Response of human uterine arteries to local anesthetics.

The in vitro effects of local anesthetics and norepinephrine upon strips of early gestation and term pregnancy uterine arteries were studied in eight cases. In another case, the effect upon uterine veins was studied with a standard organ bath used to record isometric contractions. Histologic preparations were made to verify the type of vessel studied. An artery obtained from an eight-week gestation did not respond to either lidocaine or mepivacaine. All other arterial specimens (radial and helicoidal strips) responded with slow, rising, strong contractions to diluted concentrations of both of these substances. Likewise they responded with rapid contractions when exposed to norepinephrine. Alpha blockers were unable to prevent the contractions triggered by the local anesthetics. The vein specimens did not respond to local anesthetics but contracted when stimulated by norepinephrine. Based on these observations and after brief review of some hypotheses advanced to explain post-paracervical anesthesia fetal bradycardia, it is postulated that this bradycardia is probably due to uterine artery spasm, causing decreased intervillous space blood flow and fetal hypoxia.