Optimizing Local Anesthetic Concentration for Continuous Popliteal-Sciatic Nerve Blocks

Study Summary

A continuous peripheral nerve block-also termed "perineural local anesthetic infusion"-involves the insertion of a tiny tube (a "catheter") through the skin and adjacent to a peripheral nerve, followed by local anesthetic (numbing medicine) administration via the catheter, providing pain control following surgery. Continuous peripheral nerve blocks may be provided in the hospital setting, but the use of lightweight, portable pumps permits infusion at home as well. However, it remains unknown if the concentration of the local anesthetic influences the block effects; or, is it rather simply the total dose of medication that is important. If it is the latter, then the concentration of local anesthetic could be increased, allowing a decreased basal infusion rate, which would allow patients at home to receive twice the duration of potent pain control since their infusion pump local anesthetic reservoir would last twice as long as current practice. In addition, if one concentration/dose combination results in less muscle weakness, but with at least equivalent analgesia, then the risk of falling might be decreased as well. The investigators will test the hypothesis that providing ropivacaine at different concentrations and rates (0.1% at 8 mL/hour vs. 0.4% at 2 mL/hour)-but at an equivalent total basal (8 mg/hour)-produces comparable effects when used in a continuous popliteal-sciatic nerve block.

Primary Outcome

Electrocardiogram pads are placed on the lateral aspect of the plantar surface of the foot which is covered by the sciatic nerve distribution; and, the tolerance to cutaneous electrical current is obtained using a nerve stimulator. The current is increased from 0 mA until subjects detect the electrical current (up to a maximum of 80 mA), at which time the current is recorded and the nerve stimulator turned off.

Interventions

• DRUG Ropivacaine 0.1%
• DRUG Ropivacaine 0.4%