Plasma atropine concentrations determined by radioimmunoassay after single-dose i.v. and i.m. administration

The plasma concentrations of atropine following i.v. or i.m. administration to surgical patients were determined by radioimmunoassay. When atropine sulphate 1 mg was given i.v. there was a rapid initial removal of the drug from the circulation in the first 10 min; thereafter the plasma concentration decreased more slowly. Atropine i.m. was rapidly absorbed with peak concentrations occurring at 30 min following injection. The plasma atropine concentration then decreased slowly, probably because of uptake of atropine by muscarinic cholinergic receptors. The chronotropic effect of atropine appeared to correspond to the concentration in plasma following i.m. administration. We conclude that i.m. atropine, as a premedication, should be given not later than 30 min before induction of anaesthesia.