In vitro effects of opioids on pregnant uterine muscle.

Opioids are often used for obstetric analgesia. Ideal obstetric analgesia is attained with optimal pain relief and minimal risk for the parturient. Therefore, investigators in the present study explored the effects of different opioids on the myometrium of pregnant rats. Myometrial strips were exposed to increased concentrations of fentanyl (10(-8) M to 10(-6) M), alfentanil (10(-8) M to 10(-4) M), remifentanil (10(-8) M to 10(-4) M), and meperidine (10(-8) M to 10(-4) M). Decreased contractile activity was observed in myometrial strips isolated from pregnant rats at cumulative concentrations of fentanyl, alfentanil, remifentanil, and meperidine. The amplitude of contractions was reduced with increasing concentrations of opioids; this effect was statistically significant at a concentration of 10(-4) M. When administered at higher concentrations, opioids may decrease contractions in pregnant rat myometrium.