The effect of local anaesthetic agents on the contractility of human myometrium in late pregnancy. An in vitro study.

Four local anaesthetic agents, lidocaine, prilocaine, bupivacaine and etidocaine with and without adrenaline, were tested regarding their influence on the contraction amplitude, tone and frequency of contractions of human myometrium in vitro. All four drugs had a dose-related inhibitory effect on the amplitude. This was enhanced by adrenaline. The two short-acting and the two long-acting agents were tested in equianalgesic doses in a crossover test. Lidocaine and etidocaine had a significantly greater inhibitory effect on the spontaneous myometrial contractions than prilocaine and bupivacaine. With all drugs, except etidocaine, the tone increased with increasing drug concentrations in the bath. The changes in frequency were inconsistent. Based on the concentrations required to produce 50% inhibition, lidocaine, bupivacaine and etidocaine were, respectively, 1.3, 6.5 and 8.3 times more potent than prilocaine; the corresponding figures for the anaesthetic effect on isolated nerves are reported to be 1.5, 5.3, and 5.3. The inhibitory effect of etidocaine declined with time, in contrast to the other local anaesthetic agents tested.