Local Anesthetics

Local Anesthetics

Simply stated, local anesthesia refers to loss of sensation in a limited region of the body. This is accomplished by disruption of afferent neural traffic via inhibition of impulse generation or propagation. Such blockade may bring with it other physiologic changes such as muscle paralysis and suppression of somatic or visceral reflexes, and these effects might be desirable or undesirable depending on the particular circumstances. Nonetheless, in most cases, it is the loss of sensation, or at least the achievement of localized analgesia, that is the primary goal.

Although local anesthetics are often used as analgesics, it is their ability to provide complete loss of all sensory modalities that is their distinguishing characteristic. The contrast with general anesthesia should be obvious, but it is perhaps worthwhile to emphasize that with local anesthesia the drug is delivered directly to the target organ, and the systemic circulation serves only to diminish or terminate its effect. Local anesthesia can also be produced by various chemical or physical means. However, in routine clinical practice, it is achieved with a rather narrow spectrum of compounds, and recovery is normally spontaneous, predictable, and without residual effects. The development of these compounds has a rich history (see Box: Historical Development of Local Anesthesia), punctuated by serendipitous observations, delayed starts, and an evolution driven more by concerns for safety than improvements in efficacy.

CASE STUDY

67-year-old woman is scheduled for elective total knee arthroplasty. What local anesthetic agents would be most appropriate if surgical anesthesia were to be administered using a spinal or an epidural technique, and what potential complications might arise from their use? What anesthet-ics would be most appropriate for providing postoperative analgesia via an indwelling epidural or peripheral nerve catheter?

CASE STUDY ANSWER

If a spinal anesthetic technique were selected, bupivacaine would be an excellent choice. It has an adequately long duration of action and a relatively unblemished record with respect to neurotoxic injury and transient neurologic symptoms, which are the complications of most concern with spinal anesthetic technique. Although bupivacaine has greater potential for cardiotoxicity, this is not a concern when the drug is used for spinal anesthesia because of the extremely low doses required for intrathecal administra-tion. If an epidural technique were chosen for the surgical procedure, the potential for systemic toxicity would need to be considered, making lidocaine or mepivacaine (gener-ally with epinephrine) preferable to bupivacaine (or even ropivacaine or levobupivacaine) because of their better therapeutic indexes with respect to cardiotoxicity. However, this does not apply to epidural administration for postop-erative pain control, which involves administration of more dilute anesthetic at a slower rate. The most common agents used for this indication are bupivacaine, ropiva-caine, and levobupivacaine.