An evaluation of muscle relaxants in cesarean sections.

The passage of drugs across the placenta has been the subject of a number of comprehensive reviews. The best and most prudent approach to the use of muscle relaxants often presents controversial views. The purpose of this article is to investigate and review the various muscle relaxants available to the anesthetist, and to determine which of these are best indicated for cesarean sections. It has been my observation, that at the Hurley Medical Center in Flint, Michigan, succinylcholine is used and advocated more frequently than any other muscle relaxant. However, from various studies done and available literature on this topic, I intend to explore other relaxants, their possibilities, and their potential use for cesarean sections. Of primary importance when considering which drugs are to be used in obstetric anesthesia is the risk of placental transfer of these drugs. The essential factors to be considered when administering drugs to the pregnant mother are: 1. The molecular weight of the drug. 2. The fat solubility of the drug. 3. The degree of ionization. 4. The concentration gradient. 5. The binding to maternal proteins. 6. The surface area and thickness of the membrane. It is generally believed that all drugs used for the relief of apprehension and pain during labor and delivery are capable of passing through the placenta and affecting the infant. With some drugs, this has been proved; with others, it has been inferred. This is particularly important in light of recent work showing that, in animals, the fetus is four to five times as sensitive to depressant drugs as is the adult. The newborn's response to drugs is qualitatively and quantitatively different from that of the mother. The newborn shows marked susceptibility to the depressant effect of drugs. This heightened sensitivity is related, in part. to increased permeability of the blood brain barrier, inefficient renal function, absence of microsomal enzymes necessary for drug metabolism, and asphyxia and physical trauma associated with the delivery process. The concept of the lipoid barrier appears to apply greatly to the factors affecting the diffusion of drugs across the placenta. The rate of transfer across the blood placental barrier is governed chiefly by the lipoid solubility of the nonionized or undissociated drug molecule. Other factors, such as concentration gradient, maternal and fetal flow, and molecular size, are usually of secondary importance.