The choice of the anesthetic solution, its concentration, and total dose administered must be tailored to the individual patient’s requirement to minimize the onset time and the risk for overdosage. When a catheter is placed close to the nerve, we do not have the problem of prolonging the effects of initial injection, thus we can select a short-onset, intermediate-duration anesthetic solution, such as mepivacaine or lidocaine in concentrations ranging between 1.5% and 2.0%, for the initial bolus. Some authors recommend the use of a combination of anesthetic solutions with different pharmacokinetic properties. The most frequent of these mixtures is a combination of two local anesthetics, one with a short onset and one with a long onset, such as a combination of mepivacaine with either bupivacaine, ropivacaine, or levobupivacaine. The rationale for using these mixtures is to achieve a compromise between the onset of the shorter-acting drug and the duration of the longer-acting agent. However, it must be considered that diluting the two local anesthetics actually reduces the drive of diffusion of each agent into the nerves by reducing the final concentration of each agent. Moreover, it has been reported that when two different local anesthetics are mixed, there is a competitive binding of the two different agents to the
protein carriers, and the free concentration of the more toxic local anesthetic is similar to that produced by using it alone in a volume similar to the volume actually injected.

The volumes and doses of local anesthetics will depend on the type of surgery (e.g., single block or a combination of different blocks, such as for the lower limb) as well as on whether light general anesthesia is used during surgery. Accordingly, the maximum doses suggested for each anesthetic drug must always be considered (Table 25-1).

From a theoretical standpoint, it is possible to use either a short-onset and short-duration local anesthetic or a long-acting one also for continuous postoperative infusion. The use of a short-acting anesthetic, such as lidocaine or mepivacaine, is mainly based on the reduced toxicity potential as compared with more potent and lipid-soluble agents. Moreover, short-acting agents also have the theoretical potential for allowing a fast recovery of sensory and motor function after the infusion is stopped, to enable quick and easy neurologic evaluation. The main disadvantage of shorter-acting agents is that they have been demonstrated to be less effective in providing a good differentiation between sensory and motor blocks. Evaluating the use of either 0.2% ropivacaine or 1% lidocaine for continuous interscalene analgesia after open shoulder surgery, it has been reported that both agents provide a similarly adequate analgesia, but a more efficient recovery of motor function was observed in patients receiving 0.2% ropivacaine. This finding is similar to that previously reported with epidural blockade, and can be explained with the different pKa of the two drugs. For this reason, most authors prefer to use long-acting local anesthetics (e.g., bupivacaine, ropivacaine, or levobupivacaine) for continuous perineural infusions.