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Anesthesiology Department, Pontificia Universidad Católica de Chile, Santiago, Chile, falterma{at}med.puc.cl
To the Editor:
Peripheral nerve blocks are excellent alternatives for postoperative analgesia in major knee surgery (1). However, the use of large doses of local anesthetics in these techniques carries the potential risk of systemic toxicity. Although levobupivacaine might represent a safer alternative than bupivacaine in these cases, the pharmacokinetic profile of this drug in combined posterior lumbar plexus and sciatic nerve blocks is unknown. We report preliminary results of a study on the pharmacokinetics of levobupivacaine after combined posterior lumbar plexus and sciatic nerve block.
After IRB approval and written informed consent were obtained, we studied ASA physical status I–II adult patients scheduled for unilateral major knee surgery. Before performing the blocks, all patients were given midazolam 1–2 mg IV and fentanyl 1–2 µg/kg IV. After lateral decubitus positioning, a posterior lumbar plexus block was performed using an insulated 100 mm Stimuplex® needle according to the landmarks described by Winnie et al. (2). After quadriceps muscle response ("patellar jump") with a current of 0.5 mA and negative blood aspiration, 30 mL of 0.5% plain levobupivacaine was administered in incremental doses. With the patients in the same position, the sciatic nerve block was subsequently performed using the landmarks described by Labat (3). After eliciting a distal response (plantar flexion or dorsal flexion of toes) with a current of 0.4–0.5 mA, and negative blood aspiration, 20 mL of 0.5% plain levobupivacaine was injected. After finishing the last block arterial blood samples were withdrawn at 2, 5, 10, 30, 60, 90, 120, 240, and 360 min to measure levobupivacaine total plasma concentrations by means of high performance liquid chromatography.
After 5 patients had been studied, all of whom had clinically effective blocks as measured by long-lasting postoperative analgesia, preliminary results showed that median (range) maximum plasma concentrations of levobupivacaine measured were 2.2 µg/mL (1.95–3.10 µg/mL). These peak levels occurred 5 to 10 min after finishing the blocks, demonstrating a very rapid absorption phase. (Fig. 1) These values are higher than we expected and are in the range of previously reported toxic levels (4,5). Although we did not find toxic systemic complications in any patient and the real toxic threshold for levobupivacaine has not been definitively described (4,5), we decided to stop the study and report our results.
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Our data confirm the fact that plasma levels reached after a peripheral block will not only depend on the total amount of local anesthetic used but also on the type of nerve block performed. Anatomically, the posterior lumbar plexus block lies over a highly irrigated muscular region. This might be the reason for the higher plasma levels after this block compared with those obtained after a femoral nerve block despite using the same dose of local anesthetic (6). In addition, our results suggest that safer options could be either decreasing the dose of levobupivacaine or the addition of epinephrine to levobupivacaine during the posterior lumbar plexus block. This latter option might delay levobupivacaine's systemic absorption and reduce peak plasma concentrations, as it has been reported with ropivacaine after thoracic paravertebral block (7).
References
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