Relationship of fetal bradycardia to maternal administration of lidocaine in sheep.

The casual relationship between the use of lidocaine and fetal bradycardia and the effect of the drug on maternal and fetal hemodynamics were studied on 13 chronically instrumented pregnant sheep. Lidocaine was infused intravenously to the mother for 60 minutes during arterial lidocaine concentrations were maintained at 2 to 5 microgram per milliliter in the mother and at less than 2 microgram per milliliter in the fetus. A decrease in uterine blood flow and an increase in uterine vascular resistance and uterine activity occurred immediately following the administration of lidocaine to the ewe. These changes were followed by a transient fetal bradycardia in 12 out of 17 experiments, accompanied by a decrease in fetal PaO2 values. These phenomena were seen in the absence of such predisposing conditions as maternal hypotension and fetal acidosis. It would appear that the mechanism responsible for a transient fetal bradycardia following regional obstetric anesthesia, particularly paracervical block anesthesia, in the initially nonasphyxiated fetus may in part be related to a brief decrease in perfusion of intervillous spaces. The bradycardia can occur at low lidocaine concentrations in both the maternal and fetal blood in a range similar to that observed in clinical practice.