Response of arterial segments from gravid human uterus to multiple concentrations of lignocaine

Nine uterine artery segments were obtained during Caesarean hysterotomy. The amount of isometric tension developed by each segment when exposed to 3, 6, 10, 20, 50, 100, 1000 and 2000 microng/ml of lignocaine hydrochloride (Xylocaine) was measured. All nine segments contracted in response to the 1000- and 2000-microng/ml concentrations. Eight of nine, six of nine, and three of nine responded to the 100-, 50- and 20-microng/ml concentrations, respectively; one segment contracted in response to all concentrations. A dose-response curve indicated that maximum responses were obtained at the 1000-microng/ml concentration (0.1%). Although local anaesthetics are usually considered vasodilators, this study indicates that lignocaine causes the uterine artery to constrict in concentrations in excess of 20 microng/ml. Such concentrations might occur after paracervical block or accidental intravascular injection, but would not be expected in association with extradural analgesia.