Effective peripheral nerve blocks provide excellent anesthesia and analgesia. Local anesthetics inhibit propagation of painful impulses along peripheral nerves by blocking the rapid influx of sodium that is required to initiate an action potential. Local anesthetics can reduce the amount of inhalation anesthetic needed to perform a procedure and can mitigate the "wind-up" response of the CNS to noxious stimuli. The duration of effect varies with the type of local anesthetic. Shorter-acting agents provide 1 hour of anesthesia, and longer-acting agents provide up to 6 hours. The duration of analgesia, or reduced sensation of pain, may be longer. Opioid analgesics, such as morphine, can be administered locally along with lidocaine or bupivacaine to augment the duration of analgesia. In addition to the CNS, opioid receptors are present in the periphery if tissues are inflamed.
Local anesthetic agents can be administered as topical, infiltrative, intraarticular or peripheral nerve blocks. Topical administration involves deposition of the agent on intact mucous membranes or skin. Infiltrative anesthesia involves injection into the subcutaneous tissues around an incision or wound or at the site of catheter placement. Intraarticular blocks target sensory nerve endings in the synovium. Peripheral nerve blocks are performed by injecting local anesthetic in the immediate vicinity of a peripheral nerve or nerve plexus. Common peripheral nerve blocks in small animal patients include epidural, brachial plexus, dental, and forefoot blocks. Intraarticular stifle blocks are also used. This article presents all but the dental block, which will be covered in an article in a future issue. Local anesthetic blocks can be given to heavily sedated or anesthetized animals. Greatest efficacy is obtained when local blocks are given before surgery is begun.