CHOICE OF LOCAL ANESTHETIC

The choice of local anesthetic for a peripheral nerve block obviously depends to some degree on the duration of the surgical procedure, although other factors are also important (see Chapter 14 ). Prolonged blockade for up to 24 hours often occurs with long-acting agents such as bupivacaine or ropivacaine. Although this feature results in superb postoperative pain relief for the inpatient, it may be undesirable for the ambulatory patient because of the possible risk of nerve or tissue injury in a partially blocked limb. A short- or medium-acting agent, such as lidocaine or mepivacaine, may be more appropriate in the outpatient setting. Whatever drug is chosen, the total dosage should be calculated for each patient and should be kept within acceptable safe limits (see Chapter 14 ).

The highest concentrations of local anesthetic drugs are not appropriate for peripheral neural blockade; therefore, 0.75% bupivacaine or ropivacaine, 2% lidocaine, 2% mepivacaine, and 3% 2-chloroprocaine are not recommended. The lowest concentrations of the same agents (i.e., 0.25% bupivacaine or ropivacaine and 0.5% mepivacaine or lidocaine) may not provide complete motor blockade.

Vasoconstrictors, usually epinephrine, can be added to the chosen local anesthetic to improve onset of action, to decrease drug uptake, and to prolong action. A concentration of 1:200,000 epinephrine is usually recommended. Ideally, the epinephrine should be added to the local anesthetic at the time the block is to be performed. Commercially prepared solutions with epinephrine have a lower pH than those in which it is freshly added, resulting in a higher percentage of ionized drug molecules. These ionized molecules do not readily cross the neural membrane, delaying the onset of drug action after injection. Epinephrine should not be added to the local anesthetic for blocks of the digits or penis because tissue ischemia may result. Various other additives, including clonidine, opioids, and ketamine, have been reported to enhance or prolong local anesthetic peripheral nerve blockade. ^[7]