Ropivacaine plasma concentrations are similar during continuous lumbar plexus blockade using the anterior three-in-one and the posterior psoas compartment techniques

Purpose: To compare ropivacaine blood concentrations obtained after a continuous lumbar plexus block performed either by the anterior three-in-one femoral (FEM) technique or the posterior (psoas compartment; PSOAS) technique. Methods: As a substudy of a larger clinical trial, 24 patients were randomly allocated to receive a bolus of 30 mL of ropivacaine 0.5% plus epinephrine 1:200,000 followed by an infusion of ropivacaine 0.2% at 12 mL·hr-1 for 48 hr via one of the two continuous lumbar plexus block techniques. Plasma ropivacaine concentrations, up to 48 hr, were measured by high performance liquid chromatography. Results: Mean plasma ropivacaine concentrations were higher in the PSOAS group at 15, 30, and 60 min (two-way analysis of variance, P < 0.0001) but areas under the curve were similar for both groups (FEM 452.4 ± 253.6 mg·hr-1·L-1, PSOAS 433.4 ± 99.0 mg·hr-1·L-1). Mean maximal plasma concentrations were observed at 48 hr and were comparable for the two techniques (FEM 2630.9 ± 1470.3 ng·L-1, PSOAS 2325.1 ± 604.2 ng·mL-1). There was no correlation between blood concentrations at 48 hr and body weight (r2 = 0.085, P = 0.21. One patient in the FEM group achieved a concentration of 6201 ng·mL-1 at 48 hr. Conclusions: Athough the posterior PSOAS block results in higher early plasma concentrations of local anesthetic than the anterior three-in-one FEM block, both techniques are equivalent with regards to their potential toxicity when a continuous infusion is administered. Local anesthetic accumulation occurs with an infusion of ropivacaine 0.2% at 12 mL·hr-1 and can lead to potentially dangerous concentrations at 48 hr.