As long as total mepivacaine dose stayed the same, sciatic nerve block onset and offset times were not different. (Image source: Thinkstock)

As long as total mepivacaine dose stayed the same, sciatic nerve block onset and offset times were not different. (Image source: Thinkstock)

The success of nerve blocks depends in part on local anesthetic concentration and volume.  What is the relationship between these factors?  Dr. Gianluca Cappelleri, Department of Anesthesia, Azienda Ospedaliera Istituto Ortopedico Gaetano Pini, Piazza Cardinal Ferrari 1, Milano, Italy, and colleagues used a double-injection Labat sciatic nerve block with variable volumes and concentrations of mepivacaine to determine onset time and nerve block duration.  The results of their analysis are published in this month’s issue of Anesthesia & Analgesia in the article “The Effect of Local Anesthetic Dilution on the Onset Time and Duration of Double-Injection Sciatic Nerve Block: A Prospective, Randomized, Blinded Evaluation.”

Ninety patients scheduled for foot surgery with major osteotomies and continuous sciatic nerve block were included in this study.  A nerve stimulator was used to facilitate block insertion.  Two components of the nerve were identified and either 6 mL of mepivacaine 2% (group 1) or 12 mL of mepivacaine 1% (group 2) were injected into each component.  The total amount of local anesthetic injected was 240 mg.

The onset time for sensory and motor block was statistically the same for the two groups: 4 min in group 1, and 6 min in group 2.  Offset time was also similar at 235 min for group 1, and 240 min in group II.  The authors concluded that 240 mg of mepivacaine produced successful nerve block without a meaningful difference in the onset times and duration between 6 mL of 2% or 12 mL of 1% mepivacaine.

The authors are to be congratulated for this well-designed and executed study.  In fact, not all have found the same the same results. An earlier study in Anesthesia & Analgesia found that a higher concentration improved success rate and decreased onset time.  However, those authors injected only the tibial component of the sciatic nerve.  Studies for other types of blocks support the findings in this current study.