Continuous peripheral nerve blocks are accomplished by infusion or intermittent boluses of local anesthetic solutions. Overwhelming plethora of options are available for nearly every aspect of continuous infusion administration, from the choice of infusate to choice of infusion rate and bolus regimen, to infusion pump selection.1 An ideal perineural local anesthetic solution would provide analgesia while minimizing sensory, motor, and proprioception deficits. In addition, desirable attributes include a favorable toxicity profile and cost-efficacy. The optimal infusion strategy may be modified for the large number of clinical scenarios that the regional anesthesiologist will encounter in daily practice. Considerations include the indication for perineural catheter placement, the number and location of catheters, patient weight, and ambulatory versus inpatient status.
INFUSATES AND LOCAL ANESTHETIC CONCENTRATION
Local anesthetics were described2 in continuous perineural infusions as early as 1946. Intermediate-duration local anesthetics such as mepivacaine have been used,3 but long-acting local anesthetics such as ropivacaine, bupivacaine, and levobupivacaine are most frequently described.1 These long-acting agents provide a favorable differential sensory-to-motor block. At the termination of an infusion, it is desirable for sensory and motor block to resolve quickly and predictably. Studies have suggested that sensory-and-motor block regresses faster with ropivacaine than with bupivacaine.4
It is currently unclear if local anesthetic concentration—or simply the total delivered dose—influences continuous block effects.5,6 While the evidence suggests that for infusions involving the femoral nerve, local anesthetic concentration is of minimal importance compared with total dose, data for the sciatic nerve are lacking, and the brachial plexus information is conflicting.7,8,9 Therefore, at this time it remains unknown if there is an “optimal” concentration of local anesthetic. Commonly described concentrations include ropivacaine 0.1%–0.4%,4,6,10-13 bupivacaine 0.125%–0.15%,4,14 and levobupivacaine 0.1%–0.125%.15,16
LOCAL ANESTHETIC DELIVERY STRATEGIES
Infusates are typically delivered using an infusion pump with a basal infusion, bolus dose, or combination of the two modalities. Regimens are often reported as basal rate (mL/hour)/bolus volume (mL)/bolus lockout time (minutes). The delivery regimen should minimize total local anesthetic consumption, supplemental opioid requirements, and disturbance of daily functioning/sleep. No single delivery regimen has proven ideal for all anatomic locations and clinical situations. In many cases, providing a basal infusion minimizes breakthrough pain and supplemental analgesic requirements.1 Adding a patient-controlled bolus usually decreases the required basal infusion rate,4,17,18 incidence of an insensate extremity,19 and local anesthetic consumption, the last allowing for a longer infusion duration in the ambulatory setting.20,21 For example, in the upper extremity, an interscalene infusion ...