Comparison of the effects of meperidine and fentanyl on the contractile responses in human pregnant myometrium

Abstract

In order to characterize and compare the effects of meperidine and fentanyl or pregnant human myometrium during labor, in vitro contractility studies were performed or myometrial strips obtained at caesarean section. The strips were pre-exposed to oxytocin (1 mU/ml) to stimulate labor and then were exposed to increasing concentrations of meperidine (10-7 to 10-4 M) or fentanyl (10-9 to 10-6 M). Both drugs lengthened the period of contraction at three concentrations (meperidine 10-6 - 10-4 M, fentanyl 10-8 - 10-6 M), while higher concentrations of meperidine sometimes increased the frequency of contractions. Meperidine decreased the contraction strength at 10-4 M, but other concentrations had no lowering effect on this parameter. Fentanyl did not produce any effect on contraction strength. Meperidine produced a net reduction in the effectiveness of uterine contraction as measured by the average force, at higher concentration (10- 5 and 10-4 M) while fentanyl had a nonsignificant effect on it. The difference between average forces obtained at higher equipotent concentrations c meperidine and fentanyl was found to be statistically significant, but this was not the same at clinically achieved concentrations (2.5*10-7 M - 8.2*10-7 M for meperidine and 2.1*10-9 M 3.3*10-9 M for fentanyl). None of the drugs produced a reduction in the effectiveness of labor at doses given for the relief of pain during labor.