Effects of local anesthetic agents on the uterine vasculatures and myometrium

Intravascular procaine, lidocaine, mepivacaine, and bupivacaine decrease blood flow to the placental and nonplacental vascular beds of gravid ewes by stimulating vasoconstriction and myometrial contractility. These effects appear to be direct ones since they are not affected by alpha-adrenergic blockade. Dose-response curves determined in nonpregnant ewes indicate that significant decreases in blood flow may occur at arterial blood concentrations encountered clinically. It is proposed that reduced placental blood flow is the cause of fetal bradycardia following paracervical block anesthesia. The implications of these findings in obstetric anesthesia are discussed.