was repeated after delivery. The mean steady state plasma concentration of terbutaline was about 30% lower during pregnancy than after delivery. A third group of women in preterm labour (n = 8) was treated with an i.v. infusion of terbutaline. The concentrations of terbutaline found on cessation of uterine contractions ranged between 12.8 and 31.5 ng/ml. At present there is no basis for formulation of a "therapeutic plasma level" of terbutaline for the treatment of preterm labour.