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ANALGESIA

The Effects of Varying Local Anesthetic Concentration and Volume on Continuous Popliteal Sciatic Nerve Blocks: A Dual-Center, Randomized, Controlled Study

Brian M. Ilfeld, MD, MS^*, Vanessa J. Loland, MD^*, J. C. Gerancher, MD, Anupama N. Wadhwa, MD, Elizabeth M. Renehan, MSc, MD, Daniel I. Sessler, MD^||, Jonathan J. Shuster, PhD^¶, Douglas W. Theriaque, MS^#, Rosalita C. Maldonado, BS^*, Edward R. Mariano, MD^* For the PAINfRE^TM Investigators

From the *Department of Anesthesiology, University of California San Diego, San Diego, California; Department of Anesthesiology, Wake Forest Medical Center; Department of Anesthesiology, University of Louisville, Louisville, Kentucky; Department of Anesthesiology, University of Ottawa, Ottawa, Ontario, Canada; ||Department of Outcomes Research, The Cleveland Clinic, Cleveland, Ohio; ¶Department of Epidemiology and Health Policy Research, and #General Clinical Research Center, University of Florida, Gainesville, Florida.

Address correspondence to Brian M. Ilfeld, MD, MS, Department of Anesthesiology, UCSD Center for Pain Medicine, 9300 Campus Point Dr., MC 7651, LA Jolla, CA 92037-7651. Address e-mail to bilfeld{at}ucsd.edu.

Abstract

BACKGROUND: It remains unknown whether local anesthetic concentration, or simply total drug dose, is the primary determinant of continuous peripheral nerve block effects. We therefore tested the null hypothesis that providing different concentrations and rates of ropivacaine, but at equal total doses, produces comparable effects when used in a continuous sciatic nerve block in the popliteal fossa.

METHODS: Preoperatively, a perineural catheter was inserted adjacent to the sciatic nerve using a posterior popliteal approach in patients undergoing moderately painful orthopedic surgery at or distal to the ankle. Postoperatively, patients were randomly assigned to receive a perineural ropivacaine infusion of either 0.2% (basal 8 mL/h, bolus 4 mL) or 0.4% (basal 4 mL/h, bolus 2 mL) through the second postoperative day. Therefore, both groups received 16 mg of ropivacaine each hour with a possible addition of 8 mg every 30 min via a patient-controlled bolus dose. The primary end point was the incidence of an insensate limb, considered undesirable, during the 24-h period beginning the morning after surgery. Secondary end points included analgesia and patient satisfaction.

RESULTS: Patients given 0.2% ropivacaine (n = 25) experienced an insensate limb with a mean (sd) of 1.8 (1.8) times, compared with 0.6 (1.1) times for subjects receiving 0.4% ropivacaine (n = 25; estimated difference = 1.2 episodes, 95% confidence interval, 0.3–2.0 episodes; P = 0.009). In contrast, analgesia and satisfaction were similar in each group.

CONCLUSIONS: For continuous popliteal-sciatic nerve blocks, local anesthetic concentration and volume influence block characteristics. Insensate limbs were far more common with larger volumes of relatively dilute ropivacaine. During continuous sciatic nerve block in the popliteal fossa, a relatively concentrated solution in smaller volume thus appears preferable.

Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999