Pharmacologic Aspects of Local Anesthetic Use

Abstract

Summary LAs reversibly interrupt transmission of nerve impulses by inhibiting function of the sodium channel. Large myelinated rapidly conducting A fibers are most sensitive, and small unmyelinated slowly conducting C fibers, least sensitive to the block of transmission. Pregnancy may increase even further the A-fiber sensitivity to conduction block. Of the clinically useful LAs, bupivacaine has become extremely popular in obstetric anesthesia because of its ability to create high quality analgesia with a minimum degree of motor block. The ability to cause this type of differential block is highly desirable and new LAs such as ropivacaine are evaluated with this in mind. Epinephrine is a useful addition to LA solutions for purposes of detecting accidental intravascular injections and for improving the quality and duration of block, reducing the total amount of LA needed. However, there is a risk of temporary decrease in IVBF when accidental intravascular injection of epinephrine-containing solutions occurs. Alkalinization of LA solutions with sodium bicarbonate enhances nerve block experimentally and may have a beneficial effect clinically. Similarly, carbonated lidocaine may have clinical advantages over lidocaine hydrochloride. LA effects on the neonate have been evaluated by neurobehavioral testing. Inconsistent results and a failure to consider the multiplicity of factors surrounding parturition make it difficult to determine the clinical significance of these data. Regarding the adverse effects of LAs, bupivacaine cardiotoxicity has been related to its fast-in, slow-out blocking of myocardial sodium channels. The pregnant animal may be more sensitive to this effect than the nonpregnant animal. Chloroprocaine has been reformulated without the anti-oxidant bisulfite, which along with the low pH of the earlier preparation is thought to have been responsible for the cases of neurotoxicity associated with accidental intrathecal injection of the drug. Whenever large amounts of LA are used, there is the potential for the development of toxicity. The practitioner must strive to prevent LA toxicity but must remain alert to the possibility of its occurrence and be prepared to initiate effective treatment.