Local Anesthetic Drugs -Effects on the Baby

Local Anesthetic Drugs Used During Labor –Effects on the Baby

Conduction anesthesia has become the most commonly used in modern obstetric practice, both to provide analgesia for labor pain and anesthesia for surgical delivery. Epidural blocks give a versatile range of anesthesia from a mild sensory block for normal labor to a dense blockade of most of the nerve fibers required for cesarean section.

                The local anesthetics commonly used for obstetrics are bupivicaine and lignocaine. When injected, these drugs are absorbed from the epidural space into the maternal blood stream. This results in a significant level of circulating drug which will cross the placenta rapidly by passive diffusion. Many factors affect the placental transfer such as the total dose of drug injection, route of administration, presence of adrenalin, maternal metabolism and exertion, maternal protein binding and intervillous blood flow. These factors determine the fetal-to-maternal concentration ratio. Pathological conditions of the placenta such as eclampsia, diabetes, hyper-tension, and Rhesus disease may also affect this transfer but the extent to which they do so is unknown. The local anesthetic is taken up by the fetus and metabolized and excreted by the baby. Similar, though delayed, decay curves of drug activity are a seen in neonatal and maternal circulations.

                Local anesthetics act on all conduction tissue and toxicity in the fetus and neonate is seen in the central nervous system, peripheral blood vessels and the heart. No fetal heart rate changes occur at modest plasma levels of local anesthetic.

                Conduction anesthesia also has indirect effects on the fetus, which requires an adequate delivery of oxygenated maternal blood to the intervillous space. This can be affected by changes in the uterine blood flow, which varies directly with perfusion pressure across the uterine vascular bed, and in aversely with uterine vascular resistance. So, if hypotension results from an epidural block, a reduction in mean uterine pressure will reduce uterine blood flow and thereby impair fetal oxygenation and well-being. Hypotension is a common complication of epidural block, especially at term when pooling of blood in the lower limbs in encouraged not only by sympathetic blockade but by aortocaval compression.

                Uterine vascular resistance is maintained by the intrinsic vasomotor state of uterine vessels. These are fully dilated at term but ill constrict with high catecholamine levels, general anesthesia and toxic levels of local anesthetic drugs.