Background: Intramuscular scopolamine plus morphine pre-medication is traditionally used when prominent sedative or antisialogogue effect is needed. Knowledge of the pharmacokinetics of scopolamine is limited due to low plasma concentrations found after therapeutic doses. This investigation compares the pharmacokinetics and the clinical responses of this drug combination injected into two commonly used injection sites.
Methods: Twelve ASA class 1 patients scheduled for minor surgery under spinal anaesthesia received scopolamine 6 μg/kg plus morphine 200 μg/kg injected in either deltoid (group D, n=6) or gluteal (group G, n=6) muscle.
Results: The peak plasma concentrations of scopolamine after deltoid or gluteal injection (2.2 vs 1.6 μg/1) and the time they were reached (17 vs 19 min) were comparable. The absorption of morphine was similar in both groups (Tmax 16 min), but the peak plasma concentrations were higher after deltoid injection (71 vs 49 μg/1). The individual variation in the elimination half-lives of both scopolamine and morphine was smaller after deltoid injection (T½ scopolamine 1.9±0.7 vs 2.1±1.1 h, morphine 1.3±0.7 vs 2.3±1.5 h). Moderate slowing (25%) of heart rate was found in both groups. A heavy sedation and antisialogogue effect (VAS) was found in both groups with faster occurrence of maximal effect in group D (60 vs 120–180 min).
Conclusion: More predictable pharmacokinetics and clinical effects of intramuscular scopolamine plus morphine premedication can be achieved after an injection into deltoid muscle.