Combined lumbar-plexus and sciatic-nerve blocks: An analysis of plasma ropivacaine concentrations

Background and objectives: Lumbar-plexus and sciatic-nerve blocks are commonly combined for lower-extremity anesthesia using large doses of ropivacaine. Limited information is available about the pharmacokinetics of this practice. We analyzed plasma ropivacaine concentrations after single-injection lumbar-plexus blocks with and without sciatic-nerve blocks. Methods: Twenty patients having lower-extremity surgery using a lumbar-plexus block with 0.5% ropivacaine with 1:400,000 epinephrine (35 mL, n = 10) or the same lumbar-plexus block with the addition of a sciatic-nerve block (25 mL, n = 10, 60 mL total) using the same solution were enrolled. Venous blood samples were collected at 5, 15, 30, 45, 60, 120, and 240 minutes after block placement and analyzed for total ropivacaine concentration by use of gas chromatography. Individual timepoints, maximum concentrations (C-max), and time to C-max (T-max) were compared. Values are mean +/- SD. Results: Both groups demonstrated a rapid increase in plasma concentration over the first 30 to 45 minutes. Concentrations were greater for those who received both blocks (P = .0005) at all timepoints. The lumbar-plexus block C-max was less (986 +/- 221 ng/mL) than for the combined blocks (1,560 +/- 351 ng/mL, P = .0004). The T-max was greater for the lumbar plexus (80 +/- 49 min) than for the combined blocks (38 +/- 22 min, P = .03). There was no relationship between the C-max and patient age, weight, or body mass index. Conclusions: The results of this study demonstrate that the plasma ropivacaine concentrations increase quicker when a sciatic-nerve block is added to a lumbar-plexus block, but C-max remains below the toxicity threshold.