Muscle relaxants are usually used in surgical anesthesia because most anesthetics alone do not produce sufficient relaxation of the skeletal muscles. In contrast to general and local anesthetics, muscle relaxants cross the blood-brain barrier and the placenta only in limited amounts due to their high degree of ionization and limited lipid solubility. Thus, in fetal tissue these agents achieve only 5-10% of the concentration measured in the maternal blood (Demetriou 1982, Abouleish 1980). Hence, under regular anesthesia, it is presumed that the concentrations of these agents in fetal blood are well below the effective dose for production of fetal muscle relaxation. Indeed, no effects were observed in the fetuses of 25 women treated with tubocu-rarine in the second or third trimester of pregnancy (Moise 1987). On the other hand, there is one case report of an infant born with multiple congenital joint contractures whose mother was treated with multiple doses of tubocurarine (Jago 1970). Even if this case is indeed a result of tubocurarine injection, it has no resemblance to the doses and mode of administration in surgery. Of the different muscle relaxants in use, alcuronium, atracurium, cisatracurium, mivacurium, pancuronium, rocuronium, and vecuronium seem to be used to the same extent as tubocurarine. They are all known to block, completely and reversibly, the neuromuscular junctions. Umbilical serum concentrations at term arc 10-20% of maternal serum concentrations, which means that transfer to the fetus, although reduced, docs take place. However, their use during labor is well tolerated.
There arc no human studies on pancuronium in pregnancy. This agent is particularly useful during childbirth. After a maternal dose of 0.05 mg/kg, no side effects were observed in newborns in 800 deliveries (Langanke 1987). It was also used to directly induce fetal paralysis for intrauterine fetal transfusion by injections to the fetus (Moise 1987). Repeated use in rats may result in joint contractures; single daily injections in rats and rabbits seem to have no effect. As stated, this has little relevance to man.
Atracurium's use as a fetal muscle relaxant to arrest fetal activity during intrauterine intravascular transfusions has also been described in a small number of cases. One group of investigators concluded that atracurium was superior to pancuronium for this purpose (Mouw 1999).
Some investigators point out the advantage of vecuronium for cesarean sections over other muscle relaxants, as it seems to have negligible residual effects on the fetal heart function and on the newborn (Watson 1996, Das 1993).
Suxamethonium (succinylcholine) is a depolarizing muscle relaxant. No anomalies were observed in 26 infants born to mothers treated with succinylcholine during pregnancy (Heinonen 1977). Transient respiratory depression has been described after succinylcholine use during labor, but this complication is apparently rare. If the plasma cholinesterase activity is low (as found in 3-4°/o of the population), the muscle relaxant effect of this agent may be prolonged and can result in apnea in the newborn infant (Cherala 1989). Proper care should be taken to avoid these complications, and hence the lowest effective doses should be administered.
Recommendation. The common muscle relaxants may be used as part of anesthetic regimens during pregnancy and labor, as well as for the purpose of relaxation of fetal muscles. Use of multiple recurrent doses (i.e. for the treatment of tetanus) may cause some harm, but in this indication, the benefit seems to outweigh the possible hazard to the fetus. Use of low doses may reduce the likelihood of fetal side effects.
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