Local anesthesia produces a loss of sensation in a specific body part or region. Medical practitioners use it to perform relatively minor surgical procedures. The approach involves introducing an anesthetic drug near the peripheral nerve that innervates the desired area. The basic goal is to block afferent neural transmission along the peripheral nerve so that the procedure is painless. When a local anesthetic is introduced in the vicinity of the spinal cord, transmission of impulses may be effectively blocked at a specific level of the cord, allowing for more extensive surgical procedures (e.g., cesarean delivery) because a larger region of the body is being anesthetized. This approach is still considered a local anesthetic because the drug acts locally at the spinal cord and the patient remains conscious during the surgical procedure.
Using a local anesthetic during a surgical procedure offers several advantages over the use of general anesthesia, including a relatively rapid recovery and lack of residual effects.1,2 There is a virtual absence of the postoperative confusion and lethargy often seen after general anesthesia. In most cases of minor surgery, patients are able to leave the practitioner's office or hospital almost as soon as the procedure is completed. In more extensive procedures, local anesthesia offers the advantage of not interfering with cardiovascular, respiratory, and renal functioning. This fact can be important in patients with problems in these physiological systems. During childbirth, local (spinal) anesthesia imposes a lesser risk to the mother and neonate than general anesthesia.3
The primary disadvantages of local anesthesia are the length of time required to establish an anesthetic effect and the risk that analgesia will be incomplete or insufficient for the procedure.4 The latter problem can usually be resolved by administering more local anesthesia if the procedure is relatively minor or by switching to a general anesthetic during a major procedure if an emergency arises during surgery.
In nonsurgical situations, local anesthetics are sometimes used to provide analgesia. These drugs may be used for short-term pain relief in conditions such as musculoskeletal and joint pain (e.g., bursitis, tendinitis) or in more long-term situations such as pain relief in cancer or treatment of chronic pain. In addition, local anesthetics may be used to block efferent sympathetic activity in conditions such as complex regional pain syndrome.
During these nonsurgical applications, you will often be directly involved in treating the patient while the local anesthetic is in effect. Physical therapists may actually administer the local anesthetic via phonophoresis or iontophoresis (if prescribed by a physician). Consequently, you should have adequate knowledge of the pharmacology of local anesthetics.
TYPES OF LOCAL ANESTHETICS
Commonly used local anesthetics are listed in Table 12-1. Most of these drugs share a common chemical strategy consisting of both a lipophilic and hydrophilic group connected by an intermediate chain (Fig. 12-1). ...