One of the important factors in patient satisfaction after a surgical procedure is pain relief.  A successful regional block may provide 12 hours of pain relief with the local anesthetics currently available.  Dr. Brian M. Ilfeld, Department of Anesthesiology, University of California San Diego, San Diego, CA and colleagues designed a dose-response cohort study using different doses of a liposomal bupivacaine formulation injected adjacent to the femoral nerve to examine sensory and motor block.  Their findings are published this month in Anesthesia & Analgesia in the article “Liposomal Bupivacaine as a Single-Injection Peripheral Nerve Block: A Dose-Response Study.

The authors used 0-80 mg liposomal bupivacaine in 30 mL of injectate. In a randomized design fourteen volunteers received two different doses, one on each side.  Volunteers were tested up to 120 hours after injection.  Muscle strength, measured at the quadriceps muscle, was measured using an isometric force. Sensory effect was measured using transcutaneous electrical stimulation.

Block onset occurred within the first 20 min for 95% of subjects. Onset time was not a function of bupivacaine dose.  Peak effect was seen in 75% of subjects within 24 hours. Time to maximum motor block or sensory block was not associated with bupivacaine dose.  The dose response was paradoxical, in that a higher dose of bupivacaine was associated with a lower magnitude of voluntary isometric contractile force and toleration to electrical stimulation.   Maximum voluntary isometric contraction (MVIC) did not return to within 20% of baseline until after 24 hours for 90% of subjects. For every subject receiving bupivacaine >40 mg, tolerance to cutaneous current did not return to within 20% above baseline until more than 24 hours had elapsed.  Motor block duration was not correlated with bupivacaine dose.

Use of a liposomal bupivacaine formulation should still be considered experimental.  Indeed, the liposomal bupivacaine formulation is currently only approved for surgical infiltration.  Also, this study has some limitations.  Volunteers received payment based on the length of time they spent in the research center.  These volunteers also did not undergo a procedure that causes pain.  In addition, there were some protocol issues: MVIC data was missing for four volunteers.  Given that higher doses are used normally for infiltration, could a larger infiltration dose have been utilized?

As noted by Dr. John C. Rowlingson, Department of Anesthesiology, University of Virginia Health System, Charlottesville, VA in the article’s accompanying editorial titled “We’re on the Road to Depo-Local Anesthetics, But We Aren’t There Yet,” “As health care moves towards a model in which patient satisfaction and patient outcome will be rewarded, the persistent challenge to effectively manage POP (postoperative pain) in and away from hospital settings looms large as an obstacle to our achieving these lofty goals. The drive to create a useful depo-local anesthetic preparation must continue and be successful.”