Pharmacokinetics of scopolamine during caesarean section: relationship between serum concentration and effect

The pharmacokinetics (radioreceptor assay, RRA) and some of the clinical effects of the anticholinergic agent, scopolamine, were studied in 16 parturients during caesarean section. Following a single 0.005 mg/kg intramuscular injection (deltoid muscle), a very fast rate of absorption was found with mean peak serum concentrations occurring after only 10 min (n = 6). Due to severe bradycardia, 0.5 mg of atropine i.v. had to be given in addition to the i.m. scopolamine treatment to one parturient. The RRA measured the total concentration produced by the two anticholinergic agents in both serum and urine. There was a fundamental difference in the diffusion of scopolamine through the placenta and the blood-lumbar (CSF-barrier (n = 15). There was significant drug penetration in the foeto-placental unit, indicating an efficient drug transfer to the child, but there were measurable levels of the drug in the lumbar CSF in only three cases. The apparent elimination phase half-life of scopolamine in serum was only around 1 h. The urinary excretion of scopolamine and/or its antimuscarinic metabolites lasted only for 6 h (2.63 +/- 1.14% of the dose). The onset of the clinical effects of scopolamine appeared to be delayed, but long-lasting in contrast to the rapid absorption and quick disappearance from the serum. Both the heart rate changes, sedative and antisialogogue effects and serum concentrations did not show any correlation. There appears to be a surprisingly great difference between the pharmacokinetic parameters and the clinical effects of scopolamine.